多线程高并发编程(8) -- Fork/Join源码分析

一.概念

  Fork/Join就是将一个大任务分解(fork)成许多个独立的小任务,然后多线程并行去处理这些小任务,每个小任务处理完得到结果再进行合并(join)得到最终的结果。

多线程高并发编程(8) -- Fork/Join源码分析_第1张图片

  流程:任务继承RecursiveTask,重写compute方法,使用ForkJoinPool的submit提交任务,任务在某个线程中运行,工作任务中的compute方法的代码开始对任务进行分析,如果符合条件就进行任务拆分,拆分成多个子任务,每个子任务进行数据的计算或操作,得到结果返回给上一层任务开启线程进行合并,最终通过get获取整体处理结果。【只能将任务1个切分为两个,不能切分为3个或其他数量

多线程高并发编程(8) -- Fork/Join源码分析_第2张图片

  • ForkJoinTask:代表fork/join里面的任务类型,一般用它的两个子类RecursiveTask(任务有返回值)和RecursiveAction(任务没有返回值),任务的处理逻辑包括任务的切分都是在重写compute方法里面进行处理。只有ForkJoinTask任务可以被拆分运行和合并运行。【可查看上篇Future源码分析的类图结构】【ForkJoinTask使用了模板模式进行设计,将ForkJoinTask的执行相关代码进行隐藏,通过提供抽象类(即子类RecursiveTask、RecursiveAction)暴露用户的实际业务处理。】
    • RecursiveTask:在进行exec之后会使用一个result的变量进行接受返回的结果;
      public abstract class RecursiveTask extends ForkJoinTask {
          V result;
          protected abstract V compute();
      
          public final V getRawResult() {
              return result;
          }
      
          protected final void setRawResult(V value) {
              result = value;
          }
          protected final boolean exec() {
              result = compute();
              return true;
          }
      
      }
    • RecursiveAction:在进行exec之后没有返回结果;
      public abstract class RecursiveAction extends ForkJoinTask {
         
          protected abstract void compute();
      
          public final Void getRawResult() { return null; }
      
          protected final void setRawResult(Void mustBeNull) { }
      
          protected final boolean exec() {
              compute();
              return true;
          }
      
      } 
  • ForkJoinPool:fork/join框架的管理者,最原始的任务都要交给它来处理。它负责控制整个fork/join有多少个工作线程,工作线程的创建、机会都是由它来控制。它还负责workQueue队列的创建和分配,每当创建一个工作线程,它负责分配对应的workQueue,然后它把接到的活都交给工作线程去处理。是整个fork/join的容器。
    • ForkJoinPool.WorkQueue:双端队列,负责存储接收的任务;
  • ForkJoinWorkerThread:fork/join里面真正干活的”工人“,它继承了Thread,所以本质是一个线程。它有一个ForkJoinPool.WorkQueue的队列存放着它要干的活,接活之前它要向ForkJoinPool注册(registerWorker),拿到相应的workQueue,然后就从workQueue里面拿任务出来处理。它是依附于ForkJoinPool而存活,如果ForkJoinPool销毁了,它也会跟着结束。【每一个ForkJoinWorkerThread线程都具有一个独立的任务等待队列workQueue。】
    • 当使用ForkJoinPool进行submit任务提交时,创建1个workQueue将任务放进去,然后进行fork任务切分,如果切分后的任务放的进去之前的workQueue就放进去,不行就随机选取workQueue放进去,如果还放不了就创建一个新的workQueue放进去;多线程高并发编程(8) -- Fork/Join源码分析_第3张图片
      public class ForkJoinWorkerThread extends Thread {
          final ForkJoinPool pool;
          final ForkJoinPool.WorkQueue workQueue;
          protected ForkJoinWorkerThread(ForkJoinPool pool) {
              super("aForkJoinWorkerThread");
              this.pool = pool;
              this.workQueue = pool.registerWorker(this);//向ForkJoinPool执行池注册当前工作线程,ForkJoinPool为其分配一个工作队列
          }
      }

