ThreadPoolExecutor提供了四個構造方法：

我們以最後一個構造方法（引數最多的那個），對其引數進行解釋：

 public ThreadPoolExecutor(int corePoolSize, // 1
                              int maximumPoolSize,  // 2
                              long keepAliveTime,  // 3
                              TimeUnit unit,  // 4
                              BlockingQueue<Runnable> workQueue, // 5
                              ThreadFactory threadFactory,  // 6
                              RejectedExecutionHandler handler ) { //7
        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;
    }

自定義執行緒池：

public class ThreadPoolTest {
    public static void main(String[] args) throws IOException {
        int corePoolSize = 2;// 核心執行緒大小
        int maximumPoolSize = 4;// 最大執行緒池大小
        long keepAliveTime = 10; // 執行緒空閒10s後自動結束
        TimeUnit unit = TimeUnit.SECONDS;
        // ArrayBlockingQueue，該阻塞佇列底層維護了一個定長陣列（建立物件必須指定容量）
        BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<>(2);
        ThreadFactory threadFactory = new NameTreadFactory();// 自定義執行緒工廠
        RejectedExecutionHandler handler = new MyIgnorePolicy(); // 自定義拒絕策略
        // handler = new ThreadPoolExecutor.AbortPolicy();// 無法處理新任務會丟擲RejectedExecutionException異常
        // 建立執行緒池物件
        ThreadPoolExecutor executor = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime,
                unit, workQueue, threadFactory, handler);
        executor.prestartAllCoreThreads(); // 預啟動所有核心執行緒
        // 建立10個任務物件提交到執行緒池中
        for (int i = 1; i <= 10; i++) {
            MyTask task = new MyTask(String.valueOf(i));
            executor.execute(task);
        }
        System.in.read(); //阻塞主執行緒
    }

    // 任務物件
    static class MyTask implements Runnable {
        private String name;

        public MyTask(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            try {
                System.out.println(this.toString() + " is running!");
                Thread.sleep(3000); //讓任務執行慢點
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        public String getName() {
            return name;
        }

        @Override
        public String toString() {
            return "MyTask [name=" + name + "]";
        }
    }

    // 自定義拒絕策略
    static class MyIgnorePolicy implements RejectedExecutionHandler {

        @Override
        public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
            doLog(r, executor);
        }

        private void doLog(Runnable r, ThreadPoolExecutor e) {
            // 可做日誌記錄等
            System.err.println(r.toString() + " rejected");
//          System.out.println("completedTaskCount: " + e.getCompletedTaskCount());
        }
    }

    // 執行緒工廠類
    static class NameTreadFactory implements ThreadFactory {
        private final AtomicInteger mThreadNum = new AtomicInteger(1);

        @Override
        public Thread newThread(Runnable r) {
            Thread t = new Thread(r, "my-thread-" + mThreadNum.getAndIncrement());
            System.out.println(t.getName() + " has been created");
            return t;
        }
    }

}

輸出結果如下：

該執行緒池同一時間最多處理4個任務，2個任務處於等待對列中，其餘的任務都會被拒絕執行。

執行緒池的處理流程主要分為3步：

• 當在execute(Runnable)方法中提交的新任務後，執行緒池先判斷執行緒數是否達到了核心執行緒數（corePoolSize）。如果未達到執行緒數，則建立核心執行緒處理任務；否則，就執行下一步；

• 接著執行緒池判斷任務佇列是否滿了。如果沒滿，則將任務新增到任務佇列中；否則，執行下一步；

• 接著因為任務佇列滿了，執行緒池就判斷執行緒數是否達到了最大執行緒數。如果未達到，則建立非核心執行緒處理任務；否則，就執行飽和策略，預設會丟擲RejectedExecutionException異常。