Annotation Error Decoder

This module allows to annotate Feign's interfaces with annotations to generate Exceptions based on error codes

To use AnnotationErrorDecoder with Feign, add the Annotation Error Decoder module to your classpath. Then, configure Feign to use the AnnotationErrorDecoder:

GitHub github = Feign.builder()
                     .errorDecoder(
                         AnnotationErrorDecoder.builderFor(GitHub.class).build()
                     )
                     .target(GitHub.class, "https://api.github.com");

Leveraging the annotations and priority order

For annotation decoding to work, the class must be annotated with @ErrorHandling tags or meta-annotations. The tags are valid in both the class level as well as method level. They will be treated from 'most specific' to 'least specific' in the following order:

• A code specific exception defined on the method
• A code specific exception defined on the class
• The default exception of the method
• The default exception of the class

@ErrorHandling(codeSpecific =
    {
        @ErrorCodes( codes = {401}, generate = UnAuthorizedException.class),
        @ErrorCodes( codes = {403}, generate = ForbiddenException.class),
        @ErrorCodes( codes = {404}, generate = UnknownItemException.class),
    },
    defaultException = ClassLevelDefaultException.class
)
interface GitHub {

    @ErrorHandling(codeSpecific =
        {
            @ErrorCodes( codes = {404}, generate = NonExistentRepoException.class),
            @ErrorCodes( codes = {502, 503, 504}, generate = RetryAfterCertainTimeException.class),
        },
        defaultException = FailedToGetContributorsException.class
    )
    @RequestLine("GET /repos/{owner}/{repo}/contributors")
    List<Contributor> contributors(@Param("owner") String owner, @Param("repo") String repo);
}

In the above example, error responses to 'contributors' would hence be mapped as follows by status codes:

Code	Exception	Reason
401	UnAuthorizedException	from Class definition
403	ForbiddenException	from Class definition
404	NonExistenRepoException	from Method definition, note that the class generic exception won't be thrown here
502,503,504	RetryAfterCertainTimeException	from method definition. Note that you can have multiple error codes generate the same type of exception
Any Other	FailedToGetContributorsException	from Method default

For a class level default exception to be thrown, the method must not have a defaultException defined, nor must the error code be mapped at either the method or class level.

If the return code cannot be mapped to any code and no default exceptions have been configured, then the decoder will drop to a default decoder (by default, the standard one provided by feign). You can change the default drop-into decoder as follows:

GitHub github = Feign.builder()
                     .errorDecoder(
                         AnnotationErrorDecoder.builderFor(GitHub.class)
                            .withDefaultDecoder(new MyOtherErrorDecoder())
                            .build()
                     )
                     .target(GitHub.class, "https://api.github.com");

Complex Exceptions

Any exception can be used if they have a default constructor:

class DefaultConstructorException extends Exception {}

However, if you want to have parameters (such as the feign.Request object or response body or response headers), you have to annotate its constructor appropriately (the body annotation is optional, provided there aren't paramters which will clash)

All the following examples are valid exceptions:

class JustBody extends Exception {

    @FeignExceptionConstructor
    public JustBody(String body) {

    }
}
class JustRequest extends Exception {

    @FeignExceptionConstructor
    public JustRequest(Request request) {

    }
}
class RequestAndResponseBody extends Exception {

    @FeignExceptionConstructor
    public RequestAndResponseBody(Request request, String body) {

    }
}
//Headers must be of type Map<String, Collection<String>>
class BodyAndHeaders extends Exception {

    @FeignExceptionConstructor
    public BodyAndHeaders(@ResponseBody String body, @ResponseHeaders Map<String, Collection<String>> headers) {

    }
}
class RequestAndResponseBodyAndHeaders extends Exception {

    @FeignExceptionConstructor
    public RequestAndResponseBodyAndHeaders(Request request, @ResponseBody String body, @ResponseHeaders Map<String, Collection<String>> headers) {

    }
}
class JustHeaders extends Exception {

    @FeignExceptionConstructor
    public JustHeaders(@ResponseHeaders Map<String, Collection<String>> headers) {

    }
}

If you want to have the body decoded, you'll need to pass a decoder at construction time (just as for normal responses):

GitHub github = Feign.builder()
                     .errorDecoder(
                         AnnotationErrorDecoder.builderFor(GitHub.class)
                            .withResponseBodyDecoder(new JacksonDecoder())
                            .build()
                     )
                     .target(GitHub.class, "https://api.github.com");

