轉(zhuǎn)載請(qǐng)以鏈接形式標(biāo)明出處：
本文出自:103style的博客

本文是 看到 這篇文章中 “volatile 的意義痹束？” 那一小節(jié)提供的一個(gè)例子引發(fā)的測(cè)試软驰。

volatile 的意義觅廓？

• 防止CPU指令重排序

volatile有兩條關(guān)鍵的語義：

• 保證被volatile修飾的變量對(duì)所有線程都是可見的
• 禁止進(jìn)行指令重排序

volatile變量具有下列特性:

• 可見性：總是能看到（任意線程）對(duì)這個(gè)volatile變量最后的寫入。
• 原子性：對(duì)任意單個(gè)volatile變量的讀/寫具有原子性驼侠，但類似于volatile++這種復(fù)合操作不具有原子性零聚。

下面的例子是用來證明下面這個(gè)觀點(diǎn)的后半句是錯(cuò)誤的。

由于volatile修飾的變量在各個(gè)線程里都是一致的拉队，所以基于volatile變量的運(yùn)算在多線程并發(fā)的情況下是安全的弊知。

原測(cè)試示例

例子是這樣的：

public class Test {
    private volatile int start = 0;
    public static void main(String[] args) {
        new Test().test();
    }
    private void test() {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                add();
            }
        };
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(runnable);
            thread.start();
        }
        System.out.println("start = " + start);
    }
    void add() {
        for (int i = 0; i < 10; i++) {
            start++;
        }
    }
}

大家可以看看 最后運(yùn)行的結(jié)果輸出的 start 值是多少 ?

給 add 方法加上 synchronized 之后

給 add() 方法加上 synchronized 之后輸出的 start 值又是多少呢 ？

public class Test {
    private volatile int start = 0;
    public static void main(String[] args) {
        new Test().test();
    }
    private void test() {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                add();
            }
        };
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(runnable);
            thread.start();
        }
        System.out.println("start = " + start);
    }
    synchronized void add() {
        for (int i = 0; i < 10; i++) {
            start++;
        }
    }
}

兩個(gè)測(cè)試的結(jié)果

兩個(gè)結(jié)果都是 100粱快？ 還是只有第二個(gè)是100 ?

其實(shí)兩個(gè)結(jié)果都不是100秩彤，這是因?yàn)?main方法對(duì)應(yīng)的線程 不會(huì)等待 新創(chuàng)建的線程執(zhí)行完。

我們可以加上時(shí)間輸出看看試試：

public class Test {
    private volatile int start = 0;
    public static void main(String[] args) {
        new Test().test();
    }
    private void test() {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                add();
                System.out.println("new thread: " + System.currentTimeMillis());
            }
        };
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(runnable);
            thread.start();
        }
        System.out.println("start = " + start);
        System.out.println("main: " + System.currentTimeMillis());
    }
    void add() {
        for (int i = 0; i < 10; i++) {
            start++;
        }
    }
}

控制臺(tái)打印的結(jié)果：

new thread: 1583915204005
new thread: 1583915204005
new thread: 1583915204007
new thread: 1583915204007
new thread: 1583915204007
new thread: 1583915204007
start = 60
main: 1583915204007
new thread: 1583915204008
new thread: 1583915204008
new thread: 1583915204008
new thread: 1583915204008

可以看到 main方法對(duì)應(yīng)的線程 先執(zhí)行完了事哭。

然后為了解決 main方法對(duì)應(yīng)的線程 先執(zhí)行完漫雷， 我們加上 Thread.sleep(1000); 看看：

public class Test {
    private volatile int start = 0;
    public static void main(String[] args) {
        new Test().test();
    }
    private void test() {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                add();
                System.out.println("new thread: " + System.currentTimeMillis());
            }
        };
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(runnable);
            thread.start();
        }
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("start = " + start);
        System.out.println("main: " + System.currentTimeMillis());
    }
    void add() {
        for (int i = 0; i < 10; i++) {
            start++;
        }
    }
}

查看控制臺(tái)輸出：

new thread: 1583915390819
new thread: 1583915390821
new thread: 1583915390822
new thread: 1583915390823
new thread: 1583915390823
new thread: 1583915390823
new thread: 1583915390823
new thread: 1583915390823
new thread: 1583915390823
new thread: 1583915390823
start = 100
main: 1583915391822

去掉修飾 start 的 volatile 修飾符

然后我們?cè)囋嚢?volatile 修飾符 去掉試試，運(yùn)行的結(jié)果輸出的 start 值是多少 ?

