《Java多线程编程核心技术》读后感（三）

 

synchronized同步语句块

用synchronized声明方法在某些情况下是有弊端的，比如A线程调用同步方法执行以一个长时间的任务，那么B线程则必须等待比较较长的时间。在这样的情况下可以使用synchronized同步语句块来解决

synchronized方法的弊端

package Second;

public class Task {

    private String getData1;
    private String getData2;

    public synchronized void doLongTimeTask() {
        try {
            System.out.println("begin task");
            Thread.sleep(3000);
            getData1 = "长时间处理任务后从远程返回的值1threadName="
                    + Thread.currentThread().getName();
            getData2 = "长时间处理任务后从远程返回的值2threadName="
                    + Thread.currentThread().getName();
            System.out.println(getData1);
            System.out.println(getData2);
            System.out.println("end task");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

package Second;

public class CommonUtils {

    public static long beginTime1;
    public static long endTime1;

    public static long beginTime2;
    public static long endTime2;
}

package Second;

public class MyThread1 extends Thread {

    private Task task;

    public MyThread1(Task task) {
        super();
        this.task = task;
    }

    @Override
    public void run() {
        super.run();
        CommonUtils.beginTime1 = System.currentTimeMillis();
        task.doLongTimeTask();
        CommonUtils.endTime1 = System.currentTimeMillis();
    }

}

package Second;

public class MyThread2 extends Thread {

    private Task task;

    public MyThread2(Task task) {
        super();
        this.task = task;
    }

    @Override
    public void run() {
        super.run();
        CommonUtils.beginTime2 = System.currentTimeMillis();
        task.doLongTimeTask();
        CommonUtils.endTime2 = System.currentTimeMillis();
    }

}

package Second;

public class Run {

    public static void main(String[] args) {
        Task task = new Task();

        MyThread1 thread1 = new MyThread1(task);
        thread1.start();

        MyThread2 thread2 = new MyThread2(task);
        thread2.start();

        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        long beginTime = CommonUtils.beginTime1;
        if (CommonUtils.beginTime2 < CommonUtils.beginTime1) {
            beginTime = CommonUtils.beginTime2;
        }

        long endTime = CommonUtils.endTime1;
        if (CommonUtils.endTime2 > CommonUtils.endTime1) {
            endTime = CommonUtils.endTime2;
        }

        System.out.println("耗时：" + ((endTime - beginTime) / 1000));
    }
}

synchronized同步代码块的使用

当两个并发线程访问同一个对象object中的synchronized（this）同步代码块时，一段时间内只能有一个线程被执行，另一个线程必须等待当前线程执行完这个代码块以后才能执行该代码块

package Second;

public class ObjectService {

    public void serviceMethod() {
        try {
            synchronized (this) {
                System.out.println("begin time=" + System.currentTimeMillis());
                Thread.sleep(2000);
                System.out.println("end    end=" + System.currentTimeMillis());
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

package Second;

public class ThreadA extends Thread {

    private ObjectService service;

    public ThreadA(ObjectService service) {
        super();
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.serviceMethod();
    }

}

package Second;

public class ThreadB extends Thread {
    private ObjectService service;

    public ThreadB(ObjectService service) {
        super();
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.serviceMethod();
    }
}

package Second;

public class Run {

    public static void main(String[] args) {
        ObjectService service = new ObjectService();

        ThreadA a = new ThreadA(service);
        a.setName("a");
        a.start();

        ThreadB b = new ThreadB(service);
        b.setName("b");
        b.start();
    }

}

虽然使用了同步代码块，但执行的效率还是没有提高

用同步代码块解决同步方法的弊端

package Second;

public class CommonUtils {

    public static long beginTime1;
    public static long endTime1;

    public static long beginTime2;
    public static long endTime2;
}

package Second;

public class MyThread1 extends Thread {

    private Task task;

    public MyThread1(Task task) {
        super();
        this.task = task;
    }

    @Override
    public void run() {
        super.run();
        CommonUtils.beginTime1 = System.currentTimeMillis();
        task.doLongTimeTask();
        CommonUtils.endTime1 = System.currentTimeMillis();
    }

}

package Second;

public class MyThread2 extends Thread {

    private Task task;

    public MyThread2(Task task) {
        super();
        this.task = task;
    }

    @Override
    public void run() {
        super.run();
        CommonUtils.beginTime2 = System.currentTimeMillis();
        task.doLongTimeTask();
        CommonUtils.endTime2 = System.currentTimeMillis();
    }

}

package Second;

public class Task {

    private String getData1;
    private String getData2;

    public void doLongTimeTask() {
        try {
            System.out.println("begin task");
            Thread.sleep(3000);

            String privateGetData1 = "长时间处理任务后从远程返回的值1 threadName="
                    + Thread.currentThread().getName();
            String privateGetData2 = "长时间处理任务后从远程返回的值2 threadName="
                    + Thread.currentThread().getName();

            synchronized (this) {
                getData1 = privateGetData1;
                getData2 = privateGetData2;
            }
            