二.用法

  以前1+2+3+...+100这样的处理可以用for循环处理,现在使用fork/join来处理:从下面结果可以看到,大任务被不断的拆分成小任务,然后添加到工作线程的队列中,每个小任务都会被工作线程从队列中取出进行运行,然后每个小任务的结果的合并也由工作线程执行,然后不断的汇总成最终结果。【task通过ForkJoinPool来执行,分割的子任务添加到当前工作线程的队列中,进入队列的头部,当一个工作线程中没有任务时,会从其他工作线程的队列尾部获取一个任务。(工作窃取:当前工作线程对应的队列中没有任务了,从其他工作线程对应的队列中取出任务进行操作,然后将操作结果返还给对应队列的线程。)】

public class MyFrokJoinTask extends RecursiveTask {
    private int begin;
    private int end;

    public MyFrokJoinTask(int begin, int end) {
        this.begin = begin;
        this.end = end;
    }

    public static void main(String[] args) throws Exception {
        ForkJoinPool pool = new ForkJoinPool();
        ForkJoinTask result = pool.submit(new MyFrokJoinTask(1, 100));//提交任务
        System.out.println("计算的值:"+result.get());//得到最终的结果

    }

    @Override
    protected Integer compute() {
        int sum = 0;
        if (end - begin <= 2) {
            for (int i = begin; i <= end; i++) {
                sum += i;
                System.out.println("i:"+i);
            }
        } else {
            MyFrokJoinTask d1 = new MyFrokJoinTask(begin, (begin + end) / 2);
            MyFrokJoinTask d2 = new MyFrokJoinTask((begin + end) / 2+1, end);
            d1.fork();//任务拆分
            d2.fork();//任务拆分
            Integer a = d1.join();//每个任务的结果
            Integer b = d2.join();//每个任务的结果
            sum = a + b;//汇总任务结果
            System.out.println("sum:" + sum + ",a:" + a + ",b:" + b);
        }
        System.out.println("name:"+Thread.currentThread().getName());
        return sum;
    }
}
//=========结果============
i:1
i:2
name:ForkJoinPool-1-worker-1
i:3
i:4
name:ForkJoinPool-1-worker-1
sum:10,a:3,b:7
name:ForkJoinPool-1-worker-1
i:5
i:6
i:7
name:ForkJoinPool-1-worker-1
sum:28,a:10,b:18
name:ForkJoinPool-1-worker-1
...............
...............
sum:91,a:28,b:63
sum:99,a:45,b:54
name:ForkJoinPool-1-worker-3
name:ForkJoinPool-1-worker-1
i:23
i:24
i:25
name:ForkJoinPool-1-worker-2
sum:135,a:63,b:72
name:ForkJoinPool-1-worker-2
sum:234,a:99,b:135
name:ForkJoinPool-1-worker-3
sum:325,a:91,b:234
name:ForkJoinPool-1-worker-1
sum:1275,a:325,b:950
name:ForkJoinPool-1-worker-1
sum:5050,a:1275,b:3775
name:ForkJoinPool-1-worker-1
计算的值:5050

三.分析

  ForkJoinPool

ForkJoinPool forkJoinPool = new ForkJoinPool();
//Runtime.getRuntime().availableProcessors()当前操作系统可以使用的CPU内核数量
public ForkJoinPool() {
    this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()),
         defaultForkJoinWorkerThreadFactory, null, false);
}
//this调用到下面这段代码
public ForkJoinPool(int parallelism,
                    ForkJoinWorkerThreadFactory factory,
                    UncaughtExceptionHandler handler,
                    boolean asyncMode) {
    this(checkParallelism(parallelism), //并行度
            checkFactory(factory), //工作线程创建工厂
            handler, //异常处理handler
            asyncMode ? FIFO_QUEUE : LIFO_QUEUE, //任务队列出队模式 异步:先进先出,同步:后进先出
            "ForkJoinPool-" + nextPoolId() + "-worker-");
    checkPermission();
}
//上面的this最终调用到下面这段代码
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);
}
  • parallelism:可并行数量,fork/join框架将依据这个并行数量的设定,决定框架内并行执行的线程数量。并行的每一个任务都会有一个线程进行处理;
  • factory当fork/join创建一个新的线程时,同样会用到线程创建工厂。它实现了ForkJoinWorkerThreadFactory接口,使用默认的的接口实现类DefaultForkJoinWorkerThreadFactory来实现newThread方法创建一个新的工作线程;
    public static interface ForkJoinWorkerThreadFactory {
            /**
             * Returns a new worker thread operating in the given pool.
             */
            public ForkJoinWorkerThread newThread(ForkJoinPool pool);
        }
    
