Java 简明教程
Java - Thread Deadlock
死锁描述一种情况，其中两个或更多线程永远被阻塞，互相等待。当 multiple threads 需要相同的锁但按不同的顺序获取它们时，就会发生死锁。Java 多线程程序可能会遇到死锁情况，因为 synchronized 关键字会导致执行线程在等待与指定对象关联的锁或监视器时阻塞。这是一个示例。
Example: Demonstrating Deadlock Situation
public class TestThread {
public static Object Lock1 = new Object();
public static Object Lock2 = new Object();
public static void main(String args[]) {
ThreadDemo1 T1 = new ThreadDemo1();
ThreadDemo2 T2 = new ThreadDemo2();
T1.start();
T2.start();
}
private static class ThreadDemo1 extends Thread {
public void run() {
synchronized (Lock1) {
System.out.println("Thread 1: Holding lock 1...");
try { Thread.sleep(10); }
catch (InterruptedException e) {}
System.out.println("Thread 1: Waiting for lock 2...");
synchronized (Lock2) {
System.out.println("Thread 1: Holding lock 1 & 2...");
}
}
}
}
private static class ThreadDemo2 extends Thread {
public void run() {
synchronized (Lock2) {
System.out.println("Thread 2: Holding lock 2...");
try { Thread.sleep(10); }
catch (InterruptedException e) {}
System.out.println("Thread 2: Waiting for lock 1...");
synchronized (Lock1) {
System.out.println("Thread 2: Holding lock 1 & 2...");
}
}
}
}
}当您编译并执行上述程序时，您会发现死锁情况，以下是程序产生的输出 −
Deadlock Solution Example
让我们更改锁的顺序并运行相同的程序，看看这两个线程是否仍在互相等待 −
Example
public class TestThread {
public static Object Lock1 = new Object();
public static Object Lock2 = new Object();
public static void main(String args[]) {
ThreadDemo1 T1 = new ThreadDemo1();
ThreadDemo2 T2 = new ThreadDemo2();
T1.start();
T2.start();
}
private static class ThreadDemo1 extends Thread {
public void run() {
synchronized (Lock1) {
System.out.println("Thread 1: Holding lock 1...");
try {
Thread.sleep(10);
} catch (InterruptedException e) {}
System.out.println("Thread 1: Waiting for lock 2...");
synchronized (Lock2) {
System.out.println("Thread 1: Holding lock 1 & 2...");
}
}
}
}
private static class ThreadDemo2 extends Thread {
public void run() {
synchronized (Lock1) {
System.out.println("Thread 2: Holding lock 1...");
try {
Thread.sleep(10);
} catch (InterruptedException e) {}
System.out.println("Thread 2: Waiting for lock 2...");
synchronized (Lock2) {
System.out.println("Thread 2: Holding lock 1 & 2...");
}
}
}
}
}因此，仅仅更改锁的顺序就可以防止程序进入死锁情况，并完成以下结果 −