-
Notifications
You must be signed in to change notification settings - Fork 420
/
build-great-native-cli-apps-in-java-with-graalvm-and-picocli.html
1375 lines (1351 loc) · 75.9 KB
/
build-great-native-cli-apps-in-java-with-graalvm-and-picocli.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="generator" content="Asciidoctor 2.0.18">
<title>Build great native CLI apps in Java with GraalVM and Picocli</title>
<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700">
<style>
/*! Asciidoctor default stylesheet | MIT License | https://asciidoctor.org */
/* Uncomment the following line when using as a custom stylesheet */
/* @import "https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700"; */
html{font-family:sans-serif;-webkit-text-size-adjust:100%}
a{background:none}
a:focus{outline:thin dotted}
a:active,a:hover{outline:0}
h1{font-size:2em;margin:.67em 0}
b,strong{font-weight:bold}
abbr{font-size:.9em}
abbr[title]{cursor:help;border-bottom:1px dotted #dddddf;text-decoration:none}
dfn{font-style:italic}
hr{height:0}
mark{background:#ff0;color:#000}
code,kbd,pre,samp{font-family:monospace;font-size:1em}
pre{white-space:pre-wrap}
q{quotes:"\201C" "\201D" "\2018" "\2019"}
small{font-size:80%}
sub,sup{font-size:75%;line-height:0;position:relative;vertical-align:baseline}
sup{top:-.5em}
sub{bottom:-.25em}
img{border:0}
svg:not(:root){overflow:hidden}
figure{margin:0}
audio,video{display:inline-block}
audio:not([controls]){display:none;height:0}
fieldset{border:1px solid silver;margin:0 2px;padding:.35em .625em .75em}
legend{border:0;padding:0}
button,input,select,textarea{font-family:inherit;font-size:100%;margin:0}
button,input{line-height:normal}
button,select{text-transform:none}
button,html input[type=button],input[type=reset],input[type=submit]{-webkit-appearance:button;cursor:pointer}
button[disabled],html input[disabled]{cursor:default}
input[type=checkbox],input[type=radio]{padding:0}
button::-moz-focus-inner,input::-moz-focus-inner{border:0;padding:0}
textarea{overflow:auto;vertical-align:top}
table{border-collapse:collapse;border-spacing:0}
*,::before,::after{box-sizing:border-box}
html,body{font-size:100%}
body{background:#fff;color:rgba(0,0,0,.8);padding:0;margin:0;font-family:"Noto Serif","DejaVu Serif",serif;line-height:1;position:relative;cursor:auto;-moz-tab-size:4;-o-tab-size:4;tab-size:4;word-wrap:anywhere;-moz-osx-font-smoothing:grayscale;-webkit-font-smoothing:antialiased}
a:hover{cursor:pointer}
img,object,embed{max-width:100%;height:auto}
object,embed{height:100%}
img{-ms-interpolation-mode:bicubic}
.left{float:left!important}
.right{float:right!important}
.text-left{text-align:left!important}
.text-right{text-align:right!important}
.text-center{text-align:center!important}
.text-justify{text-align:justify!important}
.hide{display:none}
img,object,svg{display:inline-block;vertical-align:middle}
textarea{height:auto;min-height:50px}
select{width:100%}
.subheader,.admonitionblock td.content>.title,.audioblock>.title,.exampleblock>.title,.imageblock>.title,.listingblock>.title,.literalblock>.title,.stemblock>.title,.openblock>.title,.paragraph>.title,.quoteblock>.title,table.tableblock>.title,.verseblock>.title,.videoblock>.title,.dlist>.title,.olist>.title,.ulist>.title,.qlist>.title,.hdlist>.title{line-height:1.45;color:#7a2518;font-weight:400;margin-top:0;margin-bottom:.25em}
div,dl,dt,dd,ul,ol,li,h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6,pre,form,p,blockquote,th,td{margin:0;padding:0}
a{color:#2156a5;text-decoration:underline;line-height:inherit}
a:hover,a:focus{color:#1d4b8f}
a img{border:0}
p{line-height:1.6;margin-bottom:1.25em;text-rendering:optimizeLegibility}
p aside{font-size:.875em;line-height:1.35;font-style:italic}
h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{font-family:"Open Sans","DejaVu Sans",sans-serif;font-weight:300;font-style:normal;color:#ba3925;text-rendering:optimizeLegibility;margin-top:1em;margin-bottom:.5em;line-height:1.0125em}
h1 small,h2 small,h3 small,#toctitle small,.sidebarblock>.content>.title small,h4 small,h5 small,h6 small{font-size:60%;color:#e99b8f;line-height:0}
h1{font-size:2.125em}
h2{font-size:1.6875em}
h3,#toctitle,.sidebarblock>.content>.title{font-size:1.375em}
h4,h5{font-size:1.125em}
h6{font-size:1em}
hr{border:solid #dddddf;border-width:1px 0 0;clear:both;margin:1.25em 0 1.1875em}
em,i{font-style:italic;line-height:inherit}
strong,b{font-weight:bold;line-height:inherit}
small{font-size:60%;line-height:inherit}
code{font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;font-weight:400;color:rgba(0,0,0,.9)}
ul,ol,dl{line-height:1.6;margin-bottom:1.25em;list-style-position:outside;font-family:inherit}
ul,ol{margin-left:1.5em}
ul li ul,ul li ol{margin-left:1.25em;margin-bottom:0}
ul.circle{list-style-type:circle}
ul.disc{list-style-type:disc}
ul.square{list-style-type:square}
ul.circle ul:not([class]),ul.disc ul:not([class]),ul.square ul:not([class]){list-style:inherit}
ol li ul,ol li ol{margin-left:1.25em;margin-bottom:0}
dl dt{margin-bottom:.3125em;font-weight:bold}
dl dd{margin-bottom:1.25em}
blockquote{margin:0 0 1.25em;padding:.5625em 1.25em 0 1.1875em;border-left:1px solid #ddd}
blockquote,blockquote p{line-height:1.6;color:rgba(0,0,0,.85)}
@media screen and (min-width:768px){h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{line-height:1.2}
h1{font-size:2.75em}
h2{font-size:2.3125em}
h3,#toctitle,.sidebarblock>.content>.title{font-size:1.6875em}
h4{font-size:1.4375em}}
table{background:#fff;margin-bottom:1.25em;border:1px solid #dedede;word-wrap:normal}
table thead,table tfoot{background:#f7f8f7}
table thead tr th,table thead tr td,table tfoot tr th,table tfoot tr td{padding:.5em .625em .625em;font-size:inherit;color:rgba(0,0,0,.8);text-align:left}
table tr th,table tr td{padding:.5625em .625em;font-size:inherit;color:rgba(0,0,0,.8)}
table tr.even,table tr.alt{background:#f8f8f7}
table thead tr th,table tfoot tr th,table tbody tr td,table tr td,table tfoot tr td{line-height:1.6}
h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{line-height:1.2;word-spacing:-.05em}
h1 strong,h2 strong,h3 strong,#toctitle strong,.sidebarblock>.content>.title strong,h4 strong,h5 strong,h6 strong{font-weight:400}
.center{margin-left:auto;margin-right:auto}
.stretch{width:100%}
.clearfix::before,.clearfix::after,.float-group::before,.float-group::after{content:" ";display:table}
.clearfix::after,.float-group::after{clear:both}
:not(pre).nobreak{word-wrap:normal}
:not(pre).nowrap{white-space:nowrap}
:not(pre).pre-wrap{white-space:pre-wrap}
:not(pre):not([class^=L])>code{font-size:.9375em;font-style:normal!important;letter-spacing:0;padding:.1em .5ex;word-spacing:-.15em;background:#f7f7f8;border-radius:4px;line-height:1.45;text-rendering:optimizeSpeed}
