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法律研究Java 多线程中的死锁
死锁表示两个或多个线程永久阻塞，互相等待的情况。当多个线程需要相同的锁，但按不同的顺序获取锁时，就会发生死锁。Java 多线程程序可能会遭遇死锁状态，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。下面是一个示例。
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...");
}
}
}
}
}当您编译并执行上述程序时，会发现一个死锁情况，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...
上述程序会永久挂起，因为没有一个处于可以继续的位置的线程在等待彼此释放锁，因此您可以按 CTRL+C 退出程序。
死锁解决方案示例
让我们更改锁的顺序并运行相同的程序，以查看这两个线程是否仍互相等待 −
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...");
}
}
}
}
}因此，只需更改锁的顺序就可以防止程序进入死锁状态，并以以下结果完成 −
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...
更新于： 19-06-2020
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