JAVA線程池之 Executors （二）原理分析

時間 2019-12-13

原文原文鏈接

1、線程池狀態java

　　 private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    private static final int COUNT_BITS = Integer.SIZE - 3;
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

    // 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;

    // 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; }

　　RUNNING : 該狀態的線程池會接收新的任務，並處理阻塞隊列中的任務。ide

　　SHUTDOWN : 該狀態的線程池不會接收新的任務，但會處理阻塞隊列中的任務。oop

　　STOP : 該狀態的線程池不會接收新的任務，也不會處理阻塞隊列中的任務，並且會中斷正在執行的任務。ui

2、任務提交方式this

　　　一、executespa

　　　　提交的任務必須實現Runnable接口，接口不帶返回值線程

public void execute(Runnable command) {

　　　二、submit
code

　　　　父類AbstractExecutorService提供有submit接口，可獲取線程執行返回值。　　　　blog

 public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<Void> ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }

public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

3、任務執行 -- execute繼承

　　execute 方法

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        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);
    }

　　大體流程爲：

　　　　一、經過workerCountOf方法獲得線程池的當前線程數，若是當前線程數小於corePoolSize，則執行addWorker方法建立一個新的核心線程執行任務。

　　　　二、若是當前線程數大於等於corePoolSize時，檢查線程池的運行狀態，若是線程池運行狀態爲RUNNING，則嘗試將任務加入阻塞隊列。

　　　　三、再次檢查線程池的運行狀態，若是運行狀態不爲RUNNING，則從阻塞隊列中刪除任務並執行reject方法調用處理機制。

　　　　四、在2的基礎上，若是加入阻塞隊列失敗，則會執行addWorker方法建立一個新的非核心線程執行任務。

　　　　五、在3的基礎上，若是addWorker執行失敗，則會調用reject調用處理機制。

　　addWorker方法

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;
    }

　　　　大體流程爲：

　　　　　　一、自旋檢測線程池狀態，若是狀態大於SHUTDOWN，或者 firstTask爲空或隊列爲空時，返回任務加入隊列失敗。

　　　　　　二、獲取線程池當前線程數，經過core判斷是不是建立核心線程，若是爲true，而且當前線程數wc小於corePoolSize時，跳出循環建立新的線程。若是core爲false,

　　　　　　　　則判斷當前線程數wc是否小於maximumPoolSize，小於跳出循環。

　　　　　　三、線程池的工做線程時候經過Worker實現的，經過ReentrantLock加鎖，再次經過線程池狀態監測以後，將worker加入到HashSet<Worker> workers 裏面

　　　　　　四、若是加入成功，則啓動Worker中的線程。

　　Worker類

private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
    {
        /**
         * This class will never be serialized, but we provide a
         * serialVersionUID to suppress a javac warning.
         */
        private static final long serialVersionUID = 6138294804551838833L;

        /** Thread this worker is running in.  Null if factory fails. */
        final Thread thread;
        /** Initial task to run.  Possibly null. */
        Runnable firstTask;
        /** Per-thread task counter */
        volatile long completedTasks;

        /**
         * Creates with given first task and thread from ThreadFactory.
         * @param firstTask the first task (null if none)
         */
        Worker(Runnable firstTask) {
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }

　　　　Worker類繼承了AbstractQueuedSynchronizer（AQS）類，能夠方便的實現工做線程的停止操做。

　　　　而且自己實現了Runnable接口，可單獨做爲任務在工做線程中執行。

　　runWorker 方法

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            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();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

　　runWorker流程：

　　一、線程啓動以後，經過unlock方法釋放鎖，設置AQS的state爲0，表示運行中斷；

　　二、獲取第一個任務firstTask，並執行task的run方法，在執行run方法前，會對Worker加鎖，任務執行完釋放鎖。

　　三、在任務執行先後，可根據業務自定義實現beforeExecute(wt, task); 和 afterExecute(task, thrown);。

　　四、任務執行完以後，調用getTask從阻塞隊列中獲取等待的任務，若是隊列中沒有任務，getTask方法會被阻塞並掛起，不會佔用CPU資源。

　　getTask方法

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;
            }
        }
    }

　　getTask流程：

Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();

　　一、若是設定了超時機制，則經過 workQueue.poll（）方法來獲取阻塞隊列中的任務，若是隊列中沒有任務，則會在keepAliveTime時間後返回null。

　　二、若是未設置超時機制，而且當前線程數小於核心線程時，同時未設置容許核心線程超時的狀況下，經過workQueue.take(); 方法來獲取阻塞隊列中的任務，若是沒有任務，

　　　　則會一直等待並掛起，直到有新任務提交時，則會環信等待的隊列並返回新的任務。

　　三、阻塞隊列使用生產者與消費者模式，使用等待與喚醒使線程池線程掛起與喚起。


4、任務執行 -- submit

　　　　submit重載了多種實現方式

　　　　一、Callable　

public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

　　　　二、Runnable

public <T> Future<T> submit(Runnable task, T result) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task, result);
        execute(ftask);
        return ftask;
    }

　　在實際業務中，Future和Callable是成雙出現的，Callable負責產生結果，Future負責獲取結果。

　　一、Callable相似於Runnable，只是Callable附帶返回值。

　　二、Callable除了正常返回以外，若是線程出現異常，該異常也會返回，即Future的get方法能夠獲取到異常結果。

　　三、Future的get()方法會致使主線程阻塞，直到Callable執行完成。

　　FutureTask

　　futureTask內部狀態

 * Possible state transitions:
     * NEW -> COMPLETING -> NORMAL
     * NEW -> COMPLETING -> EXCEPTIONAL
     * NEW -> CANCELLED
     * NEW -> INTERRUPTING -> INTERRUPTED
     */
    private volatile int state;
    private static final int NEW          = 0;
    private static final int COMPLETING   = 1;
    private static final int NORMAL       = 2;
    private static final int EXCEPTIONAL  = 3;
    private static final int CANCELLED    = 4;
    private static final int INTERRUPTING = 5;
    private static final int INTERRUPTED  = 6;

　　　　FutureTask 實現了Runnable接口，提交的任務能夠交由工做線程處理，執行run方法。

　　get方法

 public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

　　調用get方法時，若是task的狀態處於執行中或初始化，調用awaitDone方法對線程進行阻塞。

　　awaitDone方法

 private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING) // cannot time out yet
                Thread.yield();
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (timed) {
                nanos = deadline - System.nanoTime();
                if (nanos <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                LockSupport.parkNanos(this, nanos);
            }
            else
                LockSupport.park(this);
        }
    }

　　經過對Task的狀態檢測，若是Callable未執行完成，使用 LockSupport.park(this); 對當前線程進行阻塞。等待喚起，並將主線程封裝成WaitNode 並存放在 waiters 鏈表中。

　　run方法

public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

　　　　run方法流程：

　　　　　　經過對task的state判斷，若是task爲初始New狀態，則執行call方法，獲取call方法返回結果，並調用set方法

　　　　　　若是執行失敗，則調用setException方法。

setException方法

　　　設置狀態 EXCEPTIONAL

protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

　　　set方法

　　　　設置狀態 NORMAL

protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }

　　finishCompletion();方法

private void finishCompletion() {
        // assert state > COMPLETING;
        for (WaitNode q; (q = waiters) != null;) {
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }

        done();

        callable = null;        // to reduce footprint
    }

　　若是finishCompletion 檢測到經過get方法被阻塞的線程集 waiters 不爲空時，獲取的每個節點，並使用 LockSupport.unpark(t); 對其喚醒。

　　最終使用report返回結果。