局部变量和成员变量的安全问题

原创

罗罗的1024 2021-08-27 10:14:48 博主文章分类：多线程与并发 ©著作权

文章标签 java 多线程 thread ide jar 文章分类 代码人生

©著作权归作者所有：来自51CTO博客作者罗罗的1024的原创作品，请联系作者获取转载授权，否则将追究法律责任

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。