        static final class DefaultForkJoinWorkerThreadFactory
            implements ForkJoinWorkerThreadFactory {
            public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
                return new ForkJoinWorkerThread(pool);
            }
        }
  • handler:异常捕获处理器。当执行的任务出现异常,并从任务中被抛出时,就会被handler捕获;
  • asyncMode:fork/join为每一个独立的工作线程准备了对应的待执行任务队列,这个任务队列是使用数组进行组合的双向队列。即可以使用先进先出的工作模式,也可以使用后进先出的工作模式;多线程高并发编程(8) -- Fork/Join源码分析_第4张图片

   Fork()和Join()

  fork/join框架中提供的fork()和join()是最重要的两个方法,它们和parallelism(”可并行任务数量“)配合工作,可以导致拆分的子任务T1.1、T1.2甚至TX在fork/join中不同的运行效果(上面1+2....+100的每次运行的子任务都是不同的)。即TX子任务或等待其他已存在的线程运行关联的子任务(sum操作),或在运行TX的线程中”递归“执行其他任务(将1-50进行拆分后的子任务递归运行),或启动一个新的线程执行子任务(运行1-50另一边拆分的任务,即50-100的子任务)。

  fork()用于将新创建的子任务放入当前线程的workQueue队列中,fork/join框架将根据当前正在并发执行ForkJoinTask任务的ForkJoinWorkerThread线程状态,决定是让这个任务在队列中等待,还是创建一个新的ForkJoinWorkedThread线程运行它,又或者是唤起其他正在等待任务的ForkJoinWorkerThread线程运行它。

  join()用于让当前线程阻塞,直到对应的子任务完成运行并返回执行结果。或者,如果这个子任务存在于当前线程的任务等待队列workQueue中,则取出这个子任务进行”递归“执行,其目的是尽快得到当前子任务的运行结果,然后继续执行。

  提交任务:

  1.  sumbit的第一次提交:ForkJoinPool.submit(ForkJoinTask task) -> externalPush(task) -> externalSubmit(task)

    1. submit:

      public  ForkJoinTask submit(ForkJoinTask task) {
              if (task == null)
                  throw new NullPointerException();
              externalPush(task);
              return task;
          }
      
          public  ForkJoinTask submit(Callable task) {
              ForkJoinTask job = new ForkJoinTask.AdaptedCallable(task);
              externalPush(job);
              return job;
          }
      
          public  ForkJoinTask submit(Runnable task, T result) {
              ForkJoinTask job = new ForkJoinTask.AdaptedRunnable(task, result);
              externalPush(job);
              return job;
          }
      
          public ForkJoinTask submit(Runnable task) {
              if (task == null)
                  throw new NullPointerException();
              ForkJoinTask job;
              if (task instanceof ForkJoinTask) // avoid re-wrap
                  job = (ForkJoinTask) task;
              else
                  job = new ForkJoinTask.AdaptedRunnableAction(task);
              externalPush(job);
              return job;
          }
    2. externalPush:将任务添加到随机选取的队列中或新创建的队列中;
      final void externalPush(ForkJoinTask task) {
              WorkQueue[] ws; WorkQueue q; int m;
              int r = ThreadLocalRandom.getProbe();//当前线程的一个随机数
              int rs = runState;//当前容器的状态
              //如果随机选取的队列还有空位置可以存放、队列加锁锁定成功,任务就放入队列中
              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;
                      U.putOrderedObject(a, j, task);//任务加入队列中
                      U.putOrderedInt(q, QTOP, s + 1);//挪动下次任务存放的槽的位置
                      U.putIntVolatile(q, QLOCK, 0);//队列解锁
                      if (n <= 1)//当前数组元素少时,进行唤醒当前线程;或者当没有活动线程或线程数较少时,添加新的线程
                          signalWork(ws, q);
                      return;
                  }
                  U.compareAndSwapInt(q, QLOCK, 1, 0);//队列解锁
              }
              externalSubmit(task);//升级版的externalPush
          }
      
