Java多線程4 初識線程池

Java多線程目錄

前言

Java為什么引入線程池？
創(chuàng)建線程示例

    new Thread(new Runnable() {

            @Override
            public void run() {
            
            }
        }).start();
new Thread的弊端
  1. 每次new Thread新建對象性能差。
  2. 線程缺乏統(tǒng)一管理，可能無限制新建線程，相互之間競爭，及可能占用過多系統(tǒng)資源導(dǎo)致死機(jī)或oom。
  3. 缺乏更多功能，如定時執(zhí)行、定期執(zhí)行、線程中斷。
相比new Thread，Java提供的四種線程池的好處在于：
  1. 重用存在的線程，減少對象創(chuàng)建、消亡的開銷，性能佳。
  2. 可有效控制最大并發(fā)線程數(shù)，提高系統(tǒng)資源的使用率，同時避免過多資源競爭，避免堵塞。

  3. 提供定時執(zhí)行、定期執(zhí)行、單線程、并發(fā)數(shù)控制等功能。


    image.png

1.Java四種線程池

Java通過Executors提供四種線程池，分別為：
newFixedThreadPool 創(chuàng)建一個定長線程池，可控制線程最大并發(fā)數(shù)，超出的線程會在隊(duì)列中等待。
newSingleThreadExecutor 創(chuàng)建一個單線程化的線程池，它只會用唯一的工作線程來執(zhí)行任務(wù)，保證所有任務(wù)按照指定順序(FIFO, LIFO, 優(yōu)先級)執(zhí)行。
newCachedThreadPool創(chuàng)建一個可緩存線程池，如果線程池長度超過處理需要，可靈活回收空閑線程，若無可回收，則新建線程。
newScheduledThreadPool 創(chuàng)建一個定長線程池，支持定時及周期性任務(wù)執(zhí)行。

1.1 newFixedThreadPool

創(chuàng)建一個定長線程池，可控制線程最大并發(fā)數(shù)，超出的線程會在隊(duì)列中等待。

public class ThreadPoolTest {
    public static void main(String[] args) {

        //固定線程池
        ExecutorService threadPool = Executors.newFixedThreadPool(3);
        for (int i = 1; i < 10; i++) {
            int finalI = i;
            threadPool.execute(new Runnable() {
                @Override
                public void run() {
                    for (int j = 1; j < 10; j++) {
                        try {
                            Thread.sleep(20);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        System.out.println(Thread.currentThread().getName() + " is looping of " + j + " for task of " + finalI

                        );

