标签：lis 实现 col 执行 代码结构 经典 monit wait 现象
java线程死锁是一个经典的多线程问题，因为不同的线程都在等待根本不可能释放的锁，从而导致所有的任务无法继续完成，在多线程技术中，死锁是必须避免的，因为会造成线程的假死。
package com.cky.thread; /** * Created by edison on 2017/12/9. */ public class DeadThread implements Runnable{ public String username; public Object object1 = new Object(); public Object object2 = new Object(); public void setFlag(String username) { this.username = username; } @Override public void run() { if (username.equals("a")) { synchronized (object1) { try { System.out.println("username="+username); Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } synchronized (object2) { System.out.println("按lock1-》lock2代码顺序执行了"); } } } if (username.equals("b")) { synchronized (object2) { try { System.out.println("username="+username); Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } synchronized (object1) { System.out.println("按lock2-》lock1代码顺序执行了"); } } } } }
package com.cky.test; import com.cky.thread.DeadThread; /** * Created by edison on 2017/12/9. */ public class Run { public static void main(String[] args) { try { DeadThread t1 = new DeadThread(); t1.setFlag("a"); Thread thread1 = new Thread(t1); thread1.start(); Thread.sleep(100); t1.setFlag("b"); Thread t2 = new Thread(t1); t2.start(); } catch (InterruptedException e) { e.printStackTrace(); } } }
username=a
username=b
程序运行结果如上。
可以用jdk自带的工具来监测是否有死锁现象
C:\Users\edison>cd C:\itsoft\jdk\bin C:\itsoft\jdk\bin>jps 18016 Launcher 14392 Jps 20696 4936 AppMain
C:\itsoft\jdk\bin>jstack -l 4936
2017-12-09 13:49:50
Full thread dump Java HotSpot(TM) Client VM (25.144-b01 mixed mode, sharing):
============================= "Thread-1": waiting to lock monitor 0x00d7ad94 (object 0x04d01230, a java.lang.Object), which is held by "Thread-0" "Thread-0": waiting to lock monitor 0x00d793c4 (object 0x04d01238, a java.lang.Object), which is held by "Thread-1" Java stack information for the threads listed above: =================================================== "Thread-1": at com.cky.thread.DeadThread.run(DeadThread.java:44) - waiting to lock <0x04d01230> (a java.lang.Object) - locked <0x04d01238> (a java.lang.Object) at java.lang.Thread.run(Thread.java:748) "Thread-0": at com.cky.thread.DeadThread.run(DeadThread.java:25) - waiting to lock <0x04d01238> (a java.lang.Object) - locked <0x04d01230> (a java.lang.Object) at java.lang.Thread.run(Thread.java:748) Found 1 deadlock.
监测出现死锁
死锁是程序设计的bug,在程序设计时就要避免双方互相持有对方锁的情况，需要说明的是，本实验使用synchronized嵌套的代码结构来实现死锁，
其实不用嵌套的synchronized代码结构也会出现死锁，与是否嵌套无任何关系，不要被代码结构所误导，只要互相等待双方释放就有可能出现死锁。
标签：lis 实现 col 执行 代码结构 经典 monit wait 现象
原文地址：http://www.cnblogs.com/edison20161121/p/8011168.html