pre{color:rgba(0,0,0,.9);font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;line-height:1.45;text-rendering:optimizeSpeed}
pre code,pre pre{color:inherit;font-size:inherit;line-height:inherit}
pre>code{display:block}
pre.nowrap,pre.nowrap pre{white-space:pre;word-wrap:normal}
em em{font-style:normal}
strong strong{font-weight:400}
.keyseq{color:rgba(51,51,51,.8)}
kbd{font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;display:inline-block;color:rgba(0,0,0,.8);font-size:.65em;line-height:1.45;background:#f7f7f7;border:1px solid #ccc;border-radius:3px;box-shadow:0 1px 0 rgba(0,0,0,.2),inset 0 0 0 .1em #fff;margin:0 .15em;padding:.2em .5em;vertical-align:middle;position:relative;top:-.1em;white-space:nowrap}
.keyseq kbd:first-child{margin-left:0}
.keyseq kbd:last-child{margin-right:0}
.menuseq,.menuref{color:#000}
.menuseq b:not(.caret),.menuref{font-weight:inherit}
.menuseq{word-spacing:-.02em}
.menuseq b.caret{font-size:1.25em;line-height:.8}
.menuseq i.caret{font-weight:bold;text-align:center;width:.45em}
b.button::before,b.button::after{position:relative;top:-1px;font-weight:400}
b.button::before{content:"[";padding:0 3px 0 2px}
b.button::after{content:"]";padding:0 2px 0 3px}
p a>code:hover{color:rgba(0,0,0,.9)}
#header,#content,#footnotes,#footer{width:100%;margin:0 auto;max-width:62.5em;*zoom:1;position:relative;padding-left:.9375em;padding-right:.9375em}
#header::before,#header::after,#content::before,#content::after,#footnotes::before,#footnotes::after,#footer::before,#footer::after{content:" ";display:table}
#header::after,#content::after,#footnotes::after,#footer::after{clear:both}
#content{margin-top:1.25em}
#content::before{content:none}
#header>h1:first-child{color:rgba(0,0,0,.85);margin-top:2.25rem;margin-bottom:0}
#header>h1:first-child+#toc{margin-top:8px;border-top:1px solid #dddddf}
#header>h1:only-child,body.toc2 #header>h1:nth-last-child(2){border-bottom:1px solid #dddddf;padding-bottom:8px}
#header .details{border-bottom:1px solid #dddddf;line-height:1.45;padding-top:.25em;padding-bottom:.25em;padding-left:.25em;color:rgba(0,0,0,.6);display:flex;flex-flow:row wrap}
#header .details span:first-child{margin-left:-.125em}
#header .details span.email a{color:rgba(0,0,0,.85)}
#header .details br{display:none}
#header .details br+span::before{content:"\00a0\2013\00a0"}
#header .details br+span.author::before{content:"\00a0\22c5\00a0";color:rgba(0,0,0,.85)}
#header .details br+span#revremark::before{content:"\00a0|\00a0"}
#header #revnumber{text-transform:capitalize}
#header #revnumber::after{content:"\00a0"}
#content>h1:first-child:not([class]){color:rgba(0,0,0,.85);border-bottom:1px solid #dddddf;padding-bottom:8px;margin-top:0;padding-top:1rem;margin-bottom:1.25rem}
#toc{border-bottom:1px solid #e7e7e9;padding-bottom:.5em}
#toc>ul{margin-left:.125em}
#toc ul.sectlevel0>li>a{font-style:italic}
#toc ul.sectlevel0 ul.sectlevel1{margin:.5em 0}
#toc ul{font-family:"Open Sans","DejaVu Sans",sans-serif;list-style-type:none}
#toc li{line-height:1.3334;margin-top:.3334em}
#toc a{text-decoration:none}
#toc a:active{text-decoration:underline}
#toctitle{color:#7a2518;font-size:1.2em}
@media screen and (min-width:768px){#toctitle{font-size:1.375em}
body.toc2{padding-left:15em;padding-right:0}
#toc.toc2{margin-top:0!important;background:#f8f8f7;position:fixed;width:15em;left:0;top:0;border-right:1px solid #e7e7e9;border-top-width:0!important;border-bottom-width:0!important;z-index:1000;padding:1.25em 1em;height:100%;overflow:auto}
#toc.toc2 #toctitle{margin-top:0;margin-bottom:.8rem;font-size:1.2em}
#toc.toc2>ul{font-size:.9em;margin-bottom:0}
#toc.toc2 ul ul{margin-left:0;padding-left:1em}
#toc.toc2 ul.sectlevel0 ul.sectlevel1{padding-left:0;margin-top:.5em;margin-bottom:.5em}
body.toc2.toc-right{padding-left:0;padding-right:15em}
body.toc2.toc-right #toc.toc2{border-right-width:0;border-left:1px solid #e7e7e9;left:auto;right:0}}
@media screen and (min-width:1280px){body.toc2{padding-left:20em;padding-right:0}
#toc.toc2{width:20em}
#toc.toc2 #toctitle{font-size:1.375em}
#toc.toc2>ul{font-size:.95em}
#toc.toc2 ul ul{padding-left:1.25em}
body.toc2.toc-right{padding-left:0;padding-right:20em}}
#content #toc{border:1px solid #e0e0dc;margin-bottom:1.25em;padding:1.25em;background:#f8f8f7;border-radius:4px}
#content #toc>:first-child{margin-top:0}
#content #toc>:last-child{margin-bottom:0}
#footer{max-width:none;background:rgba(0,0,0,.8);padding:1.25em}
#footer-text{color:hsla(0,0%,100%,.8);line-height:1.44}
#content{margin-bottom:.625em}
.sect1{padding-bottom:.625em}
@media screen and (min-width:768px){#content{margin-bottom:1.25em}
.sect1{padding-bottom:1.25em}}
.sect1:last-child{padding-bottom:0}
.sect1+.sect1{border-top:1px solid #e7e7e9}
#content h1>a.anchor,h2>a.anchor,h3>a.anchor,#toctitle>a.anchor,.sidebarblock>.content>.title>a.anchor,h4>a.anchor,h5>a.anchor,h6>a.anchor{position:absolute;z-index:1001;width:1.5ex;margin-left:-1.5ex;display:block;text-decoration:none!important;visibility:hidden;text-align:center;font-weight:400}
#content h1>a.anchor::before,h2>a.anchor::before,h3>a.anchor::before,#toctitle>a.anchor::before,.sidebarblock>.content>.title>a.anchor::before,h4>a.anchor::before,h5>a.anchor::before,h6>a.anchor::before{content:"\00A7";font-size:.85em;display:block;padding-top:.1em}
#content h1:hover>a.anchor,#content h1>a.anchor:hover,h2:hover>a.anchor,h2>a.anchor:hover,h3:hover>a.anchor,#toctitle:hover>a.anchor,.sidebarblock>.content>.title:hover>a.anchor,h3>a.anchor:hover,#toctitle>a.anchor:hover,.sidebarblock>.content>.title>a.anchor:hover,h4:hover>a.anchor,h4>a.anchor:hover,h5:hover>a.anchor,h5>a.anchor:hover,h6:hover>a.anchor,h6>a.anchor:hover{visibility:visible}
#content h1>a.link,h2>a.link,h3>a.link,#toctitle>a.link,.sidebarblock>.content>.title>a.link,h4>a.link,h5>a.link,h6>a.link{color:#ba3925;text-decoration:none}
#content h1>a.link:hover,h2>a.link:hover,h3>a.link:hover,#toctitle>a.link:hover,.sidebarblock>.content>.title>a.link:hover,h4>a.link:hover,h5>a.link:hover,h6>a.link:hover{color:#a53221}
details,.audioblock,.imageblock,.literalblock,.listingblock,.stemblock,.videoblock{margin-bottom:1.25em}
details{margin-left:1.25rem}
details>summary{cursor:pointer;display:block;position:relative;line-height:1.6;margin-bottom:.625rem;outline:none;-webkit-tap-highlight-color:transparent}
details>summary::-webkit-details-marker{display:none}
details>summary::before{content:"";border:solid transparent;border-left:solid;border-width:.3em 0 .3em .5em;position:absolute;top:.5em;left:-1.25rem;transform:translateX(15%)}
details[open]>summary::before{border:solid transparent;border-top:solid;border-width:.5em .3em 0;transform:translateY(15%)}
details>summary::after{content:"";width:1.25rem;height:1em;position:absolute;top:.3em;left:-1.25rem}
