Forkjointask and forkjoinpool of j.u.c extension component in Java

Entry station 2021-01-20 21:43:06
forkjointask forkjoinpool j.u.c extension component

Fork/Join Two core classes in the framework ForkJoinTask And ForkJoinPool, Statement ForkJoinTask after , Add it to ForkJoinPool in , And return a Future object .
  • ForkJoinPool:ForkJoinTask Need to pass through ForkJoinPool To execute , The subtasks of task segmentation will be added to the double ended queue of current work maintenance , The head of the queue . When there are no tasks in the queue of a worker thread , It randomly fetches a task from the end of the queue of other worker threads .
  • ForkJoinTask: We need to use ForkJoin frame , First, create a ForkJoin Mission . It provides for performing... In tasks Fork() and Join() The mechanism of operation , In general, you don't need to inherit directly ForkJoinTask class , You just need to inherit its subclasses ,Fork/Join The framework provides the following two subclasses .
  • RecursiveAction: For tasks with no return value .
  • RecursizeTask: For tasks with return values .



ForkJoinTask Exceptions may be thrown during execution , But we can't catch exceptions directly in the main thread , therefore ForkJoinTask Provides isCompletedAbnormally() Method to check whether the task has thrown an exception or has been cancelled , And through ForkJoinTask Of getException Method to catch exceptions .
public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
/** ForkJoinTask Running state */
volatile int status; // Directly be ForkJoin Pool and worker access
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
* @Ruturn Whether the task throws an exception or is cancelled
public final boolean isCompletedAbnormally() {
return status < NORMAL;
* If the calculation throws an exception , Then return an exception
* If the task is cancelled, return to CancellationException. If the task is not completed or an exception is not thrown, it returns null
public final Throwable getException() {
int s = status & DONE_MASK;
return ((s >= NORMAL) ? null :
(s == CANCELLED) ? new CancellationException() :

ForkJoinPool Source code

public class ForkJoinPool extends AbstractExecutorService {
* ForkJoinPool, It is the same as ThreadPoolExecutor equally , It has also been realized. Executor and ExecutorService Interface . It has been used.
* An infinite queue to hold the tasks that need to be performed , The number of threads is passed in through the constructor , If you don't pass in the constructor
* Number of threads expected , So the current computer available CPU The number is set to the number of threads as the default .
public ForkJoinPool() {
defaultForkJoinWorkerThreadFactory, null, false);
public ForkJoinPool(int parallelism) {
this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
// There are multiple constructors , It is omitted here
volatile WorkQueue[] workQueues; // main registry
static final class WorkQueue {
final ForkJoinWorkerThread owner; // The worker thread
ForkJoinTask<?>[] array; // Mission
// What's coming in is ForkJoinPool With a specified worker thread
WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner) {
this.pool = pool;
this.owner = owner;
// Place indices in the center of array (that is not yet allocated)
base = top = INITIAL_QUEUE_CAPACITY >>> 1;

FrokJoinPool work stealing Algorithm


ForkJoinPool Maintained a group of WorkQueue, That's the work queue , Another worker thread is maintained in the work queue ForkJoinWorkerThread With a group of tasks ForkJoinTask

WorkQueue It's a double ended queue Deque(Double Ended Queue),Deque It's a data structure with the properties of queue and stack , Elements in a two terminal queue can pop up from both ends , It limits the insertion and deletion operations on both sides of the table .

Each worker thread creates a new task at run time ( Usually because it calls fork()) when , Will be placed at the end of the work queue , And when the worker thread is processing its own work queue , It uses LIFO, That is to say, every time you take a task from the end of the queue to execute .

Each worker thread is processing its own work queue at the same time , Will try to steal a task ( Or from just submitted to pool The task of , Or work queues from other worker threads ), The stolen task is at the head of the work queue of other threads , That is to say, when a worker thread steals tasks from other workers , It uses FIFO The way .

In case of Join() when , if necessary Join Tasks for have not been completed , Priority will be given to other tasks , And wait for it to finish .

When you don't have a mission of your own , And there's nothing to steal , Then go to sleep .

public class ForkJoinPool extends AbstractExecutorService {
public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {}
public <T> ForkJoinTask<T> submit(Callable<T> task) {}
public <T> ForkJoinTask<T> submit(Runnable task, T result) {}
public ForkJoinTask<?> submit(Runnable task) {}
ForkJoinPool It has its own work queue , These work queues are used to receive messages from external threads ( Not ForkJoinThread Threads ) The task submitted , And these work queues are called submitting queue .


The operation of the task , It is important to fork() and join().
public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
* Execute this task asynchronously in the pool where the current task is running ( If applicable )
* Or use ForkJoinPool.commonPool()( If not ForkJoinWorkerThread example ) To execute asynchronously
public final ForkJoinTask<V> fork() {
Thread t;
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
return this;
public final V join() {
int s;
if ((s = doJoin() & DONE_MASK) != NORMAL)
return getRawResult();
private int doJoin() {
int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
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) :
fork() There's only one thing to do , Push the current task into the work queue of the current thread .

join() My work is more complicated , It's also join() The reason why the thread can be prevented from being blocked .

  • Check call join() Is the thread of ForkJoinThread Threads . If not ( for example main Threads ), Block the current thread , Waiting for the task to complete . If it is , It doesn't block .
  • Check the completion status of the task , If completed , Then directly return the result .
  • If the task is not completed , But dealing with your own work queue , Then complete it .
  • If the task has been stolen by another thread , Then the tasks of this work queue are executed in a first in first out manner , Help thief thread finish as soon as possible join
  • If the thief who stole the task has finished all his tasks , Waiting for the need to join The task of , Find the thief ( Recursive execution ), Help it do its job .


ForkJoinPool.submit Method

public static void main(String[] args) throws ExecutionException, InterruptedException {
// Generate a pool
ForkJoinPool forkJoinPool=new ForkJoinPool();
ForkJoinTask task=new ForkJoinExample(1, 100000);
ForkJoinTask<Integer> submit = forkJoinPool.submit(task);
Integer sum = submit.get();
System.out.println(" The end result is :"+sum);
Each worker thread has its own work queue , ForkJoinPool It has its own work queue , These work queues are used to receive external threads ( Not ForkJoinPool) The task submitted , And these work queues are called submitting queue.

submit() and fork() There is no essential difference , It's just that the submitted object becomes submitting queue( And some initialization , Synchronous operation ).submitting queue And other work queue equally , Is the object that worker thread steals , So when one of the tasks is successfully stolen by a worker thread , This means that the submitted task really begins to enter the implementation phase .

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