《Java多线程编程核心技术》读后感（四）

 

 

 

 

将任意对象作为对象监视器

 

synchronized同步代码块还支持任意对象，使用格式为synchronized（非this对象）

package Second;

public class Service {

    private String usernameParam;
    private String passwordParam;
 　private String anyString =  new String();
    public void setUsernamePassword(String username, String password) {
        try {
            synchronized (anyString) {
                System.out.println("线程名称为：" + Thread.currentThread().getName()
                        + "在" + System.currentTimeMillis() + "进入同步块");
                usernameParam = username;
                Thread.sleep(3000);
                passwordParam = password;
                System.out.println("线程名称为：" + Thread.currentThread().getName()
                        + "在" + System.currentTimeMillis() + "离开同步块");
            }
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

package Second;

public class ThreadA extends Thread {
    private Service service;

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

    @Override
    public void run() {
        service.setUsernamePassword("a", "aa");

    }

}

package Second;

public class ThreadB extends Thread {

    private Service service;

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

    @Override
    public void run() {
        service.setUsernamePassword("b", "bb");

    }

}

package Second;

public class Run {

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

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

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

    }

}

package Second;

public class Service {

    private String usernameParam;
    private String passwordParam;
    public void setUsernamePassword(String username, String password) {
        try {
            String anyString = new String();
            synchronized (anyString) {
                System.out.println("线程名称为：" + Thread.currentThread().getName()
                        + "在" + System.currentTimeMillis() + "进入同步块");
                usernameParam = username;
                Thread.sleep(3000);
                passwordParam = password;
                System.out.println("线程名称为：" + Thread.currentThread().getName()
                        + "在" + System.currentTimeMillis() + "离开同步块");
            }
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

 

package Second;

public class Service {

    private String anyString = new String();

    public void a() {
        try {
            synchronized (anyString) {
                System.out.println("a begin");
                Thread.sleep(3000);
                System.out.println("a   end");
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    synchronized public void b() {
        System.out.println("b begin");
        System.out.println("b   end");
    }

}

package Second;

public class ThreadA extends Thread {
    private Service service;

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

    @Override
    public void run() {
        service.a();

    }

}

package Second;

public class ThreadB extends Thread {

    private Service service;

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

    @Override
    public void run() {
        service.b();

    }

}

package Second;

public class Run {

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

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

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

    }

}

由于对象监视器不同，所以运行结果就是异步的

下面验证多个线程调用同一个方法是随机的

package Second;

import java.util.ArrayList;
import java.util.List;

public class MyList {

    private List list = new ArrayList();

    synchronized public void add(String username) {
        System.out.println("ThreadName=" + Thread.currentThread().getName()
                + "执行了add方法！");
        list.add(username);
        System.out.println("ThreadName=" + Thread.currentThread().getName()
                + "退出了add方法！");
    }

    synchronized public int getSize() {
        System.out.println("ThreadName=" + Thread.currentThread().getName()
                + "执行了getSize方法！");
        int sizeValue = list.size();
        System.out.println("ThreadName=" + Thread.currentThread().getName()
                + "退出了getSize方法！");
        return sizeValue;
    }

}

package Second;

public class MyThreadA extends Thread {

    private MyList list;

    public MyThreadA(MyList list) {
        super();
        this.list = list;
    }

    @Override
    public void run() {
        for (int i = 0; i < 100000; i++) {
            list.add("threadA" + (i + 1));
        }
    }

}

package Second;

public class MyThreadB extends Thread {

    private MyList list;

    public MyThreadB(MyList list) {
        super();
        this.list = list;
    }

    @Override
    public void run() {
        for (int i = 0; i < 100000; i++) {
            list.add("threadB" + (i + 1));
        }
    }
}

package Second;

public class Test {

    public static void main(String[] args) {
        MyList mylist = new MyList();

        MyThreadA a = new MyThreadA(mylist);
        a.setName("A");
        a.start();

        MyThreadB b = new MyThreadB(mylist);
        b.setName("B");
        b.start();
    }

}

下面展示多线程出现脏读的情况

package Second;

import java.util.ArrayList;
import java.util.List;

public class MyOneList {

    private List list = new ArrayList();

    synchronized public void add(String data) {
        list.add(data);
    };

    synchronized public int getSize() {
        return list.size();
    };

}

package Second;

public class MyService {

    public MyOneList addServiceMethod(MyOneList list, String data) {
        try {
            synchronized (list) {
                if (list.getSize() < 1) {
                    Thread.sleep(2000);
                    list.add(data);
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return list;
    }

}

package Second;

public class MyThread1 extends Thread {

    private MyOneList list;

    public MyThread1(MyOneList list) {
        super();
        this.list = list;
    }

    @Override
    public void run() {
        MyService msRef = new MyService();
        msRef.addServiceMethod(list, "A");
    }

}

package Second;

public class MyThread2 extends Thread {

    private MyOneList list;

    public MyThread2(MyOneList list) {
        super();
        this.list = list;
    }

    @Override
    public void run() {
        MyService msRef = new MyService();
        msRef.addServiceMethod(list, "B");
    }

}

package Second;

public class Run {

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

        MyThread1 thread1 = new MyThread1(list);
        thread1.setName("A");
        thread1.start();

        MyThread2 thread2 = new MyThread2(list);
        thread2.setName("B");
        thread2.start();
        
        Thread.sleep(6000);
        
        System.out.println("listSize=" + list.getSize());

    }

}

"脏读"出现了。出现的原因是两个线程以异步的方式返回list参数的size()大小。解决办法是“”同步化

package Second;

public class MyService {

    public MyOneList addServiceMethod(MyOneList list, String data) {
        try {
            synchronized (list) {
                if (list.getSize() < 1) {
                    Thread.sleep(2000);
                    list.add(data);
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return list;
    }

}

由于list参数对象在项目中是一份实例，是单例的，而且也正需要对list参数的getsize()方法做同步的调用，所以就对list参数进行同步处理