害，又是一個炒冷飯的時間。fork/join是在jdk1.7中出現的一個併發工作包，其特點是可以將一個大的任務拆分成多個子任務進行並行處理，最後將子任務結果合併成最後的計算結果，並進行輸出。從而達到多執行緒分發任務，達到高效處理的目的。
1. 關於fork/join的一點想法
以上說法，也許大家沒什麼感覺。但換個說法可能會更讓人體會深切。總體上，相當於一個map階段資料拆分，一個reduce階段資料收集。即一個mapreduce過程，是不是有大資料的思想在了。只不過這fork/join的拆分難度可見性更大（自己手動拆，mapreduce由shuffle元件自動拆），另外fork/join是在一個機器上執行，而大資料的框架，則是在分散式系統中執行的。
從這個點說來，好像研究fork/join就顯得有些意義了。
只是，按照fork/join的語義解釋，是將任務拆分，然後處理，然後再合併結果。如果沒有了合併結果這一步，那麼，它就等同於執行緒池了，這也就是有人說它與執行緒池有啥差別的疑惑所在了。再說有需要收集結果的這一語義，其實我們也是可以通過執行緒池去執行任務，然後再用get()得到結果，然後在外部做合併，也是一樣咯。
2. fork/join的幾個核心類
fork/join被稱作執行框架，自然不會是一個單一元件問題了。
首先，它會有一個 ForkJoinPool, 相當於執行緒池, 所有的任務都要通過它來進行提交，然後由其進行統一排程。
然後，每個任務都會有許多相同的程式碼，只有業務實現是不一樣的，所以它會有一個基類： RecursiveTask . 實現上還有一個無返回結果的類：RecursiveAction, 只是沒有返回結果時，往往又可能可以使用普通執行緒池執行替代了。（沒有絕對）
ForkJoinWorkerThreadFactory, 是fork/join框架的執行緒工廠類，原本含義與普通的執行緒工廠類一致，只是它的入參不再是一個個 Runnable 任務，而是 ForkJoinPool, 因為它們所處的上下文是不一樣的。
ForkJoinWorkerThread, 執行fork/join的具體執行緒，它可能在執行過程中，再去主動新增task。而它自身擁有一個佇列，它的主要任務就是獲取佇列任務，然後執行。但當其自身的佇列完成時，它可以通過work-steal演算法竊取其他執行緒的佇列任務。這也是fork/join的核心所在。
sun.misc.Unsafe, 之所以要提到這個jdk類，是因為在fork/join框架中，對於佇列的管理，不是通過普通的list或陣列來實現，而是通過 U.putOrderedObject(a, j, task); 來存放，雖然效果與陣列是一樣的，但它會更簡單地實現執行緒安全的操作。只是，其中有許多的位操作，值得學習的同時，也顯得有些麻煩了。
3. fork/join使用樣例
我們通過對一個陣列的排序過程，使用fork/join來實現看看如何使用這框架。尤其對於大陣列的排序，顯得還是有用的。這種大陣列的排序，一般都會使用快速排序或者歸併排序來處理。此處使用fork/join框架來處理，也是暗合了歸併排序的道理了。
import java.util.Arrays; import java.util.Random; import java.util.concurrent.ExecutionException; import java.util.concurrent.ForkJoinPool; import java.util.concurrent.ForkJoinTask; import java.util.concurrent.RecursiveTask; /** * Fork/join框架測試 */ public class TestForkJoinFramework { public static void main(String[] args) { long beginTime = System.currentTimeMillis(); ForkJoinPool pool = new ForkJoinPool(); int mockArrLen = 1000_0000; int[] arr = new int[mockArrLen]; Random r = new Random(); for (int index = 1; index <= mockArrLen; index++) { arr[index - 1] = r.nextInt(1000_0000); } FJOrderTask task = new FJOrderTask(arr); ForkJoinTask<int[]> taskResult = pool.submit(task); try { // 等待結果完成 taskResult.get(); } catch (InterruptedException | ExecutionException e) { e.printStackTrace(); } long endTime = System.currentTimeMillis(); System.out.println("耗時=" + (endTime - beginTime)); } /** * 單個排序的子任務 */ private static class FJOrderTask extends RecursiveTask<int[]> { /** * 當前排序的陣列值 */ private final int[] source; public FJOrderTask(int[] source) { this.source = source; } /** * 真正的業務計算邏輯 * * @see java.util.concurrent.RecursiveTask#compute() */ @Override protected int[] compute() { int sourceLen = source.length; // 如果條件成立，說明任務中要進行排序的集合還不夠小 System.out.println(Thread.currentThread()); if (sourceLen > 2) { int midIndex = sourceLen / 2; // 拆分成兩個子任務, 0 -> mid - 1, mid -> len FJOrderTask task1 = new FJOrderTask( Arrays.copyOf(source, midIndex)); task1.fork(); FJOrderTask task2 = new FJOrderTask( Arrays.copyOfRange(source, midIndex, sourceLen)); task2.fork(); // 將兩個有序的陣列，合併成一個有序的陣列 int[] result1 = task1.join(); int[] result2 = task2.join(); return insertMerge(result1, result2); } // 否則說明集合中只有一個或者兩個元素，可以進行這兩個元素的比較排序了 else { // 如果條件成立，說明陣列中只有一個元素，或者是陣列中的元素都已經排列好位置了 if (sourceLen == 1 || source[0] <= source[1]) { return source; } else { int[] orderedArr = new int[sourceLen]; orderedArr[0] = source[1]; orderedArr[1] = source[0]; return orderedArr; } } } /** * 使用插入排序，將兩個有序陣列合並起來 * * @param arr1 有序陣列1 * @param arr2 有序陣列2 * @return 合併後的有序陣列 */ private int[] insertMerge(int[] arr1, int[] arr2) { int[] result = new int[arr1.length + arr2.length]; int arr1Len = arr1.length; int arr2Len = arr2.length; int destLen = result.length; // 簡單插入排序 for (int i = 0, array1Index = 0, array2Index = 0; i < destLen; i++) { int value1 = array1Index >= arr1Len ? Integer.MAX_VALUE : arr1[array1Index]; int value2 = array2Index >= arr2Len ? Integer.MAX_VALUE : arr2[array2Index]; if (value1 < value2) { array1Index++; result[i] = value1; } else { array2Index++; result[i] = value2; } } return result; } } }