.admonitionblock td.content>.title,.audioblock>.title,.exampleblock>.title,.imageblock>.title,.listingblock>.title,.literalblock>.title,.stemblock>.title,.openblock>.title,.paragraph>.title,.quoteblock>.title,table.tableblock>.title,.verseblock>.title,.videoblock>.title,.dlist>.title,.olist>.title,.ulist>.title,.qlist>.title,.hdlist>.title{text-rendering:optimizeLegibility;text-align:left;font-family:"Noto Serif","DejaVu Serif",serif;font-size:1rem;font-style:italic}
table.tableblock.fit-content>caption.title{white-space:nowrap;width:0}
.paragraph.lead>p,#preamble>.sectionbody>[class=paragraph]:first-of-type p{font-size:1.21875em;line-height:1.6;color:rgba(0,0,0,.85)}
.admonitionblock>table{border-collapse:separate;border:0;background:none;width:100%}
.admonitionblock>table td.icon{text-align:center;width:80px}
.admonitionblock>table td.icon img{max-width:none}
.admonitionblock>table td.icon .title{font-weight:bold;font-family:"Open Sans","DejaVu Sans",sans-serif;text-transform:uppercase}
.admonitionblock>table td.content{padding-left:1.125em;padding-right:1.25em;border-left:1px solid #dddddf;color:rgba(0,0,0,.6);word-wrap:anywhere}
.admonitionblock>table td.content>:last-child>:last-child{margin-bottom:0}
.exampleblock>.content{border:1px solid #e6e6e6;margin-bottom:1.25em;padding:1.25em;background:#fff;border-radius:4px}
.exampleblock>.content>:first-child{margin-top:0}
.exampleblock>.content>:last-child{margin-bottom:0}
.sidebarblock{border:1px solid #dbdbd6;margin-bottom:1.25em;padding:1.25em;background:#f3f3f2;border-radius:4px}
.sidebarblock>:first-child{margin-top:0}
.sidebarblock>:last-child{margin-bottom:0}
.sidebarblock>.content>.title{color:#7a2518;margin-top:0;text-align:center}
.exampleblock>.content>:last-child>:last-child,.exampleblock>.content .olist>ol>li:last-child>:last-child,.exampleblock>.content .ulist>ul>li:last-child>:last-child,.exampleblock>.content .qlist>ol>li:last-child>:last-child,.sidebarblock>.content>:last-child>:last-child,.sidebarblock>.content .olist>ol>li:last-child>:last-child,.sidebarblock>.content .ulist>ul>li:last-child>:last-child,.sidebarblock>.content .qlist>ol>li:last-child>:last-child{margin-bottom:0}
.literalblock pre,.listingblock>.content>pre{border-radius:4px;overflow-x:auto;padding:1em;font-size:.8125em}
@media screen and (min-width:768px){.literalblock pre,.listingblock>.content>pre{font-size:.90625em}}
@media screen and (min-width:1280px){.literalblock pre,.listingblock>.content>pre{font-size:1em}}
.literalblock pre,.listingblock>.content>pre:not(.highlight),.listingblock>.content>pre[class=highlight],.listingblock>.content>pre[class^="highlight "]{background:#f7f7f8}
.literalblock.output pre{color:#f7f7f8;background:rgba(0,0,0,.9)}
.listingblock>.content{position:relative}
.listingblock code[data-lang]::before{display:none;content:attr(data-lang);position:absolute;font-size:.75em;top:.425rem;right:.5rem;line-height:1;text-transform:uppercase;color:inherit;opacity:.5}
.listingblock:hover code[data-lang]::before{display:block}
.listingblock.terminal pre .command::before{content:attr(data-prompt);padding-right:.5em;color:inherit;opacity:.5}
.listingblock.terminal pre .command:not([data-prompt])::before{content:"$"}
.listingblock pre.highlightjs{padding:0}
.listingblock pre.highlightjs>code{padding:1em;border-radius:4px}
.listingblock pre.prettyprint{border-width:0}
.prettyprint{background:#f7f7f8}
pre.prettyprint .linenums{line-height:1.45;margin-left:2em}
pre.prettyprint li{background:none;list-style-type:inherit;padding-left:0}
pre.prettyprint li code[data-lang]::before{opacity:1}
pre.prettyprint li:not(:first-child) code[data-lang]::before{display:none}
table.linenotable{border-collapse:separate;border:0;margin-bottom:0;background:none}
table.linenotable td[class]{color:inherit;vertical-align:top;padding:0;line-height:inherit;white-space:normal}
table.linenotable td.code{padding-left:.75em}
table.linenotable td.linenos,pre.pygments .linenos{border-right:1px solid;opacity:.35;padding-right:.5em;-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none}
pre.pygments span.linenos{display:inline-block;margin-right:.75em}
.quoteblock{margin:0 1em 1.25em 1.5em;display:table}
.quoteblock:not(.excerpt)>.title{margin-left:-1.5em;margin-bottom:.75em}
.quoteblock blockquote,.quoteblock p{color:rgba(0,0,0,.85);font-size:1.15rem;line-height:1.75;word-spacing:.1em;letter-spacing:0;font-style:italic;text-align:justify}
.quoteblock blockquote{margin:0;padding:0;border:0}
.quoteblock blockquote::before{content:"\201c";float:left;font-size:2.75em;font-weight:bold;line-height:.6em;margin-left:-.6em;color:#7a2518;text-shadow:0 1px 2px rgba(0,0,0,.1)}
.quoteblock blockquote>.paragraph:last-child p{margin-bottom:0}
.quoteblock .attribution{margin-top:.75em;margin-right:.5ex;text-align:right}
.verseblock{margin:0 1em 1.25em}
.verseblock pre{font-family:"Open Sans","DejaVu Sans",sans-serif;font-size:1.15rem;color:rgba(0,0,0,.85);font-weight:300;text-rendering:optimizeLegibility}
.verseblock pre strong{font-weight:400}
.verseblock .attribution{margin-top:1.25rem;margin-left:.5ex}
.quoteblock .attribution,.verseblock .attribution{font-size:.9375em;line-height:1.45;font-style:italic}
.quoteblock .attribution br,.verseblock .attribution br{display:none}
.quoteblock .attribution cite,.verseblock .attribution cite{display:block;letter-spacing:-.025em;color:rgba(0,0,0,.6)}
.quoteblock.abstract blockquote::before,.quoteblock.excerpt blockquote::before,.quoteblock .quoteblock blockquote::before{display:none}
.quoteblock.abstract blockquote,.quoteblock.abstract p,.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{line-height:1.6;word-spacing:0}
.quoteblock.abstract{margin:0 1em 1.25em;display:block}
.quoteblock.abstract>.title{margin:0 0 .375em;font-size:1.15em;text-align:center}
.quoteblock.excerpt>blockquote,.quoteblock .quoteblock{padding:0 0 .25em 1em;border-left:.25em solid #dddddf}
.quoteblock.excerpt,.quoteblock .quoteblock{margin-left:0}
.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{color:inherit;font-size:1.0625rem}
.quoteblock.excerpt .attribution,.quoteblock .quoteblock .attribution{color:inherit;font-size:.85rem;text-align:left;margin-right:0}
p.tableblock:last-child{margin-bottom:0}
td.tableblock>.content{margin-bottom:1.25em;word-wrap:anywhere}
td.tableblock>.content>:last-child{margin-bottom:-1.25em}
table.tableblock,th.tableblock,td.tableblock{border:0 solid #dedede}
table.grid-all>*>tr>*{border-width:1px}
table.grid-cols>*>tr>*{border-width:0 1px}
table.grid-rows>*>tr>*{border-width:1px 0}
table.frame-all{border-width:1px}
table.frame-ends{border-width:1px 0}
table.frame-sides{border-width:0 1px}
table.frame-none>colgroup+*>:first-child>*,table.frame-sides>colgroup+*>:first-child>*{border-top-width:0}
table.frame-none>:last-child>:last-child>*,table.frame-sides>:last-child>:last-child>*{border-bottom-width:0}