                    }
                }
            });
        }
    }
}

pool-1-thread-3 is looping of 1 for task of 3
pool-1-thread-2 is looping of 1 for task of 2
pool-1-thread-1 is looping of 1 for task of 1
pool-1-thread-3 is looping of 2 for task of 3
pool-1-thread-1 is looping of 2 for task of 1
pool-1-thread-2 is looping of 2 for task of 2
pool-1-thread-3 is looping of 3 for task of 3
pool-1-thread-1 is looping of 3 for task of 1
pool-1-thread-2 is looping of 3 for task of 2
pool-1-thread-3 is looping of 4 for task of 3
pool-1-thread-2 is looping of 4 for task of 2
pool-1-thread-1 is looping of 4 for task of 1
pool-1-thread-1 is looping of 5 for task of 1
pool-1-thread-2 is looping of 5 for task of 2
pool-1-thread-3 is looping of 5 for task of 3
pool-1-thread-1 is looping of 6 for task of 1
pool-1-thread-3 is looping of 6 for task of 3
pool-1-thread-2 is looping of 6 for task of 2
pool-1-thread-1 is looping of 7 for task of 1
pool-1-thread-3 is looping of 7 for task of 3
pool-1-thread-2 is looping of 7 for task of 2
pool-1-thread-1 is looping of 8 for task of 1
pool-1-thread-3 is looping of 8 for task of 3
pool-1-thread-2 is looping of 8 for task of 2
pool-1-thread-1 is looping of 9 for task of 1
pool-1-thread-2 is looping of 9 for task of 2
pool-1-thread-3 is looping of 9 for task of 3
pool-1-thread-1 is looping of 1 for task of 4
pool-1-thread-2 is looping of 1 for task of 5
pool-1-thread-3 is looping of 1 for task of 6
pool-1-thread-1 is looping of 2 for task of 4
pool-1-thread-2 is looping of 2 for task of 5
pool-1-thread-3 is looping of 2 for task of 6
pool-1-thread-1 is looping of 3 for task of 4
pool-1-thread-2 is looping of 3 for task of 5
pool-1-thread-3 is looping of 3 for task of 6
pool-1-thread-3 is looping of 4 for task of 6
pool-1-thread-1 is looping of 4 for task of 4
pool-1-thread-2 is looping of 4 for task of 5
pool-1-thread-3 is looping of 5 for task of 6
pool-1-thread-1 is looping of 5 for task of 4
pool-1-thread-2 is looping of 5 for task of 5
pool-1-thread-2 is looping of 6 for task of 5
pool-1-thread-1 is looping of 6 for task of 4
pool-1-thread-3 is looping of 6 for task of 6
pool-1-thread-2 is looping of 7 for task of 5
pool-1-thread-3 is looping of 7 for task of 6
pool-1-thread-1 is looping of 7 for task of 4
pool-1-thread-1 is looping of 8 for task of 4
pool-1-thread-3 is looping of 8 for task of 6
pool-1-thread-2 is looping of 8 for task of 5
pool-1-thread-3 is looping of 9 for task of 6
pool-1-thread-2 is looping of 9 for task of 5
pool-1-thread-1 is looping of 9 for task of 4
pool-1-thread-2 is looping of 1 for task of 8
pool-1-thread-1 is looping of 1 for task of 9
pool-1-thread-3 is looping of 1 for task of 7
pool-1-thread-1 is looping of 2 for task of 9
pool-1-thread-2 is looping of 2 for task of 8
pool-1-thread-3 is looping of 2 for task of 7
pool-1-thread-1 is looping of 3 for task of 9
pool-1-thread-2 is looping of 3 for task of 8
pool-1-thread-3 is looping of 3 for task of 7
pool-1-thread-1 is looping of 4 for task of 9
pool-1-thread-2 is looping of 4 for task of 8
pool-1-thread-3 is looping of 4 for task of 7
pool-1-thread-2 is looping of 5 for task of 8
pool-1-thread-3 is looping of 5 for task of 7
pool-1-thread-1 is looping of 5 for task of 9
pool-1-thread-2 is looping of 6 for task of 8
pool-1-thread-1 is looping of 6 for task of 9
pool-1-thread-3 is looping of 6 for task of 7
pool-1-thread-3 is looping of 7 for task of 7
pool-1-thread-1 is looping of 7 for task of 9
pool-1-thread-2 is looping of 7 for task of 8
pool-1-thread-3 is looping of 8 for task of 7
pool-1-thread-1 is looping of 8 for task of 9
pool-1-thread-2 is looping of 8 for task of 8
pool-1-thread-3 is looping of 9 for task of 7
pool-1-thread-2 is looping of 9 for task of 8
pool-1-thread-1 is looping of 9 for task of 9

定長線程池的大小最好根據(jù)系統(tǒng)資源進(jìn)行設(shè)置。如Runtime.getRuntime().availableProcessors()

1.2 newFixedThreadPool

創(chuàng)建一個單線程化的線程池，它只會用唯一的工作線程來執(zhí)行任務(wù)，保證所有任務(wù)按照指定順序(FIFO, LIFO, 優(yōu)先級)執(zhí)行。

public class ThreadPoolTest {
    public static void main(String[] args) {
        //固定線程池
        ExecutorService threadPool = Executors.newSingleThreadExecutor();
        for (int i = 1; i < 10; i++) {
            int finalI = i;
            threadPool.execute(new Runnable() {
                @Override
                public void run() {
                    for (int j = 1; j < 10; j++) {
                        try {
                            Thread.sleep(20);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        System.out.println(Thread.currentThread().getName() + " is looping of " + j + " for task of " + finalI

