java 死锁检测工具_Java精通并发-死锁检测与相关工具详解

关于死锁其实在之前https://www.cnblogs.com/webor2006/p/10659938.html的jvm学习中已经详细举过例子了，不过这里再来复习一下，另外是从并发这个专题领域的角度再来看下它，这里先来阐述一下相关的概念：

死锁：线程1等待线程2互斥持有的资源，而线程2也在等待线程1互斥持有的资源，两个线程都无法继续执行。

活锁：线程持续重试一个总是失败的操作，导致无法继续执行。

饿死：线程一直被调度器延迟访问其赖以执行的资源，也许是调度器先于低优先级的线程而执行高优先级的线程，同时总是会有一个高优先级的线程可以执行，饿死也叫无限延迟。

以上是在Java并发中会遇到的三种情况，这次主要是来探讨死锁，先来编写一个死锁程序：

然后再创建线程调用一下：

public classMyTest6 {private Object lock1 = newObject();private Object lock2 = newObject();public voidmyMethod1() {synchronized(lock1) {synchronized(lock2) {

System.out.println("myMethod1 invoked");

}

}

}public voidmyMethod2() {synchronized(lock2) {synchronized(lock1) {

System.out.println("myMethod2 invoked");

}

}

}public static void main(String[] args) {

MyTest6 myTest6 = new MyTest6();

Runnable runnable1 = () -> {

while (true) {

myTest6.myMethod1();

try {

Thread.sleep(100);

} catch (InterruptedException e) {

e.printStackTrace();

}

}

};

Thread thread1 = new Thread(runnable1, "myThread1");

Runnable runnable2 = () -> {

while (true) {

myTest6.myMethod2();

try {

Thread.sleep(250);

} catch (InterruptedException e) {

e.printStackTrace();

}

}

};

Thread thread2 = new Thread(runnable2, "myThread2");

thread1.start();

thread2.start();

}

}

然后运行一段时间后，死锁就会出现了：

而到底是不是死锁了呢？可以采用jvisualvm工具可以进行死锁的检测，具体工具如何使用这里就不多说了，之前在JVM详细学习了，如下：

此时我们可以点击“线程dump”来进行详细死锁的跟踪：

其中看到的"BLOCKED"状态其实是在Thread类中的枚举值所定义的，如下：

public enumState {/*** Thread state for a thread which has not yet started.*/NEW,/*** Thread state for a runnable thread. A thread in the runnable

* state is executing in the Java virtual machine but it may

* be waiting for other resources from the operating system

* such as processor.*/RUNNABLE,/*** Thread state for a thread blocked waiting for a monitor lock.

* A thread in the blocked state is waiting for a monitor lock

* to enter a synchronized block/method or

* reenter a synchronized block/method after calling

* {@linkObject#wait() Object.wait}.*/BLOCKED,/*** Thread state for a waiting thread.

* A thread is in the waiting state due to calling one of the

* following methods:

*

*

{@linkObject#wait() Object.wait} with no timeout

*

{@link#join() Thread.join} with no timeout

*

{@linkLockSupport#park() LockSupport.park}

*

*

*

A thread in the waiting state is waiting for another thread to

* perform a particular action.

*

* For example, a thread that has called Object.wait()

* on an object is waiting for another thread to call

* Object.notify() or Object.notifyAll() on

* that object. A thread that has called Thread.join()

* is waiting for a specified thread to terminate.*/WAITING,/*** Thread state for a waiting thread with a specified waiting time.

* A thread is in the timed waiting state due to calling one of

* the following methods with a specified positive waiting time:

*

*

{@link#sleep Thread.sleep}

*

{@linkObject#wait(long) Object.wait} with timeout

*

{@link#join(long) Thread.join} with timeout

*

{@linkLockSupport#parkNanos LockSupport.parkNanos}

*

{@linkLockSupport#parkUntil LockSupport.parkUntil}

*

*/TIMED_WAITING,/*** Thread state for a terminated thread.

* The thread has completed execution.*/TERMINATED;

}

另外，在dump信息最后还有一个对死锁的总述：

这是用的可视化的工具来查看死锁的工具，但有时可能操作系统不支持查看可视化的工具，比如一些服务器只支持命令的方式来查看，此时就可以换另外一个JDK自带的工具来查看，如下：

其中的输出信息跟在jvisualvm所看到的是一样的，只是是在命令行中来查看的：

另外还可以用强大的JMS工具来查看：