1. 實現第一版基礎的線程池

1.1 首先我們定義一個線程池類ThreadPool，然后線程池有一個容器存放我們創建的線程，另一個容器則是存放當前線程池需要處理的任務隊列，線程容器用ArrayList實現，任務隊列容器用LinkedList實現。

1.2 線程池功能分析：創建線程池時，往線程容器中加入默認數量的線程，但是由于此時的任務隊列為空，所以線程處于等待狀態；當往線程池提交任務時，喚醒正在等待的線程來處理任務，處理完任務后如果任務隊列為空，線程又進入等待狀態，代碼及運行結果如下。

import java.util.ArrayList; import java.util.LinkedList;public class ThreadPoolTest {public static void main(String[] args) {// 創建線程池ThreadPool threadPool = new ThreadPool();// 提交任務給線程池處理for (int i = 0; i < 40; i++) {threadPool.submit(String.valueOf(i));}} }class ThreadPool{private static final int DEFAULT_SIZE = 10;private int size;private final static ThreadGroup GROUP = new ThreadGroup("MyThreadGroup");private final LinkedList<String> taskQueue = new LinkedList<>();private ArrayList<Thread> threadLst = new ArrayList<>();ThreadPool(){this(DEFAULT_SIZE);}ThreadPool(int size){this.size = size;for (int i = 0; i < size; i++) {Thread t = new Thread(GROUP, ()->{while(true){synchronized (taskQueue) {// 任務隊列為空，線程則進入等待狀態while (taskQueue.isEmpty()) {try {taskQueue.wait();} catch (InterruptedException e) {e.printStackTrace();}}// 模擬處理任務System.out.println(Thread.currentThread().getName() + " deal task!!");taskQueue.removeFirst();}// 如果這一段業務代碼處于synchronized內，那么會導致提交任務時搶不到鎖，// 而造成提交任務被打斷try {Thread.sleep(20);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName() + " finish task!");}});threadLst.add(t);t.start();}}public void submit(String task){// 多線程下提交任務和處理任務，需要加synchronized保證線程安全。// 提交任務后需要喚醒線程處理任務synchronized (taskQueue){taskQueue.add(task);taskQueue.notify();System.out.println("提交了一個任務========");}} }

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2. 增加任務數過多和線程池銷毀功能的改進版線程池

2.1 正常運行的代碼及結果

import java.util.ArrayList; import java.util.LinkedList;public class ThreadPoolTest {public static void main(String[] args) {// 創建線程池ThreadPool threadPool = new ThreadPool();// 提交任務給線程池處理new Thread(){@Overridepublic void run() {for (int i = 0; i < 20; i++) {threadPool.submit(String.valueOf(i));}}}.start();// 銷毀線程池new Thread(){@Overridepublic void run() {threadPool.destry();}}.start();// 線程池被銷毀后，再提交任務會拋出異常threadPool.submit("w");} }class ThreadPool{private static final int DEFAULT_SIZE = 10;private int size;private boolean destry = false;private final static ThreadGroup GROUP = new ThreadGroup("MyThreadGroup");private final LinkedList<String> taskQueue = new LinkedList<>();private static final int MAX_TASK_SIZE = 2000;private final ArrayList<Worker> threadLst = new ArrayList<>();private class Worker extends Thread{private volatile boolean dead = false;public Worker(ThreadGroup group, String name){super(group, name);}@Overridepublic void run() {end:while(!dead){// 模擬多線程，等待搶taskQueue鎖 // try { // Thread.sleep(1); // } catch (InterruptedException e) { // }synchronized (taskQueue) {while (taskQueue.isEmpty()) {try {taskQueue.wait();} catch (InterruptedException e) {break end;}}System.out.println(Thread.currentThread().getName() + " deal task!!");taskQueue.removeFirst();}// 如果這一段業務代碼處于synchronized內，那么會導致提交任務時搶不到鎖，// 而造成提交任務被阻塞try {Thread.sleep(20);} catch (InterruptedException e) {}System.out.println(Thread.currentThread().getName() + " finish task!");}}public void terminate(){dead = true;}}ThreadPool(){this(DEFAULT_SIZE);}ThreadPool(int size){this.size = size;for (int i = 0; i < size; i++) {Worker worker = new Worker(GROUP, "t"+i);threadLst.add(worker);worker.start();}}public void submit(String task) {// 多線程下提交任務和處理任務，需要加synchronized保證線程安全。// 提交任務后需要喚醒線程處理任務synchronized (taskQueue){// 線程被銷毀，如果還有任務，那么拋出異常if(destry){throw new RuntimeException("ThreadPool has been destry!!");}if(taskQueue.size() >= MAX_TASK_SIZE){throw new RuntimeException("Task is too much!!!");}taskQueue.add(task);taskQueue.notify();System.out.println("提交了一個任務========");}}public void destry() {destry = true;/*// 當然可以加上這么一段，保證任務處理結束再銷毀線程池，但是需要等待的時間也變長了。synchronized(taskQueue){while(!taskQueue.isEmpty()){try {Thread.sleep(50);} catch (InterruptedException e) {e.printStackTrace();}}}*/for (int i = 0; i < size; i++) {Worker t = threadLst.get(i);t.interrupt();t.terminate();}// 等待50ms，再去查看線程池活躍線程數，因為還是要等run方法執行結束才行try {Thread.sleep(50);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("銷毀線程池后剩余的活躍線程數：" + GROUP.activeCount());} }