table.frame-none>*>tr>:first-child,table.frame-ends>*>tr>:first-child{border-left-width:0}
table.frame-none>*>tr>:last-child,table.frame-ends>*>tr>:last-child{border-right-width:0}
table.stripes-all>*>tr,table.stripes-odd>*>tr:nth-of-type(odd),table.stripes-even>*>tr:nth-of-type(even),table.stripes-hover>*>tr:hover{background:#f8f8f7}
th.halign-left,td.halign-left{text-align:left}
th.halign-right,td.halign-right{text-align:right}
th.halign-center,td.halign-center{text-align:center}
th.valign-top,td.valign-top{vertical-align:top}
th.valign-bottom,td.valign-bottom{vertical-align:bottom}
th.valign-middle,td.valign-middle{vertical-align:middle}
table thead th,table tfoot th{font-weight:bold}
tbody tr th{background:#f7f8f7}
tbody tr th,tbody tr th p,tfoot tr th,tfoot tr th p{color:rgba(0,0,0,.8);font-weight:bold}
p.tableblock>code:only-child{background:none;padding:0}
p.tableblock{font-size:1em}
ol{margin-left:1.75em}
ul li ol{margin-left:1.5em}
dl dd{margin-left:1.125em}
dl dd:last-child,dl dd:last-child>:last-child{margin-bottom:0}
li p,ul dd,ol dd,.olist .olist,.ulist .ulist,.ulist .olist,.olist .ulist{margin-bottom:.625em}
ul.checklist,ul.none,ol.none,ul.no-bullet,ol.no-bullet,ol.unnumbered,ul.unstyled,ol.unstyled{list-style-type:none}
ul.no-bullet,ol.no-bullet,ol.unnumbered{margin-left:.625em}
ul.unstyled,ol.unstyled{margin-left:0}
li>p:empty:only-child::before{content:"";display:inline-block}
ul.checklist>li>p:first-child{margin-left:-1em}
ul.checklist>li>p:first-child>.fa-square-o:first-child,ul.checklist>li>p:first-child>.fa-check-square-o:first-child{width:1.25em;font-size:.8em;position:relative;bottom:.125em}
ul.checklist>li>p:first-child>input[type=checkbox]:first-child{margin-right:.25em}
ul.inline{display:flex;flex-flow:row wrap;list-style:none;margin:0 0 .625em -1.25em}
ul.inline>li{margin-left:1.25em}
.unstyled dl dt{font-weight:400;font-style:normal}
ol.arabic{list-style-type:decimal}
ol.decimal{list-style-type:decimal-leading-zero}
ol.loweralpha{list-style-type:lower-alpha}
ol.upperalpha{list-style-type:upper-alpha}
ol.lowerroman{list-style-type:lower-roman}
ol.upperroman{list-style-type:upper-roman}
ol.lowergreek{list-style-type:lower-greek}
.hdlist>table,.colist>table{border:0;background:none}
.hdlist>table>tbody>tr,.colist>table>tbody>tr{background:none}
td.hdlist1,td.hdlist2{vertical-align:top;padding:0 .625em}
td.hdlist1{font-weight:bold;padding-bottom:1.25em}
td.hdlist2{word-wrap:anywhere}
.literalblock+.colist,.listingblock+.colist{margin-top:-.5em}
.colist td:not([class]):first-child{padding:.4em .75em 0;line-height:1;vertical-align:top}
.colist td:not([class]):first-child img{max-width:none}
.colist td:not([class]):last-child{padding:.25em 0}
.thumb,.th{line-height:0;display:inline-block;border:4px solid #fff;box-shadow:0 0 0 1px #ddd}
.imageblock.left{margin:.25em .625em 1.25em 0}
.imageblock.right{margin:.25em 0 1.25em .625em}
.imageblock>.title{margin-bottom:0}
.imageblock.thumb,.imageblock.th{border-width:6px}
.imageblock.thumb>.title,.imageblock.th>.title{padding:0 .125em}
.image.left,.image.right{margin-top:.25em;margin-bottom:.25em;display:inline-block;line-height:0}
.image.left{margin-right:.625em}
.image.right{margin-left:.625em}
a.image{text-decoration:none;display:inline-block}
a.image object{pointer-events:none}
sup.footnote,sup.footnoteref{font-size:.875em;position:static;vertical-align:super}
sup.footnote a,sup.footnoteref a{text-decoration:none}
sup.footnote a:active,sup.footnoteref a:active{text-decoration:underline}
#footnotes{padding-top:.75em;padding-bottom:.75em;margin-bottom:.625em}
#footnotes hr{width:20%;min-width:6.25em;margin:-.25em 0 .75em;border-width:1px 0 0}
#footnotes .footnote{padding:0 .375em 0 .225em;line-height:1.3334;font-size:.875em;margin-left:1.2em;margin-bottom:.2em}
#footnotes .footnote a:first-of-type{font-weight:bold;text-decoration:none;margin-left:-1.05em}
#footnotes .footnote:last-of-type{margin-bottom:0}
#content #footnotes{margin-top:-.625em;margin-bottom:0;padding:.75em 0}
div.unbreakable{page-break-inside:avoid}
.big{font-size:larger}
.small{font-size:smaller}
.underline{text-decoration:underline}
.overline{text-decoration:overline}
.line-through{text-decoration:line-through}
.aqua{color:#00bfbf}
.aqua-background{background:#00fafa}
.black{color:#000}
.black-background{background:#000}
.blue{color:#0000bf}
.blue-background{background:#0000fa}
.fuchsia{color:#bf00bf}
.fuchsia-background{background:#fa00fa}
.gray{color:#606060}
.gray-background{background:#7d7d7d}
.green{color:#006000}
.green-background{background:#007d00}
.lime{color:#00bf00}
.lime-background{background:#00fa00}
.maroon{color:#600000}
.maroon-background{background:#7d0000}
.navy{color:#000060}
.navy-background{background:#00007d}
.olive{color:#606000}
.olive-background{background:#7d7d00}
.purple{color:#600060}
.purple-background{background:#7d007d}
.red{color:#bf0000}
.red-background{background:#fa0000}
.silver{color:#909090}
.silver-background{background:#bcbcbc}
.teal{color:#006060}
.teal-background{background:#007d7d}
.white{color:#bfbfbf}
.white-background{background:#fafafa}
.yellow{color:#bfbf00}
.yellow-background{background:#fafa00}
span.icon>.fa{cursor:default}
a span.icon>.fa{cursor:inherit}
.admonitionblock td.icon [class^="fa icon-"]{font-size:2.5em;text-shadow:1px 1px 2px rgba(0,0,0,.5);cursor:default}
.admonitionblock td.icon .icon-note::before{content:"\f05a";color:#19407c}
.admonitionblock td.icon .icon-tip::before{content:"\f0eb";text-shadow:1px 1px 2px rgba(155,155,0,.8);color:#111}
.admonitionblock td.icon .icon-warning::before{content:"\f071";color:#bf6900}
.admonitionblock td.icon .icon-caution::before{content:"\f06d";color:#bf3400}
.admonitionblock td.icon .icon-important::before{content:"\f06a";color:#bf0000}
.conum[data-value]{display:inline-block;color:#fff!important;background:rgba(0,0,0,.8);border-radius:50%;text-align:center;font-size:.75em;width:1.67em;height:1.67em;line-height:1.67em;font-family:"Open Sans","DejaVu Sans",sans-serif;font-style:normal;font-weight:bold}
.conum[data-value] *{color:#fff!important}
.conum[data-value]+b{display:none}
.conum[data-value]::after{content:attr(data-value)}
pre .conum[data-value]{position:relative;top:-.125em}
b.conum *{color:inherit!important}
.conum:not([data-value]):empty{display:none}
dt,th.tableblock,td.content,div.footnote{text-rendering:optimizeLegibility}
h1,h2,p,td.content,span.alt,summary{letter-spacing:-.01em}
p strong,td.content strong,div.footnote strong{letter-spacing:-.005em}
p,blockquote,dt,td.content,span.alt,summary{font-size:1.0625rem}
p{margin-bottom:1.25rem}
.sidebarblock p,.sidebarblock dt,.sidebarblock td.content,p.tableblock{font-size:1em}
.exampleblock>.content{background:#fffef7;border-color:#e0e0dc;box-shadow:0 1px 4px #e0e0dc}
.print-only{display:none!important}
@page{margin:1.25cm .75cm}
