死锁描述了一种情况，在这种情况下，两个或更多的线程永远被阻塞，相互等待。死锁发生在多个线程需要相同的锁但是以不同的顺序获取它们的时候。一个Java多线程程序可能会遭受死锁条件的影响，因为synchronized关键字会导致执行线程在等待特定对象的锁或监视器时阻塞。以下是一个例子。
示例：演示死锁情况
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...");
}
}
}
}
}
当你编译并执行上述程序时，你会发现一个死锁的情况，并且程序产生的输出如下：
Output
Thread 1: Holding lock 1...
Thread 2: Holding lock 2...
Thread 1: Waiting for lock 2...
Thread 2: Waiting for lock 1...
上述程序将会永远挂起，因为没有一个线程可以继续执行并且都在等待对方释放锁，因此你可以通过按下CTRL+C来退出程序。
死锁解决方案示例
让我们改变锁的顺序并运行相同的程序来看这两个线程是否仍然互相等待。
示例
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...");
}
}
}
}
}
仅仅是改变锁的顺序就能防止程序陷入死锁的情况，并且程序会以以下结果完成：
Output
Thread 1: Holding lock 1...
Thread 1: Waiting for lock 2...
Thread 1: Holding lock 1 & 2...
Thread 2: Holding lock 1...
Thread 2: Waiting for lock 2...
Thread 2: Holding lock 1 & 2...
以上示例只是为了澄清概念，然而，这确实是一个复杂的概念，在开发应用程序以处理死锁情况之前，你应该深入了解这一主题。