对象及变量的并发访问-----synchronized同步（1）-----方法内变量线程安全

一、方法内变量为线程安全

　　创建该类实例对象并且多线程调用，此时若是变量在方法内部，则是线程安全。

package service;

/**
 * Created by zhc on 2017/10/10
 */
public class HasSelfPrivateNum {

    public void addI(String username) {
        try {
            int num = 0;          /////////////////////////////////变量方法内部
            if (username.equals("a")) {
                num = 100;
                System.out.println("a is set over");
            } else {
                num = 200;
                System.out.println("b is set over");
            }
            System.out.println(username + " num = " + num);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

package extthread;

import service.HasSelfPrivateNum;

/**
 * Created by zhc on 2017/10/10
 */
public class ThreadA extends Thread {
    private HasSelfPrivateNum numRef;
    public ThreadA(HasSelfPrivateNum hasSelfPrivateNum){
        super();
        this.numRef = hasSelfPrivateNum;
    }

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

package extthread;

import service.HasSelfPrivateNum;

/**
 * Created by zhc on 2017/10/10
 */
public class ThreadB extends Thread {
    private HasSelfPrivateNum numRef;
    public ThreadB(HasSelfPrivateNum hasSelfPrivateNum){
        super();
        this.numRef = hasSelfPrivateNum;
    }

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

　　线程分别调用结果还是安全的，

package test;

import extthread.ThreadA;
import extthread.ThreadB;
import service.HasSelfPrivateNum;

/**
 * Created by zhc on 2017/10/10
 */
public class Run {
    public static void main(String[] args){
        HasSelfPrivateNum hasSelfPrivateNum = new HasSelfPrivateNum();
        ThreadB t2 = new ThreadB(hasSelfPrivateNum);
        ThreadA t1= new ThreadA(hasSelfPrivateNum);
        t1.start();
        t2.start();
    }
}

结果如下：

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

二、如果变量在方法外部，可能出现如下情况，造成线程不安全

b is set over
a is set over
b num = 100
a num = 100