思路很簡單，就是將陣列一直拆分，直到最後一個或者兩個時，從最下面來開始排序，然後依次往上回溯，使用插入排序歸併結果集，最終返回排好序的值。如果除去任務拆分的過程，則時間複雜度還是非常好的 O(nlog(n)), 只是這任務拆分的過程，需要大量的空間複雜度，也不見得是什麼好事。且不管它。
4. fork/join框架的實現原理
我們以上面的demo為出發點，觀察fork/join的工作過程，不知道100%，也八九不離十了。上面主要有幾個動作，一ForkJoinPool例項化，submit一個Task, get()等待最終結果完成。這三個看得見的動作好辦，只是其核心也許還在背後。
4.1. ForkJoinPool構造器
每個要呼叫框架的應用，必先初始化一個pool例項，這是自然。如上使用無參構造器，實際上是使用了框架的各種預設值而已, 這種預設值往往是能夠滿足大部分的場景的，從而體現其易用性。
// java.util.concurrent.ForkJoinPool#ForkJoinPool() /** * Creates a {@code ForkJoinPool} with parallelism equal to {@link * java.lang.Runtime#availableProcessors}, using the {@linkplain * #defaultForkJoinWorkerThreadFactory default thread factory}, * no UncaughtExceptionHandler, and non-async LIFO processing mode. * * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public ForkJoinPool() { // 並行度預設是cpu的核數 this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()), defaultForkJoinWorkerThreadFactory, null, false); } /** * Creates a {@code ForkJoinPool} with the given parameters. * * @param parallelism the parallelism level. For default value, * use {@link java.lang.Runtime#availableProcessors}. * @param factory the factory for creating new threads. For default value, * use {@link #defaultForkJoinWorkerThreadFactory}. * @param handler the handler for internal worker threads that * terminate due to unrecoverable errors encountered while executing * tasks. For default value, use {@code null}. * @param asyncMode if true, * establishes local first-in-first-out scheduling mode for forked * tasks that are never joined. This mode may be more appropriate * than default locally stack-based mode in applications in which * worker threads only process event-style asynchronous tasks. * For default value, use {@code false}. * @throws IllegalArgumentException if parallelism less than or * equal to zero, or greater than implementation limit * @throws NullPointerException if the factory is null * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory, UncaughtExceptionHandler handler, boolean asyncMode) { this(checkParallelism(parallelism), checkFactory(factory), handler, // FIFO_QUEUE = 1 << 16, LIFO_QUEUE = 0 asyncMode ? FIFO_QUEUE : LIFO_QUEUE, "ForkJoinPool-" + nextPoolId() + "-worker-"); checkPermission(); } /** * Creates a {@code ForkJoinPool} with the given parameters, without * any security checks or parameter validation. Invoked directly by * makeCommonPool. */ private ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory, UncaughtExceptionHandler handler, int mode, String workerNamePrefix) { this.workerNamePrefix = workerNamePrefix; this.factory = factory; this.ueh = handler; this.config = (parallelism & SMASK) | mode; long np = (long)(-parallelism); // offset ctl counts this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK); }
構造器自然沒啥好說的，就是設定幾個並行度，初始化執行緒工廠，標識等等。為下文做準備。
4.2. 任務submit過程
上面的例子中，submit只有一次呼叫，而實際應用中則不一定。但即使如此，一次submit, 其實背後也是有許多的動作的。因為這一個task裡，又會生出許多task來。