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2.2 模擬多線程出錯的情況

2.3 解決：既然一次銷毀不了，你還會進入等待狀態，那么就兩次唄，為了簡單實現而已。實際操作肯定不行，有待改進。

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3. 簡單地擴充線程數量及閑時自動回收邏輯的改進版

package ThreadTest;import java.util.ArrayList; import java.util.LinkedList;public class ThreadPoolTest {public static void main(String[] args) throws InterruptedException {// 創建線程池ThreadPool threadPool = new ThreadPool();new Thread(threadPool).start();// 提交任務給線程池處理new Thread(){@Overridepublic void run() {for (int i = 0; i < 1000; i++) {threadPool.submit(String.valueOf(i));}}}.start();// 銷毀線程池new Thread(){@Overridepublic void run() {threadPool.destry();}}.start();// 線程池被銷毀后，再提交任務會拋出異常threadPool.submit("w");} }class ThreadPool implements Runnable{private static int cnt = 1;private boolean destry = false;private final static ThreadGroup GROUP = new ThreadGroup("MyThreadGroup");private final LinkedList<String> taskQueue = new LinkedList<>();private static final int MAX_TASK_SIZE = 2000;private final ArrayList<Worker> threadLst = new ArrayList<>();private int min;private int idle;private int max;private class Worker extends Thread{private volatile boolean dead = false;public Worker(ThreadGroup group, String name){super(group, name);}@Overridepublic void run() {end:while(!dead){// 模擬多線程，等待搶taskQueue鎖try {Thread.sleep(1);} catch (InterruptedException e) {}synchronized (taskQueue) {while (taskQueue.isEmpty()) {try {taskQueue.wait();} catch (InterruptedException e) {break end;}}System.out.println(Thread.currentThread().getName() + " deal task!!");taskQueue.removeFirst();}// 如果這一段業務代碼處于synchronized內，那么會導致提交任務時搶不到鎖，// 而造成提交任務被阻塞try {Thread.sleep(20);} catch (InterruptedException e) {}System.out.println(Thread.currentThread().getName() + " finish task!");}}public void terminate(){dead = true;}}ThreadPool(){this(5, 10, 50);}ThreadPool(int min, int idle, int max){this.min = min;this.idle = idle;this.max = max;for (int i = 0; i < min; i++) {createWorker();}}private void createWorker(){Worker worker = new Worker(GROUP, "t"+(cnt++));threadLst.add(worker);worker.start();}public void submit(String task) {// 多線程下提交任務和處理任務，需要加synchronized保證線程安全。// 提交任務后需要喚醒線程處理任務synchronized (taskQueue){// 線程被銷毀，如果還有任務，那么拋出異常if(destry){throw new RuntimeException("ThreadPool has been destry!!");}if(taskQueue.size() >= MAX_TASK_SIZE){throw new RuntimeException("Task is too much!!!");}taskQueue.add(task);taskQueue.notify();System.out.println("提交了一個任務========");}}public void destry() {destry = true;/*// 當然可以加上這么一段，保證任務處理結束再銷毀線程池，但是需要等待的時間也變長了。synchronized(taskQueue){while(!taskQueue.isEmpty()){try {Thread.sleep(50);} catch (InterruptedException e) {e.printStackTrace();}}}*/synchronized (threadLst){for (int i = 0; i < threadLst.size(); i++) {Worker t = threadLst.get(i);t.interrupt();t.terminate();}// 等待50ms，再去查看線程池活躍線程數，因為還是要等run方法執行結束才行try {Thread.sleep(50);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("第一次銷毀線程池后剩余的活躍線程數：" + GROUP.activeCount());for (int i = 0; i < threadLst.size(); i++) {Worker t = threadLst.get(i);t.interrupt();t.terminate();}try {Thread.sleep(30);} catch (InterruptedException e) {e.printStackTrace();}System.out.println("第二次銷毀線程池后剩余的活躍線程數：" + GROUP.activeCount());}}@Overridepublic void run() {while(! destry){System.out.println("線程池當前的線程個數：" + threadLst.size());try {Thread.sleep(50);// taskQueue只是讀取size大小，不涉及更改；而且size的不安全不造成問題。// 但是threadLst涉及增加元素，存在同步安全問題；而且線程池大小有設定，所以這里對threadLst對象加鎖synchronized (threadLst){while (!destry){int size = threadLst.size();if(taskQueue.size() > 3 * min && size < idle){for (int i = size; i < idle; i++) {createWorker();}}else if(taskQueue.size() > 5 * idle && size < max){for (int i = size; i < max; i++) {createWorker();}}if(taskQueue.size() < idle && size > idle){for (int i = idle; i < size; i++) {Worker worker = threadLst.get(0);worker.interrupt();worker.terminate();threadLst.remove(0);}}}}} catch (InterruptedException e) {e.printStackTrace();}}} }

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總結：寫得可能有點亂，甚至有bug，請多多指教。

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