今天面试有一道题，写一个死锁的程序，自己也是短路了，没写出来，回来写下。

死锁常见的情况是A线程持有a锁、阻塞于b锁，B线程持有b锁，阻塞于a锁，形成一个循环阻塞的状态。

import java.util.concurrent.TimeUnit;

public class CyclicLock {

	private static Object o1 = new Object();

	private static Object o2 = new Object();

	public static void main(String[] args) {

		new Thread(new Runnable() {

			@Override

			public void run() {

				synchronized(o1) {

					try {

						TimeUnit.SECONDS.sleep(5);

					} catch (InterruptedException e) {

						e.printStackTrace();

					}

					synchronized(o2) {

					}

				}

			}

		}).start();

		new Thread(new Runnable() {

			@Override

			public void run() {

				synchronized(o2) {

					try {

						TimeUnit.SECONDS.sleep(5);

					} catch (InterruptedException e) {

						e.printStackTrace();

					}

					synchronized(o1) {

					}

				}

			}

		}).start();

	}

}

用了下Java栈跟踪工具jstack，可以生成当前虚拟机中的线程快照，此文件称为thread dump。（而用jmap -dump生成的是堆转储快照，heap dump）

这里居然还自动检测出死锁，略牛略牛。

C:\Users\Administrator>jps

11168 CyclicLock

3504

10204 Jps

C:\Users\Administrator>

C:\Users\Administrator>jstack -l 11168

2017-03-22 23:24:24

Full thread dump Java HotSpot(TM) Client VM (25.121-b13 mixed mode, sharing):

"DestroyJavaVM" #10 prio=5 os_prio=0 tid=0x001ac800 nid=0x20f0 waiting on condition [0

x00000000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"Thread-1" #9 prio=5 os_prio=0 tid=0x00a1f400 nid=0x2a28 waiting for monitor entry [0x

15f2f000]

   java.lang.Thread.State: BLOCKED (on object monitor)

        at CyclicLock$2.run(CyclicLock.java:37)

        - waiting to lock <0x044ab368> (a java.lang.Object)

        - locked <0x044ab370> (a java.lang.Object)

        at java.lang.Thread.run(Unknown Source)

   Locked ownable synchronizers:

        - None

"Thread-0" #8 prio=5 os_prio=0 tid=0x00a1e400 nid=0x2bf0 waiting for monitor entry [0x

15bff000]

   java.lang.Thread.State: BLOCKED (on object monitor)

        at CyclicLock$1.run(CyclicLock.java:20)

        - waiting to lock <0x044ab370> (a java.lang.Object)

        - locked <0x044ab368> (a java.lang.Object)

        at java.lang.Thread.run(Unknown Source)

   Locked ownable synchronizers:

        - None

"Service Thread" #7 daemon prio=9 os_prio=0 tid=0x009e6c00 nid=0x2698 runnable [0x0000

0000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"C1 CompilerThread0" #6 daemon prio=9 os_prio=2 tid=0x009dfc00 nid=0x2278 waiting on c

ondition [0x00000000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x009de800 nid=0x27d8 waiting on cond

ition [0x00000000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x009db400 nid=0x52c runnable [0x00

000000]

   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:

        - None

"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x009c0c00 nid=0x2214 in Object.wait() [0x1

56bf000]

   java.lang.Thread.State: WAITING (on object monitor)

        at java.lang.Object.wait(Native Method)

        - waiting on <0x04408978> (a java.lang.ref.ReferenceQueue$Lock)

        at java.lang.ref.ReferenceQueue.remove(Unknown Source)

        - locked <0x04408978> (a java.lang.ref.ReferenceQueue$Lock)

        at java.lang.ref.ReferenceQueue.remove(Unknown Source)

        at java.lang.ref.Finalizer$FinalizerThread.run(Unknown Source)

   Locked ownable synchronizers:

        - None

"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x00965000 nid=0x2624 in Object.wa

it() [0x155ff000]

   java.lang.Thread.State: WAITING (on object monitor)

        at java.lang.Object.wait(Native Method)

        - waiting on <0x04406a90> (a java.lang.ref.Reference$Lock)

        at java.lang.Object.wait(Unknown Source)

        at java.lang.ref.Reference.tryHandlePending(Unknown Source)

        - locked <0x04406a90> (a java.lang.ref.Reference$Lock)

        at java.lang.ref.Reference$ReferenceHandler.run(Unknown Source)

   Locked ownable synchronizers:

        - None

"VM Thread" os_prio=2 tid=0x00960c00 nid=0x2874 runnable

"VM Periodic Task Thread" os_prio=2 tid=0x009e8000 nid=0x1664 waiting on condition

JNI global references: 6

Found one Java-level deadlock:

=============================

"Thread-1":

  waiting to lock monitor 0x009681c4 (object 0x044ab368, a java.lang.Object),

  which is held by "Thread-0"

"Thread-0":

  waiting to lock monitor 0x00969b94 (object 0x044ab370, a java.lang.Object),

  which is held by "Thread-1"

Java stack information for the threads listed above:

===================================================

"Thread-1":

        at CyclicLock$2.run(CyclicLock.java:37)

        - waiting to lock <0x044ab368> (a java.lang.Object)

        - locked <0x044ab370> (a java.lang.Object)

        at java.lang.Thread.run(Unknown Source)

"Thread-0":

        at CyclicLock$1.run(CyclicLock.java:20)

        - waiting to lock <0x044ab370> (a java.lang.Object)

        - locked <0x044ab368> (a java.lang.Object)

        at java.lang.Thread.run(Unknown Source)

Found 1 deadlock.