// java.util.concurrent.ForkJoinPool#submit /** * Submits a ForkJoinTask for execution. * * @param task the task to submit * @param <T> the type of the task's result * @return the task * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) { if (task == null) throw new NullPointerException(); // submit主要是向pool中加入任務佇列 externalPush(task); return task; } /** * Tries to add the given task to a submission queue at * submitter's current queue. Only the (vastly) most common path * is directly handled in this method, while screening for need * for externalSubmit. * * @param task the task. Caller must ensure non-null. */ final void externalPush(ForkJoinTask<?> task) { WorkQueue[] ws; WorkQueue q; int m; int r = ThreadLocalRandom.getProbe(); int rs = runState; // 如果執行緒不是第一次進入，且獲得鎖，則直接放佇列即可 // 否則走普通加入佇列邏輯 if ((ws = workQueues) != null && (m = (ws.length - 1)) >= 0 && (q = ws[m & r & SQMASK]) != null && r != 0 && rs > 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) { ForkJoinTask<?>[] a; int am, n, s; if ((a = q.array) != null && (am = a.length - 1) > (n = (s = q.top) - q.base)) { int j = ((am & s) << ASHIFT) + ABASE; // 通過 putOrderedObject 新增任務到佇列中 U.putOrderedObject(a, j, task); U.putOrderedInt(q, QTOP, s + 1); U.putIntVolatile(q, QLOCK, 0); if (n <= 1) signalWork(ws, q); return; } U.compareAndSwapInt(q, QLOCK, 1, 0); } // 初始化時的submit或者通用 submit externalSubmit(task); } /** * Full version of externalPush, handling uncommon cases, as well * as performing secondary initialization upon the first * submission of the first task to the pool. It also detects * first submission by an external thread and creates a new shared * queue if the one at index if empty or contended. * * @param task the task. Caller must ensure non-null. */ private void externalSubmit(ForkJoinTask<?> task) { int r; // initialize caller's probe if ((r = ThreadLocalRandom.getProbe()) == 0) { ThreadLocalRandom.localInit(); r = ThreadLocalRandom.getProbe(); } for (;;) { WorkQueue[] ws; WorkQueue q; int rs, m, k; boolean move = false; // 停止執行 if ((rs = runState) < 0) { tryTerminate(false, false); // help terminate throw new RejectedExecutionException(); } // 未被初始化，先執行初始化 else if ((rs & STARTED) == 0 || // initialize ((ws = workQueues) == null || (m = ws.length - 1) < 0)) { int ns = 0; // 上鎖初始化 rs = lockRunState(); try { if ((rs & STARTED) == 0) { U.compareAndSwapObject(this, STEALCOUNTER, null, new AtomicLong()); // create workQueues array with size a power of two int p = config & SMASK; // ensure at least 2 slots int n = (p > 1) ? p - 1 : 1; n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; n |= n >>> 16; n = (n + 1) << 1; // 佇列數量初始化 workQueues = new WorkQueue[n]; ns = STARTED; } } finally { unlockRunState(rs, (rs & ~RSLOCK) | ns); } } // 當前執行緒已新增過佇列 else if ((q = ws[k = r & m & SQMASK]) != null) { // 上鎖新增到佇列中 if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) { ForkJoinTask<?>[] a = q.array; // 取出棧頂指標，向其中放入任務 int s = q.top; boolean submitted = false; // initial submission or resizing try { // locked version of push if ((a != null && a.length > s + 1 - q.base) || (a = q.growArray()) != null) { int j = (((a.length - 1) & s) << ASHIFT) + ABASE; U.putOrderedObject(a, j, task); U.putOrderedInt(q, QTOP, s + 1); submitted = true; } } finally { U.compareAndSwapInt(q, QLOCK, 1, 0); } // 如果佇列新增成功，則喚醒一個 worker, 返回 // 否則進入下一次嘗試新增過程 if (submitted) { signalWork(ws, q); return; } } move = true; // move on failure } else if (((rs = runState) & RSLOCK) == 0) { // create new queue q = new WorkQueue(this, null); q.hint = r; q.config = k | SHARED_QUEUE; q.scanState = INACTIVE; rs = lockRunState(); // publish index if (rs > 0 && (ws = workQueues) != null && k < ws.length && ws[k] == null) ws[k] = q; // else terminated unlockRunState(rs, rs & ~RSLOCK); } else move = true; // move if busy // 如有必要，為當前執行緒生成新的標識 if (move) r = ThreadLocalRandom.advanceProbe(r); } }