                        );

                    }
                }
            });
        }

    }
}

pool-1-thread-1 is looping of 1 for task of 1
pool-1-thread-1 is looping of 2 for task of 1
pool-1-thread-1 is looping of 3 for task of 1
pool-1-thread-1 is looping of 4 for task of 1
pool-1-thread-1 is looping of 5 for task of 1
pool-1-thread-1 is looping of 6 for task of 1
pool-1-thread-1 is looping of 7 for task of 1
pool-1-thread-1 is looping of 8 for task of 1
pool-1-thread-1 is looping of 9 for task of 1
pool-1-thread-1 is looping of 1 for task of 2
pool-1-thread-1 is looping of 2 for task of 2
pool-1-thread-1 is looping of 3 for task of 2
pool-1-thread-1 is looping of 4 for task of 2
pool-1-thread-1 is looping of 5 for task of 2
pool-1-thread-1 is looping of 6 for task of 2
pool-1-thread-1 is looping of 7 for task of 2
pool-1-thread-1 is looping of 8 for task of 2
pool-1-thread-1 is looping of 9 for task of 2
pool-1-thread-1 is looping of 1 for task of 3
pool-1-thread-1 is looping of 2 for task of 3
pool-1-thread-1 is looping of 3 for task of 3
pool-1-thread-1 is looping of 4 for task of 3
pool-1-thread-1 is looping of 5 for task of 3
pool-1-thread-1 is looping of 6 for task of 3
pool-1-thread-1 is looping of 7 for task of 3
pool-1-thread-1 is looping of 8 for task of 3
pool-1-thread-1 is looping of 9 for task of 3
pool-1-thread-1 is looping of 1 for task of 4
pool-1-thread-1 is looping of 2 for task of 4
pool-1-thread-1 is looping of 3 for task of 4
pool-1-thread-1 is looping of 4 for task of 4
pool-1-thread-1 is looping of 5 for task of 4
pool-1-thread-1 is looping of 6 for task of 4
pool-1-thread-1 is looping of 7 for task of 4
pool-1-thread-1 is looping of 8 for task of 4
pool-1-thread-1 is looping of 9 for task of 4
pool-1-thread-1 is looping of 1 for task of 5
pool-1-thread-1 is looping of 2 for task of 5
pool-1-thread-1 is looping of 3 for task of 5
pool-1-thread-1 is looping of 4 for task of 5
pool-1-thread-1 is looping of 5 for task of 5
pool-1-thread-1 is looping of 6 for task of 5
pool-1-thread-1 is looping of 7 for task of 5
pool-1-thread-1 is looping of 8 for task of 5
pool-1-thread-1 is looping of 9 for task of 5
pool-1-thread-1 is looping of 1 for task of 6
pool-1-thread-1 is looping of 2 for task of 6
pool-1-thread-1 is looping of 3 for task of 6
pool-1-thread-1 is looping of 4 for task of 6
pool-1-thread-1 is looping of 5 for task of 6
pool-1-thread-1 is looping of 6 for task of 6
pool-1-thread-1 is looping of 7 for task of 6
pool-1-thread-1 is looping of 8 for task of 6
pool-1-thread-1 is looping of 9 for task of 6
pool-1-thread-1 is looping of 1 for task of 7
pool-1-thread-1 is looping of 2 for task of 7
pool-1-thread-1 is looping of 3 for task of 7
pool-1-thread-1 is looping of 4 for task of 7
pool-1-thread-1 is looping of 5 for task of 7
pool-1-thread-1 is looping of 6 for task of 7
pool-1-thread-1 is looping of 7 for task of 7
pool-1-thread-1 is looping of 8 for task of 7
pool-1-thread-1 is looping of 9 for task of 7
pool-1-thread-1 is looping of 1 for task of 8
pool-1-thread-1 is looping of 2 for task of 8
pool-1-thread-1 is looping of 3 for task of 8
pool-1-thread-1 is looping of 4 for task of 8
pool-1-thread-1 is looping of 5 for task of 8
pool-1-thread-1 is looping of 6 for task of 8
pool-1-thread-1 is looping of 7 for task of 8
pool-1-thread-1 is looping of 8 for task of 8
pool-1-thread-1 is looping of 9 for task of 8
pool-1-thread-1 is looping of 1 for task of 9
pool-1-thread-1 is looping of 2 for task of 9
pool-1-thread-1 is looping of 3 for task of 9
pool-1-thread-1 is looping of 4 for task of 9
pool-1-thread-1 is looping of 5 for task of 9
pool-1-thread-1 is looping of 6 for task of 9
pool-1-thread-1 is looping of 7 for task of 9
pool-1-thread-1 is looping of 8 for task of 9
pool-1-thread-1 is looping of 9 for task of 9

