变量的并发访问

synchronized的同步方法

方法内的变量为线程安全的

"非线程安全" 问题存在于“实例变量”中，如果是方法内部的私有变量，则不存在“非线程安全”问题，也就是线程安全。

public class HasSelfPrivateNum  {


    public void addI(String name) {
        try {
            int num = 0;
            if ("a".equals(name)) {
                num=100;
                System.out.println("a set over!");
                Thread.sleep(2000);
            } else {
                num = 200;
                System.out.println("b set over!");
            }
            System.out.println(name +" num="+num);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}
线程A
public class ThreadA extends  Thread {
    private HasSelfPrivateNum hasSelfPrivateNum;

    public ThreadA(HasSelfPrivateNum hasSelfPrivateNum) {
        this.hasSelfPrivateNum = hasSelfPrivateNum;
    }

    @Override
    public void run() {
        hasSelfPrivateNum.addI("a");
    }
}

线程B
public class ThreadB  extends  Thread{
    private HasSelfPrivateNum hasSelfPrivateNum;

    public ThreadB(HasSelfPrivateNum hasSelfPrivateNum) {
        this.hasSelfPrivateNum = hasSelfPrivateNum;
    }

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

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

运行结果：

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