由上可知，submit主要初始化佇列以及向佇列中新增任務，並在喚醒worker處理任務。但實際上，worker Thread 我們還沒有看到被啟用，只是看到有隊workQueue的初始化。那麼，worker又是在哪進行初始化的呢？只可能是在 signal 的時候了。
4.3. worker的初始化
worker是真正執行任務的執行緒，前面光看到新增佇列，以及喚醒worker了。只是這時還未見worker被初始化，實際上它是在被喚醒的邏輯中進行初始化的。
// java.util.concurrent.ForkJoinPool#signalWork /** * Tries to create or activate a worker if too few are active. * * @param ws the worker array to use to find signallees * @param q a WorkQueue --if non-null, don't retry if now empty */ final void signalWork(WorkQueue[] ws, WorkQueue q) { long c; int sp, i; WorkQueue v; Thread p; while ((c = ctl) < 0L) { // too few active，一個標識，分兩段使用，低位為0代表worker還可以新增 if ((sp = (int)c) == 0) { // no idle workers if ((c & ADD_WORKER) != 0L) // too few workers tryAddWorker(c); break; } if (ws == null) // unstarted/terminated break; if (ws.length <= (i = sp & SMASK)) // terminated break; if ((v = ws[i]) == null) // terminating break; int vs = (sp + SS_SEQ) & ~INACTIVE; // next scanState int d = sp - v.scanState; // screen CAS long nc = (UC_MASK & (c + AC_UNIT)) | (SP_MASK & v.stackPred); if (d == 0 && U.compareAndSwapLong(this, CTL, c, nc)) { v.scanState = vs; // activate v if ((p = v.parker) != null) U.unpark(p); break; } if (q != null && q.base == q.top) // no more work break; } } /** * Tries to add one worker, incrementing ctl counts before doing * so, relying on createWorker to back out on failure. * * @param c incoming ctl value, with total count negative and no * idle workers. On CAS failure, c is refreshed and retried if * this holds (otherwise, a new worker is not needed). */ private void tryAddWorker(long c) { boolean add = false; do { long nc = ((AC_MASK & (c + AC_UNIT)) | (TC_MASK & (c + TC_UNIT))); if (ctl == c) { int rs, stop; // check if terminating if ((stop = (rs = lockRunState()) & STOP) == 0) add = U.compareAndSwapLong(this, CTL, c, nc); unlockRunState(rs, rs & ~RSLOCK); if (stop != 0) break; // 新增標識成功，再建立worker if (add) { createWorker(); break; } } } while (((c = ctl) & ADD_WORKER) != 0L && (int)c == 0); } /** * Tries to construct and start one worker. Assumes that total * count has already been incremented as a reservation. Invokes * deregisterWorker on any failure. * * @return true if successful */ private boolean createWorker() { ForkJoinWorkerThreadFactory fac = factory; Throwable ex = null; ForkJoinWorkerThread wt = null; try { // 呼叫執行緒工廠建立新的worker, 並立即啟動worker if (fac != null && (wt = fac.newThread(this)) != null) { wt.start(); return true; } } catch (Throwable rex) { ex = rex; } // 建立失敗，處理異常 deregisterWorker(wt, ex); return false; } /** * Default ForkJoinWorkerThreadFactory implementation; creates a * new ForkJoinWorkerThread. */ static final class DefaultForkJoinWorkerThreadFactory implements ForkJoinWorkerThreadFactory { public final ForkJoinWorkerThread newThread(ForkJoinPool pool) { return new ForkJoinWorkerThread(pool); } }
果然在signal時，建立worker。值得一提的，為了實現安全地新增worker，它會先更新成功ctl，然後再執行真正的create操作。避免多建立出worker來。
4.4. worker的工作原理
前面看到worker建立過程，傳入了pool的例項，即當前上下文都是被worker可見的。所以，它能很好地複用當前的配置資訊，而它自身是一個非同步執行緒，在建立之後，立即被啟動起來了。那它後續則必然嘗試從佇列獲取任務，進行執行了。具體如何？
1. WorkerThread 構造方法