結(jié)果依次輸出，相當(dāng)于順序執(zhí)行各個任務(wù)。
現(xiàn)行大多數(shù)GUI程序都是單線程的。Android中單線程可用于數(shù)據(jù)庫操作，文件操作，應(yīng)用批量安裝，應(yīng)用批量刪除等不適合并發(fā)但可能IO阻塞性及影響UI線程響應(yīng)的操作。

1.3 newCachedThreadPool

創(chuàng)建一個可緩存線程池，如果線程池長度超過處理需要，可靈活回收空閑線程，若無可回收，則新建線程

public class ThreadPoolTest {
    public static void main(String[] args) {
        //固定線程池
        ExecutorService threadPool = Executors.newCachedThreadPool();
        for (int i = 1; i < 10; i++) {
            int finalI = i;
            threadPool.execute(new Runnable() {
                @Override
                public void run() {
                    for (int j = 1; j < 10; j++) {
                        try {
                            Thread.sleep(20);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        System.out.println(Thread.currentThread().getName() + " is looping of " + j + " for task of " + finalI

                        );

                    }
                }
            });
        }

    }
}

pool-1-thread-2 is looping of 1 for task of 2
pool-1-thread-1 is looping of 1 for task of 1
pool-1-thread-8 is looping of 1 for task of 8
pool-1-thread-9 is looping of 1 for task of 9
pool-1-thread-6 is looping of 1 for task of 6
pool-1-thread-3 is looping of 1 for task of 3
pool-1-thread-7 is looping of 1 for task of 7
pool-1-thread-4 is looping of 1 for task of 4
pool-1-thread-5 is looping of 1 for task of 5
pool-1-thread-6 is looping of 2 for task of 6
pool-1-thread-5 is looping of 2 for task of 5
pool-1-thread-2 is looping of 2 for task of 2
pool-1-thread-1 is looping of 2 for task of 1
pool-1-thread-7 is looping of 2 for task of 7
pool-1-thread-8 is looping of 2 for task of 8
pool-1-thread-3 is looping of 2 for task of 3
pool-1-thread-9 is looping of 2 for task of 9
pool-1-thread-4 is looping of 2 for task of 4
pool-1-thread-5 is looping of 3 for task of 5
pool-1-thread-9 is looping of 3 for task of 9
pool-1-thread-1 is looping of 3 for task of 1
pool-1-thread-2 is looping of 3 for task of 2
pool-1-thread-8 is looping of 3 for task of 8
pool-1-thread-3 is looping of 3 for task of 3
pool-1-thread-7 is looping of 3 for task of 7
pool-1-thread-6 is looping of 3 for task of 6
pool-1-thread-4 is looping of 3 for task of 4
pool-1-thread-2 is looping of 4 for task of 2
pool-1-thread-6 is looping of 4 for task of 6
pool-1-thread-4 is looping of 4 for task of 4
pool-1-thread-7 is looping of 4 for task of 7
pool-1-thread-1 is looping of 4 for task of 1
pool-1-thread-9 is looping of 4 for task of 9
pool-1-thread-3 is looping of 4 for task of 3
pool-1-thread-5 is looping of 4 for task of 5
pool-1-thread-8 is looping of 4 for task of 8
pool-1-thread-6 is looping of 5 for task of 6
pool-1-thread-9 is looping of 5 for task of 9
pool-1-thread-5 is looping of 5 for task of 5
pool-1-thread-8 is looping of 5 for task of 8
