3.死锁
避免死锁的几个常见方法
- 避免一个线程同时获取多个锁
- 尽量保证每个锁只占用一个资源
- 尝试使用定时锁,使用lock.tryLock(timeout)来替代使用内部锁机制
- 对于数据库锁,加锁和解锁必须在一个数据库连接里
一个死锁的例子
public class DeadLockDemo { privat static String A = "A"; private static String B = "B"; public static void main(String[] args) { new DeadLockDemo().deadLock(); }
private void deadLock() { Thread t1 = new Thread(new Runnable() { @Override publicvoid run() { synchronized (A) { try {
Thread.currentThread().sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); }
synchronized (B) { System.out.println("1"); } } } }); Thread t2 = new Thread(new Runnable() { @Override publicvoid run() { synchronized (B) { synchronized (A) { System.out.println("2"); } } } }); t1.start(); t2.start(); } }
在一些更为复杂的场景:t1拿到锁之后,因为一些异常情况没有释放锁(死循环)。又或者是t1拿到一个数据库锁,释放锁的时候抛出了异常,没释放掉。
通过dump线程查看到底是哪个线程出现了问题,以下线程信息告诉我们是DeadLockDemo类的第42行和第31行引起的死锁
"Thread-2" prio=5 tid=7fc0458d1000 nid=0x116c1c000 waiting for monitor entry [116c1b000java.lang.Thread.State: BLOCKED (on object monitor)
at com.ifeve.book.forkjoin.DeadLockDemo$2.run(DeadLockDemo.java:42)
- waiting to lock <7fb2f3ec0> (a java.lang.String)
- locked <7fb2f3ef8> (a java.lang.String)
at java.lang.Thread.run(Thread.java:695)
"Thread-1" prio=5 tid=7fc0430f6800 nid=0x116b19000 waiting for monitor entry [116b18000java.lang.Thread.State: BLOCKED (on object monitor)
at com.ifeve.book.forkjoin.DeadLockDemo$1.run(DeadLockDemo.java:31)
- waiting to lock <7fb2f3ef8> (a java.lang.String)
- locked <7fb2f3ec0> (a java.lang.String)
at java.lang.Thread.run(Thread.java:695)