// java.util.concurrent.ForkJoinWorkerThread#ForkJoinWorkerThread /** * Creates a ForkJoinWorkerThread operating in the given pool. * * @param pool the pool this thread works in * @throws NullPointerException if pool is null */ protected ForkJoinWorkerThread(ForkJoinPool pool) { // Use a placeholder until a useful name can be set in registerWorker super("aForkJoinWorkerThread"); this.pool = pool; // workQueue 臨時向 pool 中進行註冊所得 this.workQueue = pool.registerWorker(this); } /** * Callback from ForkJoinWorkerThread constructor to establish and * record its WorkQueue. * * @param wt the worker thread * @return the worker's queue */ final WorkQueue registerWorker(ForkJoinWorkerThread wt) { UncaughtExceptionHandler handler; wt.setDaemon(true); // configure thread if ((handler = ueh) != null) wt.setUncaughtExceptionHandler(handler); WorkQueue w = new WorkQueue(this, wt); int i = 0; // assign a pool index int mode = config & MODE_MASK; int rs = lockRunState(); try { WorkQueue[] ws; int n; // skip if no array if ((ws = workQueues) != null && (n = ws.length) > 0) { int s = indexSeed += SEED_INCREMENT; // unlikely to collide int m = n - 1; i = ((s << 1) | 1) & m; // odd-numbered indices if (ws[i] != null) { // collision int probes = 0; // step by approx half n int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2; while (ws[i = (i + step) & m] != null) { if (++probes >= n) { workQueues = ws = Arrays.copyOf(ws, n <<= 1); m = n - 1; probes = 0; } } } w.hint = s; // use as random seed w.config = i | mode; w.scanState = i; // publication fence ws[i] = w; } } finally { unlockRunState(rs, rs & ~RSLOCK); } wt.setName(workerNamePrefix.concat(Integer.toString(i >>> 1))); return w; }
重點則是在 pool 中註冊自身，得到一個 workQueue. 而其具體業務，則是在run方法中實現。
// java.util.concurrent.ForkJoinWorkerThread#run /** * This method is required to be public, but should never be * called explicitly. It performs the main run loop to execute * {@link ForkJoinTask}s. */ public void run() { if (workQueue.array == null) { // only run once Throwable exception = null; try { onStart(); pool.runWorker(workQueue); } catch (Throwable ex) { exception = ex; } finally { try { onTermination(exception); } catch (Throwable ex) { if (exception == null) exception = ex; } finally { pool.deregisterWorker(this, exception); } } } } // java.util.concurrent.ForkJoinPool#runWorker /** * Top-level runloop for workers, called by ForkJoinWorkerThread.run. */ final void runWorker(WorkQueue w) { w.growArray(); // allocate queue int seed = w.hint; // initially holds randomization hint int r = (seed == 0) ? 1 : seed; // avoid 0 for xorShift for (ForkJoinTask<?> t;;) { // 取任務，執行 if ((t = scan(w, r)) != null) w.runTask(t); else if (!awaitWork(w, r)) break; r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift } } /** * Executes the given task and any remaining local tasks. */ final void runTask(ForkJoinTask<?> task) { if (task != null) { scanState &= ~SCANNING; // mark as busy (currentSteal = task).doExec(); U.putOrderedObject(this, QCURRENTSTEAL, null); // release for GC execLocalTasks(); ForkJoinWorkerThread thread = owner; if (++nsteals < 0) // collect on overflow transferStealCount(pool); scanState |= SCANNING; if (thread != null) thread.afterTopLevelExec(); } } // java.util.concurrent.ForkJoinTask#doExec /** * Primary execution method for stolen tasks. Unless done, calls * exec and records status if completed, but doesn't wait for * completion otherwise. * * @return status on exit from this method */ final int doExec() { int s; boolean completed; if ((s = status) >= 0) { try { completed = exec(); } catch (Throwable rex) { return setExceptionalCompletion(rex); } if (completed) s = setCompletion(NORMAL); } return s; } // java.util.concurrent.RecursiveTask#exec /** * Implements execution conventions for RecursiveTask. */ protected final boolean exec() { // 即呼叫具體業務類的 compute 方法 result = compute(); return true; }
