并发(九)：检查死锁与Locked ownable synchronizers

通过jstack可快速检查线程的死锁信息，用法如下：

# 获取JVM ID(JAVA 进程ID)，通过参数lv可以获取更详细的JAVA方法调用信息
jps -lv
# 得到JVM ID后，执行jstack命令，这里假定为123
# 将结果重定向到文件，更便于查看
jstack -l 123 >> d:/123.txt

翻到页底，可发现明显的死锁信息：

Java stack information for the threads listed above:
===================================================
"Thread-1":
    at com.mirana.concurrent.LockedOwnThread.run(LockedOwnThread.java:47)
    - waiting to lock <0x000000076c5806f8> (a java.lang.Class for java.lang.Object)
    - locked <0x000000076c636568> (a java.lang.Class for com.mirana.concurrent.LockedOwnThread)
    - locked <0x000000076c6392f0> (a com.mirana.concurrent.LockedOwnThread)
"Thread-0":
    at com.mirana.concurrent.LockedOwnThread$AThread.run(LockedOwnThread.java:27)
    - waiting to lock <0x000000076c636568> (a java.lang.Class for com.mirana.concurrent.LockedOwnThread)
    - locked <0x000000076c5806f8> (a java.lang.Class for java.lang.Object)

Found 1 deadlock.

从输出信息中，可明显发觉两个线程彼此锁定了对方需要的锁（0x000000076c636568与0x000000076c5806f8）。

在打印的堆栈信息中，我们还发现了如下信息：

"Thread-0" #11 prio=5 os_prio=0 tid=0x000000001e902000 nid=0x5b34c waiting for monitor entry [0x000000001f5bf000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.mirana.concurrent.LockedOwnThread$AThread.run(LockedOwnThread.java:27)
    - waiting to lock <0x000000076c636538> (a java.lang.Class for com.mirana.concurrent.LockedOwnThread)
    - locked <0x000000076c5806f8> (a java.lang.Class for java.lang.Object)

   Locked ownable synchronizers:
    - None

Locked ownable synchronizers是个什么东东？按照官方定义

一个可持有的同步器多半是线程独有并且使用了AbstractOwnableSynchronizer(或是其子类)去实现它的同步特性，ReentrantLock与ReentrantReadWriteLock就是JAVA平台提供的两个例子。

我们将程序改为如下形式，依旧为死锁形式：
```java
static class AThread extends Thread {

    private ReentrantLock lock1;

    private ReentrantLock lock2;

    /**
     * @param lock1
     * @param lock2
     */
    public AThread(ReentrantLock lock1, ReentrantLock lock2) {
        super();
        this.lock1 = lock1;
        this.lock2 = lock2;
    }



    public void run() {
        try {
            lock1.lock();
            Thread.sleep(3000); 
            //  必须获取两个锁后才执行操作
            lock2.lock();
            System.out.println("A: I have all Locks!");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock2.unlock();
            lock1.unlock();
        }
    }

}

static class BThread extends Thread {

    private ReentrantLock lock1;

    private ReentrantLock lock2;

    /**
     * @param lock1
     * @param lock2
     */
    public BThread(ReentrantLock lock1, ReentrantLock lock2) {
        super();
        this.lock1 = lock1;
        this.lock2 = lock2;
    }



    public void run() {
        try {
            lock2.lock();
            Thread.sleep(1000); 
            //  必须获取两个锁后才执行操作
            lock1.lock();
            System.out.println("B: I have all Locks!");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock1.unlock();
            lock2.unlock();
        }
    }

}

//  测试程序主函数
public static void main(String[] args) throws InterruptedException {
    final ReentrantLock lock1 = new ReentrantLock();
    final ReentrantLock lock2 = new ReentrantLock();
    new AThread(lock1, lock2).start();
    new BThread(lock1, lock2).start();
}




<div class="se-preview-section-delimiter"></div>

最后打印线程的堆栈信息，出现的内容如下：

"Thread-1" #12 prio=5 os_prio=0 tid=0x000000001ef5a000 nid=0x1c2c waiting on condition [0x000000001fcbf000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x000000076c637ca0> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199)
    at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:209)
    at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:285)
    at com.mirana.concurrent.LockedOwnThread$BThread.run(LockedOwnThread.java:70)

   Locked ownable synchronizers:
    - <0x000000076c637cd0> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)

发现ReentrantLock与内置锁有如下3点不同：
1. 等待的对象不同，内置锁是“monitor entry”（监视器进入点），而ReentrantLock是“condition”(条件)；
2. 线程的状态不同，内置锁是“BLOCKED”，而ReentrantLock是“WAITING”；
3. 锁定的同步器不同，内置锁没有，而ReentrantLock则指向持有的同步器；

结论

通过jstack可快速检查到死锁情况，并能给出冲突的锁与监测对象，另外，内置锁对象与ReentrantLock在运行中表现出截然不同的状态。