Java，线程死锁？

Question

我的一个 friend 向我展示了他下面的代码，我认为这两个线程可能会死锁，因为它们在尝试获取不同变量的锁时可能会死锁:sb1 和 sb2 。

当我运行代码时，它们似乎没有死锁，因为我能够看到输出:

A
B
second thread: AB
second thread: BA

代码如下:

public static void main(String[] args) {
        StringBuilder sb1 = new StringBuilder();
        StringBuilder sb2 = new StringBuilder();

        new Thread() {
            public void run() {
                synchronized (sb1) {
                    sb1.append("A");
                    synchronized (sb2) {
                        sb2.append("B");
                        System.out.println(sb1.toString());
                        System.out.println(sb2.toString());
                    }
                }
            }
        }.start();

        new Thread() {
            public void run() {
                synchronized (sb2) {
                    sb2.append("A");
                    synchronized (sb1) {
                        sb1.append("B");
                        System.out.println("second thread: " + sb1.toString());
                        System.out.println("second thread: " + sb2.toString());
                    }
                }
            }
        }.start();
    }

那么这两个线程会不会死锁呢？

Answer 1

您发布的代码有一个潜在的死锁。如果它运行成功，那就意味着你很幸运。

为了演示潜在的死锁，您可以操纵时间以确保发生死锁。

public static void main(String[] args) {
    final StringBuilder sb1 = new StringBuilder();
    final StringBuilder sb2 = new StringBuilder();

    new Thread() {
        public void run() {
            synchronized (sb1) {
                sb1.append("A");
                System.out.println("Thread 1 has sync sb1");
                try { Thread.sleep(700); }
                catch (InterruptedException e) { e.printStackTrace(); return; }
                System.out.println("Waiting for thread 1 to sync sb2");
                synchronized (sb2) {
                    sb2.append("B");
                    System.out.println(sb1.toString());
                    System.out.println(sb2.toString());
                }
            }
        }
    }.start();

    new Thread() {
        public void run() {
            try { Thread.sleep(500); }
            catch (InterruptedException e) { e.printStackTrace(); return; }
            synchronized (sb2) {
                System.out.println("Thread 2 has sync sb2");
                sb2.append("A");
                System.out.println("Waiting for thread 2 to sync sb1");
                synchronized (sb1) {
                    sb1.append("B");
                    System.out.println("second thread: " + sb1.toString());
                    System.out.println("second thread: " + sb2.toString());
                }
            }
        }
    }.start();
}

现在第一个线程肯定会在sb1上同步，第二个线程会在sb2上同步，然后就会出现死锁。

输出:

Thread 1 has sync sb1
Thread 2 has sync sb2
Waiting for thread 2 to sync sb1
Waiting for thread 1 to sync sb2