我們們草草看了 worker 如何執行任務。這和執行緒池沒多少差別，大致仍是從佇列獲取任務，然後執行業務方法compute . 我們暫時略去了如何獲取任務，以及如何執行work-steal了。且看下節。
4.5. 任務獲取實現
主要是通過scan處理。
// java.util.concurrent.ForkJoinPool#scan /** * Scans for and tries to steal a top-level task. Scans start at a * random location, randomly moving on apparent contention, * otherwise continuing linearly until reaching two consecutive * empty passes over all queues with the same checksum (summing * each base index of each queue, that moves on each steal), at * which point the worker tries to inactivate and then re-scans, * attempting to re-activate (itself or some other worker) if * finding a task; otherwise returning null to await work. Scans * otherwise touch as little memory as possible, to reduce * disruption on other scanning threads. * * @param w the worker (via its WorkQueue) * @param r a random seed * @return a task, or null if none found */ private ForkJoinTask<?> scan(WorkQueue w, int r) { WorkQueue[] ws; int m; if ((ws = workQueues) != null && (m = ws.length - 1) > 0 && w != null) { int ss = w.scanState; // initially non-negative for (int origin = r & m, k = origin, oldSum = 0, checkSum = 0;;) { WorkQueue q; ForkJoinTask<?>[] a; ForkJoinTask<?> t; int b, n; long c; // 首次獲取時，是從自身佇列中獲取 if ((q = ws[k]) != null) { if ((n = (b = q.base) - q.top) < 0 && (a = q.array) != null) { // non-empty long i = (((a.length - 1) & b) << ASHIFT) + ABASE; if ((t = ((ForkJoinTask<?>) U.getObjectVolatile(a, i))) != null && q.base == b) { if (ss >= 0) { if (U.compareAndSwapObject(a, i, t, null)) { q.base = b + 1; if (n < -1) // signal others signalWork(ws, q); return t; } } else if (oldSum == 0 && // try to activate w.scanState < 0) tryRelease(c = ctl, ws[m & (int)c], AC_UNIT); } if (ss < 0) // refresh ss = w.scanState; r ^= r << 1; r ^= r >>> 3; r ^= r << 10; origin = k = r & m; // move and rescan oldSum = checkSum = 0; continue; } checkSum += b; } if ((k = (k + 1) & m) == origin) { // continue until stable if ((ss >= 0 || (ss == (ss = w.scanState))) && oldSum == (oldSum = checkSum)) { if (ss < 0 || w.qlock < 0) // already inactive break; int ns = ss | INACTIVE; // try to inactivate long nc = ((SP_MASK & ns) | (UC_MASK & ((c = ctl) - AC_UNIT))); w.stackPred = (int)c; // hold prev stack top U.putInt(w, QSCANSTATE, ns); if (U.compareAndSwapLong(this, CTL, c, nc)) ss = ns; else w.scanState = ss; // back out } checkSum = 0; } } } return null; }
要安全高效地實現一個獲取佇列還是不易啊。
4.6. task.fork 實現
一般地，能用上fork一詞的場景，一般是對於當前環境的一個copy. 難道這裡的fork也是這樣嗎？新開一個執行緒？不然又是如何找到需要處理的佇列的呢？
// java.util.concurrent.ForkJoinTask#fork /** * Arranges to asynchronously execute this task in the pool the * current task is running in, if applicable, or using the {@link * ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}. While * it is not necessarily enforced, it is a usage error to fork a * task more than once unless it has completed and been * reinitialized. Subsequent modifications to the state of this * task or any data it operates on are not necessarily * consistently observable by any thread other than the one * executing it unless preceded by a call to {@link #join} or * related methods, or a call to {@link #isDone} returning {@code * true}. * * @return {@code this}, to simplify usage */ public final