一、线程

在Android开发中，你不可能都在主线程中开发，毕竟要联网，下载数据，保存数据等操作，当然这就离不开线程。

（当然你可以在Android4.0以前的手机里在主线程请求网络，我最早开发的时候，用的手机比较古老。。。）
在Android中你可以随意创建线程，于是就会造成线程不可控，内存泄漏，创建线程消耗资源，线程太多了消耗资源等问题。
具体线程怎么创建我就不在文章里描述了，毕竟这主要将并发编程。。。。

大家知道线程不可控就好了。。。于是就需要对线程进行控制，防止一系列问题出现，这就用到了如下要讲的东西。

二、线程池

线程池：顾名思义，就是放线程的大池子。

如何创建一个线程池？

先说说几个系统的线程池：

• FixedThreadPool 创建定长线程的线程池
• CachedThreadPool 需要的时候建立新的线程，超时线程销毁
• SingleThreadPool 单个线程的线程池
• ScheduledThreadPool 可以定时的线程池，创建周期性的任务

这几个线程池不做多余阐述，因为这些线程池的原理都与我下面要讲的有关。。。。

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如何自定义线程池（先来了解几个必须知道的参数）：

corePoolSize：

核心线程池大小，线程池中主要工作的线程的多少。

maximumPoolSize：

线程池最大线程数。

keepAliveTime：

空闲线程可保持的时间是多久，如果你启用了allowCoreThreadTimeOut方法，你的线程池里的空闲线程在这个时间段后会自动销毁，如果没启用，则只要不超过corePoolSize，空闲线程也不会销毁。

Unit：

keepAliveTime的时间单位

workQueue：

阻塞队列，当任务达到corePoolSize，就会被放入这个队列

常见几种BlockingQueue实现

1. ArrayBlockingQueue : 有界的数组队列
2. LinkedBlockingQueue : 可支持有界/无界的队列，使用链表实现
3. PriorityBlockingQueue : 优先队列，可以针对任务排序
4. SynchronousQueue : 队列长度为1的队列，和Array有点区别就是：client thread提交到block queue会是一个阻塞过程，直到有一个worker thread连接上来poll task。

threadFactory：

线程工厂，主要用来创建线程；

handler：

表示当拒绝处理任务时的策略，也就是参数maximumPoolSize达到后丢弃处理的方法。有以下四种取值：
ThreadPoolExecutor.AbortPolicy:丢弃任务并抛出RejectedExecutionException异常。
ThreadPoolExecutor.DiscardPolicy：也是丢弃任务，但是不抛出异常。
ThreadPoolExecutor.DiscardOldestPolicy：丢弃队列最前面的任务，然后重新尝试执行任务（重复此过程）
ThreadPoolExecutor.CallerRunsPolicy：由调用线程处理该任务
用户也可以实现接口RejectedExecutionHandler定制自己的策略。

代码展示：

//线程工厂
public class TaskThreadFactory  implements ThreadFactory {

    private final AtomicInteger mThreadNumber = new AtomicInteger(1);

    private final String mNamePrefix;

    TaskThreadFactory(String name) {
        mNamePrefix = name + "#";
    }

    public Thread newThread(Runnable r) {
        Thread t = new Thread(r,mNamePrefix + mThreadNumber.getAndIncrement());

//        if (t.isDaemon())
//            t.setDaemon(false);
//
//        if (t.getPriority() != Thread.NORM_PRIORITY)
//            t.setPriority(Thread.NORM_PRIORITY);

        return t;
    }
}

//重写runnable
public class PRunnable implements Runnable {

    public static final int HIGH = 1;//优先级高
    public static final int NORMAL = 2;//优先级中等
    public static final int LOW = 3;//优先级低
    @IntDef({HIGH,NORMAL,LOW})
    @Retention(RetentionPolicy.SOURCE)
    public @interface Priority{}

    public final int priority;
    private final Runnable runnable;
    public int serial;

    public PRunnable(Runnable runnable){
        this(NORMAL,runnable);
    }

    public PRunnable(@Priority int priority,Runnable runnable){
        this.priority = priority;
        this.runnable = runnable;
    }

    @Override
    public void run() {
        if (runnable != null) {
            runnable.run();
        }
    }

    /**
     * 线程队列方式 先进先出
     * @param r1
     * @param r2
     * @return
     */
    public static final int compareFIFO(PRunnable r1, PRunnable r2) {
        int result = r1.priority-r2.priority;
        return result==0?r1.serial-r2.serial:result;
    }

    /**
     * 线程队列方式 后进先出
     * @param r1
     * @param r2
     * @return
     */
    public static final int compareLIFO(PRunnable r1, PRunnable r2) {
        int result = r1.priority-r2.priority;
        return result==0?r2.serial-r1.serial:result;
    }
}

