一定要使用-server参数来调试并发程序

在阅读JCIP的时候想手工测试一下，结果发现总是没有出现书中描述的并发问题

后来我琢磨，以前记得书上说过，在debugging的环境下，JVM是低并发的，一定要在server的环境下测试，让JVM在高并发的情况下使用，才容易测试出并发BUG，最简便的方法就是给JVM添加上-server的运行参数

代码如下，JCIP Listing3.15

class Holder {
    private int n;
    private static Random random = new Random();
    public Holder(int n) {
        this.n = n;
    }
    public void changeValue() {
        n = random.nextInt();
    }
    public void assertSanity() {
        if (n != n) {
            throw new AssertionError("n != n");
        }
    }
}

class Checker implements Runnable {
    private boolean stop = false;
    private Holder holder;
    public Checker(Holder holder) {
        this.holder = holder;
    }
    public synchronized void stop() {
        stop = true;
    }
    @Override
    public void run() {
        while (!stop) {
            try {
                // System.out.println("checking");
                holder.assertSanity(); //! Possible to throw exception in concurrent circumstances
            } catch (Throwable e) {
                System.out.println(e);
                stop();
            }
        }
    }
}

class Modifier implements Runnable {
    private boolean stop = false;
    private Holder holder;
    public Modifier(Holder holder) {
        this.holder = holder;
    }
    public synchronized void stop() {
        stop = true;
    }
    @Override
    public void run() {
        while (!stop) {
            // System.out.println("modifying");
            holder.changeValue();
        }
    }
}

public class Listing3_15 {
    public static void main(String[] args) {
        ExecutorService exec = null;
        Holder holder = null;
        Modifier m1 = null, m2 = null;
        Checker checker = null;
        Scanner scanner = null;
        try {
            exec = Executors.newCachedThreadPool();
            holder = new Holder(10);
            m1 = new Modifier(holder);
            m2 = new Modifier(holder);
            checker = new Checker(holder);
            exec.execute(checker);
            exec.execute(m1);
            exec.execute(m2);
            scanner = new Scanner(System.in);
            scanner.next(); // pause
        } finally {
            m1.stop();
            m2.stop();
            checker.stop();
            exec.shutdown();
            scanner.close();
        }
    }
}

果然，在debug的环境下，JVM跑不出这个并发BUG，添加-server参数之后，JVM就跑出这个并发BUG了，在eclipse中设置-server参数的方法如下：Run>Run Configurations>Arguments>VM Arguments，写上-server即可