ForkJoinTask<V> fork() { Thread t; // ForkJoinWorkerThread 中持有workQueue例項，可直接向其新增任務 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ((ForkJoinWorkerThread)t).workQueue.push(this); else // 如果是外部執行緒，則新增到一共享pool中即可，後續將其各空閒執行緒處理 ForkJoinPool.common.externalPush(this); return this; } // java.util.concurrent.ForkJoinPool.WorkQueue#push /** * Pushes a task. Call only by owner in unshared queues. (The * shared-queue version is embedded in method externalPush.) * * @param task the task. Caller must ensure non-null. * @throws RejectedExecutionException if array cannot be resized */ final void push(ForkJoinTask<?> task) { ForkJoinTask<?>[] a; ForkJoinPool p; int b = base, s = top, n; if ((a = array) != null) { // ignore if queue removed int m = a.length - 1; // fenced write for task visibility U.putOrderedObject(a, ((m & s) << ASHIFT) + ABASE, task); U.putOrderedInt(this, QTOP, s + 1); if ((n = s - b) <= 1) { if ((p = pool) != null) p.signalWork(p.workQueues, this); } else if (n >= m) growArray(); } } /** * A thread managed by a {@link ForkJoinPool}, which executes * {@link ForkJoinTask}s. * This class is subclassable solely for the sake of adding * functionality -- there are no overridable methods dealing with * scheduling or execution. However, you can override initialization * and termination methods surrounding the main task processing loop. * If you do create such a subclass, you will also need to supply a * custom {@link ForkJoinPool.ForkJoinWorkerThreadFactory} to * {@linkplain ForkJoinPool#ForkJoinPool use it} in a {@code ForkJoinPool}. * * @since 1.7 * @author Doug Lea */ public class ForkJoinWorkerThread extends Thread { /* * ForkJoinWorkerThreads are managed by ForkJoinPools and perform * ForkJoinTasks. For explanation, see the internal documentation * of class ForkJoinPool. * * This class just maintains links to its pool and WorkQueue. The * pool field is set immediately upon construction, but the * workQueue field is not set until a call to registerWorker * completes. This leads to a visibility race, that is tolerated * by requiring that the workQueue field is only accessed by the * owning thread. * * Support for (non-public) subclass InnocuousForkJoinWorkerThread * requires that we break quite a lot of encapsulation (via Unsafe) * both here and in the subclass to access and set Thread fields. */ final ForkJoinPool pool; // the pool this thread works in final ForkJoinPool.WorkQueue workQueue; // work-stealing mechanics ... }
可見，fork的過程，即是向當前執行緒中新增當前任務而已，並沒有所謂的上下文copy過程。
4.7. task.join 實現
join的語義是，等待任務完成後返回。與 Thread.join()一致。只是有一個問題，即如果某個執行緒阻塞等待結果去了，那當前執行緒自然就相當於無法再被利用了。那後續的任務又何從談起呢？想來只有遞迴能夠解決這個問題了。但是遞迴往往又是在單執行緒中完成的，這豈不無法利用併發特性了？
實際上，之所以被分作fork/join兩個步驟，意義就是在這。上一節我們看到，fork的過程是向佇列中新增了任務，隨後就返回了。這時，如果當前worker比較繁忙（在做任務拆分），則這些任務就會被其他worker竊取過去處理了。而其他任務在處理時，又會遇到自己的遞迴，從而將一個單執行緒的遞迴變為多執行緒的遞迴了。
下面我們主要看一個執行緒的遞迴過程。join的本義只是等待當前任務完成，但是當前任務完成又要依賴於其子任務完成join, 子任務又要等待其子任務join, 因此形成遞迴。而join()返回的表象是compute()完成，所以這過程其實是伴隨著compute的運算的。
// java.util.concurrent.ForkJoinTask#join /** * Returns the result of the computation when it {@link #isDone is * done}. This method differs from {@link #get()} in that * abnormal completion results in {@code RuntimeException} or * {@code Error}, not {@code ExecutionException}, and that * interrupts of the calling thread do <em>not</em> cause the * method to abruptly return by throwing {@code * InterruptedException}. * * @return the computed result */ public final V join() { int s; if ((s = doJoin() & DONE_MASK) != NORMAL) reportException(s); // 