pool-1-thread-3 is looping of 5 for task of 3
pool-1-thread-1 is looping of 5 for task of 1
pool-1-thread-7 is looping of 5 for task of 7
pool-1-thread-4 is looping of 5 for task of 4
pool-1-thread-2 is looping of 5 for task of 2
pool-1-thread-6 is looping of 6 for task of 6
pool-1-thread-2 is looping of 6 for task of 2
pool-1-thread-4 is looping of 6 for task of 4
pool-1-thread-7 is looping of 6 for task of 7
pool-1-thread-3 is looping of 6 for task of 3
pool-1-thread-1 is looping of 6 for task of 1
pool-1-thread-5 is looping of 6 for task of 5
pool-1-thread-8 is looping of 6 for task of 8
pool-1-thread-9 is looping of 6 for task of 9
pool-1-thread-4 is looping of 7 for task of 4
pool-1-thread-3 is looping of 7 for task of 3
pool-1-thread-5 is looping of 7 for task of 5
pool-1-thread-1 is looping of 7 for task of 1
pool-1-thread-9 is looping of 7 for task of 9
pool-1-thread-8 is looping of 7 for task of 8
pool-1-thread-7 is looping of 7 for task of 7
pool-1-thread-6 is looping of 7 for task of 6
pool-1-thread-2 is looping of 7 for task of 2
pool-1-thread-9 is looping of 8 for task of 9
pool-1-thread-5 is looping of 8 for task of 5
pool-1-thread-7 is looping of 8 for task of 7
pool-1-thread-1 is looping of 8 for task of 1
pool-1-thread-3 is looping of 8 for task of 3
pool-1-thread-4 is looping of 8 for task of 4
pool-1-thread-8 is looping of 8 for task of 8
pool-1-thread-6 is looping of 8 for task of 6
pool-1-thread-2 is looping of 8 for task of 2
pool-1-thread-5 is looping of 9 for task of 5
pool-1-thread-2 is looping of 9 for task of 2
pool-1-thread-4 is looping of 9 for task of 4
pool-1-thread-6 is looping of 9 for task of 6
pool-1-thread-8 is looping of 9 for task of 8
pool-1-thread-3 is looping of 9 for task of 3
pool-1-thread-1 is looping of 9 for task of 1
pool-1-thread-7 is looping of 9 for task of 7
pool-1-thread-9 is looping of 9 for task of 9

線程池為無限大，當(dāng)執(zhí)行第二個任務(wù)時第一個任務(wù)已經(jīng)完成，會復(fù)用執(zhí)行第一個任務(wù)的線程，而不用每次新建線程。

1.4 newScheduledThreadPool
public class ThreadPoolTest {
    public static void main(String[] args) {

        System.out.println("init");

        Executors.newScheduledThreadPool(3).schedule(
                new Runnable() {
                    @Override
                    public void run() {
                        System.out.println("Done");
                    }
                }, 3, TimeUnit.SECONDS
        );
    }
}

init
Done

表示延遲3秒執(zhí)行。

定期執(zhí)行
public class ThreadPoolTest {
    public static void main(String[] args) {

        Executors.newScheduledThreadPool(3).scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {

                System.out.println("每三秒執(zhí)行一次");
            }
        },2,3,TimeUnit.SECONDS);

    }
}

每三秒執(zhí)行一次
每三秒執(zhí)行一次
每三秒執(zhí)行一次
每三秒執(zhí)行一次
省略...

