一个线程问题的记录

DH镔

发布于 2019-12-19 22:44:18

3690

发布于 2019-12-19 22:44:18

不阻塞

/**
 * @author donghaibin
 */
public class ThreadDemo {

    private static boolean running = true;

    public static void main(String[] args) throws InterruptedException {
        newThread();
        running = false;
    }

    private static void newThread() {
        Thread thread = new Thread() {
            @Override
            public void run() {
                while (running) {
                    
                }
            }
        };
        thread.start();
    }
}

这里不阻塞的原因：main函数是程序的入口，先执行，不让出CPU子线程无法执行。当执行newThread()创建一个子线程时，running已经等于false。

阻塞

/**
 * @author donghaibin
 */
public class ThreadDemo {

    private static boolean running = true;

    public static void main(String[] args) throws InterruptedException {
        newThread();
        Thread.sleep(100);
        running = false;
    }

    private static void newThread() {
        Thread thread = new Thread() {
            @Override
            public void run() {
                while (running) {
         
                }
            }
        };
        thread.start();
    }
}

这里原因和上面同理可得，主线程停止了100ms，让出了cpu执行子线程，running变量没有加volatile，无法保证可见性，当执行running = false时，对其他线程不可见。所以无法跳出while循环。

不阻塞

/**
 * @author donghaibin
 */
public class ThreadDemo {

    private static boolean running = true;

    public static void main(String[] args) throws InterruptedException {
        newThread();
        Thread.sleep(100);
        running = false;
    }

    private static void newThread() {
        Thread thread = new Thread() {
            @Override
            public void run() {
                while (running) {
                    System.out.println("---");
                }
            }
        };
        thread.start();
    }
}

这里不阻塞的原因：

先看下System.out.println的源码

    /**
     * Prints an integer and then terminate the line.  This method behaves as
     * though it invokes <code>{@link #print(int)}</code> and then
     * <code>{@link #println()}</code>.
     *
     * @param x  The <code>int</code> to be printed.
     */
    public void println(int x) {
        synchronized (this) {
            print(x);
            newLine();
        }
    }

源码中存在synchronized