线程间的互斥与通信

1.多线程编程中操作共享资源会涉及到多线程安全问题，生活中我们可以用银行转账的例子，用代码来展示如下：

public class MutexTest {
    public static void main(String[] args) {
        final Writer writer = new Writer();
        new Thread(new Runnable() {
            public void run() {
                try {
                    Thread.sleep(10);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                while (true) {
                    writer.write("aaaaaaaaaa");
                }
            }
        }).start();
        new Thread(new Runnable() {
            public void run() {
                try {
                    Thread.sleep(10);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                while (true) {
                    writer.write("bbbbbbb");
                }
            }
        }).start();
    }


}

class Writer {
    public synchronized void write(String string) {
        for (int i = 0; i < string.length(); i++) {
            System.out.print(string.charAt(i));
        }
        System.out.println();
    }
}

运行结果：

我们的理想状态为：aaaaaa打印完以后在打印bbbbbb 现在出现了隔断现象。

解决：在write方法上添加synchronized或者把需要互斥的代码放在synchronized块中。方法上和同步代码块上加的锁对象都是this，要实现互斥那么加的锁对象一定要相同。如果方法上加上了static关键字此时的锁对象是该类的字节码对象。

2.线程间的通信

在1中已经解决了线程安全问题，但是如果我们希望 线程A和线程B友好的运行，即希望线程A打印一次接着线程B打印一次如此循环下去。

代码改造：

public class MutexTest {
    public static void main(String[] args) {
        final Writer writer = new Writer();
        new Thread(new Runnable() {
            public void run() {
                for (int i = 0; i < 10; i++) {
                    writer.sub(i + 1);
                }
            }
        }).start();

        for (int i = 0; i < 10; i++) {
            writer.main(i + 1);
        }
    }
}
class Writer {
    boolean isSubThread = true;
    public synchronized void main(int num) {
        while (!isSubThread) {
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        for (int i = 1; i <= 100; i++) {
            System.out.println("main thread sequece of " + i + ",loop of " + num);
        }
        isSubThread = false;
        this.notify();
    }
    public synchronized void sub(int num) {
        while (isSubThread) {
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        for (int i = 1; i <= 10; i++) {
            System.out.println("sub thread sequece of " + i + ",loop of " + num);
        }
        isSubThread = true;
        this.notify();
    }
}

执行结果：