任務完成後，主動獲取結果 return getRawResult(); } /** * Throws exception, if any, associated with the given status. */ private void reportException(int s) { if (s == CANCELLED) throw new CancellationException(); if (s == EXCEPTIONAL) rethrow(getThrowableException()); } // java.util.concurrent.RecursiveTask#getRawResult public final V getRawResult() { return result; } /** * Implementation for join, get, quietlyJoin. Directly handles * only cases of already-completed, external wait, and * unfork+exec. Others are relayed to ForkJoinPool.awaitJoin. * * @return status upon completion */ private int doJoin() { int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w; return (s = status) < 0 ? s : ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? // 取當前任務執行， doExec 執行任務，awaitJoin 等待執行完成 (w = (wt = (ForkJoinWorkerThread)t).workQueue). tryUnpush(this) && (s = doExec()) < 0 ? s : wt.pool.awaitJoin(w, this, 0L) : externalAwaitDone(); } // java.util.concurrent.ForkJoinPool#awaitJoin /** * Helps and/or blocks until the given task is done or timeout. * * @param w caller * @param task the task * @param deadline for timed waits, if nonzero * @return task status on exit */ final int awaitJoin(WorkQueue w, ForkJoinTask<?> task, long deadline) { int s = 0; if (task != null && w != null) { ForkJoinTask<?> prevJoin = w.currentJoin; U.putOrderedObject(w, QCURRENTJOIN, task); CountedCompleter<?> cc = (task instanceof CountedCompleter) ? (CountedCompleter<?>)task : null; for (;;) { if ((s = task.status) < 0) break; if (cc != null) helpComplete(w, cc, 0); // 遞迴新增任務等待完成 else if (w.base == w.top || w.tryRemoveAndExec(task)) helpStealer(w, task); if ((s = task.status) < 0) break; long ms, ns; if (deadline == 0L) ms = 0L; else if ((ns = deadline - System.nanoTime()) <= 0L) break; else if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) <= 0L) ms = 1L; if (tryCompensate(w)) { task.internalWait(ms); U.getAndAddLong(this, CTL, AC_UNIT); } } U.putOrderedObject(w, QCURRENTJOIN, prevJoin); } return s; } // java.util.concurrent.ForkJoinPool.WorkQueue#tryRemoveAndExec /** * If present, removes from queue and executes the given task, * or any other cancelled task. Used only by awaitJoin. * * @return true if queue empty and task not known to be done */ final boolean tryRemoveAndExec(ForkJoinTask<?> task) { ForkJoinTask<?>[] a; int m, s, b, n; if ((a = array) != null && (m = a.length - 1) >= 0 && task != null) { while ((n = (s = top) - (b = base)) > 0) { for (ForkJoinTask<?> t;;) { // traverse from s to b long j = ((--s & m) << ASHIFT) + ABASE; if ((t = (ForkJoinTask<?>)U.getObject(a, j)) == null) return s + 1 == top; // shorter than expected else if (t == task) { boolean removed = false; if (s + 1 == top) { // pop if (U.compareAndSwapObject(a, j, task, null)) { U.putOrderedInt(this, QTOP, s); removed = true; } } else if (base == b) // replace with proxy removed = U.compareAndSwapObject( a, j, task, new EmptyTask()); // 執行子任務 if (removed) task.doExec(); break; } else if (t.status < 0 && s + 1 == top) { if (U.compareAndSwapObject(a, j, t, null)) U.putOrderedInt(this, QTOP, s); break; // was cancelled } if (--n == 0) return false; } if (task.status < 0) return false; } } return true; }
可見，最終fork/join還是使用遞迴完成join任務等待。差別在於其利用了多執行緒的優勢，同時執行多個任務。這有兩個好處，一是減輕了單執行緒的任務處理壓力，二是讓遞迴的深度也分擔到了多個點上。避免了棧早早溢位的可能。
只是每個執行緒被分配的任務數是多少，join需要等待的結果有多少，就不太好說了。比如最上層的執行緒如果任務被別的執行緒搶走，則它就只需一直在等結果就行了。而最下面的執行緒，則需要承擔最深的遞迴深度，以保證程式的最終出口。其實從這個點，我們自己可以做個猜想，如果沒有做好控制，讓執行緒之間任意執行任務，是否會造成死鎖呢？這恐怕是個問題。