JAVA基礎學習之-ThreadPoolExecutor的實現原理

池技術是性能優化的重要手段：鏈接池，線程池已是開發中的標配了。面試中這個知識點也是高頻問題。抽空學習了Java的ThreadPoolExecutor, 把學習的思路記錄一下。

因爲線程的建立和銷燬都是系統層面的操做，涉及到系統資源的佔用和回收，因此建立線程是一個重量級的操做。爲了提高性能，就引入了線程池；即線程複用。Java不只提供了線程池，還提供了線程池的操做工具類。 咱們由淺入深瞭解一下。

import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ThreadDemo {

    static class Worker implements Runnable{

        public void run(){
                System.out.println("run work "+Thread.currentThread().getName() );
        }
    }

    public static void main(String[] args) {

        Worker w1 = new Worker();

        ExecutorService service = Executors.newFixedThreadPool(10);

        service.submit(w1);

        service.shutdown();
    }
}

看Executors的源碼，發現其使用的是ThreadPoolExecutor。 研究一下ThreadPoolExecutor, 發現其默認的參數Executors.defaultThreadFactory(), defaultHandler。線程池的建立工廠默認以下:

public Thread newThread(Runnable r) {
            Thread t = new Thread(group, r,
                                  namePrefix + threadNumber.getAndIncrement(),
                                  0);
            if (t.isDaemon())
                t.setDaemon(false);
            if (t.getPriority() != Thread.NORM_PRIORITY)
                t.setPriority(Thread.NORM_PRIORITY);
            return t;
        }

也就是自定義了一下線程的名字，將線程歸到了同一個組。
線程池的defaultHandler以下:

public static class AbortPolicy implements RejectedExecutionHandler {
        /**
         * Creates an {@code AbortPolicy}.
         */
        public AbortPolicy() { }

        /**
         * Always throws RejectedExecutionException.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         * @throws RejectedExecutionException always.
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            throw new RejectedExecutionException("Task " + r.toString() +
                                                 " rejected from " +
                                                 e.toString());
        }
    }

也就是說，當提交的任務超過線程池的容量，那麼就會拋出RejectedExecutionException異常。 可是使用Executors會發現，並無拋出異常。這是由於Executors建立BlockingQueue時沒有指定隊列的容量。

換言之，線程池能容納的任務數量最多爲maximumPoolSize + queueSize。 好比線程池以下new ThreadPoolExecutor(10, 11, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(5)); 則最大任務數量爲16個，超過16個就會拋出異常。

線程池中線程數量有多少呢？先運行以下的代碼:

import java.util.concurrent.*;
import java.util.concurrent.locks.ReentrantLock;

public class ThreadDemo {

    static class Worker implements Runnable{

        public void run(){

            try {
                Thread.sleep(1000);
                System.out.println("done work "+Thread.currentThread().getName() );
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {

        Worker w1 = new Worker();

        ThreadPoolExecutor executor = new ThreadPoolExecutor(3, 4,
                0L, TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<Runnable>(5));
        for(int i=0;i<9;i++) {
            executor.submit(w1);
        }

        executor.shutdown();
    }
}

能夠發現最多開啓了4個線程。 這4個線程就對應了4個Worker的實例。
看worker的源碼能夠發現，它兼備AQS和Runnable兩個特性。 咱們只關注它Runnable的特性。

while (task != null || (task = getTask()) != null) {
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted.  This
                // requires a recheck in second case to deal with
                // shutdownNow race while clearing interrupt
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        task.run();
                    } catch (RuntimeException x) {
                        thrown = x; throw x;
                    } catch (Error x) {
                        thrown = x; throw x;
                    } catch (Throwable x) {
                        thrown = x; throw new Error(x);
                    } finally {
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }

在這個線程中不斷從隊列中獲取任務，而後執行。 worker中反覆出現的ctl又是什麼呢？

ctl是兩個變量組合，一個32位的int, 高3位用於控制線程池的狀態，低29位用於記錄線程池啓動線程的數量。
因此有這麼幾個方法

// Packing and unpacking ctl
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    private static int ctlOf(int rs, int wc) { return rs | wc; }

整個線程池的核心，就worker和ctl的理解。 有點複雜，主要是集中了:

1. AQS
2. BlockingQueue

這也是爲何我建議先學AQS,後學線程池的實現原理。