//线程池实现
public class TaskExecutor implements Executor {

    private final static int QUEUE_INIT_CAPACITY = 20;

    private static final int CORE = 3;

    private static final int MAX = 5;

    private static final int TIMEOUT = 30 * 1000;

    private AtomicInteger SERIAL = new AtomicInteger(0);//主要获取添加任务

    public static class Config {
        public int core;

        public int max;

        public int timeout;

        public boolean allowCoreTimeOut;

        public boolean fifo;

        public Config(int core, int max, int timeout, boolean allowCoreTimeOut,boolean fifo) {
            this.core = core;
            this.max = max;
            this.timeout = timeout;
            this.allowCoreTimeOut = allowCoreTimeOut;
            this.fifo = fifo;
        }
    }

    public static Config defaultConfig = new Config(CORE, MAX, TIMEOUT, true,true);

    private final String name;

    private final Config config;

    private ExecutorService service;

    public TaskExecutor(String name) {
        this(name, defaultConfig);
    }

    public TaskExecutor(String name, Config config) {
        this(name, config, true);
    }

    public TaskExecutor(String name, Config config, boolean startup) {
        this.name = name;
        this.config = config;

        if (startup) {
            startup();
        }
    }

    public void startup() {
        synchronized (this) {
            if (service != null && !service.isShutdown()) {
                return;
            }

            service = createExecutor(config);
        }
    }

    public void shutdown() {
        ExecutorService executor = null;

        synchronized (this) {
            // 交换变量
            if (service != null) {
                executor = service;
                service = null;
            }
        }

        if (executor != null) {
            // 停止线程
            if (!executor.isShutdown()) {
                executor.shutdown();
            }

            // 回收变量
            executor = null;
        }
    }

    private void executeRunnable(PRunnable runnable) {
        synchronized (this) {
            if (service == null || service.isShutdown()) {
                return;
            }
            runnable.serial = SERIAL.getAndIncrement();
            service.execute(runnable);
        }
    }

    @Override
    public void execute(Runnable runnable) {
        if (runnable instanceof PRunnable) {
            executeRunnable((PRunnable) runnable);
        }else{
            executeRunnable(new PRunnable(runnable));
        }
    }

    public Future<?> submit(Runnable runnable) {
        synchronized (this) {
            if (service == null || service.isShutdown()) {
                return null;
            }
            if (runnable instanceof PRunnable) {
                ((PRunnable) runnable).serial = SERIAL.getAndIncrement();
                return service.submit(runnable);
            }else{
                PRunnable pRunnable = new PRunnable(runnable);
                pRunnable.serial = SERIAL.getAndIncrement();
                return service.submit(pRunnable);
            }
        }
    }

    public void execute(Runnable runnable, @PRunnable.Priority int priority) {
        executeRunnable(new PRunnable(priority,runnable));
    }

    private ExecutorService createExecutor(Config config) {
        ThreadPoolExecutor service = new ThreadPoolExecutor(config.core, config.max, config.timeout,
                TimeUnit.MILLISECONDS, new PriorityBlockingQueue<Runnable>(QUEUE_INIT_CAPACITY, config.fifo ? mQueueFIFOComparator : mQueueLIFOComparator),
                new TaskThreadFactory(name), new ThreadPoolExecutor.DiscardPolicy());

        allowCoreThreadTimeOut(service, config.allowCoreTimeOut);

        return service;
    }

    public boolean isBusy() {
        synchronized (this) {
            if (service == null || service.isShutdown()) {
                return false;
            }
            if(service instanceof ThreadPoolExecutor){
                ThreadPoolExecutor tService = (ThreadPoolExecutor) service;
                return tService.getActiveCount() >= tService.getCorePoolSize();
            }
            return false;
        }
    }

    private static final void allowCoreThreadTimeOut(ThreadPoolExecutor service, boolean value) {
        if (Build.VERSION.SDK_INT >= 9) {
            allowCoreThreadTimeOut9(service, value);
        }
    }

    @TargetApi(9)
    private static final void allowCoreThreadTimeOut9(ThreadPoolExecutor service, boolean value) {
        service.allowCoreThreadTimeOut(value);
    }

    Comparator<Runnable> mQueueFIFOComparator = new Comparator<Runnable>() {

        @Override
        public int compare(Runnable lhs, Runnable rhs) {
            PRunnable r1 = (PRunnable) lhs;
            PRunnable r2 = (PRunnable) rhs;

            return PRunnable.compareFIFO(r1, r2);
        }
    };

    Comparator<Runnable> mQueueLIFOComparator = new Comparator<Runnable>() {

        @Override
        public int compare(Runnable lhs, Runnable rhs) {
            PRunnable r1 = (PRunnable) lhs;
            PRunnable r2 = (PRunnable) rhs;

            return PRunnable.compareLIFO(r1, r2);
        }
    };

}

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