什么是死锁
死锁问题是多线程特有的问题，它可以被认为是线程间切换消耗系统性能的一种极端情况。 在死锁时，线程间相互等待资源，而又不释放自身的资源，导致无穷无尽的等待，其结果是系统任务永远无法执行完成。 死锁问题是在多线程开发中应该坚决避免和杜绝的问题.
死锁示例代码
package com.rumenz.learn.deadLock;
public class RumenzThread implements Runnable{
int a,b;
public RumenzThread(int a, int b) {
this.a = a;
this.b = b;
}
@Override
public void run() {
//Integer.valueOf(a) 包装成对象
synchronized (Integer.valueOf(a)){
try{
//睡眠3秒,增加死锁的几率
Thread.sleep(3000);
}catch (Exception e){
e.printStackTrace();
}
synchronized (Integer.valueOf(b)){
System.out.println("a+b="+(a+b));
}
}
}
}
package com.rumenz.learn.deadLock;
public class DeadLock {
public static void main(String[] args) {
new Thread(new RumenzThread(1, 2)).start();
new Thread(new RumenzThread(2, 1)).start();
}
}
运行程序使用jstack -l pid来定位死锁
先找到死锁程序的进程id
> jps
56993 Jps
56636 Launcher
57066 DeadLock //这个就是死锁的进程
使用jstack -l 57066来定位死锁
> jstack -l 57066
Found one Java-level deadlock:
=============================
"Thread-1":
waiting to lock monitor 0x00007fbe6d80de18 (object 0x000000076ab33988, a java.lang.Integer),
which is held by "Thread-0"
"Thread-0":
waiting to lock monitor 0x00007fbe6d8106a8 (object 0x000000076ab33998, a java.lang.Integer),
which is held by "Thread-1"
Java stack information for the threads listed above:
===================================================
"Thread-1":
at com.rumenz.learn.deadLock.RumenzThread.run(RumenzThread.java:27)
- waiting to lock <0x000000076ab33988> (a java.lang.Integer)
- locked <0x000000076ab33998> (a java.lang.Integer)
at java.lang.Thread.run(Thread.java:748)
"Thread-0":
at com.rumenz.learn.deadLock.RumenzThread.run(RumenzThread.java:27)
- waiting to lock <0x000000076ab33998> (a java.lang.Integer)
- locked <0x000000076ab33988> (a java.lang.Integer)
at java.lang.Thread.run(Thread.java:748)
Found 1 deadlock. //发现一个死锁
RumenzThread.java:27定位到大概的代码文件位置。
jconsole定位死锁问题
- 找到死锁进程
-
链接,不安全的链接
-
选择线程
-
点击检测死锁
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