并发编程之守护线程和线程状态

1. 守护线程

用户线程：我们平常创建的普通线程。
守护线程（即 Daemon thread）：是个服务线程，用来服务于用户线程；不需要上层逻辑介入，当然我们也可以手动创建一个守护线程。在JVM中，所有非守护线程都执行完毕后，无论有没有守护线程，虚拟机都会自动退出。

  我们在main方法中创建一个死循环的用户线程并执行main方法，会发现程序不会自己结束；如果将创建的线程设置为守护线程，则main方法执行完毕后，程序将自动推出。

示例代码：

package org.example;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.TimeUnit;

@Slf4j
public class Main {
    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(() -> {
            while (true) {

            }
        }, "t1");
        
        // 将t1设置为守护线程，如果t1不是守护线程，程序将无法结束
        t1.setDaemon(true);

        t1.start();
        TimeUnit.SECONDS.sleep(10L);
    }
}

2. 进程状态

    Thread的getgetState可以回去线程的状态，JVM中定义了5中状态，定义如下：

public enum State {
        /**
         * Thread state for a thread which has not yet started.
         */
        NEW,

        /**
         * Thread state for a runnable thread.  A thread in the runnable
         * state is executing in the Java virtual machine but it may
         * be waiting for other resources from the operating system
         * such as processor.
         */
        RUNNABLE,

        /**
         * Thread state for a thread blocked waiting for a monitor lock.
         * A thread in the blocked state is waiting for a monitor lock
         * to enter a synchronized block/method or
         * reenter a synchronized block/method after calling
         * {@link Object#wait() Object.wait}.
         */
        BLOCKED,

        /**
         * Thread state for a waiting thread.
         * A thread is in the waiting state due to calling one of the
         * following methods:
         * 

         *   
{@link Object#wait() Object.wait} with no timeout
         *   
{@link #join() Thread.join} with no timeout
         *   
{@link LockSupport#park() LockSupport.park}
         * 
         *
         * 
A thread in the waiting state is waiting for another thread to
         * perform a particular action.
         *
         * For example, a thread that has called {@code Object.wait()}
         * on an object is waiting for another thread to call
         * {@code Object.notify()} or {@code Object.notifyAll()} on
         * that object. A thread that has called {@code Thread.join()}
         * is waiting for a specified thread to terminate.
         */
        WAITING,

        /**
         * Thread state for a waiting thread with a specified waiting time.
         * A thread is in the timed waiting state due to calling one of
         * the following methods with a specified positive waiting time:
         * 

         *   
{@link #sleep Thread.sleep}
         *   
{@link Object#wait(long) Object.wait} with timeout
         *   
{@link #join(long) Thread.join} with timeout
         *   
{@link LockSupport#parkNanos LockSupport.parkNanos}
         *   
{@link LockSupport#parkUntil LockSupport.parkUntil}
         * 
         */
        TIMED_WAITING,

        /**
         * Thread state for a terminated thread.
         * The thread has completed execution.
         */
        TERMINATED;
    }