This will enable you to create exceptions where the body is a complex pojo:

class ComplexPojoException extends Exception {

    @FeignExceptionConstructor
    public ComplexPojoException(GithubExceptionResponse body) {
        if (body != null) {
            // extract data
        } else {
            // fallback code
        }
    }
}
//The pojo can then be anything you'd like provided the decoder can manage it
class GithubExceptionResponse {
    public String message;
    public int githubCode;
    public List<String> urlsForHelp;
}

It's worth noting that at setup/startup time, the generators are checked with a null value of the body. If you don't do the null-checker, you'll get an NPE and startup will fail.

Inheriting from other interface definitions

You can create a client interface that inherits from a different one. However, there are some limitations that you should be aware of (for most cases, these shouldn't be an issue):

• The inheritance is not natural java inheritance of annotations - as these don't work on interfaces
• Instead, the error looks at the class and if it finds the @ErrorHandling annotation, it uses that one.
• If not, it will look at all the interfaces the main interface extends - but it does so in the order the java API gives it - so order is not guaranteed.
• If it finds the annotation in one of those parents, it uses that definition, without looking at any other
• That means that if more than one interface was extended which contained the @ErrorHandling annotation, we can't really guarantee which one of the parents will be selected and you should really do handling at the child interface
  • so far, the java API seems to return in order of definition after the extends, but it's a really bad practice if you have to depend on that... so our suggestion: don't.

That means that as long as you only ever extend from a base interface (where you may decide that all 404's are "NotFoundException", for example) then you should be ok. But if you get complex in polymorphism, all bets are off - so don't go crazy!

Example: In the following code:

• The base FeignClientBase interface defines a default set of exceptions at class level
• the GitHub1 and GitHub2 interfaces will inherit the class-level error handling, which means that any 401/403/404 will be handled correctly (provided the method doesn't specify a more specific exception)
• the GitHub3 interface however, by defining its own error handling, will handle all 401's, but not the 403/404's since there's no merging/etc (not really in the plan to implement either...)

@ErrorHandling(codeSpecific =
    {
        @ErrorCodes( codes = {401}, generate = UnAuthorizedException.class),
        @ErrorCodes( codes = {403}, generate = ForbiddenException.class),
        @ErrorCodes( codes = {404}, generate = UnknownItemException.class),
    },
    defaultException = ClassLevelDefaultException.class
)
interface FeignClientBase {}

interface GitHub1 extends FeignClientBase {

    @ErrorHandling(codeSpecific =
        {
            @ErrorCodes( codes = {404}, generate = NonExistentRepoException.class),
            @ErrorCodes( codes = {502, 503, 504}, generate = RetryAfterCertainTimeException.class),
        },
        defaultException = FailedToGetContributorsException.class
    )
    @RequestLine("GET /repos/{owner}/{repo}/contributors")
    List<Contributor> contributors(@Param("owner") String owner, @Param("repo") String repo);
}

interface GitHub2 extends FeignClientBase {

    @ErrorHandling(codeSpecific =
        {
            @ErrorCodes( codes = {404}, generate = NonExistentRepoException.class),
            @ErrorCodes( codes = {502, 503, 504}, generate = RetryAfterCertainTimeException.class),
        },
        defaultException = FailedToGetContributorsException.class
    )
    @RequestLine("GET /repos/{owner}/{repo}/contributors")
    List<Contributor> contributors(@Param("owner") String owner, @Param("repo") String repo);
}

@ErrorHandling(codeSpecific =
    {
        @ErrorCodes( codes = {401}, generate = UnAuthorizedException.class)
    },
    defaultException = ClassLevelDefaultException.class
)
interface GitHub3 extends FeignClientBase {

    @ErrorHandling(codeSpecific =
        {
            @ErrorCodes( codes = {404}, generate = NonExistentRepoException.class),
            @ErrorCodes( codes = {502, 503, 504}, generate = RetryAfterCertainTimeException.class),
        },
        defaultException = FailedToGetContributorsException.class
    )
    @RequestLine("GET /repos/{owner}/{repo}/contributors")
    List<Contributor> contributors(@Param("owner") String owner, @Param("repo") String repo);
}

Meta-annotations

When you want to share the same configuration of one @ErrorHandling annotation the @ErrorHandling annotation can be moved to a meta-annotation. Then later on this meta-annotation can be used on a method or at class level to reduce the amount duplicated code. A meta-annotation is a special annotation that contains the @ErrorHandling annotation and possibly other annotations, e.g. Spring-Rest annotations.