@media print{*{box-shadow:none!important;text-shadow:none!important}
html{font-size:80%}
a{color:inherit!important;text-decoration:underline!important}
a.bare,a[href^="#"],a[href^="mailto:"]{text-decoration:none!important}
a[href^="http:"]:not(.bare)::after,a[href^="https:"]:not(.bare)::after{content:"(" attr(href) ")";display:inline-block;font-size:.875em;padding-left:.25em}
abbr[title]{border-bottom:1px dotted}
abbr[title]::after{content:" (" attr(title) ")"}
pre,blockquote,tr,img,object,svg{page-break-inside:avoid}
thead{display:table-header-group}
svg{max-width:100%}
p,blockquote,dt,td.content{font-size:1em;orphans:3;widows:3}
h2,h3,#toctitle,.sidebarblock>.content>.title{page-break-after:avoid}
#header,#content,#footnotes,#footer{max-width:none}
#toc,.sidebarblock,.exampleblock>.content{background:none!important}
#toc{border-bottom:1px solid #dddddf!important;padding-bottom:0!important}
body.book #header{text-align:center}
body.book #header>h1:first-child{border:0!important;margin:2.5em 0 1em}
body.book #header .details{border:0!important;display:block;padding:0!important}
body.book #header .details span:first-child{margin-left:0!important}
body.book #header .details br{display:block}
body.book #header .details br+span::before{content:none!important}
body.book #toc{border:0!important;text-align:left!important;padding:0!important;margin:0!important}
body.book #toc,body.book #preamble,body.book h1.sect0,body.book .sect1>h2{page-break-before:always}
.listingblock code[data-lang]::before{display:block}
#footer{padding:0 .9375em}
.hide-on-print{display:none!important}
.print-only{display:block!important}
.hide-for-print{display:none!important}
.show-for-print{display:inherit!important}}
@media amzn-kf8,print{#header>h1:first-child{margin-top:1.25rem}
.sect1{padding:0!important}
.sect1+.sect1{border:0}
#footer{background:none}
#footer-text{color:rgba(0,0,0,.6);font-size:.9em}}
@media amzn-kf8{#header,#content,#footnotes,#footer{padding:0}}
</style>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
<style>
/*! Stylesheet for CodeRay to loosely match GitHub themes | MIT License */
pre.CodeRay{background:#f7f7f8}
.CodeRay .line-numbers{border-right:1px solid;opacity:.35;padding:0 .5em 0 0;-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none}
.CodeRay span.line-numbers{display:inline-block;margin-right:.75em}
.CodeRay .line-numbers strong{color:#000}
table.CodeRay{border-collapse:separate;border:0;margin-bottom:0;background:none}
table.CodeRay td{vertical-align:top;line-height:inherit}
table.CodeRay td.line-numbers{text-align:right}
table.CodeRay td.code{padding:0 0 0 .75em}
.CodeRay .debug{color:#fff!important;background:navy!important}
.CodeRay .annotation{color:#007}
.CodeRay .attribute-name{color:navy}
.CodeRay .attribute-value{color:#700}
.CodeRay .binary{color:#509}
.CodeRay .comment{color:#998;font-style:italic}
.CodeRay .char{color:#04d}
.CodeRay .char .content{color:#04d}
.CodeRay .char .delimiter{color:#039}
.CodeRay .class{color:#458;font-weight:bold}
.CodeRay .complex{color:#a08}
.CodeRay .constant,.CodeRay .predefined-constant{color:teal}
.CodeRay .color{color:#099}
.CodeRay .class-variable{color:#369}
.CodeRay .decorator{color:#b0b}
.CodeRay .definition{color:#099}
.CodeRay .delimiter{color:#000}
.CodeRay .doc{color:#970}
.CodeRay .doctype{color:#34b}
.CodeRay .doc-string{color:#d42}
.CodeRay .escape{color:#666}
.CodeRay .entity{color:#800}
.CodeRay .error{color:#808}
.CodeRay .exception{color:inherit}
.CodeRay .filename{color:#099}
.CodeRay .function{color:#900;font-weight:bold}
.CodeRay .global-variable{color:teal}
.CodeRay .hex{color:#058}
.CodeRay .integer,.CodeRay .float{color:#099}
.CodeRay .include{color:#555}
.CodeRay .inline{color:#000}
.CodeRay .inline .inline{background:#ccc}
.CodeRay .inline .inline .inline{background:#bbb}
.CodeRay .inline .inline-delimiter{color:#d14}
.CodeRay .inline-delimiter{color:#d14}
.CodeRay .important{color:#555;font-weight:bold}
.CodeRay .interpreted{color:#b2b}
.CodeRay .instance-variable{color:teal}
.CodeRay .label{color:#970}
.CodeRay .local-variable{color:#963}
.CodeRay .octal{color:#40e}
.CodeRay .predefined{color:#369}
.CodeRay .preprocessor{color:#579}
.CodeRay .pseudo-class{color:#555}
.CodeRay .directive{font-weight:bold}
.CodeRay .type{font-weight:bold}
.CodeRay .predefined-type{color:inherit}
.CodeRay .reserved,.CodeRay .keyword{color:#000;font-weight:bold}
.CodeRay .key{color:#808}
.CodeRay .key .delimiter{color:#606}
.CodeRay .key .char{color:#80f}
.CodeRay .value{color:#088}
.CodeRay .regexp .delimiter{color:#808}
.CodeRay .regexp .content{color:#808}
.CodeRay .regexp .modifier{color:#808}
.CodeRay .regexp .char{color:#d14}
.CodeRay .regexp .function{color:#404;font-weight:bold}
.CodeRay .string{color:#d20}
.CodeRay .string .string .string{background:#ffd0d0}
.CodeRay .string .content{color:#d14}
.CodeRay .string .char{color:#d14}
.CodeRay .string .delimiter{color:#d14}
.CodeRay .shell{color:#d14}
.CodeRay .shell .delimiter{color:#d14}
.CodeRay .symbol{color:#990073}
.CodeRay .symbol .content{color:#a60}
.CodeRay .symbol .delimiter{color:#630}
.CodeRay .tag{color:teal}
.CodeRay .tag-special{color:#d70}
.CodeRay .variable{color:#036}
.CodeRay .insert{background:#afa}
.CodeRay .delete{background:#faa}
.CodeRay .change{color:#aaf;background:#007}
.CodeRay .head{color:#f8f;background:#505}
.CodeRay .insert .insert{color:#080}
.CodeRay .delete .delete{color:#800}
.CodeRay .change .change{color:#66f}
.CodeRay .head .head{color:#f4f}
</style>
<style>
.hidden {
display: none;
}
.switch {
border-width: 1px 1px 0 1px;
border-style: solid;
border-color: #7a2518;
display: inline-block;
}
.switch--item {
padding: 10px;
background-color: #ffffff;
color: #7a2518;
display: inline-block;
cursor: pointer;
}
.switch--item.selected {
background-color: #7a2519;
color: #ffffff;
}
</style>
<script src="https://cdnjs.cloudflare.com/ajax/libs/zepto/1.2.0/zepto.min.js"></script>
<script type="text/javascript">
function addBlockSwitches() {
$('.primary').each(function() {
primary = $(this);
createSwitchItem(primary, createBlockSwitch(primary)).item.addClass("selected");
primary.children('.title').remove();
});
$('.secondary').each(function(idx, node) {
secondary = $(node);
primary = findPrimary(secondary);
switchItem = createSwitchItem(secondary, primary.children('.switch'));
switchItem.content.addClass('hidden');
findPrimary(secondary).append(switchItem.content);
secondary.remove();
});
}
function createBlockSwitch(primary) {
blockSwitch = $('<div class="switch"></div>');
primary.prepend(blockSwitch);
return blockSwitch;
}
function findPrimary(secondary) {
candidate = secondary.prev();
while (!candidate.is('.primary')) {
candidate = candidate.prev();
}
return candidate;
}
function createSwitchItem(block, blockSwitch) {
blockName = block.children('.title').text();
