Java 多线程中的死锁
java programming java8object oriented programming
死锁描述了两个或多个线程被永远阻塞，相互等待的情况。当多个线程需要相同的锁但以不同的顺序获取它们时，就会发生死锁。Java 多线程程序可能会遭遇死锁情况，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。下面是一个例子。
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...");
}
}
}
}
}
当你编译并执行上述程序时，你会发现一个死锁的情况，以下是程序产生的输出 −
输出
Thread 1: Holding lock 1... Thread 2: Holding lock 2... Thread 1: Waiting for lock 2... Thread 2: Waiting for lock 1...
上述程序将永远挂起，因为两个线程都无法继续执行，并且都在等待对方释放锁，因此您可以按 CTRL+C 退出程序。
死锁解决方案示例
让我们改变同一程序的锁定和运行顺序，看看两个线程是否仍在等待对方 −
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，只需改变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
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...