      
          volatile int runState;               // lockable status锁定状态
          // runState: SHUTDOWN为负数,其他的为2的次幂
          private static final int  RSLOCK     = 1;
          private static final int  RSIGNAL    = 1 << 1;//唤醒
          private static final int  STARTED    = 1 << 2;//启动
          private static final int  STOP       = 1 << 29;//停止
          private static final int  TERMINATED = 1 << 30;//结束
          private static final int  SHUTDOWN   = 1 << 31;//关闭
    3. externalSubmit:队列添加任务失败,进行升级版操作,即创建队列数组和创建队列后,将任务放入新创建的队列中;
      private void externalSubmit(ForkJoinTask task) {
          int r;                                    // initialize caller's probe
          if ((r = ThreadLocalRandom.getProbe()) == 0) {
              ThreadLocalRandom.localInit();
              r = ThreadLocalRandom.getProbe();
          }
          for (;;) {//自旋
              WorkQueue[] ws; WorkQueue q; int rs, m, k;
              boolean move = false;
              /**
              *ForkJoinPool执行器停止工作了,抛出异常
              *ForkJoinPool extends AbstractExecutorService
              *abstract class AbstractExecutorService implements ExecutorService
              *interface ExecutorService extends Executor
              *interface Executor执行提交的对象Runnable任务
              */
              if ((rs = runState) < 0) {
                  tryTerminate(false, false);    // help terminate
                  throw new RejectedExecutionException();
              }
              //第一次遍历,队列数组未创建,进行创建
              else if ((rs & STARTED) == 0 ||     // initialize初始化
                       ((ws = workQueues) == null || (m = ws.length - 1) < 0)) {
                  int ns = 0;
                  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,config是CPU核数
                          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;
                          workQueues = new WorkQueue[n];//创建
                          ns = STARTED;
                      }
                  } finally {
                      unlockRunState(rs, (rs & ~RSLOCK) | ns);
                  }
              }
              //第三次遍历,把任务放入队列中
              else if ((q = ws[k = r & m & SQMASK]) != null) {
                  if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) {
                      ForkJoinTask[] a = q.array;
                      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);
                      }
                      if (submitted) {
                          signalWork(ws, q);
                          return;
                      }
                  }
                  move = true;                   // move on failure
              }
              //第二次遍历,队列数组为空,创建队列
              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
              if (move)
                  r = ThreadLocalRandom.advanceProbe(r);
          }
      }
  2. fork任务切分的提交:ForkJoinTask.fork() -> ForkJoinWorkerThread.workQueue.push(task)/ForkJoinPool.common.externalPush(task) -> ForkJoinPool.push(task)/externalPush(task)

    1. fork:
      public final ForkJoinTask fork() {
              Thread t;
              if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)//当前线程是workerThread,任务直接放入workerThread当前的workQueue
                  ((ForkJoinWorkerThread)t).workQueue.push(this);
              else
                  ForkJoinPool.common.externalPush(this);//将任务添加到随机选取的队列中或新创建的队列中
              return this;
          }
    2.  push:

