java多线程核心技术——synchronized同步方法与synchronized同步快

Posted 2020-09-03 Vip灬cnblog

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一、synchronized同步方法

论：“线程安全”与“非线程安全”是多线程的经典问题。synchronized（）方法就是解决非线程安全的。

1、方法内的变量为线程安全

public void addI(String username) {
        try {
            int num = 0;     \\\\方法内的变量为线程安全
            if (username.equals("a")) {
                num = 100;
                System.out.println("a set over!");
                Thread.sleep(2000);
            } else {
                num = 200;
                System.out.println("b set over!");
            }
            System.out.println(username + " num=" + num);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

2、实例变量非线程安全

public class HasSelfPrivateNum {
    private int num = 0;    \\\\实例变量非线程安全
    public void addI(String username) {
        try {
            if (username.equals("a")) {
                num = 100;
                System.out.println("a set over!");
                Thread.sleep(2000);
            } else {
                num = 200;
                System.out.println("b set over!");
            }
            System.out.println(username + " num=" + num);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

解决方法：方法前加synchronized关键字。

public class HasSelfPrivateNum {

    private int num = 0;

    synchronized public void addI(String username) {
       ..............
    }

}

3、多个对象多个锁

HasSelfPrivateNum.java

public class HasSelfPrivateNum {

    private int num = 0;

    synchronized public void addI(String username) {
        try {
            if (username.equals("a")) {
                num = 100;
                System.out.println("a set over!");
                Thread.sleep(2000);
            } else {
                num = 200;
                System.out.println("b set over!");
            }
            System.out.println(username + " num=" + num);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

ThreadA

    
    public class ThreadA extends Thread {
    
        private HasSelfPrivateNum numRef;
    
        public ThreadA(HasSelfPrivateNum numRef) {
            super();
            this.numRef = numRef;
        }
    
        @Override
        public void run() {
            super.run();
            numRef.addI("a");
        }
    
    }

ThreadB

public class ThreadB extends Thread {

    private HasSelfPrivateNum numRef;

    public ThreadB(HasSelfPrivateNum numRef) {
        super();
        this.numRef = numRef;
    }

    @Override
    public void run() {
        super.run();
        numRef.addI("b");
    }

}

RUN

public class Run {

    public static void main(String[] args) {

        HasSelfPrivateNum numRef1 = new HasSelfPrivateNum();
        HasSelfPrivateNum numRef2 = new HasSelfPrivateNum();

        ThreadA athread = new ThreadA(numRef1);
        athread.start();

        ThreadB bthread = new ThreadB(numRef2);
        bthread.start();

    }

}

结果：创建了2个业务实例，产生2个锁，所以运行结果是异步的。同步为synchronized 异步：asynchronized

4、synchronized 锁重入

当一个线程得到一个对象锁时，再次请求该对象锁时是可以再次得到该对象的锁的。继承关系也可重入锁。

当一个线程执行发生异常时，其持有的锁会自动释放。

同步不具有继承性。的在子类方法中添加synchronized的关键字。

public class HasSelfPrivateNum {
    synchronized public void service1() {
        System.out.println("service1");
        service2();
    }

    synchronized public void service2() {
        System.out.println("service2");
        service3();
    }

    synchronized public void service3() {
        System.out.println("service3");
    }
}

HasSelfPrivateNum.java

public class mYThread extends Thread {
    @Override
    public void run() {
        HasSelfPrivateNum hspn=new HasSelfPrivateNum();
        hspn.service1();    
    }
   public static void main(String[] args) {
       mYThread mYThread=new mYThread();
       mYThread.start();
 }
}

mYThread.java

二、synchronized同步快

synchronized同步方法持有锁后，如果长时间不释放，那另一个线程就必须长时间等待。。这种情况下synchronized同步快来解决

使用方法如图：

1、synchronized同步快之间具有同步性，当线程访问object的一个 synchronized(this)同步代码块时，其他线程对同一个object对象中所有其他的

synchronized(this)同步代码块的访问会被阻塞。

2、synchronized(this)同步代码块是锁定当前对象的。

1、synchronized同步快解决同步方法效率低得弊端

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1 threadName="
                    + Thread.currentThread().getName();
            getData2 = "长时间处理任务后从远程返回的值2 threadName="
                    + 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();
        }
    }
}

Task

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();
    }

}

MyThread1

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();
    }

}

MyThread2

public class CommonUtils {

    public static long beginTime1;
    public static long endTime1;

    public static long beginTime2;
    public static long endTime2;
}

CommonUtils

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));
    }
}

Run

修改Task

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();
        }
    }
}

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