CPU同一时间只能处理有限数目的线程，赋予线程怎样的优先级，将决定其最终在CPU上得以执行的顺序
光头哥回归啦！读信的样子简直不要太萌！
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Thread Scheduling

线程调度是现代计算机系统必不可少的功能，Android做为独特的移动系统，在线程调度上，还有一些它与众不同的特性。

Android系统中对于线程优先级的考虑，还会包括以下方面：

  • Activity生命周期
  • 是否可见
  • Activity在前台还是运行

Visible vs Invisible

CPU时间的90%左右，都是用以处理界面展示中的线程，只有约10%留给了后台线程

拼爹的线程

在线程的世界里，一样要拼爹——新创建的线程，其优先级与创建它的线程相同。

也就是说，如果UI Thread创建了20个Worker Thread，那么这20个Worker Thread都具有相同的优先级，它们会平等竞争CPU时间。

这其实是很不好的，我们知道哪些工作更重要，哪些工作相比于其它不那么重要，所以我们要主动设置线程优先级，让那些具有更高优先级的线程优先得到执行。

Priority

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android.os.Process.setThreadPriority(int);

上面的方法接受-20~24闭区间内所有的整数作为参数，数值越小，优先级越高。为甚么采用负数呢？数值越低，表示这个线程越没有耐心，情绪越暴躁，越急切希望得到执行。

android.os.Process中为我们定义了一些表示线程优先级的常量。

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/**
     * Standard priority of application threads.
     * Use with {@link #setThreadPriority(int)} and
     * {@link #setThreadPriority(int, int)}, <b>not</b> with the normal
     * {@link java.lang.Thread} class.
     */
    public static final int THREAD_PRIORITY_DEFAULT = 0;

    /*
     * ***************************************
     * ** Keep in sync with utils/threads.h **
     * ***************************************
     */
    
    /**
     * Lowest available thread priority.  Only for those who really, really
     * don't want to run if anything else is happening.
     * Use with {@link #setThreadPriority(int)} and
     * {@link #setThreadPriority(int, int)}, <b>not</b> with the normal
     * {@link java.lang.Thread} class.
     */
    public static final int THREAD_PRIORITY_LOWEST = 19;
    
    /**
     * Standard priority background threads.  This gives your thread a slightly
     * lower than normal priority, so that it will have less chance of impacting
     * the responsiveness of the user interface.
     * Use with {@link #setThreadPriority(int)} and
     * {@link #setThreadPriority(int, int)}, <b>not</b> with the normal
     * {@link java.lang.Thread} class.
     */
    public static final int THREAD_PRIORITY_BACKGROUND = 10;
    
    /**
     * Standard priority of threads that are currently running a user interface
     * that the user is interacting with.  Applications can not normally
     * change to this priority; the system will automatically adjust your
     * application threads as the user moves through the UI.
     * Use with {@link #setThreadPriority(int)} and
     * {@link #setThreadPriority(int, int)}, <b>not</b> with the normal
     * {@link java.lang.Thread} class.
     */
    public static final int THREAD_PRIORITY_FOREGROUND = -2;
    
    /**
     * Standard priority of system display threads, involved in updating
     * the user interface.  Applications can not
     * normally change to this priority.
     * Use with {@link #setThreadPriority(int)} and
     * {@link #setThreadPriority(int, int)}, <b>not</b> with the normal
     * {@link java.lang.Thread} class.
     */
    public static final int THREAD_PRIORITY_DISPLAY = -4;
    
    /**
     * Standard priority of the most important display threads, for compositing
     * the screen and retrieving input events.  Applications can not normally
     * change to this priority.
     * Use with {@link #setThreadPriority(int)} and
     * {@link #setThreadPriority(int, int)}, <b>not</b> with the normal
     * {@link java.lang.Thread} class.
     */
    public static final int THREAD_PRIORITY_URGENT_DISPLAY = -8;

    /**
     * Standard priority of audio threads.  Applications can not normally
     * change to this priority.
     * Use with {@link #setThreadPriority(int)} and
     * {@link #setThreadPriority(int, int)}, <b>not</b> with the normal
     * {@link java.lang.Thread} class.
     */
    public static final int THREAD_PRIORITY_AUDIO = -16;

    /**
     * Standard priority of the most important audio threads.
     * Applications can not normally change to this priority.
     * Use with {@link #setThreadPriority(int)} and
     * {@link #setThreadPriority(int, int)}, <b>not</b> with the normal
     * {@link java.lang.Thread} class.
     */
    public static final int THREAD_PRIORITY_URGENT_AUDIO = -19;

此外，还有两个常量用于提高／降低优先级，记住，负数越小，优先级越高（more favorable）。

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/**
 * Minimum increment to make a priority more favorable.
 */
public static final int THREAD_PRIORITY_MORE_FAVORABLE = -1;

/**
 * Minimum increment to make a priority less favorable.
 */
public static final int THREAD_PRIORITY_LESS_FAVORABLE = +1;

有了上面的知识，你就可以在UI Thread启动Worker Thread时，通过setPriority(THREAD_PRIORITY_DEFAULT + THREAD_PRIORITY_LESS_FAVORABLE * n)的方式，酌情降低Worker Thread的优先级了。

That’s How We Did It!

还记得之前我们总是提到的 AsyncTask/HandlerThread/ThreadPool/IntentService 么？其中AsyncTask和IntentService已经为自动我们实现了“降低Worker Thread优先级”的功能，我们无需额外设置。而另外两种则没有这么周到，我们需要主动设置工作线程优先级。

===Ending===