content = block.children('.content').first().append(block.next('.colist'));
item = $('<div class="switch--item">' + blockName + '</div>');
item.on('click', '', content, function(e) {
$(this).addClass('selected');
$(this).siblings().removeClass('selected');
e.data.siblings('.content').addClass('hidden');
e.data.removeClass('hidden');
});
blockSwitch.append(item);
return {'item': item, 'content': content};
}
$(addBlockSwitches);
</script>
</head>
<body class="article">
<div id="header">
<h1>Build great native CLI apps in Java with GraalVM and Picocli</h1>
<div class="details">
<span id="revnumber">version 4.7.4</span>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p><em>Let’s Put Java Back on the CLI Map</em></p>
</div>
<div class="paragraph">
<p>Takeaways:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Developers want to <strong>distribute</strong> their command line applications as a <strong>single native executable</strong>.</p>
</li>
<li>
<p><strong>GraalVM</strong> can compile your Java applications to single native images, but it has some limitations.</p>
</li>
<li>
<p><strong>Picocli</strong> is a modern library for writing CLI apps on the JVM which can help tackle GraalVM’s limitations including on Windows.</p>
</li>
<li>
<p>Setting up the GraalVM toolchain for creating native images on Windows is not well documented.</p>
</li>
</ul>
</div>
<div class="imageblock">
<div class="content">
<img src="images/fight-for-cli-apps.png" alt="fight for cli apps">
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_the_dream_java_executables">The Dream: Java Executables</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The <a href="https://golang.org/">Go</a> programming language has become popular for writing command line applications.
There may be many reasons for this, but one aspect where Go shines is the ability to compile a program to a single native executable file.
This makes the program much easier to distribute.</p>
</div>
<div class="paragraph">
<p>Java programs have traditionally been hard to distribute because they require a Java Virtual Machine to be installed on the target machine.
It is possible to bundle a recent JVM with the application but that adds roughly 200MB to the package size.
Things are moving in the right direction: the Java Module System (JPMS), introduced in Java 9, includes the <a href="https://docs.oracle.com/javase/9/tools/jlink.htm#JSWOR-GUID-CECAC52B-CFEE-46CB-8166-F17A8E9280E9">jlink</a> utility that allows an application to create a custom, minimized, JRE, which can be as small as 30-40MB, and Java 14 will include the <a href="https://openjdk.java.net/jeps/343">jpackage</a> utility, which can create an installer that includes this minimum JRE with your application.</p>
</div>
<div class="paragraph">
<p>Still, for command line applications an installer is not ideal.
Ideally, we <a href="https://dev.to/uilicious/why-we-migrated-our-cli-from-nodejs-to-golang-1ol8">want to distribute</a> our CLI utility as a "real" native executable without a packaged runtime.
GraalVM allows us to do this with programs written in Java.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_graalvm">GraalVM</h2>
<div class="sectionbody">
<div class="paragraph">
<p><a href="https://www.graalvm.org/">GraalVM</a> is a universal virtual machine that can run applications written in JavaScript, Python, Ruby, R, JVM-based languages like Java, Scala, Clojure, Kotlin, and LLVM-based languages such as C and C++.
One interesting aspect is that GraalVM allows you to mix programming languages: programs partially written in JavaScript, R, Python, or Ruby can be called from Java and can share data with each other.
Another feature is the ability to create a native image, and this is what we will explore in this article.</p>
</div>
<div class="sect2">
<h3 id="_graalvm_native_images">GraalVM Native Images</h3>
<div class="paragraph">
<p><a href="https://www.graalvm.org/docs/reference-manual/native-image/">GraalVM Native Image</a> allows you to ahead-of-time compile Java code to a standalone executable, called a native image.
This executable includes the application, the libraries, the JDK and does not run on the Java VM, but includes necessary components like memory management and thread scheduling from a different virtual machine, called “Substrate VM”.
Substrate VM is the name for the runtime components (like the deoptimizer, garbage collector, thread scheduling etc.). The resulting program has faster startup time and lower runtime memory overhead compared to a Java VM.</p>
</div>
</div>
<div class="sect2">
<h3 id="_native_image_limitations">Native Image Limitations</h3>
<div class="paragraph">
<p>To keep the implementation small and concise, and also to allow aggressive ahead-of-time optimizations, Native Image does not support all features of Java.
The <a href="https://www.graalvm.org/latest/reference-manual/native-image/metadata/Compatibility/#features-that-may-operate-differently-in-a-native-image">features</a> that may operate differently are documented in the GraalVM reference manual.</p>
</div>
<div class="paragraph">
<p>Two limitations are of particular interest:</p>
</div>
<div class="ulist">
<ul>
<li>
<p><a href="https://www.graalvm.org/latest/reference-manual/native-image/dynamic-features/Reflection/">reflection</a></p>
</li>
<li>
<p><a href="https://www.graalvm.org/latest/reference-manual/native-image/dynamic-features/Resources/">resources</a></p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Basically, to create a self-contained binary, the native image compiler needs to know up-front all the classes of your application, their dependencies, and the resources they use. Reflection and resource bundles often require configuration. We will see an example of this later on.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_picocli">Picocli</h2>
<div class="sectionbody">
<div class="paragraph">
<p><a href="https://github.com/remkop/picocli">Picocli</a> is a modern library and framework for building command line applications on the JVM.
It supports Java, Groovy, Kotlin and Scala.
It is less than 3 years old but has become quite popular with over 500,000 downloads per month.
The Groovy language uses picocli to implement its <code>CliBuilder</code> DSL.</p>
</div>
<div class="paragraph">
<p>Picocli aims to be “the easiest way to create rich command line applications that can run on and off the JVM”.
It offers colored output, TAB autocompletion, subcommands, and some unique features compared to other JVM CLI libraries such as negatable options, repeating composite argument groups, repeating subcommands and sophisticated handling of quoted arguments.
Its source code is in a single file so it can optionally be included as source to avoid adding a dependency.
