J.U.C：线程基本概念

Posted 2022-06-11 恒奇恒毅

进程与线程

进程（Process） 是计算机中的程序关于某数据集合上的一次运行活动，是系统进行资源分配和调度的基本单位，是操作系统结构的基础。 在当代面向线程设计的计算机结构中，进程是线程的容器。程序是指令、数据及其组织形式的描述，进程是程序的实体。是计算机中的程序关于某数据集合上的一次运行活动，是系统进行资源分配和调度的基本单位，是操作系统结构的基础。程序是指令、数据及其组织形式的描述，进程是程序的实体。
线程（thread） 是操作系统能够进行运算调度的最小单位。它被包含在进程之中，是进程中的实际运作单位。一条线程指的是进程中一个单一顺序的控制流，一个进程中可以并发多个线程，每条线程并行执行不同的任务。

线程的状态

/**
     * A thread state.  A thread can be in one of the following states:
     * <ul>
     * <li>@link #NEW<br>
     *     A thread that has not yet started is in this state.
     *     </li>
     * <li>@link #RUNNABLE<br>
     *     A thread executing in the Java virtual machine is in this state.
     *     </li>
     * <li>@link #BLOCKED<br>
     *     A thread that is blocked waiting for a monitor lock
     *     is in this state.
     *     </li>
     * <li>@link #WAITING<br>
     *     A thread that is waiting indefinitely for another thread to
     *     perform a particular action is in this state.
     *     </li>
     * <li>@link #TIMED_WAITING<br>
     *     A thread that is waiting for another thread to perform an action
     *     for up to a specified waiting time is in this state.
     *     </li>
     * <li>@link #TERMINATED<br>
     *     A thread that has exited is in this state.
     *     </li>
     * </ul>
     *
     * <p>
     * A thread can be in only one state at a given point in time.
     * These states are virtual machine states which do not reflect
     * any operating system thread states.
     *
     * @since   1.5
     * @see #getState
     */
    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 <tt>Object.wait()</tt>
         * on an object is waiting for another thread to call
         * <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
         * that object. A thread that has called <tt>Thread.join()</tt>
         * 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;
    

线程有NEW（新建）、RUNNABLE（就绪）、BLOCKED（阻塞）、WAITING（不见不散）、TIMED_WAITING（过时不候）、TERMINATED（终止）这六种状态。

NEW

表示线程处于新建状态，还不可以运行

RUNNABLE

表示线程处于就绪状态，可以被调度器调度运行，调用线程的start方法后进入该状态。

BLOCKED

表示线程在等待同步监视器锁，一般是在竞争syncronized的锁或者进入syncronized之后调用了Object.wait方法

WAITING

表示线程处于等待状态，一般是调用了以下三个方法之一：

Object#wait()不带参的

相应地需要对应监视器对象调用notify或者notifyAll方法或者被中断interrupted

Thread.join()不带参的

相应地需要另外的线程执行完成

LockSupport#park()方法

相应地需要调用LockSupport#unpark()方法

TIMED_WAITING

表示线程处于带时等待状态，一般是调用了以下五个方法之一：

Thread.sleep(long)

需要等待线程休眠时间到或者被中断interrupted

Object#wait(long)

相应地需要对应监视器对象调用notify或者notifyAll方法或者被中断interrupted

Thread.join(long)

相应地需要另外的线程执行完成

LockSupport#parkNanos

相应地需要调用LockSupport#unpark()方法

LockSupport#parkUntil

相应地需要调用LockSupport#unpark()方法

TERMINATED

线程的终止状态

wait和sleep

• sleep 是 Thread 的静态方法，wait 是 Object 的方法，任何对象实例都能调用。
• sleep 不会释放锁，它也不需要占用锁。wait 会释放锁，但调用它的前提是当前线程占有锁(即代码要在 synchronized 中)。
• 它们都可以被 interrupted 方法中断。

并行与并发

并行是表示同时做多件事情，对于单核CPU来说，是达不到并行的效果的。并行标示的是同时做do多件事的能力。
并发是看起来是同时在做多件事情，对于单核CPU来说，同一时刻只可能一个线程在运行。并发表示的是同时处理handle多件事情的能力。
所以，只有多核CPU才能真正利用好多线程能力。

用户线程和后台线程

用户线程:也叫前台线程，平时用到的普通线程,自定义线程setDaemon(false)，默认就是前台线程
守护线程:运行在后台,是一种特殊的线程，setDaemon(true),比如垃圾回收

• 当主线程结束后,用户线程还在运行,JVM 存活
• 如果没有用户线程,都是守护线程,JVM 结束
  这里的一个很好的利用就是：如何优雅地保证程序不退出？