JUC概述

JUC概述1：

首先是进程和线程的概念：

进程：是指系统在系统中正在运行的一个应用程序，程序一旦运行就是进程，进程是资源分配的最小单位

线程：进程之内独立执行，是程序执行的最小单位

线程的六大状态：在线程的枚举类中

 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:
          * <ul>
          *   <li>{@link Object#wait() Object.wait} with no timeout</li>
          *   <li>{@link #join() Thread.join} with no timeout</li>
          *   <li>{@link LockSupport#park() LockSupport.park}</li>
          * </ul>
          *
          * <p>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:
          * <ul>
          *   <li>{@link #sleep Thread.sleep}</li>
          *   <li>{@link Object#wait(long) Object.wait} with timeout</li>
          *   <li>{@link #join(long) Thread.join} with timeout</li>
          *   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
          *   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
          * </ul>
          */
         TIMED_WAITING,
 ​
         /**
          * Thread state for a terminated thread.
          * The thread has completed execution.
          */
         TERMINATED;
     }

状态名称	说明
new	初始状态
runnable	运行状态
blocked	阻塞状态
waiting	等待状态，一直等(不见不散)
time_waiting	超时等待，（过时不候）
terminated	终止状态

wait和sleep的区别：

1. sleep是Thread的静态方法，wait是Object的方法，任何对象实例化都能调用
2. sleep不会释放锁，他也不需要占用锁，wait会释放锁，但是调用它的前提是当前线程占有锁
3. 它们都可以interrupted被中断

并发和并行：

并发是指多个事情在同一个时间段中执行

并行是指多个事情在同一时刻执行

管程：

是一种同步机制，保证同一时间内只有一个线程访问被保护数据或者代码

jvm同步基于进入(加锁)和退出(解锁)，是管程对象实现的

大意就是进加锁，退是解锁，通过管程对象管理

用户线程：自定义线程 主线程结束了，用户线程还存在，则表示JVM还存在

 public class demo {
     public static void main(String[] args) {
         Thread a = new Thread(() -> {
             System.out.println(Thread.currentThread().getName()+"::"+Thread.currentThread().isDaemon());
             while (true){}
         }, "a");
 ​
         a.start();
         System.out.println(Thread.currentThread().getName());
     }
 }

守护线程：

ex:垃圾回收 没有用户线程了，都是守护线程，JVM结束

 public class demo {
     public static void main(String[] args) {
         Thread a = new Thread(() -> {
             System.out.println(Thread.currentThread().getName()+"::"+Thread.currentThread().isDaemon());
             while (true){}
         }, "a");
         //设置子线程为守护线程
         a.setDaemon(true);
         a.start();
         System.out.println(Thread.currentThread().getName());
     }
 }