      public class ForkJoinPool extends AbstractExecutorService {
              static final class WorkQueue {
                  final void push(ForkJoinTask task) {
                      ForkJoinTask[] a; ForkJoinPool p;
                      int b = base, s = top, n;
                      if ((a = array) != null) {    // ignore if queue removed,队列被移除忽略
                          int m = a.length - 1;     // fenced write for task visibility
                          U.putOrderedObject(a, ((m & s) << ASHIFT) + ABASE, task);//任务加入队列中
                          U.putOrderedInt(this, QTOP, s + 1);//挪动下次任务存放的槽的位置
                          if ((n = s - b) <= 1) {//当前数组元素少时,进行唤醒当前线程;或者当没有活动线程或线程数较少时,添加新的线程
                              if ((p = pool) != null)
                                  p.signalWork(p.workQueues, this);
                          }
                          else if (n >= m)//数组所有元素都满了进行2倍扩容
                              growArray();
                      }
                  }
                  final ForkJoinTask[] growArray() {
                      ForkJoinTask[] oldA = array;
                      int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;//2倍扩容或初始化
                      if (size > MAXIMUM_QUEUE_CAPACITY)
                          throw new RejectedExecutionException("Queue capacity exceeded");
                      int oldMask, t, b;
                      ForkJoinTask[] a = array = new ForkJoinTask[size];
                      if (oldA != null && (oldMask = oldA.length - 1) >= 0 &&
                          (t = top) - (b = base) > 0) {
                          int mask = size - 1;
                          do { // emulate poll from old array, push to new array遍历从旧数组中取出放到新数组中
                              ForkJoinTask x;
                              int oldj = ((b & oldMask) << ASHIFT) + ABASE;
                              int j    = ((b &    mask) << ASHIFT) + ABASE;
                              x = (ForkJoinTask)U.getObjectVolatile(oldA, oldj);//从旧数组中取出
                              if (x != null &&
                                  U.compareAndSwapObject(oldA, oldj, x, null))//将旧数组取出的位置的对象置为null
                                  U.putObjectVolatile(a, j, x);//放入新数组
                          } while (++b != t);
                      }
                      return a;
                  }
              }
          }

  任务的消费

  任务的消费的执行链路是ForkJoinTask.doExec() -> RecursiveTask.exec()/RecursiveAction.exec() -> 覆盖重写的compute()

  1.  doExec:任务的执行入口

    final int doExec() {
            int s; boolean completed;
            if ((s = status) >= 0) {
                try {
                    completed = exec();//消费任务
                } catch (Throwable rex) {
                    return setExceptionalCompletion(rex);
                }
                if (completed)
                    s = setCompletion(NORMAL);//任务执行完设置状态为NORMAL,并唤醒其他等待任务
            }
            return s;
        }
        protected abstract boolean exec();
        private int setCompletion(int completion) {
            for (int s;;) {
                if ((s = status) < 0)
                    return s;
                if (U.compareAndSwapInt(this, STATUS, s, s | completion)) {//任务状态修改为NORMAL
                    if ((s >>> 16) != 0)//状态不是SMASK
                        synchronized (this) { notifyAll(); }//唤醒其他等待任务
                    return completion;
                }
            }
        }
        /** The run status of this task 任务的运行状态*/
        volatile int status; // accessed directly by pool and workers由ForkJoinPool池或ForkJoinWorkerThread控制
        static final int DONE_MASK   = 0xf0000000;  // mask out non-completion bits
        static final int NORMAL      = 0xf0000000;  // must be negative
        static final int CANCELLED   = 0xc0000000;  // must be < NORMAL
        static final int EXCEPTIONAL = 0x80000000;  // must be < CANCELLED
        static final int SIGNAL      = 0x00010000;  // must be >= 1 << 16
        static final int SMASK       = 0x0000ffff;  // short bits for tags

  任务真正执行处理逻辑

  任务提交到ForkJoinPool,最终真正的是由继承Thread的ForkJoinWorkerThread的run方法来执行消费任务的,ForkJoinWorkerThread处理哪个任务是由join来出队的;