2 關(guān)閉線程池

shutdown()

public class ThreadPoolTest {
    public static void main(String[] args) {

        //固定線程池
        ExecutorService threadPool = Executors.newFixedThreadPool(3);

        //緩存線程池 池中的線程數(shù)量
        //ExecutorService threadPool = Executors.newCachedThreadPool();

        //單個線程
        //ExecutorService threadPool = Executors.newSingleThreadExecutor();

        for (int i = 1; i < 10; i++) {
            int finalI = i;
            threadPool.execute(new Runnable() {
                @Override
                public void run() {
                    for (int j = 1; j < 10; j++) {
                        try {
                            Thread.sleep(20);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        System.out.println(Thread.currentThread().getName() + " is looping of " + j + " for task of " + finalI

                        );

                    }
                }
            });
        }
        threadPool.shutdown();


    }
}

pool-1-thread-1 is looping of 1 for task of 1
pool-1-thread-2 is looping of 1 for task of 2
pool-1-thread-3 is looping of 1 for task of 3
pool-1-thread-1 is looping of 2 for task of 1
pool-1-thread-3 is looping of 2 for task of 3
pool-1-thread-2 is looping of 2 for task of 2
pool-1-thread-1 is looping of 3 for task of 1
pool-1-thread-2 is looping of 3 for task of 2
pool-1-thread-3 is looping of 3 for task of 3
pool-1-thread-1 is looping of 4 for task of 1
pool-1-thread-3 is looping of 4 for task of 3
pool-1-thread-2 is looping of 4 for task of 2
pool-1-thread-3 is looping of 5 for task of 3
pool-1-thread-1 is looping of 5 for task of 1
pool-1-thread-2 is looping of 5 for task of 2
pool-1-thread-3 is looping of 6 for task of 3
pool-1-thread-1 is looping of 6 for task of 1
pool-1-thread-2 is looping of 6 for task of 2
pool-1-thread-3 is looping of 7 for task of 3
pool-1-thread-1 is looping of 7 for task of 1
pool-1-thread-2 is looping of 7 for task of 2
pool-1-thread-3 is looping of 8 for task of 3
pool-1-thread-1 is looping of 8 for task of 1
pool-1-thread-2 is looping of 8 for task of 2
pool-1-thread-1 is looping of 9 for task of 1
pool-1-thread-3 is looping of 9 for task of 3
pool-1-thread-2 is looping of 9 for task of 2
pool-1-thread-1 is looping of 1 for task of 4
pool-1-thread-3 is looping of 1 for task of 5
pool-1-thread-2 is looping of 1 for task of 6
pool-1-thread-1 is looping of 2 for task of 4
pool-1-thread-3 is looping of 2 for task of 5
pool-1-thread-2 is looping of 2 for task of 6
pool-1-thread-1 is looping of 3 for task of 4
pool-1-thread-3 is looping of 3 for task of 5
pool-1-thread-2 is looping of 3 for task of 6
pool-1-thread-1 is looping of 4 for task of 4
pool-1-thread-3 is looping of 4 for task of 5
pool-1-thread-2 is looping of 4 for task of 6
pool-1-thread-3 is looping of 5 for task of 5
pool-1-thread-1 is looping of 5 for task of 4
pool-1-thread-2 is looping of 5 for task of 6
pool-1-thread-3 is looping of 6 for task of 5
pool-1-thread-1 is looping of 6 for task of 4
pool-1-thread-2 is looping of 6 for task of 6
pool-1-thread-3 is looping of 7 for task of 5
pool-1-thread-1 is looping of 7 for task of 4
pool-1-thread-2 is looping of 7 for task of 6
pool-1-thread-3 is looping of 8 for task of 5
pool-1-thread-1 is looping of 8 for task of 4
pool-1-thread-2 is looping of 8 for task of 6
pool-1-thread-3 is looping of 9 for task of 5
pool-1-thread-1 is looping of 9 for task of 4
pool-1-thread-2 is looping of 9 for task of 6
pool-1-thread-1 is looping of 1 for task of 8
pool-1-thread-2 is looping of 1 for task of 9
pool-1-thread-3 is looping of 1 for task of 7
pool-1-thread-1 is looping of 2 for task of 8
pool-1-thread-2 is looping of 2 for task of 9
pool-1-thread-3 is looping of 2 for task of 7
pool-1-thread-1 is looping of 3 for task of 8
pool-1-thread-2 is looping of 3 for task of 9
pool-1-thread-3 is looping of 3 for task of 7
pool-1-thread-1 is looping of 4 for task of 8
pool-1-thread-2 is looping of 4 for task of 9
pool-1-thread-3 is looping of 4 for task of 7
pool-1-thread-2 is looping of 5 for task of 9
pool-1-thread-3 is looping of 5 for task of 7
pool-1-thread-1 is looping of 5 for task of 8
pool-1-thread-2 is looping of 6 for task of 9
pool-1-thread-3 is looping of 6 for task of 7
pool-1-thread-1 is looping of 6 for task of 8
pool-1-thread-2 is looping of 7 for task of 9
pool-1-thread-3 is looping of 7 for task of 7
pool-1-thread-1 is looping of 7 for task of 8
pool-1-thread-3 is looping of 8 for task of 7
pool-1-thread-1 is looping of 8 for task of 8
pool-1-thread-2 is looping of 8 for task of 9
pool-1-thread-3 is looping of 9 for task of 7
pool-1-thread-1 is looping of 9 for task of 8
pool-1-thread-2 is looping of 9 for task of 9

