2.1.2實(shí)例變量非線程安全

如果多個(gè)線程共同訪問(wèn)1個(gè)對(duì)象中的實(shí)例變量，則有可能出現(xiàn)“非線程安全”問(wèn)題。

/**
 * @author wuyoushan
 * @date 2017/4/4.
 */
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(200);
            }else{
                num=200;
                System.out.println("b set over!");
            }
            System.out.println(username+" num="+num);
        }catch (Exception ex){
            ex.printStackTrace();
        }
    }
}

/**
 * @author wuyoushan
 * @date 2017/4/4.
 */
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");
    }
}

/**
 * @author wuyoushan
 * @date 2017/4/4.
 */
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");
    }
}


/**
 * @author wuyoushan
 * @date 2017/3/20.
 */
public class Run {
    public static void main(String[] args){
       HasSelfPrivateNum numRef=new HasSelfPrivateNum();
        ThreadA threadA=new ThreadA(numRef);
        threadA.start();

        ThreadB threadB=new ThreadB(numRef);
        threadB.start();
    }
}

程序運(yùn)行結(jié)果為:

a set over!
b set over!
b num=200
a num=200

本實(shí)驗(yàn)是兩個(gè)線程同時(shí)訪問(wèn)一個(gè)沒有同步的方法，如果兩個(gè)線程同時(shí)操作業(yè)務(wù)對(duì)象中的實(shí)例變量，則有可能會(huì)出現(xiàn)“非線程安全”問(wèn)題。修改HasSelfPrivateNum.java文件如下

/**
 * @author wuyoushan
 * @date 2017/4/4.
 */
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(200);
            }else{
                num=200;
                System.out.println("b set over!");
            }
            System.out.println(username+" num="+num);
        }catch (Exception ex){
            ex.printStackTrace();
        }
    }
}

重新運(yùn)行程序，運(yùn)行結(jié)果為:

a set over!
a num=100
b set over!
b num=200

摘選自 java多線程核心編程技術(shù)-2.1.2