    1. ForkJoinTask.join()

          public final V join() {
              int s;
              if ((s = doJoin() & DONE_MASK) != NORMAL)
                  reportException(s);
              return getRawResult();//得到返回结果
          }
          private int doJoin() {
              int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
              /**
               * (s = status) < 0 判断任务是否已经完成,完成直接返回s
               * 任务未完成:
               *          1)线程是ForkJoinWorkerThread,tryUnpush任务出队然后消费任务doExec
               *              1.1)出队或消费失败,执行awaitJoin进行自旋,如果任务状态是完成就退出,否则继续尝试出队,直到任务完成或超时为止;
               *          2)如果线程不是ForkJoinWorkerThread,执行externalAwaitDone进行出队消费
               */
              return (s = status) < 0 ? s :
                  ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
                  (w = (wt = (ForkJoinWorkerThread)t).workQueue).
                  tryUnpush(this) && (s = doExec()) < 0 ? s :
                  wt.pool.awaitJoin(w, this, 0L) :
                  externalAwaitDone();
          }
          private void reportException(int s) {
              if (s == CANCELLED)//取消
                  throw new CancellationException();
              if (s == EXCEPTIONAL)//异常
                  rethrow(getThrowableException());
          }
      1. awaitJoin:
            public class ForkJoinPool{
                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)//任务完成退出
                                break;
                            if (cc != null)//当前任务即将完成,检查是否还有其他的等待任务,如果有
                                //运行当前队列的其他任务,若当前的队列中没有任务了,则窃取其他队列的任务并运行
                                helpComplete(w, cc, 0);
                            //当前队列没有任务了,或遍历当前队列有没有任务,如果有且在top端取出来运行,或在队列中间使用EmptyTask替代原位置取出来运行,如果没有,执行helpStealer
                            else if (w.base == w.top || w.tryRemoveAndExec(task))
                                helpStealer(w, task);//窃取其他队列的任务
                            if ((s = task.status) < 0)
                                break;
                            long ms, ns;
                            if (deadline == 0L)
                                ms = 0L;
                            else if ((ns = deadline - System.nanoTime()) <= 0L)//超时退出
                                break;
                            else if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) <= 0L)
                                ms = 1L;
                            if (tryCompensate(w)) {//当前队列阻塞了
                                task.internalWait(ms);//进行等待
                                U.getAndAddLong(this, CTL, AC_UNIT);
                            }
                        }
                        U.putOrderedObject(w, QCURRENTJOIN, prevJoin);
                    }
                    return s;
                }
            }
      2. externalAwaitDone:
            private int externalAwaitDone() {
                /**
                *   当前任务是CountedCompleter
                *   1)是则执行ForkJoinPool.common.externalHelpComplete()
                *   2)否则执行ForkJoinPool.common.tryExternalUnpush(this)进行任务出队
                *       2.1)出队成功,进行doExec()消费,否则进行阻塞等待
                */
                int s = ((this instanceof CountedCompleter) ? // try helping
                         ForkJoinPool.common.externalHelpComplete(
                             (CountedCompleter)this, 0) :
                         ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0);
                if (s >= 0 && (s = status) >= 0) {//任务未完成
                    boolean interrupted = false;
                    do {
                        if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {//任务状态标记为SIGNAL
                            synchronized (this) {
                                if (status >= 0) {
                                    try {
                                        wait(0L);//阻塞等待
                                    } catch (InterruptedException ie) {//有中断异常
                                        interrupted = true;//设置中断标识为true
                                    }
                                }
                                else
                                    notifyAll();//任务完成唤醒其他任务
                            }
                        }
                    } while ((s = status) >= 0);
                    if (interrupted)
                        Thread.currentThread().interrupt();//当前线程进行中断
                }
                return s;
            }
            final int externalHelpComplete(CountedCompleter task, int maxTasks) {
                WorkQueue[] ws; int n;
                int r = ThreadLocalRandom.getProbe();
                //没有任务直接结束,有任务则执行helpComplete
                //helpComplete:运行随机选取的队列的任务,若选取的队列中没有任务了,则窃取其他队列的任务并运行
                return ((ws = workQueues) == null || (n = ws.length) == 0) ? 0 :
                    helpComplete(ws[(n - 1) & r & SQMASK], task, maxTasks);
            } 
  1. run和工作窃取

  任务是由workThread来窃取的,workThread是一个线程。线程的所有逻辑都是由run()方法执行:

