线程进程管程概念介绍

1.什么是进程？

总结一句话就是系统中正在运行的程序；应用程序一旦运行就是进程；进程——资源分配的最小单位。比如说你现在打开哔哩哔哩，哔哩哔哩就是运行的程序，即进程。里面的各种操作比如看视频之类的就是进程中的一个线程。一个进程里面可以有一堆线程。

2.线程？

2.1线程的概念

系统分配处理器时间资源的基本单元，或者说进程之内独立执行的一个单元执行流。线程——程序执行的最小单位。

2.2线程的状态

这是线程类中的状态枚举类，枚举了线程中的各种状态

    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 Object.wait()
         * on an object is waiting for another thread to call
         * Object.notify() or Object.notifyAll() on
         * that object. A thread that has called 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;
    }

NEW（新建），RUNNABLE（准备就绪），BLOCKED（阻塞），WAITING（不见不散），TIMED_WAITING（过时不候），这两个的区别就是一种情况下你是舔狗，跟女生出来见面到点儿了她不来，你就一直在那边等着。另一种情况就是你是直男，到点儿了不来直接走，比较硬气，不过我估计你这对象也就黄了。TERMINATED（终结）。

2.3用户线程和守护线程

平时用到的自定义的基本都是用户线程，后台中一种特殊的线程比如说垃圾回收。

public class Test {
    public static void main(String[] args) {
        new Thread(()->{
            System.out.println(Thread.currentThread().getName() + ":: " + Thread.currentThread().isDaemon());
            while(true){

            }
        },"aa").start();
        System.out.println(Thread.currentThread().getName() + " is over");
    }
}

这段代码就是对用户线程的测试。isDemon()为true说明是守护线程，false说明是用户线程我们运行一下。

可以看到他还没完全结束，这时候main线程结束了，返回值为false说明是用户线程，主线程虽然结束但用户线程没结束，jvm不退出，还在运行。

public class Test {
    public static void main(String[] args) {
        Thread aa = new Thread(() -> {
            System.out.println(Thread.currentThread().getName() + ":: " + Thread.currentThread().isDaemon());
            while (true) {

            }
        }, "aa");
        aa.setDaemon(true);
        aa.start();
        System.out.println(Thread.currentThread().getName() + " is over");
    }
}

现在我把这个用户线程改成守护线程，这样主线程结束后就没有任何用户线程了。所以jvm会退出。

2.4wait/sleep的区别

sleep是Thread的静态方法，wait是Object的方法谁都能用，sleep不会释放锁，它也不需要占用锁。wait需要释放锁所以他必须在synchronized中。他们都可以被interrupted方法中断。

3.管程

管程就是一中同步机制，保证同一个时间，只有一个线程访问被保护的数据或者代码。我们执行线程操作首先要持有管程对象，方法执行结束之后会释放管程对象。