There are some limitations and rules to keep in mind when using meta-annotation:

• inheritance for meta-annotations when using interface inheritance is supported and is following the same rules as for interface inheritance (see above)
  • @ErrorHandling has precedence over any meta-annotation when placed together on a class or method
  • a meta-annotation on a child interface (method or class) has precedence over the error handling defined in the parent interface
• having a meta-annotation on a meta-annotation is not supported, only the annotations on a type are checked for a @ErrorHandling
• when multiple meta-annotations with an @ErrorHandling annotation are present on a class or method the first one which is returned by java API is used to figure out the error handling, the others are not considered, so it is advisable to have only one meta-annotation on each method or class as the order is not guaranteed.
• no merging of configurations is supported, e.g. multiple meta-annotations on the same type, meta-annotation with @ErrorHandling on the same type

Example:

Let's assume multiple methods need to handle the response-code 404 in the same way but differently what is specified in the @ErrorHandling annotation on the class-level. In that case, to avoid also duplicate annotation definitions on the affected methods a meta-annotation can reduce the amount of code to be written to handle this 404 differently.

In the following code the status-code 404 is handled on a class level which throws an UnknownItemException for all methods inside this interface. For the methods contributors and languages a different exceptions needs to be thrown, in this case it is a NoDataFoundException. The teamsmethod will still use the exception defined by the class-level error handling annotation. To simplify the code a meta-annotation can be created and be used in the interface to keep the interface small and readable.

@ErrorHandling(
    codeSpecific = {
        @ErrorCodes(codes = {404}, generate = NoDataFoundException.class),
    },
    defaultException = GithubRemoteException.class)
@Retention(RetentionPolicy.RUNTIME)
@interface NoDataErrorHandling {
}

Having this meta-annotation in place it can be used to transform the interface into a much smaller one, keeping the same behavior.

• contributers will throw a NoDataFoundException for status code 404 as defined on method level and a GithubRemoteException for all other status codes
• languages will throw a NoDataFoundException for status code 404 as defined on method level and a GithubRemoteException for all other status codes
• teams will throw a UnknownItemException for status code 404 as defined on class level and a
  ClassLevelDefaultException for all other status codes

Before:

@ErrorHandling(codeSpecific =
    {
        @ErrorCodes( codes = {404}, generate = UnknownItemException.class)
    },
    defaultException = ClassLevelDefaultException.class
)
interface GitHub {
    @ErrorHandling(codeSpecific =
        {
            @ErrorCodes( codes = {404}, generate = NoDataFoundException.class)
        },
        defaultException = GithubRemoteException.class
    )
    @RequestLine("GET /repos/{owner}/{repo}/contributors")
    List<Contributor> contributors(@Param("owner") String owner, @Param("repo") String repo);

    @ErrorHandling(codeSpecific =
        {
            @ErrorCodes( codes = {404}, generate = NoDataFoundException.class)
        },
        defaultException = GithubRemoteException.class
    )
    @RequestLine("GET /repos/{owner}/{repo}/languages")
    Map<String, Integer> languages(@Param("owner") String owner, @Param("repo") String repo);
    
    @ErrorHandling
    @RequestLine("GET /repos/{owner}/{repo}/team")
    List<Team> languages(@Param("owner") String owner, @Param("repo") String repo);
}

After:

@ErrorHandling(codeSpecific =
    {
        @ErrorCodes( codes = {404}, generate = UnknownItemException.class)
    },
    defaultException = ClassLevelDefaultException.class
)
interface GitHub {
    @NoDataErrorHandling
    @RequestLine("GET /repos/{owner}/{repo}/contributors")
    List<Contributor> contributors(@Param("owner") String owner, @Param("repo") String repo);

    @NoDataErrorHandling
    @RequestLine("GET /repos/{owner}/{repo}/languages")
    Map<String, Integer> languages(@Param("owner") String owner, @Param("repo") String repo);
    
    @ErrorHandling
    @RequestLine("GET /repos/{owner}/{repo}/team")
    List<Team> languages(@Param("owner") String owner, @Param("repo") String repo);
}