public class ForkJoinWorkerThread extends Thread {
    public void run() {
        if (workQueue.array == null) { // only run once
            Throwable exception = null;
            try {
                onStart();//初始化状态
                pool.runWorker(workQueue);//处理任务队列
            } catch (Throwable ex) {
                exception = ex;//记录异常
            } finally {
                try {
                    onTermination(exception);
                } catch (Throwable ex) {
                    if (exception == null)
                        exception = ex;
                } finally {
                    pool.deregisterWorker(this, exception);//注销工作线程
                }
            }
        }
    }
}
    public class ForkJoinPool{
        final void runWorker(WorkQueue w) {
            w.growArray();                   // allocate queue,队列初始化
            int seed = w.hint;               // initially holds randomization hint
            int r = (seed == 0) ? 1 : seed;  // avoid 0 for xorShift
            for (ForkJoinTask t;;) {//自旋
                if ((t = scan(w, r)) != null)//从队列中窃取任务成功,scan()进行任务窃取
                    w.runTask(t);//执行任务,内部方法调用了doExec()进行任务的消费
                else if (!awaitWork(w, r))//队列没有任务了则结束
                    break;
                r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
            }
        }
    }
    1. scan:
      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;
                      if ((q = ws[k]) != null) {   //随机选中了非空队列 q
                          if ((n = (b = q.base) - q.top) < 0 &&
                              (a = q.array) != null) {      // non-empty
                              long i = (((a.length - 1) & b) << ASHIFT) + ABASE;  //从尾部出队,b是尾部下标
                              if ((t = ((ForkJoinTask)
                                        U.getObjectVolatile(a, i))) != null &&
                                  q.base == b) {
                                  if (ss >= 0) {
                                      if (U.compareAndSwapObject(a, i, t, null)) { //利用cas出队
                                          q.base = b + 1;
                                          if (n < -1)       // signal others
                                              signalWork(ws, q);
                                          return t;  //出队成功,成功窃取一个任务!
                                      }
                                  }
                                  else if (oldSum == 0 &&   // try to activate 队列没有激活,尝试激活
                                           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;
                      }
         //k = k + 1表示取下一个队列 如果(k + 1) & m == origin表示已经遍历完所有队列了 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; }
    2. ForkJoinPool.runTask:
              volatile int scanState;    // versioned, <0: inactive; odd:scanning,版本标记,小于0暂停,奇数进行扫描其他任务
              static final int SCANNING     = 1;             // false when running tasks,有任务执行是false
              /**
               * Executes the given task and any remaining local tasks.
               * 执行给定的任务和任何剩余的本地任务
               */
              final void runTask(ForkJoinTask task) {
                  if (task != null) {
                      scanState &= ~SCANNING; // mark as busy,暂停扫描,当前有任务执行
                      (currentSteal = task).doExec();//执行窃取的任务
                      U.putOrderedObject(this, QCURRENTSTEAL, null); // release for GC,窃取的任务执行完置为null
                      execLocalTasks();//执行本地的任务,即自己workQueue的任务,调用doExec执行到workQueue空为止
                      ForkJoinWorkerThread thread = owner;
                      if (++nsteals < 0)      // collect on overflow,窃取计数溢出
                          transferStealCount(pool);//重置窃取计数
                      scanState |= SCANNING;//继续扫描队列
                      if (thread != null)
                          thread.afterTopLevelExec();
                  }
              }
              static final class InnocuousForkJoinWorkerThread extends ForkJoinWorkerThread {
                      @Override // to erase ThreadLocals,清除threadLocals
                      void afterTopLevelExec() {
                          eraseThreadLocals();
                      }
                      /**
                       * Erases ThreadLocals by nulling out Thread maps.
                       */
                      final void eraseThreadLocals() {
                          U.putObject(this, THREADLOCALS, null);//threadLocals置为null
                          U.putObject(this, INHERITABLETHREADLOCALS, null);//inheritablethreadlocals置为null
                      }
              } 

四.总结

  对于fork/join来说,在使用时还是存在下面的一些问题的:

  • 在使用JVM的时候我们要考虑OOM的问题,如果我们的任务处理时间非常耗时,并且处理的数据非常大的时候,会造成OOM;
  • ForkJoin是通过多线程的方式进行处理任务,那么我们不得不考虑是否应该使用ForkJoin。因为当数据量不是特别大的时候,我们没有必要使用ForkJoin。因为多线程会涉及到上下文的切换,所以数据量不大的时候使用串行比使用多线程快;
    • 项目中进行本地测试发现,业务层Service进行excel表数据(数据量几百)的复杂处理,进行单线程for循环统计消耗时间,然后与使用fork/join进行处理统计消耗时间,发现fork/join的消耗时间是单线程for的2倍;多线程高并发编程(8) -- Fork/Join源码分析_第5张图片
 

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