Java多线程中的死锁
死锁描述了一种情况，其中两个或多个线程永远被阻塞，互相等待。当多个线程需要相同的锁但以不同的顺序获得它们时，就会发生死锁。Java多线程程序可能会遇到死锁情况，因为 synced 关键字会导致正在执行的线程在等待与指定对象关联的锁或监视器时阻塞。这是一个例子。
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 (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...");
}
}
}
}
}编译并执行上述程序时，发现死锁情况，以下是该程序产生的输出：
输出结果
Thread 1: Holding lock 1... Thread 2: Holding lock 2... Thread 1: Waiting for lock 2... Thread 2: Waiting for lock 1...
上面的程序将永远挂起，因为两个线程都无法继续进行并等待彼此释放锁，因此可以通过按CTRL+C退出程序。
死锁解决方案示例
让我们更改同一程序的锁定和运行顺序，以查看两个线程是否仍然彼此等待-
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...");
}
}
}
}
}因此，只需更改锁的顺序即可防止程序陷入死锁情况，并完成以下结果：
输出结果
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...