線程池的實現原理，絕對簡單易懂

線程池實現原理

1.線程池簡介

爲什麼要構建線程池？

1.複用已有的資源（減少cpu時間片的切換）

2.控制現場資源總數

優勢：

1.限流->控制線程數量

2.降低頻繁創建和銷燬線程。（對於任務的響應速度更快，可以直接從線程池中取，不需要創建線程）

2.Java中提供的線程池

Executors,線程池工具類 。

//主要都是ThreadPoolExecutor 構造的線程池
//ThreadPoolExecutor的構造方法
public ThreadPoolExecutor(int corePoolSize,//核心線程數
                              int maximumPoolSize,最大線程數
                              long keepAliveTime,//保持連接時間
                              TimeUnit unit,//保持連接時間單位
                              BlockingQueue<Runnable> workQueue,//阻塞隊列
                              ThreadFactory threadFactory,//線程工廠
                              RejectedExecutionHandler handler) //拒絕策略

newFixedThreadPool()

public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }

newCachedThreadPool

public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

newSingleThreadScheduledExecutor

public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
        return new DelegatedScheduledExecutorService
            (new ScheduledThreadPoolExecutor(1));
    }

newScheduledThreadPool

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }

public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue());
    }

newWorkStealingPool

public static ExecutorService newWorkStealingPool(int parallelism) {
        return new ForkJoinPool
            (parallelism,
             ForkJoinPool.defaultForkJoinWorkerThreadFactory,
             null, true);
    }

//ForkJoinPool構造方法。
public ForkJoinPool(int parallelism,
                        ForkJoinWorkerThreadFactory factory,
                        UncaughtExceptionHandler handler,
                        boolean asyncMode) {
        this(checkParallelism(parallelism),
             checkFactory(factory),
             handler,
             asyncMode ? FIFO_QUEUE : LIFO_QUEUE,
             "ForkJoinPool-" + nextPoolId() + "-worker-");
        checkPermission();
    }

問題1:猜想keepAliveTime如何去監控線程進行回收？下面我們看看線程池是如何實現的。
1.有一個是否回收核心線程的開關(allowCoreThreadTimeOut)
2.當前線程數是否大於核心線程數（wc > corePoolSize）
請參考getTask()方法，只要返回空，線程則會進行回收。

3.ThreadPoolExecutor實現原理：

execute方法實現如下：

	//線程默認運行狀態是 RUNNING
	private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    private static final int COUNT_BITS = Integer.SIZE - 3;
	//表示低29位全是1，高3位爲0
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

    // runState is stored in the high-order bits
	//高3位存儲線程運行狀態
    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;

    // Packing and unpacking ctl
	//獲取高三位，表示獲取線程運行狀態
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
	//獲取低29位，表示獲取線程數量
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    private static int ctlOf(int rs, int wc) { return rs | wc; }
//ThreadPoolExecutor.execute實現源碼
public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        /*
         * Proceed in 3 steps://分爲3步
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         */
  			//ctl默認狀態爲運行狀態 
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
               addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }

addWorker：

//firstTask 傳遞的是.execute(Runnable r) 中的r,core 表示是否創建一個核心線程數
private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                try {
                    // Recheck while holding lock.
                    // Back out on ThreadFactory failure or if
                    // shut down before lock acquired.
                    int rs = runStateOf(ctl.get());

                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                        workers.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

主要做兩件事：

//主要增加工作線程數
for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
              //最主要做的事情 CAS給ctl加1，增加一個工作線程數
                if (compareAndIncrementWorkerCount(c))
                    break retry;
              //重試獲取，再一次讀區ctl
                c = ctl.get();  // Re-read ctl
              //如果當前狀態不是之前的狀態，則繼續retry
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }

//主要創建一個Worker線程，並添加到隊列(真正意義的構建線程)
				boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            w = new Worker(firstTask);
          	/**
          	*Worker(Runnable firstTask) {
          	* 初始化state爲-1。
            *	setState(-1); // inhibit interrupts until runWorker
            *	this.firstTask = firstTask;
            *	把當前Worker對象賦值爲Worker中的thread
            *	this.thread = getThreadFactory().newThread(this);
        		*}
          	*
          	*/
            final Thread t = w.thread;
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                try {
                    // Recheck while holding lock.
                    // Back out on ThreadFactory failure or if
                    // shut down before lock acquired.
                    int rs = runStateOf(ctl.get());
										//當前狀態爲Runnable 或者（狀態爲SHUTDOWN，firstTask爲null）
                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                //private final HashSet<Worker> workers = new HashSet<Worker>();
                        //把當前worker添加到workers Set中
                      	workers.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                  	/**
                  	*	public void run() {
                    *    runWorker(this);
                    *	}
                  	*
                  	*/
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
          //如果執行失敗，需要回滾
          /**
          *			 private void addWorkerFailed(Worker w) {
          *      final ReentrantLock mainLock = this.mainLock;
          *      mainLock.lock();
          *      try {
          *          if (w != null)
          *          workers.remove(w);
          *					 //把數量減1 CAS實現
          *          decrementWorkerCount();
          *          tryTerminate();
          *      } finally {
          *          mainLock.unlock();
          *      }
          *  }
          */
                addWorkerFailed(w);
        }
        return workerStarted;

//runWorker 是核心
final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {方法後面講
          	//getTask是從阻塞隊列裏獲取，已經不是直接交給核心線程了。
            while (task != null || (task = getTask()) != null) {
                //不僅僅是加鎖着麼簡單！後面講解
              	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 {
                      	//問題2.爲什麼要調用run方法呢？
                      	//task這時候已經存在線程池中了，所以沒有必要.start方法，再創建一個線程去執行了
                        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();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

Worker：實現了Runnable，並且繼承了AQS

問題3:Worker爲什麼要實現AQS呢？爲什麼不用ReentrantLock實現呢？

我們看一下Worker裏面做了什麼？

Worker(Runnable firstTask) {
  					//設置同步狀態，
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }

protected boolean tryAcquire(int unused) {
  					//1是表示獲取鎖，0表示釋放狀態，-1是初始化狀態
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }

protected boolean tryRelease(int unused) {
            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }

//上面的runWorker 爲什麼w要加鎖呢？
//ThreadPoolExecutor.shutdown方法
    public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            advanceRunState(SHUTDOWN);
            interruptIdleWorkers();
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        } finally {
            mainLock.unlock();
        }
        tryTerminate();
    }

	//interruptIdleWorkers方法，中斷時會判斷，線程是否還在運行。
  private void interruptIdleWorkers(boolean onlyOne) {
          final ReentrantLock mainLock = this.mainLock;
          mainLock.lock();
          try {
              for (Worker w : workers) {
                  Thread t = w.thread;			
                  if (!t.isInterrupted() && /*重點*/ w.tryLock()) {
                      try {
                          t.interrupt();
                      } catch (SecurityException ignore) {
                      } finally {
                          w.unlock();
                      }
                  }
                  if (onlyOne)
                      break;
              }
          } finally {
              mainLock.unlock();
          }
      }

這就是爲什麼Worker要實現AQS的原因，爲了shuatdown時不終止正在運行的任務

//getTask從workQueue獲取執行的線程，非核心線程數是否被回收？
private Runnable getTask() {
    boolean timedOut = false; // Did the last poll() time out?

    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);

        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            decrementWorkerCount();
            return null;
        }

        int wc = workerCountOf(c);

        // Are workers subject to culling?
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

        if ((wc > maximumPoolSize || (timed && timedOut))
            && (wc > 1 || workQueue.isEmpty())) {
            if (compareAndDecrementWorkerCount(c))
                return null;
            continue;
        }

        try {
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();
            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}

執行流程：

圖例：

隊列的簡單操作
offer       添加一個元素並返回true      如果隊列已滿，則返回false
poll        移除並返問隊列頭部的元素     如果隊列爲空，則返回null
put         添加一個元素               如果隊列滿，則阻塞
take        移除並返回隊列頭部的元素     如果隊列爲空，則阻塞

1.不建議使用Executors 創建線程

2.線程的執行情況->

​ IO密集型 CPU核心數的2倍

​ CPU密集型 CPU核心數+1

Executor框架最核心的接口是Executor，它表示任務的執行器。

Executor的子接口爲ExecutorService。

ExecutorService有兩大實現類：ThreadPoolExecutor和ScheduledThreadPoolExecutor。

注：本文爲博主原創文章，轉載請附上原文出處鏈接和本聲明。