shutdownNow

public class ThreadPoolTest {
    public static void main(String[] args) {

        //固定線程池
        ExecutorService threadPool = Executors.newFixedThreadPool(3);

        //緩存線程池 池中的線程數(shù)量
        //ExecutorService threadPool = Executors.newCachedThreadPool();

        //單個線程
        //ExecutorService threadPool = Executors.newSingleThreadExecutor();

        for (int i = 1; i < 10; i++) {
            int finalI = i;
            threadPool.execute(new Runnable() {
                @Override
                public void run() {
                    for (int j = 1; j < 10; j++) {
                        try {
                            Thread.sleep(20);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        System.out.println(Thread.currentThread().getName() + " is looping of " + j + " for task of " + finalI

                        );

                    }
                }
            });
        }
        threadPool.shutdownNow();


    }
}

java.lang.InterruptedException: sleep interrupted
    at java.lang.Thread.sleep(Native Method)
    at ThreadPoolTest$1.run(ThreadPoolTest.java:23)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
java.lang.InterruptedException: sleep interrupted
pool-1-thread-3 is looping of 1 for task of 3
    at java.lang.Thread.sleep(Native Method)
    at ThreadPoolTest$1.run(ThreadPoolTest.java:23)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
pool-1-thread-2 is looping of 1 for task of 2
java.lang.InterruptedException: sleep interrupted
    at java.lang.Thread.sleep(Native Method)
    at ThreadPoolTest$1.run(ThreadPoolTest.java:23)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
pool-1-thread-1 is looping of 1 for task of 1
pool-1-thread-3 is looping of 2 for task of 3
pool-1-thread-1 is looping of 2 for task of 1
pool-1-thread-2 is looping of 2 for task of 2
pool-1-thread-2 is looping of 3 for task of 2
pool-1-thread-1 is looping of 3 for task of 1
pool-1-thread-3 is looping of 3 for task of 3
pool-1-thread-2 is looping of 4 for task of 2
pool-1-thread-3 is looping of 4 for task of 3
pool-1-thread-1 is looping of 4 for task of 1
pool-1-thread-2 is looping of 5 for task of 2
pool-1-thread-1 is looping of 5 for task of 1
pool-1-thread-3 is looping of 5 for task of 3
pool-1-thread-1 is looping of 6 for task of 1
pool-1-thread-2 is looping of 6 for task of 2
pool-1-thread-3 is looping of 6 for task of 3
pool-1-thread-1 is looping of 7 for task of 1
pool-1-thread-3 is looping of 7 for task of 3
pool-1-thread-2 is looping of 7 for task of 2
pool-1-thread-1 is looping of 8 for task of 1
pool-1-thread-3 is looping of 8 for task of 3
pool-1-thread-2 is looping of 8 for task of 2
pool-1-thread-3 is looping of 9 for task of 3
pool-1-thread-2 is looping of 9 for task of 2
pool-1-thread-1 is looping of 9 for task of 1
2.1 shutdown和shutdownNow的區(qū)別

