死锁在Java中是一种编程情境，其中两个或多个线程被永久阻塞。Java死锁情况至少涉及两个线程和两个或多个资源。在这里，我编写了一个简单的程序，将导致Java死锁场景，然后我们将看到如何分析它。
Java中的死锁
让我们看一个简单的程序，在这个程序中，我将在Java线程中创建死锁。
package com.journaldev.threads;
public class ThreadDeadlock {
public static void main(String[] args) throws InterruptedException {
Object obj1 = new Object();
Object obj2 = new Object();
Object obj3 = new Object();
Thread t1 = new Thread(new SyncThread(obj1, obj2), "t1");
Thread t2 = new Thread(new SyncThread(obj2, obj3), "t2");
Thread t3 = new Thread(new SyncThread(obj3, obj1), "t3");
t1.start();
Thread.sleep(5000);
t2.start();
Thread.sleep(5000);
t3.start();
}
}
class SyncThread implements Runnable{
private Object obj1;
private Object obj2;
public SyncThread(Object o1, Object o2){
this.obj1=o1;
this.obj2=o2;
}
@Override
public void run() {
String name = Thread.currentThread().getName();
System.out.println(name + " acquiring lock on "+obj1);
synchronized (obj1) {
System.out.println(name + " acquired lock on "+obj1);
work();
System.out.println(name + " acquiring lock on "+obj2);
synchronized (obj2) {
System.out.println(name + " acquired lock on "+obj2);
work();
}
System.out.println(name + " released lock on "+obj2);
}
System.out.println(name + " released lock on "+obj1);
System.out.println(name + " finished execution.");
}
private void work() {
try {
Thread.sleep(30000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
在上面的程序中，SyncThread正在实现Runnable接口，并通过在每个对象上一次性获取锁来处理两个对象。在主方法中，我有三个线程运行SyncThread，并且每个线程之间有一个共享资源。线程以这样的方式运行，它将能够在第一个对象上获取锁，但当它尝试在第二个对象上获取锁时，它会进入等待状态，因为它已经被另一个线程锁定。这在线程之间形成了对资源的循环依赖，导致死锁。当我执行上述程序时，生成了以下输出，但程序永远不会终止，因为Java线程中发生了死锁。
t1 acquiring lock on java.lang.Object@6d9dd520
t1 acquired lock on java.lang.Object@6d9dd520
t2 acquiring lock on java.lang.Object@22aed3a5
t2 acquired lock on java.lang.Object@22aed3a5
t3 acquiring lock on java.lang.Object@218c2661
t3 acquired lock on java.lang.Object@218c2661
t1 acquiring lock on java.lang.Object@22aed3a5
t2 acquiring lock on java.lang.Object@218c2661
t3 acquiring lock on java.lang.Object@6d9dd520
在这里，我们可以清楚地从输出中识别出死锁情况，但在实际应用中，很难找到死锁情况并对其进行调试。
如何检测Java中的死锁
要在Java中检测死锁，我们需要查看应用程序的Java线程转储，在上一篇帖子中，我解释了如何使用VisualVM分析器或使用jstack实用程序生成线程转储。以下是上述程序的线程转储。
2012-12-27 19:08:34
Full thread dump Java HotSpot(TM) 64-Bit Server VM (23.5-b02 mixed mode):
"Attach Listener" daemon prio=5 tid=0x00007fb0a2814000 nid=0x4007 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"DestroyJavaVM" prio=5 tid=0x00007fb0a2801000 nid=0x1703 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"t3" prio=5 tid=0x00007fb0a204b000 nid=0x4d07 waiting for monitor entry [0x000000015d971000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
- waiting to lock <0x000000013df2f658> (a java.lang.Object)
- locked <0x000000013df2f678> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:722)
"t2" prio=5 tid=0x00007fb0a1073000 nid=0x4207 waiting for monitor entry [0x000000015d209000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
- waiting to lock <0x000000013df2f678> (a java.lang.Object)
- locked <0x000000013df2f668> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:722)
"t1" prio=5 tid=0x00007fb0a1072000 nid=0x5503 waiting for monitor entry [0x000000015d86e000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
- waiting to lock <0x000000013df2f668> (a java.lang.Object)
- locked <0x000000013df2f658> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:722)
"Service Thread" daemon prio=5 tid=0x00007fb0a1038000 nid=0x5303 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread1" daemon prio=5 tid=0x00007fb0a1037000 nid=0x5203 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread0" daemon prio=5 tid=0x00007fb0a1016000 nid=0x5103 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Signal Dispatcher" daemon prio=5 tid=0x00007fb0a4003000 nid=0x5003 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Finalizer" daemon prio=5 tid=0x00007fb0a4800000 nid=0x3f03 in Object.wait() [0x000000015d0c0000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x000000013de75798> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)
