多线程编程核心技术-对象及变量的并发访问-synchronize同步方法(2）（上）

2022-10-09 85

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简介： 多线程编程核心技术-对象及变量的并发访问-synchronize同步方法(2）（上）

对象及变量的并发访问之synchronize同步方法：

1）方法内的变量为线程安全

例子:

public class HasSelfPrivateNum {
    public void addI(String username){
        try{
            int num = 0;
            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){
            e.printStackTrace();
        }
    }
}

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");
    }
}

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");
    }
}

public class Run {
    public static void main(String[] args) {
        HasSelfPrivateNum selfPrivateNum = new HasSelfPrivateNum();
        ThreadA threadA = new ThreadA(selfPrivateNum);
        threadA.start();
        ThreadB threadB = new ThreadB(selfPrivateNum);
        threadB.start();
    }
}

运行结果:

2）实例变量非线程安全

上面的例子,讲num从私用变量，改成成员变量。再次运行结果:

public class HasSelfPrivateNum {
    private  int num = 0;
    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){
            e.printStackTrace();
        }
    }
}

运行结果:

解决非线程安全

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){
            e.printStackTrace();
        }
    }
}

再次运行结果:

3)多个对象多个锁

接着上面的实验，改动下，Run类，声明2个对象

public class Run {
    public static void main(String[] args) {
        HasSelfPrivateNum selfPrivateNum1 = new HasSelfPrivateNum();
        HasSelfPrivateNum selfPrivateNum2 = new HasSelfPrivateNum();
        ThreadA threadA = new ThreadA(selfPrivateNum1);
        threadA.start();
        ThreadB threadB = new ThreadB(selfPrivateNum2);
        threadB.start();
    }
}

看下效果:

4)synchronize方法与锁对象

实验1

public class MyObject {
    public void methodA(){
        try{
            System.out.println("begin methodA threadName=" + Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("end");
        }catch (InterruptedException e){
            e.printStackTrace();
        }
    }
}

public class ThreadA extends Thread{
   private MyObject object;
    public ThreadA(MyObject object) {
        this.object = object;
    }
    @Override
    public void run() {
        super.run();
        object.methodA();
    }
}

public class ThreadB extends Thread {
    private MyObject object;
    public ThreadB(MyObject object) {
        this.object = object;
    }
    @Override
    public void run() {
        super.run();
        object.methodA();
    }
}

public class Run {
    public static void main(String[] args) {
        MyObject myObject = new MyObject();
        ThreadA thread1 = new ThreadA(myObject);
        thread1.setName("A");
        ThreadB thread2 = new ThreadB(myObject);
        thread1.start();
        thread2.setName("B");
        thread2.start();
    }
}

结果显示:2个线程一同进入了methodA方法。

实验2

改进上面的实验，加个synchronize关键字

synchronized public class MyObject {
    public void methodA(){
        try{
            System.out.println("begin methodA threadName=" + Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("end");
        }catch (InterruptedException e){
            e.printStackTrace();
        }
    }
}

结果：排队进入方法

实验3：

继续改进myObject类，再加一个非同步方法

public class MyObject {
    synchronized  public void methodA(){
        try{
            System.out.println("begin methodA threadName=" + Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("end");
        }catch (InterruptedException e){
            e.printStackTrace();
        }
    }
    public void methodB(){
        try {
            System.out.println("begin methodB threadName=" +
                                Thread.currentThread().getName() + "begin time=" +
                                System.currentTimeMillis());
            Thread.sleep(5000);
            System.out.println("end");
        }catch (Exception e){
        }
    }
}

同样，让ThreadB调用非同步方法B

public class ThreadB extends Thread {
    private MyObject object;
    public ThreadB(MyObject object) {
        this.object = object;
    }
    @Override
    public void run() {
        super.run();
        object.methodB();
    }
}