shutdown只是將線程池的狀態(tài)設(shè)置為SHUTWDOWN狀態(tài)，正在執(zhí)行的任務(wù)會繼續(xù)執(zhí)行下去，沒有被執(zhí)行的則中斷。而shutdownNow則是將線程池的狀態(tài)設(shè)置為STOP，正在執(zhí)行的任務(wù)則被停止，沒被執(zhí)行任務(wù)的則返回。

2.3 源碼分析shutdown和shutdownNow

1.shutdown源碼

    public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            advanceRunState(SHUTDOWN);
            interruptIdleWorkers();
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
    }

2.shutdownNow源碼

    public List<Runnable> shutdownNow() {
        List<Runnable> tasks;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            advanceRunState(STOP);
            interruptWorkers();
            tasks = drainQueue();
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
        return tasks;
    }

從源碼可以很清晰的看出兩者的區(qū)別，shutdown使用了以后會置狀態(tài)為SHUTDOWN，而shutdownNow為STOP。此外，shutdownNow會返回任務(wù)列表。

2.3 線程狀態(tài)知識延伸

在ThreadPoolExecutor中定義了關(guān)于線程狀態(tài)的幾個變量如下：

// runState is stored in the high-order bits
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;

1.當(dāng)創(chuàng)建線程池后，初始時，線程池處于RUNNING狀態(tài)，此時線程池中的任務(wù)為0；
2.如果調(diào)用了shutdown()方法，則線程池處于SHUTDOWN狀態(tài)，此時線程池不能夠接受新的任務(wù)，它會等待所有任務(wù)執(zhí)行完畢；
3.如果調(diào)用了shutdownNow()方法，則線程池處于STOP狀態(tài)，此時線程池不能接受新的任務(wù)，并且會去嘗試終止正在執(zhí)行的任務(wù)；
4.當(dāng)所有的任務(wù)已終止，ctl記錄的”任務(wù)數(shù)量”為0，線程池會變?yōu)門IDYING狀態(tài)。接著會執(zhí)行terminated()函數(shù)。
5.線程池處在TIDYING狀態(tài)時，執(zhí)行完terminated()之后，就會由 TIDYING -> TERMINATED，線程池被設(shè)置為TERMINATED狀態(tài)。

總結(jié)

線程池的作用：

線程池作用就是限制系統(tǒng)中執(zhí)行線程的數(shù)量。
根據(jù)系統(tǒng)的環(huán)境情況，可以自動或手動設(shè)置線程數(shù)量，達(dá)到運(yùn)行的最佳效果；少了浪費(fèi)了系統(tǒng)資源，多了造成系統(tǒng)擁擠效率不高。用線程池控制線程數(shù)量，其他線程排隊(duì)等候。一個任務(wù)執(zhí)行完畢，再從隊(duì)列的中取最前面的任務(wù)開始執(zhí)行。若隊(duì)列中沒有等待進(jìn)程，線程池的這一資源處于等待。當(dāng)一個新任務(wù)需要運(yùn)行時，如果線程池 中有等待的工作線程，就可以開始運(yùn)行了；否則進(jìn)入等待隊(duì)列。

為什么要用線程池:

1.減少了創(chuàng)建和銷毀線程的次數(shù)，每個工作線程都可以被重復(fù)利用，可執(zhí)行多個任務(wù)。
2.可以根據(jù)系統(tǒng)的承受能力，調(diào)整線程池中工作線線程的數(shù)目，防止因?yàn)橄倪^多的內(nèi)存，而把服務(wù)器累趴下(每個線程需要大約1MB內(nèi)存，線程開的越多，消耗的內(nèi)存也就越大，最后死機(jī))。

特別感謝：

星辰之力Java 四種線程池
一句話說明白Java線程池中shutdown和shutdownNow的區(qū)別

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