方法内的变量为线程安全

Java栈：线程私有，用于存放局部变量，方法参数，同时和java方法的调用返回密切相关。

栈上分配的基本思想，是将线程私有的对象，打散分配到栈上，分配在栈上的函数调用结束后对象会自行销毁，不需要垃圾回收接入:方法内部变量线程安全

public class Text {

public static void main(String[] args) throws InterruptedException {
// TODO Auto-generated method stub
        HasSelfPrivateNum num = new HasSelfPrivateNum();
        ThreadA a = new ThreadA(num);
        ThreadB b = new ThreadB(num);
        a.start();
        b.start();
    }
}

public class HasSelfPrivateNum {

public void addI(String username) {
int num;
if (username.equals("a")) {
            num = 100;
            System.out.println("a set over");
try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
// TODO Auto-generated catch block
                e.printStackTrace();
            }
        } else {
            num = 200;
            System.out.println("b set over");
        }
        System.out.println(username + "num = " + num);
    }
}

public class ThreadA extends Thread {
public HasSelfPrivateNum num;
public ThreadA(HasSelfPrivateNum num) {
// TODO Auto-generated constructor stub
this.num = num;
    }
public void run() {
// TODO Auto-generated method stub
        num.addI("a");
    }
}

public class ThreadB extends Thread{
private HasSelfPrivateNum num;
public ThreadB(HasSelfPrivateNum num) {
// TODO Auto-generated constructor stub
this.num = num;
    }

public void run() {
// TODO Auto-generated method stub
        num.addI("b");
    }
}