Thread's guardian thread and thread safety issues

Daemon thread

---

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);
        }
    }
}

Thread safe

Verify thread safety of local variables

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");
        }
    }
}

Verify that member variables are not thread safe

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");
        }
    }
}

in conclusion:

• Local variables are private inside the method. This means that it is always thread-safe.
• Member variables are owned by the class, it is not thread-safe.