- locked <0x000000013de75798> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:177)
"Reference Handler" daemon prio=5 tid=0x00007fb0a4002000 nid=0x3e03 in Object.wait() [0x000000015cfbd000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x000000013de75320> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:503)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133)
- locked <0x000000013de75320> (a java.lang.ref.Reference$Lock)
"VM Thread" prio=5 tid=0x00007fb0a2049800 nid=0x3d03 runnable
"GC task thread#0 (ParallelGC)" prio=5 tid=0x00007fb0a300d800 nid=0x3503 runnable
"GC task thread#1 (ParallelGC)" prio=5 tid=0x00007fb0a2001800 nid=0x3603 runnable
"GC task thread#2 (ParallelGC)" prio=5 tid=0x00007fb0a2003800 nid=0x3703 runnable
"GC task thread#3 (ParallelGC)" prio=5 tid=0x00007fb0a2004000 nid=0x3803 runnable
"GC task thread#4 (ParallelGC)" prio=5 tid=0x00007fb0a2005000 nid=0x3903 runnable
"GC task thread#5 (ParallelGC)" prio=5 tid=0x00007fb0a2005800 nid=0x3a03 runnable
"GC task thread#6 (ParallelGC)" prio=5 tid=0x00007fb0a2006000 nid=0x3b03 runnable
"GC task thread#7 (ParallelGC)" prio=5 tid=0x00007fb0a2006800 nid=0x3c03 runnable
"VM Periodic Task Thread" prio=5 tid=0x00007fb0a1015000 nid=0x5403 waiting on condition
JNI global references: 114
Found one Java-level deadlock:
=============================
"t3":
waiting to lock monitor 0x00007fb0a1074b08 (object 0x000000013df2f658, a java.lang.Object),
which is held by "t1"
"t1":
waiting to lock monitor 0x00007fb0a1010f08 (object 0x000000013df2f668, a java.lang.Object),
which is held by "t2"
"t2":
waiting to lock monitor 0x00007fb0a1012360 (object 0x000000013df2f678, a java.lang.Object),
which is held by "t3"
Java stack information for the threads listed above:
===================================================
"t3":
at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
- waiting to lock <0x000000013df2f658> (a java.lang.Object)
- locked <0x000000013df2f678> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:722)
"t1":
at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
- waiting to lock <0x000000013df2f668> (a java.lang.Object)
- locked <0x000000013df2f658> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:722)
"t2":
at com.journaldev.threads.SyncThread.run(ThreadDeadlock.java:41)
- waiting to lock <0x000000013df2f678> (a java.lang.Object)
- locked <0x000000013df2f668> (a java.lang.Object)
at java.lang.Thread.run(Thread.java:722)
Found 1 deadlock.
线程转储输出清楚地显示了死锁情况以及导致死锁的线程和资源。为了分析死锁，我们需要查找状态为BLOCKED的线程，然后查看它等待锁定的资源。每个资源都有一个唯一的ID，我们可以使用它找到哪个线程已经持有对象的锁。例如，线程“t3”正在等待锁定0x000000013df2f658，但它已经被线程“t1”锁定。一旦分析了死锁情况并找到了导致死锁的线程，我们需要进行代码更改以避免死锁情况。
如何在Java中避免死锁
以下是一些建议，可以帮助我们避免大多数死锁情况。
-
避免嵌套锁：这是死锁最常见的原因，如果已经持有一个资源，请避免锁定另一个资源。如果您只使用一个对象锁定，几乎不可能出现死锁的情况。例如，这里是一个没有嵌套锁的`run()`方法的另一种实现，程序可以成功运行且不会出现死锁的情况。
public void run() { String name = Thread.currentThread().getName(); System.out.println(name + " 正在锁定 " + obj1); synchronized (obj1) { System.out.println(name + " 已锁定 " + obj1); work(); } System.out.println(name + " 释放了 " + obj1 + " 的锁"); System.out.println(name + " 正在锁定 " + obj2); synchronized (obj2) { System.out.println(name + " 已锁定 " + obj2); work(); } System.out.println(name + " 释放了 " + obj2 + " 的锁"); System.out.println(name + " 执行完成。"); } -
只锁定必要的内容: 您应该仅在必须处理的资源上获取锁定，例如，在上述程序中，我正在锁定完整的对象资源，但如果我们只对其中的一个字段感兴趣，那么我们应该只锁定该特定字段而不是整个对象。
-
避免无限等待: 如果两个线程正在无限期地等待彼此完成，使用 线程 join 可能会导致死锁。如果您的线程必须等待另一个线程完成，最好始终使用 join，并设置您希望等待线程完成的最长时间。
关于 Java 线程中的死锁就是这些了。
Source:
https://www.digitalocean.com/community/tutorials/deadlock-in-java-example