Java 学习笔记之 实例变量非线程安全

实例变量非线程安全：

如果多个线程共同访问1个对象中的实例变量，则可能出现“非线程安全”问题。

public class UnSafeHasSelfPrivateNum {
    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();
        }
    }
}
 
public class UnSafeHasSelfPrivateNumThreadA extends Thread{
    private UnSafeHasSelfPrivateNum numRef;
 
    public UnSafeHasSelfPrivateNumThreadA(UnSafeHasSelfPrivateNum numRef) {
        this.numRef = numRef;
    }
 
    @Override
    public void run() {
        super.run();
        numRef.addI("a");
    }
}
 
public class UnSafeHasSelfPrivateNumThreadB extends Thread {
    private UnSafeHasSelfPrivateNum numRef;
 
    public UnSafeHasSelfPrivateNumThreadB(UnSafeHasSelfPrivateNum numRef) {
        this.numRef = numRef;
    }
 
    @Override
    public void run() {
        super.run();
        numRef.addI("b");
    }
}
 
public class ThreadRunMain {
    public static void main(String[] args) {
        testUnSafeHasSelfPrivateNumThread();
    }
 
    public static void testUnSafeHasSelfPrivateNumThread(){
        UnSafeHasSelfPrivateNum numRef = new UnSafeHasSelfPrivateNum();
        UnSafeHasSelfPrivateNumThreadA athread = new UnSafeHasSelfPrivateNumThreadA(numRef);
        athread.start();
        UnSafeHasSelfPrivateNumThreadB bthread = new UnSafeHasSelfPrivateNumThreadB(numRef);
        bthread.start();
    }
}

运行结果：

a的值本应该是100，却变成了200. 用synchronized关键字解决问题：

public class UnSafeHasSelfPrivateNum {
    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();
        }
    }
}

运行结果：

在两个线程访问同一个对象中的同步方法时是线程安全的，先执行a，在执行b。