Picocli prides itself on its extensive and meticulous <a href="https://picocli.info/">documentation</a>.</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/picocli.Demo.png" alt="picocli.Demo">
</div>
</div>
<div class="paragraph">
<p>Picocli uses reflection, so it is vulnerable to GraalVM’s Java native image limitations, but it offers an annotation processor that generates the configuration files that address this limitation at compile time.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_a_concrete_use_case">A Concrete Use Case</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Let’s take a concrete example of a command line utility that we will write in Java and compile to a single native executable.
Along the way we will look at some features of the picocli library that help make our utility easy to use.</p>
</div>
<div class="paragraph">
<p>We will build a <code>checksum</code> CLI utility, that takes a named option <code>-a</code> or <code>--algorithm</code>, and a positional parameter, which is the file whose checksum to compute.</p>
</div>
<div class="paragraph">
<p>We want our users to be able to use our Java <code>checksum</code> utility just like they use applications written in C++ or other languages. Something like this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ echo hi > hi.txt
$ checksum -a md5 hi.txt
764efa883dda1e11db47671c4a3bbd9e
$ checksum -a sha1 hi.txt
55ca6286e3e4f4fba5d0448333fa99fc5a404a73</pre>
</div>
</div>
<div class="paragraph">
<p>This is the minimum we expect from a command line application, but we are not going to be satisfied with a lowest common denominator app, we want to create a great CLI application that delights our users. What does that mean and how do we do it?</p>
</div>
<div class="sect2">
<h3 id="_great_cli_apps_are_helpful">Great CLI Apps are Helpful</h3>
<div class="paragraph">
<p>We made a trade-off: by choosing a command line interface (CLI) instead of a graphical user interface (GUI), our application is less easy to learn to use for new users. We can partially make up for that by providing good online help.</p>
</div>
<div class="paragraph">
<p>Our application should show a usage help message when the user requests help with the <code>-h</code> or <code>--help</code> option, or when invalid user input is specified. It should also show version information when requested with <code>-V</code> or <code>--version</code>. We will see how picocli makes it easy to do this.</p>
</div>
<div class="sect3">
<h4 id="_user_experience">User Experience</h4>
<div class="paragraph">
<p>We can make our application more user-friendly by using colors on supported platforms. This doesn’t just look good, it also reduces the cognitive load on the user: the contrast makes the important information like commands, options, and parameters stand out from the surrounding text.</p>
</div>
<div class="paragraph">
<p>The usage help message generated by a picocli-based application uses colors by default.
Our <code>checksum</code> example looks something like this:</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/checksum-usage-help.png" alt="checksum usage help">
</div>
</div>
<div class="paragraph">
<p>In general, applications should only output colors when they are used interactively; when executing a script we don’t want the log file cluttered with ANSI escape codes. Fortunately, picocli takes care of this automatically. This brings us to the next topic: good CLI apps are designed to be combined with other commands.</p>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_great_cli_apps_work_well_with_others">Great CLI Apps Work Well with Others</h3>
<div class="sect3">
<h4 id="_stdout_vs_stderr">Stdout vs stderr</h4>
<div class="paragraph">
<p>Many CLI utilities use the standard I/O streams so they can be combined with other utilities. The devil is often in the details. When the user <em>requested</em> help, the application should print the usage help message to standard output. This allows users to pipe the output to another tool like <code>grep</code> or <code>less</code>.</p>
</div>
<div class="paragraph">
<p>On the other hand, on invalid input, the error message and usage help message should be printed to the standard error stream: in case the output of our program is used as input for another program, we don’t want our error message to disrupt things.</p>
</div>
</div>
<div class="sect3">
<h4 id="_exit_code">Exit Code</h4>
<div class="paragraph">
<p>When your program ends, it returns an exit status code. An exit code of zero is often used to indicate success, and a non-zero exit code often indicates a failure of some kind.</p>
</div>
<div class="paragraph">
<p>This allows users to chain together a number of commands using <code>&&</code>, knowing that if any command in the sequence fails, the whole sequence will stop.</p>
</div>
<div class="paragraph">
<p>By default, picocli returns <code>2</code> for invalid user input, <code>1</code> when an exception occurred in the business logic of the application, and zero otherwise (when everything went well).
Of course it is easy to configure other exit codes in your application, but for our <code>checksum</code> example the default values are fine.</p>
</div>
<div class="paragraph">
<p>Note that the picocli library will not call <code>System.exit</code>; it just returns an integer and it is up to the application to call <code>System.exit</code> or not.</p>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_compact_code">Compact Code</h3>
<div class="paragraph">
<p>The above sections describe quite a bit of functionality.
You would think that this would require a lot of code to accomplish, but most of the "standard CLI behavior" is provided by the picocli library.
In our application, all we need to do is define our options and positional parameters, and implement the business logic by making our class <code>Callable</code> or <code>Runnable</code>.
We can bootstrap the application in our <code>main</code> method in one line of code:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="CodeRay highlight nowrap"><code data-lang="java"><table class="CodeRay"><tr>
<td class="line-numbers"><pre>1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
</pre></td>
<td class="code"><pre><span class="keyword">import</span> <span class="include">picocli.CommandLine</span>;
<span class="keyword">import</span> <span class="include">picocli.CommandLine.Command</span>;
<span class="keyword">import</span> <span class="include">picocli.CommandLine.Option</span>;
<span class="keyword">import</span> <span class="include">picocli.CommandLine.Parameters</span>;
<span class="keyword">import</span> <span class="include">java.io.File</span>;
<span class="keyword">import</span> <span class="include">java.math.BigInteger</span>;
<span class="keyword">import</span> <span class="include">java.nio.file.Files</span>;
<span class="keyword">import</span> <span class="include">java.security.MessageDigest</span>;
<span class="keyword">import</span> <span class="include">java.util.concurrent.Callable</span>;
<span class="annotation">@Command</span>(name = <span class="string"><span class="delimiter">"</span><span class="content">checksum</span><span class="delimiter">"</span></span>, mixinStandardHelpOptions = <span class="predefined-constant">true</span>,
version = <span class="string"><span class="delimiter">"</span><span class="content">checksum 4.0</span><span class="delimiter">"</span></span>,
description = <span class="string"><span class="delimiter">"</span><span class="content">Prints the checksum (MD5 by default) of a file to STDOUT.</span><span class="delimiter">"</span></span>)
<span class="type">class</span> <span class="class">CheckSum</span> <span class="directive">implements</span> <span class="predefined-type">Callable</span><<span class="predefined-type">Integer</span>> {
<span class="annotation">@Parameters</span>(index = <span class="string"><span class="delimiter">"</span><span class="content">0</span><span class="delimiter">"</span></span>, arity = <span class="string"><span class="delimiter">"</span><span class="content">1</span><span class="delimiter">"</span></span>,
description = <span class="string"><span class="delimiter">"</span><span class="content">The file whose checksum to calculate.</span><span class="delimiter">"</span></span>)
<span class="directive">private</span> <span class="predefined-type">File</span> file;
<span class="annotation">@Option</span>(names = {<span class="string"><span class="delimiter">"</span><span class="content">-a</span><span class="delimiter">"</span></span>, <span class="string"><span class="delimiter">"</span><span class="content">--algorithm</span><span class="delimiter">"</span></span>},
description = <span class="string"><span class="delimiter">"</span><span class="content">MD5, SHA-1, SHA-256, ...</span><span class="delimiter">"</span></span>)
<span class="directive">private</span> <span class="predefined-type">String</span> algorithm = <span class="string"><span class="delimiter">"</span><span class="content">MD5</span><span class="delimiter">"</span></span>;
<span class="comment">// this example implements Callable, so parsing, error handling</span>
<span class="comment">// and handling user requests for usage help or version help</span>
<span class="comment">// can be done with one line of code.</span>
<span class="directive">public</span> <span class="directive">static</span> <span class="type">void</span> main(<span class="predefined-type">String</span>... args) {
<span class="type">int</span> exitCode = <span class="keyword">new</span> CommandLine(<span class="keyword">new</span> CheckSum()).execute(args);
<span class="predefined-type">System</span>.exit(exitCode);
}
<span class="annotation">@Override</span>
<span class="directive">public</span> <span class="predefined-type">Integer</span> call() <span class="directive">throws</span> <span class="exception">Exception</span> { <span class="comment">// the business logic...</span>
<span class="type">byte</span><span class="type">[]</span> data = Files.readAllBytes(file.toPath());
<span class="type">byte</span><span class="type">[]</span> digest = <span class="predefined-type">MessageDigest</span>.getInstance(algorithm).digest(data);
<span class="predefined-type">String</span> format = <span class="string"><span class="delimiter">"</span><span class="content">%0</span><span class="delimiter">"</span></span> + (digest.length*<span class="integer">2</span>) + <span class="string"><span class="delimiter">"</span><span class="content">x%n</span><span class="delimiter">"</span></span>;
<span class="predefined-type">System</span>.out.printf(format, <span class="keyword">new</span> <span class="predefined-type">BigInteger</span>(<span class="integer">1</span>, digest));
<span class="keyword">return</span> <span class="integer">0</span>;
}
}</pre></td>
</tr></table></code></pre>
</div>
</div>
<div class="paragraph">
<p>We have an example of a realistic Java utility program.
