Java 同步代码块 - Synchronized Blocks

 java锁实现原理：

http://blog.csdn.net/endlu/article/details/51249156

 

The synchronized keyword can be used to mark four different types of blocks:

1. Instance methods
2. Static methods
3. Code blocks inside instance methods
4. Code blocks inside static methods

Instance methods & Code blocks inside instance methods

Java实例方法同步是同步在拥有该方法的对象上

同步构造器中用括号括起来的对象叫做监视器对象

public class SyncBlockTest {

    public static void main(String[] args) {
        ExecutorService es = Executors.newFixedThreadPool(2);
        final MySyncBlockClass syncBlockClass = new MySyncBlockClass();
        es.submit(new Runnable() {
            @Override
            public void run() {
                try {
                    syncBlockClass.mehtod1();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        es.submit(new Runnable() {
            @Override
            public void run() {
                try {
                    syncBlockClass.mehtod2();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        es.shutdown();
    }

}

class MySyncBlockClass {

    public synchronized void mehtod1() throws InterruptedException {
        TimeUnit.SECONDS.sleep(1);
        System.out.println(System.currentTimeMillis() + ":method1 run.");
    }

    public synchronized void mehtod2() throws InterruptedException {
        TimeUnit.SECONDS.sleep(4);
        System.out.println(System.currentTimeMillis() + ":method2 run.");
    }
}

// method1较method2延迟了2000ms
// 1479350064132:method2 run. 
// 1479350066132:method1 run.

/**
     * method1 与method2等效
     * 同步构造器中用括号括起来的对象叫做监视器对象
     * 
     * @throws InterruptedException
     */
    public synchronized void mehtod1() throws InterruptedException {
        TimeUnit.SECONDS.sleep(1);
        System.out.println(System.currentTimeMillis() + ":method1 run.");
    }

    public synchronized void mehtod2() throws InterruptedException {
        synchronized (this) {
            TimeUnit.SECONDS.sleep(4);
            System.out.println(System.currentTimeMillis() + ":method2 run.");
        }
    }

 

Static methods & Code blocks inside static methods

静态方法的同步是指同步在该方法所在的类对象上。因为在Java虚拟机中一个类只能对应一个类对象，所以同时只允许一个线程执行同一个类中的静态同步方法。

public class SyncBlockTest {

    public static void main(String[] args) {
        ExecutorService es = Executors.newFixedThreadPool(2);
        final MySyncBlockClass syncBlockClass1 = new MySyncBlockClass();
        final MySyncBlockClass syncBlockClass2 = new MySyncBlockClass();
        es.submit(new Runnable() {
            @Override
            public void run() {
                try {
                    syncBlockClass1.mehtod1();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        es.submit(new Runnable() {
            @Override
            public void run() {
                try {
                    syncBlockClass2.mehtod2();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        es.shutdown();
    }

}

class MySyncBlockClass {

    public static synchronized void mehtod1() throws InterruptedException {
        TimeUnit.SECONDS.sleep(1);
        System.out.println(System.currentTimeMillis() + ":method1 run.");
    }

    public static synchronized void mehtod2() throws InterruptedException {
        TimeUnit.SECONDS.sleep(4);
        System.out.println(System.currentTimeMillis() + ":method2 run.");
    }
}

// method1较method2延迟了3000ms
// 1479358310630:method1 run.
// 1479358314631:method2 run.

/**
     * method1 与method2等效
     * 静态方法中的同步块
     *
     * @throws InterruptedException
     */
    public static synchronized void mehtod1() throws InterruptedException {
        TimeUnit.SECONDS.sleep(1);
        System.out.println(System.currentTimeMillis() + ":method1 run.");
    }

    public static synchronized void mehtod2() throws InterruptedException {
        synchronized (MySyncBlockClass.class) {
            TimeUnit.SECONDS.sleep(4);
            System.out.println(System.currentTimeMillis() + ":method2 run.");
        }
    }

 

示例：

以下代码对应的Block关系如下：

synchronized(class)与 static synchronized 等效

public class SyncMethod2 {
    private int value = 0;
    private final Object mutex = new Object();

    public synchronized int incAndGet0() {
        try {
            TimeUnit.SECONDS.sleep(4);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("static synchronized void incAndGet0");
        return ++value;
    }

    public int incAndGet1() {
        synchronized(this){
            try {
                TimeUnit.SECONDS.sleep(4);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("static synchronized void incAndGet1");
            return ++value;
        }
    }

    public int incAndGet2() {
        synchronized(SyncMethod.class){
            ++value;
            try {
                TimeUnit.SECONDS.sleep(4);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("static synchronized void incAndGet4");
        }
        return 0;
    }

    public int incAndGet3() {
        synchronized(mutex){
            return ++value;
        }
    }

    public static synchronized void incAndGet4() {
        try {
            TimeUnit.SECONDS.sleep(4);
            System.out.println("static synchronized void incAndGet4");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}