Thread之守护线程和线程安全问题

守护线程

---

package com.hanyxx.thread;
import lombok.SneakyThrows;

/**
 * @author layman
 * @date 2021/2/5
 */
public class ThreadDaemon04 {
    public static void main(String[] args) throws InterruptedException {
        demo04 demo04 = new demo04();
        //当把demo04设置为守护线程时，主线程结束时，demo04就会结束。
        demo04.setDaemon(true);
        demo04.start();
        Thread.sleep(2000);
        System.out.println("main线程结束");
    }
}
class demo04 extends  Thread{
    @SneakyThrows
    @Override
    public void run() {
        while(true){
            System.out.println("time= "+System.currentTimeMillis());
            Thread.sleep(500);
        }
    }
}

线程安全

验证局部变量线程安全

package com.hanyxx.thread;

import lombok.SneakyThrows;

/**
 * @author layman
 * @date 2021/2/5
 */
public class Demo05 {
    public static void main(String[] args) {
        Demo05Service service = new Demo05Service();
        Demo05ThreadA threadA = new Demo05ThreadA(service);
        threadA.start();
        Demo05ThreadB threadB = new Demo05ThreadB(service);
        threadB.start();
    }
}
class Demo05Service{
    @SneakyThrows
    public void changeNum(String username){
        int number;
        if("layman".equals(username)){
            number = 100;
            //线程睡眠，让后续线程可以修改number的值
            Thread.sleep(2000);
        }else{
            number = 200;
        }
        System.out.println("username: ["+username+"] number: ["+number+"]");
    }
}
class Demo05ThreadA extends  Thread{
    private Demo05Service service;
    public Demo05ThreadA(Demo05Service service){
        this.service = service;
    }
    @SneakyThrows
    @Override
    public void run() {
        while (true) {
            Thread.sleep(1000);
            service.changeNum("layman");
        }
    }
}
class Demo05ThreadB extends  Thread{
    private Demo05Service service;
    public Demo05ThreadB(Demo05Service service){
        this.service = service;
    }
    @SneakyThrows
    @Override
    public void run() {
        while (true) {
            Thread.sleep(1000);
            service.changeNum("菠菜饭团");
        }
    }
}

验证成员变量线程不安全

package com.hanyxx.thread;

import lombok.SneakyThrows;

/**
 * @author layman
 * @date 2021/2/5
 */
public class Demo05 {
    public static void main(String[] args) {
        Demo05Service service = new Demo05Service();
        Demo05ThreadA threadA = new Demo05ThreadA(service);
        threadA.start();
        Demo05ThreadB threadB = new Demo05ThreadB(service);
        threadB.start();
    }
}
class Demo05Service{
    int number;
    @SneakyThrows
    public void changeNum(String username){
        if("layman".equals(username)){
            number = 100;
            //线程睡眠，让后续线程可以修改number的值
            Thread.sleep(2000);
        }else{
            number = 200;
        }
        System.out.println("username: ["+username+"] number: ["+number+"]");
    }
}
class Demo05ThreadA extends  Thread{
    private Demo05Service service;
    public Demo05ThreadA(Demo05Service service){
        this.service = service;
    }
    @SneakyThrows
    @Override
    public void run() {
        while (true) {
            Thread.sleep(1000);
            service.changeNum("layman");
        }
    }
}
class Demo05ThreadB extends  Thread{
    private Demo05Service service;
    public Demo05ThreadB(Demo05Service service){
        this.service = service;
    }
    @SneakyThrows
    @Override
    public void run() {
        while (true) {
            Thread.sleep(1000);
            service.changeNum("菠菜饭团");
        }
    }
}

结论：

• 局部变量在方法内部，是私有的。这意味着它永远是线程安全的。
• 成员变量为类所有，它不是线程安全的。