線程、進(jìn)程、協(xié)程

• 進(jìn)程 進(jìn)程是計(jì)算機(jī)資源調(diào)度和分配的單位。是程序的實(shí)體。計(jì)算機(jī)將自己的內(nèi)存資源，CPU資源分配給不同的進(jìn)程，這樣程序才得以運(yùn)行。
• 線程 線程是計(jì)算機(jī)運(yùn)算的最小調(diào)度單位。線程依附于進(jìn)程，一個(gè)進(jìn)程至少有一個(gè)線程。線程沒有自己的內(nèi)存，而使用進(jìn)程的內(nèi)存。進(jìn)程調(diào)度一個(gè)或多個(gè)線程進(jìn)行運(yùn)算。
• 協(xié)程 協(xié)程其實(shí)沒有統(tǒng)一的定義。一般來說，協(xié)程是單線程內(nèi)實(shí)現(xiàn)并發(fā)、非阻塞執(zhí)行。kotlin中的協(xié)程實(shí)際上線程、只不過與傳統(tǒng)的線程看起來不太一樣。

線程的創(chuàng)建方式

線程的創(chuàng)建需要有系統(tǒng)的支持，所以在代碼中，創(chuàng)建代碼的方式本質(zhì)上就幾種，以及在其上做的一些封裝。

• 使用Thread類創(chuàng)建線程。Thread#start()是native方法，調(diào)用這個(gè)方法后，進(jìn)程會創(chuàng)建一個(gè)新線程，并且在新的線程里面執(zhí)行Thread#run()方法。

  /**
   * If this thread was constructed using a separate
   * {@code Runnable} run object, then that
   * {@code Runnable} object's {@code run} method is called;
   * otherwise, this method does nothing and returns.
   * <p>
   * Subclasses of {@code Thread} should override this method.
   *
   * @see     #start()
   * @see     #stop()
   * @see     #Thread(ThreadGroup, Runnable, String)
   */
  @Override
  public void run() {
      if (target != null) {
          target.run();
      }
  }
  

所以,使用Thread類創(chuàng)建線程又有兩種方式，1.創(chuàng)建一個(gè)Runnable對象作為target。2.創(chuàng)建一個(gè)Thread的子類，重寫run()方法。

• 使用線程池創(chuàng)建和管理多線程。頻繁的創(chuàng)建和切換線程，對于JVM會造成很大的負(fù)擔(dān)。使用線程池即使可以降低這種負(fù)擔(dān)。線程池本質(zhì)上也是使用Thread，但是線程池提供了線程的復(fù)用方法。當(dāng)一個(gè)線程結(jié)束后，根據(jù)之前線程池的設(shè)定，不會馬上回收，而是等待一段時(shí)間，一段時(shí)間內(nèi)如果用多線程任務(wù)的話，復(fù)用這個(gè)線程。線程池的基本構(gòu)造方法如下

  /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters.
     *
     * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the
     *        pool
     * @param keepAliveTime when the number of threads is greater than
     *        the core, this is the maximum time that excess idle threads
     *        will wait for new tasks before terminating.
     * @param unit the time unit for the {@code keepAliveTime} argument
     * @param workQueue the queue to use for holding tasks before they are
     *        executed.  This queue will hold only the {@code Runnable}
     *        tasks submitted by the {@code execute} method.
     * @param threadFactory the factory to use when the executor
     *        creates a new thread
     * @param handler the handler to use when execution is blocked
     *        because the thread bounds and queue capacities are reached
     * @throws IllegalArgumentException if one of the following holds:<br>
     *         {@code corePoolSize < 0}<br>
     *         {@code keepAliveTime < 0}<br>
     *         {@code maximumPoolSize <= 0}<br>
     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue}
     *         or {@code threadFactory} or {@code handler} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }  
  

• 以Android中的其他方式創(chuàng)建線程，如AsycTask，IntentService等，這些本質(zhì)上都是對Thread的封裝，在這里就不做詳細(xì)介紹

Android多線程（一）
Android多線程（三）
Android多線程（四）
Android多線程（五）
Android多線程（六）