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_posts/2020-03-19-what-is deadlock.md

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+---
+layout: post
+title: 死锁
+tags: Java基础
+author: neal
+---
+* content
+{:toc}
+
+## 锁顺序死锁
+例如有两个方法分别持有锁`L`,`R`。A线程获得L锁之后申请R锁，同时B线程获得了R锁申请L锁，
+由于两个两个线程调用顺序不一致导致死锁；
+这种情况需要在编码时避免多个线程以不同的顺序请求相同的锁；
+
+## 动态的锁顺序死锁
+例如
+```java
+public voidd transferMoney(Account fromAccount, Account toAccount, BigDecimal amount){
+	synchronized(fromAccount){
+		synchronized(toAcccount){
+			fromAccount.debit(toAcccount);
+			toAcccount.credit(fromAccount);
+		}
+	}
+}
+```
+看似每个线程都是按照相同的顺序获得锁，其实锁顺序是动态变化的，如果一个线程由A向B转账，
+同时另一个线程由B向A转账，就会出现顺序死锁。
+可以采用计算两个对象的hashCode，判断hashCode值的大小，始终保证大的/小的在前保证顺序；
+如果两个恰好相等又不属于同一个对象，可定义一个额外的对象锁进行锁定从而保证锁顺序；
+
+## 协作对象间的死锁
+一般出现在持有锁A的时候调用某个外部方法，如果外部方法恰好持有锁B并且会请求锁A，
+就会出现死锁。
+```java
+/**
+ * CooperatingDeadlock
+ * <p/>
+ * Lock-ordering deadlock between cooperating objects
+ *
+ * @author Brian Goetz and Tim Peierls
+ */
+@Evaluated(">_<")
+public class CooperatingDeadlock {
+    // Warning: deadlock-prone!
+    class Taxi {
+        @GuardedBy("this")
+        private Point location, destination;
+        private final Dispatcher dispatcher;
+
+        public Taxi(Dispatcher dispatcher) {
+            this.dispatcher = dispatcher;
+        }
+
+        public synchronized Point getLocation() {
+            return location;
+        }
+
+        public synchronized void setLocation(Point location) {
+            this.location = location;
+			//持有锁Taxi并调用外部方法notifyAvailable()申请锁Dispatcher
+            if (location.equals( destination ))
+                dispatcher.notifyAvailable( this );
+        }
+
+        public synchronized Point getDestination() {
+            return destination;
+        }
+
+        public synchronized void setDestination(Point destination) {
+            this.destination = destination;
+        }
+    }
+
+    class Dispatcher {
+        @GuardedBy("this")
+        private final Set< Taxi > taxis;
+        @GuardedBy("this")
+        private final Set< Taxi > availableTaxis;
+
+        public Dispatcher() {
+            taxis = new HashSet< Taxi >();
+            availableTaxis = new HashSet< Taxi >();
+        }
+
+        public synchronized void notifyAvailable(Taxi taxi) {
+            availableTaxis.add( taxi );
+        }
+
+        public synchronized Image getImage() {
+            Image image = new Image();
+			//持有锁Dispatcher并调用外部方法t.getLocation()申请锁Taxi
+            for (Taxi t : taxis)
+                image.drawMarker( t.getLocation() );
+            return image;
+        }
+    }
+
+    class Image {
+        public void drawMarker(Point p) {
+        }
+    }
+
+    interface Point {
+
+    }
+}
+```
+解决办法是将同步方法改为同步代码块，避免在持有锁时调用外部方法
+