方法内的临时变量是线程安全：

 

方法内部的私有变量，是线程安全的。

public class HasSelfPrivateNum {    public void addI(String username) {        try {            int num = 0;            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 HasSelfPrivateNumThreadA extends Thread{    private HasSelfPrivateNum numRef;    public HasSelfPrivateNumThreadA(HasSelfPrivateNum numRef) {        super();        this.numRef = numRef;    }    @Override    public void run() {        super.run();        numRef.addI("a");    }}public class HasSelfPrivateNumThreadB extends Thread {    private HasSelfPrivateNum numRef;    public HasSelfPrivateNumThreadB(HasSelfPrivateNum numRef) {        super();        this.numRef = numRef;    }    @Override    public void run() {        super.run();        numRef.addI("b");    }}public class ThreadRunMain {    public static void main(String[] args) {        testHasSelfPrivateNumThread();    }    public static void testHasSelfPrivateNumThread(){        HasSelfPrivateNum numRef = new HasSelfPrivateNum();        HasSelfPrivateNumThreadA athread = new HasSelfPrivateNumThreadA(numRef);        athread.start();        HasSelfPrivateNumThreadB bthread = new HasSelfPrivateNumThreadB(numRef);        bthread.start();    }}

运行结果：