局部变量和成员变量的安全问题

废话不多说，直接上代码
show code

/**
 * @Auther: 罗罗
 */
public class Test05 {
    public static void main(String[] args) throws InterruptedException {
        Student student = new Student();
        new T1(student).start();
        new T2(student).start();

    }
}
class T1 extends Thread{
    private Student student;
    public T1(Student student){
      this.student = student;
    }
    @Override
    public void run() {
        student.sell("a");
    }
}
class T2 extends Thread{
    private Student student;
    public T2(Student student){
        this.student = student;
    }
    @Override
    public void run() {
        student.sell("b");
    }
}
class Student{
    private int num = 0;
    public void sell(String name){
        if (name.equals("a")){
            num = 100;
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }else {
            num = 200;
        }
        System.out.println(name+" num = "+num);
    }
}

"C:\Program Files\Java\jdk1.8.0_144\bin\java.exe" "-javaagent:D:\IntelliJ IDEA\IntelliJ IDEA 2018.2.4\lib\idea_rt.jar=56700:D:\IntelliJ IDEA\IntelliJ IDEA 2018.2.4\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunec.jar;C:\Program "
b num = 200
a num = 200

Process finished with exit code 0

我们试试把变量移到方法内，看看运行的效果，多测几次

public class Test05 {
    public static void main(String[] args) throws InterruptedException {
        Student student = new Student();
        new T1(student).start();
        new T2(student).start();

    }
}
class T1 extends Thread{
    private Student student;
    public T1(Student student){
      this.student = student;
    }
    @Override
    public void run() {
        student.sell("a");
    }
}
class T2 extends Thread{
    private Student student;
    public T2(Student student){
        this.student = student;
    }
    @Override
    public void run() {
        student.sell("b");
    }
}
class Student{
    public void sell(String name){
        int num = 0;
        if (name.equals("a")){
            num = 100;
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }else {
            num = 200;
        }
        System.out.println(name+" num = "+num);
    }
}

"C:\Program Files\Java\jdk1.8.0_144\bin\java.exe" "-javaagent:D:\IntelliJ IDEA\IntelliJ IDEA Files\Java\jdk1.8.0_144\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jaccess.jar;C:\Program "
b num = 200
a num = 100

Process finished with exit code 0

为什么会这样呢？还记得前面我写的 罗罗初探JVM 吗，看过的小伙伴就知道为什么会出现上面的情况 ，感兴趣的小伙伴可以去回顾一下，下面我把关于这篇文章的需要用到的提取出来。

栈帧中主要保存3类数据：
1.本地变量(Local Variables) :输入参数和输出参数以及方法内的变量;
2.栈操作(Operand Stack) :记录出栈、入栈的操作;
3.栈帧数据(Frame Data) :包括类文件、方法等等。

线程执行一个方法就会被压入栈，而栈是每一个线程私有，所以方内的变量是线程私有的，既然是私有，肯定是安全的啊，哈哈，是不是突然就通了。