參考鏈接：https://blog.csdn.net/u012702547/article/details/52259529

場(chǎng)景：listview中的item加載圖片，通過(guò)new Thread開(kāi)啟線程請(qǐng)求網(wǎng)絡(luò)

使用new Thread()創(chuàng)建線程存在的問(wèn)題

• 針對(duì)每一個(gè)item都創(chuàng)建一個(gè)新線程，這樣會(huì)導(dǎo)致頻繁的創(chuàng)建線程，線程執(zhí)行完之后又被回收，又會(huì)導(dǎo)致頻繁的GC
• 這么多線程缺乏統(tǒng)一管理，各線程之間互相競(jìng)爭(zhēng)，降低程序的運(yùn)行效率，手機(jī)頁(yè)面卡頓，甚至?xí)?dǎo)致程序崩潰
• 如果一個(gè)item滑出頁(yè)面，則要停止該item上圖片的加載，但是如果使用這種方式來(lái)創(chuàng)建線程，則無(wú)法實(shí)現(xiàn)線程停止執(zhí)行

使用線程池的好處

• 重用已經(jīng)創(chuàng)建的好的線程，避免頻繁創(chuàng)建進(jìn)而導(dǎo)致的頻繁GC
• 控制線程并發(fā)數(shù)，合理使用系統(tǒng)資源，提高應(yīng)用性能
• 可以有效的控制線程的執(zhí)行，比如定時(shí)執(zhí)行，取消執(zhí)行等

Executor本身是一個(gè)接口

ThreadPoolExecutor Executor的實(shí)現(xiàn)類

/**
      /**
     * 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;
    }

ThreadPoolExecutor構(gòu)造方法參數(shù)的解釋

corePoolSize

核心線程，即在線程池中保留的線程數(shù)，即使它們處于空閑狀態(tài)也保留，除非設(shè)置了{(lán)@code allowCoreThreadTimeOut}

maximumPoolSize：

線程池中允許的最大線程數(shù)

keepAliveTime

非核心線程的超時(shí)時(shí)長(zhǎng)，當(dāng)系統(tǒng)中非核心線程閑置時(shí)間超過(guò)keepAliveTime之后，則會(huì)被回收。如果ThreadPoolExecutor的allowCoreThreadTimeOut屬性設(shè)置為true，則該參數(shù)也表示核心線程的超時(shí)時(shí)長(zhǎng)

unit

參數(shù)的時(shí)間單位，有納秒、微秒、毫秒、秒、分、時(shí)、天等

workQueue

線程池中的任務(wù)隊(duì)列，該隊(duì)列主要用來(lái)存儲(chǔ)已經(jīng)被提交但是尚未執(zhí)行的任務(wù)。存儲(chǔ)在這里的任務(wù)是由ThreadPoolExecutor的execute方法提交來(lái)的。
workQueue是一個(gè)BlockingQueue類型，是一個(gè)特殊的隊(duì)列，當(dāng)我們從BlockingQueue中取數(shù)據(jù)時(shí)，如果BlockingQueue是空的，則取數(shù)據(jù)的操作會(huì)進(jìn)入到阻塞狀態(tài)，當(dāng)BlockingQueue中有了新數(shù)據(jù)時(shí)，這個(gè)取數(shù)據(jù)的操作又會(huì)被重新喚醒。同理，如果BlockingQueue中的數(shù)據(jù)已經(jīng)滿了，往BlockingQueue中存數(shù)據(jù)的操作又會(huì)進(jìn)入阻塞狀態(tài)，直到BlockingQueue中又有新的空間，存數(shù)據(jù)的操作又會(huì)被沖洗喚醒。

threadFactory

在執(zhí)行時(shí)使用，為線程池提供創(chuàng)建新線程的功能，這個(gè)我們一般使用默認(rèn)即可

handler

拒絕策略，當(dāng)線程無(wú)法執(zhí)行新任務(wù)時(shí)（一般是由于線程池中的線程數(shù)量已經(jīng)達(dá)到最大數(shù)或者線程池關(guān)閉導(dǎo)致的），默認(rèn)情況下，當(dāng)線程池?zé)o法處理新線程時(shí)，會(huì)拋出一個(gè)RejectedExecutionException。

線程提交到線程池之后運(yùn)行規(guī)則，遵循如下規(guī)則：

• 1.execute一個(gè)線程之后，如果線程池中的線程數(shù)未達(dá)到核心線程數(shù)，則會(huì)立馬啟用一個(gè)核心線程去執(zhí)行
• 2.execute一個(gè)線程之后，如果線程池中的線程數(shù)已經(jīng)達(dá)到核心線程數(shù)，且workQueue未滿，則將新線程放入workQueue中等待執(zhí)行
• 3.execute一個(gè)線程之后，如果線程池中的線程數(shù)已經(jīng)達(dá)到核心線程數(shù)但未超過(guò)非核心線程數(shù)，且workQueue已滿，則開(kāi)啟一個(gè)非核心線程來(lái)執(zhí)行任務(wù)
• 4.execute一個(gè)線程之后，如果線程池中的線程數(shù)已經(jīng)超過(guò)非核心線程數(shù)，則拒絕執(zhí)行該任務(wù)