            System.out.println(getData1);
            System.out.println(getData2);
            System.out.println("end task");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

package Second;

public class MyThread1 extends Thread {

    private Task task;

    public MyThread1(Task task) {
        super();
        this.task = task;
    }

    @Override
    public void run() {
        super.run();
        CommonUtils.beginTime1 = System.currentTimeMillis();
        task.doLongTimeTask();
        CommonUtils.endTime1 = System.currentTimeMillis();
    }

}

一半异步，一半同步

package Second;

public class Task {

    public void doLongTimeTask() {
        for (int i = 0; i < 100; i++) {
            System.out.println("nosynchronized threadName="
                    + Thread.currentThread().getName() + " i=" + (i + 1));
        }
        System.out.println("");
        synchronized (this) {
            for (int i = 0; i < 100; i++) {
                System.out.println("synchronized threadName="
                        + Thread.currentThread().getName() + " i=" + (i + 1));
            }
        }

    }
}

package Second;

public class MyThread1 extends Thread {

    private Task task;

    public MyThread1(Task task) {
        super();
        this.task = task;
    }

    @Override
    public void run() {
        super.run();
        task.doLongTimeTask();
    }

}

package Second;

public class MyThread2 extends Thread {

    private Task task;

    public MyThread2(Task task) {
        super();
        this.task = task;
    }

    @Override
    public void run() {
        super.run();
        task.doLongTimeTask();
    }

}

package Second;

public class Run {

    public static void main(String[] args) {
        Task task = new Task();

        MyThread1 thread1 = new MyThread1(task);
        thread1.start();

        MyThread2 thread2 = new MyThread2(task);
        thread2.start();
    }
}

 进入代码块后则排队执行

synchronized代码块间的同步性

当一个线程访问object的一个同步代码块时，其他线程对同一个object中所有其他的同步代码块的访问将被阻塞。这说明synchronized使用的时一个“对象监视器”

package Second;

public class ObjectService {

    public void serviceMethodA() {
        try {
            synchronized (this) {
                System.out.println("A begin time=" + System.currentTimeMillis());
                Thread.sleep(2000);
                System.out.println("A end    end=" + System.currentTimeMillis());
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void serviceMethodB() {
        synchronized (this) {
            System.out.println("B begin time=" + System.currentTimeMillis());
            System.out.println("B end    end=" + System.currentTimeMillis());
        }
    }
}

package Second;

public class ThreadA extends Thread {

    private ObjectService service;

    public ThreadA(ObjectService service) {
        super();
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.serviceMethodA();
    }

}

package Second;

public class ThreadB extends Thread {
    private ObjectService service;

    public ThreadB(ObjectService service) {
        super();
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.serviceMethodB();
    }
}

package Second;

public class Run {

    public static void main(String[] args) {
        ObjectService service = new ObjectService();

        ThreadA a = new ThreadA(service);
        a.setName("a");
        a.start();

        ThreadB b = new ThreadB(service);
        b.setName("b");
        b.start();
    }

}

两个代码块按顺序执行

验证同步代码块是锁定当前对象的

package Second;

public class Task {

    public void otherMethod() {
        System.out.println("------------------------run--otherMethod");
    }

    public void doLongTimeTask() {
        synchronized (this) {
            for (int i = 0; i < 10000; i++) {
                System.out.println("synchronized threadName="
                        + Thread.currentThread().getName() + " i=" + (i + 1));
            }
        }

    }
}

package Second;

public class MyThread1 extends Thread {

    private Task task;

    public MyThread1(Task task) {
        super();
        this.task = task;
    }

    @Override
    public void run() {
        super.run();
        task.doLongTimeTask();
    }

}

package Second;

public class MyThread2 extends Thread {

    private Task task;

    public MyThread2(Task task) {
        super();
        this.task = task;
    }

    @Override
    public void run() {
        super.run();
        task.otherMethod();
    }

}

package Second;

public class Run {

    public static void main(String[] args) throws InterruptedException {
        Task task = new Task();

        MyThread1 thread1 = new MyThread1(task);
        thread1.start();

        Thread.sleep(100);

        MyThread2 thread2 = new MyThread2(task);
        thread2.start();
    }
}

package Second;

public class Task {

    synchronized public void otherMethod() {
        System.out.println("------------------------run--otherMethod");
    }

    public void doLongTimeTask() {
        synchronized (this) {
            for (int i = 0; i < 10000; i++) {
                System.out.println("synchronized threadName="
                        + Thread.currentThread().getName() + " i=" + (i + 1));
            }
        }

    }
}

实际上，synchronized(this)以及非static的synchronized方法（至于static synchronized方法请往下看），只能防止多个线程同时执行同一个对象的同步代码段。

回到本文的题目上：synchronized锁住的是代码还是对象。答案是：synchronized锁住的是括号里的对象，而不是代码。对于非static的synchronized方法，锁的就是对象本身也就是this。

参考：https://www.cnblogs.com/QQParadise/articles/5059824.html