Next, let’s take a look at turning it into a native executable.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_native_image">Native Image</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_reflection_configuration">Reflection Configuration</h3>
<div class="paragraph">
<p>We mentioned earlier that the native image compiler has some limitations: reflection is supported but <a href="https://www.graalvm.org/latest/reference-manual/native-image/overview/BuildConfiguration/">requires configuration</a>.</p>
</div>
<div class="paragraph">
<p>This impacts picocli-based applications: at runtime, picocli uses reflection to discover any <code>@Command</code>-annotated subcommands, and the <code>@Option</code> and <code>@Parameters</code>-annotated command options and positional parameters.</p>
</div>
<div class="paragraph">
<p>Therefore, we need to provide GraalVM with a configuration file specifying all annotated classes, methods and fields. Such a configuration file looks something like this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="CodeRay highlight"><code data-lang="json">[
{
<span class="key"><span class="delimiter">"</span><span class="content">name</span><span class="delimiter">"</span></span> : <span class="string"><span class="delimiter">"</span><span class="content">CheckSum</span><span class="delimiter">"</span></span>,
<span class="key"><span class="delimiter">"</span><span class="content">allDeclaredConstructors</span><span class="delimiter">"</span></span> : <span class="value">true</span>,
<span class="key"><span class="delimiter">"</span><span class="content">allPublicConstructors</span><span class="delimiter">"</span></span> : <span class="value">true</span>,
<span class="key"><span class="delimiter">"</span><span class="content">allDeclaredMethods</span><span class="delimiter">"</span></span> : <span class="value">true</span>,
<span class="key"><span class="delimiter">"</span><span class="content">allPublicMethods</span><span class="delimiter">"</span></span> : <span class="value">true</span>,
<span class="key"><span class="delimiter">"</span><span class="content">fields</span><span class="delimiter">"</span></span> : [
{ <span class="key"><span class="delimiter">"</span><span class="content">name</span><span class="delimiter">"</span></span> : <span class="string"><span class="delimiter">"</span><span class="content">algorithm</span><span class="delimiter">"</span></span> },
{ <span class="key"><span class="delimiter">"</span><span class="content">name</span><span class="delimiter">"</span></span> : <span class="string"><span class="delimiter">"</span><span class="content">file</span><span class="delimiter">"</span></span> }
]
},
{
<span class="key"><span class="delimiter">"</span><span class="content">name</span><span class="delimiter">"</span></span> : <span class="string"><span class="delimiter">"</span><span class="content">picocli.CommandLine$AutoHelpMixin</span><span class="delimiter">"</span></span>,
<span class="key"><span class="delimiter">"</span><span class="content">allDeclaredConstructors</span><span class="delimiter">"</span></span> : <span class="value">true</span>,
<span class="key"><span class="delimiter">"</span><span class="content">allPublicConstructors</span><span class="delimiter">"</span></span> : <span class="value">true</span>,
<span class="key"><span class="delimiter">"</span><span class="content">allDeclaredMethods</span><span class="delimiter">"</span></span> : <span class="value">true</span>,
<span class="key"><span class="delimiter">"</span><span class="content">allPublicMethods</span><span class="delimiter">"</span></span> : <span class="value">true</span>,
<span class="key"><span class="delimiter">"</span><span class="content">fields</span><span class="delimiter">"</span></span> : [
{ <span class="key"><span class="delimiter">"</span><span class="content">name</span><span class="delimiter">"</span></span> : <span class="string"><span class="delimiter">"</span><span class="content">helpRequested</span><span class="delimiter">"</span></span> },
{ <span class="key"><span class="delimiter">"</span><span class="content">name</span><span class="delimiter">"</span></span> : <span class="string"><span class="delimiter">"</span><span class="content">versionRequested</span><span class="delimiter">"</span></span> }
]
}
]</code></pre>
</div>
</div>
<div class="paragraph">
<p>This quickly becomes quite cumbersome for utilities with many options, but fortunately we don’t need to do this by hand.</p>
</div>
</div>
<div class="sect2">
<h3 id="_picocli_annotation_processor">Picocli Annotation Processor</h3>
<div class="paragraph">
<p>The <code>picocli-codegen</code> module includes an annotation processor that can build a model from the picocli annotations at compile time rather than at runtime.</p>
</div>
<div class="paragraph">
<p>The annotation processor generates Graal configuration files under <code>META-INF/native-image/picocli-generated/$project</code> during compilation, to be included in the application jar.
This includes configuration files for <a href="https://www.graalvm.org/latest/reference-manual/native-image/dynamic-features/Reflection/">reflection</a>, <a href="https://www.graalvm.org/latest/reference-manual/native-image/dynamic-features/Resources/">resources</a> and <a href="https://www.graalvm.org/latest/reference-manual/native-image/dynamic-features/DynamicProxy/">dynamic proxies</a>.
By embedding these configuration files, your jar is instantly Graal-enabled.
In most cases no further configuration is needed when generating a native image.</p>
</div>
<div class="paragraph">
<p>As a bonus, the annotation processor shows errors for invalid annotations or attributes immediately when you compile, instead of during testing at runtime, resulting in shorter feedback cycles.</p>
</div>
<div class="paragraph">
<p>So, all we need to do is compile our <code>CheckSum.java</code> source file with the <code>picocli-codegen</code> jar on the classpath:</p>
</div>
<div class="listingblock">
<div class="title">Compiling <code>CheckSum.java</code> and creating a <code>checksum.jar</code> on Linux. Replace the <code>:</code> path separator with <code>;</code> for these commands to work on Windows.</div>
<div class="content">
<pre>mkdir classes
javac -cp .:picocli-4.2.0.jar:picocli-codegen-4.2.0.jar -d classes CheckSum.java
cd classes && jar -cvef CheckSum ../checksum.jar * && cd ..</pre>
</div>
</div>
<div class="paragraph">
<p>You can see the generated configuration files are in the <code>META-INF/native-image/picocli-generated/</code> directory inside the jar:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>jar -tf checksum.jar
META-INF/
META-INF/MANIFEST.MF
CheckSum.class
META-INF/native-image/
META-INF/native-image/picocli-generated/
META-INF/native-image/picocli-generated/proxy-config.json
META-INF/native-image/picocli-generated/reflect-config.json