Java 内存模型JMM 多线程变量可见性问题验证程序及volatile关键字作用验证程序

测试程序1：两个线程对同一个变量的可见性问题

    static int n = 0;
    static void testMultiThreadVisiable(){

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                while(true){
                    if(n == 1){
                        System.out.println(System.currentTimeMillis());
                        break;
                    }
                }
            }
        });
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("start set." + System.currentTimeMillis());
                n = 1;
                System.out.println("end set." + System.currentTimeMillis());
            }
        });
        t1.start();
        t2.start();

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

   public static void main(String[] args) throws Exception {
        testMultiThreadVisiable();
    }

运行结果

/usr/local/jdk1.8.0_291/bin/java -javaagent:/home/yeqiang/program/idea-IC-192.6603.28/lib/idea_rt.jar=40477:/home/yeqiang/program/idea-IC-192.6603.28/bin -Dfile.encoding=UTF-8 -classpath /usr/local/jdk1.8.0_291/jre/lib/charsets.jar:/usr/local/jdk1.8.0_291/jre/lib/deploy.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/cldrdata.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/dnsns.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/jaccess.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/jfxrt.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/localedata.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/nashorn.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/sunec.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/sunjce_provider.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/sunpkcs11.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/zipfs.jar:/usr/local/jdk1.8.0_291/jre/lib/javaws.jar:/usr/local/jdk1.8.0_291/jre/lib/jce.jar:/usr/local/jdk1.8.0_291/jre/lib/jfr.jar:/usr/local/jdk1.8.0_291/jre/lib/jfxswt.jar:/usr/local/jdk1.8.0_291/jre/lib/jsse.jar:/usr/local/jdk1.8.0_291/jre/lib/management-agent.jar:/usr/local/jdk1.8.0_291/jre/lib/plugin.jar:/usr/local/jdk1.8.0_291/jre/lib/resources.jar:/usr/local/jdk1.8.0_291/jre/lib/rt.jar:/home/yeqiang/code/untitled/out/production/untitled com.company.Main
start set.1658303958008
end set.1658303958008

线程t1观测到的n=0，一直处于死循环状态。

测试程序2：采用volatile修饰变量n，即

static volatile int n = 0;

运行结果

/usr/local/jdk1.8.0_291/bin/java -javaagent:/home/yeqiang/program/idea-IC-192.6603.28/lib/idea_rt.jar=34637:/home/yeqiang/program/idea-IC-192.6603.28/bin -Dfile.encoding=UTF-8 -classpath /usr/local/jdk1.8.0_291/jre/lib/charsets.jar:/usr/local/jdk1.8.0_291/jre/lib/deploy.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/cldrdata.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/dnsns.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/jaccess.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/jfxrt.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/localedata.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/nashorn.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/sunec.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/sunjce_provider.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/sunpkcs11.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/zipfs.jar:/usr/local/jdk1.8.0_291/jre/lib/javaws.jar:/usr/local/jdk1.8.0_291/jre/lib/jce.jar:/usr/local/jdk1.8.0_291/jre/lib/jfr.jar:/usr/local/jdk1.8.0_291/jre/lib/jfxswt.jar:/usr/local/jdk1.8.0_291/jre/lib/jsse.jar:/usr/local/jdk1.8.0_291/jre/lib/management-agent.jar:/usr/local/jdk1.8.0_291/jre/lib/plugin.jar:/usr/local/jdk1.8.0_291/jre/lib/resources.jar:/usr/local/jdk1.8.0_291/jre/lib/rt.jar:/home/yeqiang/code/untitled/out/production/untitled com.company.Main
start set.1658304324424
end set.1658304324424
1658304324424

Process finished with exit code 0

结果分析：volatile强制变量n线程之前可见，t2设置n=1后，t1立刻退出了循环。

扩展测试1.采用Thread.sleep 让t1休眠一小段时间

static int n = 0;
    static void testMultiThreadVisiable(){

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                while(true){
                    if(n == 1){
                        System.out.println(System.currentTimeMillis());
                        break;
                    }
                    try {
                        Thread.sleep(1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        });
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("start set." + System.currentTimeMillis());
                n = 1;
                System.out.println("end set." + System.currentTimeMillis());
            }
        });
        t1.start();
        t2.start();

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }


    public static void main(String[] args) throws Exception {
        testMultiThreadVisiable();
    }

输出

/usr/local/jdk1.8.0_291/bin/java -javaagent:/home/yeqiang/program/idea-IC-192.6603.28/lib/idea_rt.jar=38567:/home/yeqiang/program/idea-IC-192.6603.28/bin -Dfile.encoding=UTF-8 -classpath /usr/local/jdk1.8.0_291/jre/lib/charsets.jar:/usr/local/jdk1.8.0_291/jre/lib/deploy.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/cldrdata.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/dnsns.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/jaccess.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/jfxrt.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/localedata.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/nashorn.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/sunec.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/sunjce_provider.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/sunpkcs11.jar:/usr/local/jdk1.8.0_291/jre/lib/ext/zipfs.jar:/usr/local/jdk1.8.0_291/jre/lib/javaws.jar:/usr/local/jdk1.8.0_291/jre/lib/jce.jar:/usr/local/jdk1.8.0_291/jre/lib/jfr.jar:/usr/local/jdk1.8.0_291/jre/lib/jfxswt.jar:/usr/local/jdk1.8.0_291/jre/lib/jsse.jar:/usr/local/jdk1.8.0_291/jre/lib/management-agent.jar:/usr/local/jdk1.8.0_291/jre/lib/plugin.jar:/usr/local/jdk1.8.0_291/jre/lib/resources.jar:/usr/local/jdk1.8.0_291/jre/lib/rt.jar:/home/yeqiang/code/untitled/out/production/untitled com.company.Main
start set.1658304812227
end set.1658304812227
1658304812228

Process finished with exit code 0

说明：t1线程每个循环都会休眠1毫秒，是CPU空闲，t2线程修改n=1，待t1线程唤醒后，可以得到最新的数据。此时并没有使用volatile修饰变量n，但这是一种低效且不安全的方法。