我們修改代碼如下：

public class Test {
    private int start = 0;
    public static void main(String[] args) {
        List<Integer> res = new ArrayList<>();
        for (int i = 0; i < 20; i++) {
            res.add(new Test().test());
        }
        System.out.println(res);
    }
    private int test() {
        Runnable runnable = () -> add();
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(runnable);
            thread.start();
        }
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return start;
    }
    void add() {
        for (int i = 0; i < 10; i++) {
            start++;
        }
    }
}

查看控制臺(tái)輸出：

[100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100]

就不賣關(guān)子了鳍咱，這里結(jié)果沒問題的原因主要是因?yàn)?創(chuàng)建線程的耗時(shí) 比 add 方法執(zhí)行的耗時(shí)還長降盹， 所以就相當(dāng)與單線程執(zhí)行了，我們可以來驗(yàn)證下谤辜。

public class Test {
    private int start = 0;
    public static void main(String[] args) {
        new Test().test();
    }
    private int test() {
        for (int i = 0; i < 10; i++) {
            long t = System.nanoTime();
            Thread thread = new Thread(new TestRunnable(i));
            System.out.println(i + "_new_thred: " + (System.nanoTime() - t));
            thread.start();
        }
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return start;
    }
    void add() {
        for (int i = 0; i < 10; i++) {
            start++;
        }
    }
    private class TestRunnable implements Runnable {
        int id;
        public TestRunnable(int i) {
            id = i;
        }
        @Override
        public void run() {
            long t = System.nanoTime();
            add();
            System.out.println(id + "_add: " + (System.nanoTime() - t));
        }
    }
}

查看控制臺(tái)輸出：

0_new_thred: 1232700
1_new_thred: 31800
2_new_thred: 18000
3_new_thred: 24500
0_add: 62100
4_new_thred: 19700
5_new_thred: 76800
3_add: 19200
6_new_thred: 22300
7_new_thred: 24500
8_new_thred: 32000
9_new_thred: 26100
4_add: 23100
8_add: 20000
7_add: 18400
1_add: 20900
2_add: 19600
5_add: 40300
9_add: 22100
6_add: 23600

當(dāng)我們修改 add 方法的次數(shù)為 10W 次之后：

public class Test {
    private int start = 0;
    public static void main(String[] args) {
        List<Integer> res = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            res.add(new Test().test());
        }
        System.out.println(res);
    }
    private int test() {
        Runnable runnable = () -> add();
        ExecutorService executorService = Executors.newFixedThreadPool(100);
        for (int i = 0; i < 100; i++) {
            executorService.execute(runnable);
        }
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        executorService.shutdown();
        return start;
    }
    void add() {
        for (int i = 0; i < 100000; i++) {
            start++;
        }
    }
}

查看控制臺(tái)輸出：

[9337838, 9957329, 10000000, 10000000, 10000000, 9925170, 10000000, 9922369, 10000000, 10000000]

修改上面的測(cè)試代碼蓄坏，給 start 添加 volatile 修飾符：

public class Test {
    private volatile int start = 0;
    public static void main(String[] args) {
        List<Integer> res = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            res.add(new Test().test());
        }
        System.out.println(res);
    }
    private int test() {
        Runnable runnable = () -> add();
        ExecutorService executorService = Executors.newFixedThreadPool(100);
        for (int i = 0; i < 100; i++) {
            executorService.execute(runnable);
        }
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        executorService.shutdown();
        return start;
    }
    void add() {
        for (int i = 0; i < 100000; i++) {
            start++;
        }
    }
}

查看控制臺(tái)輸出：

[2292403, 2449807, 2146843, 1899033, 2120498, 4689152, 2264998, 2181451, 2266435, 2443323]

可以發(fā)現(xiàn)結(jié)果也是不對(duì)的价捧。

執(zhí)行結(jié)果正確的代碼

要正確輸出結(jié)果我們可以修改代碼如下：

public class Test {
    private int start = 0;
    public static void main(String[] args) {
        List<Integer> res = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            res.add(new Test().test());
        }
        System.out.println(res);
    }

    private int test() {
        Runnable runnable = () -> add();
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        for (int i = 0; i < 100; i++) {
            executorService.execute(runnable);
        }
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        executorService.shutdown();
        return start;
    }
    synchronized void add() {
        for (int i = 0; i < 100000; i++) {
            start++;
        }
    }
}

或者

public class Test {
    private AtomicInteger start = new AtomicInteger(0);
    public static void main(String[] args) {
        List<Integer> res = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            res.add(new Test().test());
        }
        System.out.println(res);
    }
    private int test() {
        Runnable runnable = () -> add();
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        for (int i = 0; i < 100; i++) {
            executorService.execute(runnable);
        }
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        executorService.shutdown();
        return start.get();
    }
    void add() {
        for (int i = 0; i < 100000; i++) {
            start.addAndGet(1);
        }
    }
}

或者

public class Test {
    private static ReentrantLock lock = new ReentrantLock();
    private int start = 0;

    public static void main(String[] args) {
        List<Integer> res = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            res.add(new Test().test());
        }
        System.out.println(res);
    }

    private int test() {
        Runnable runnable = () -> add();
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        for (int i = 0; i < 100; i++) {
            executorService.execute(runnable);
        }
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        executorService.shutdown();
        return start;
    }

    void add() {
        lock.lock();
        try {
            for (int i = 0; i < 100000; i++) {
                start++;
            }
        } finally {
            lock.unlock();
        }
    }
}

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