ForkJoinPool一种ExecutorService的实现，运行ForkJoinTask任务。ForkJoinPool区别于其它ExecutorService，主要是因为它采用了一种工作窃取(work-stealing)的机制。所有被ForkJoinPool管理的线程尝试窃取提交到池子里的任务来执行，执行中又可产生子任务提交到池子中。
ForkJoinPool维护了一个WorkQueue的数组(数组长度是2的整数次方，自动增长)。每个workQueue都有任务队列(ForkJoinTask的数组)，并且用base、top指向任务队列队尾和队头。work-stealing机制就是工作线程挨个扫描任务队列，如果队列不为空则取队尾的任务并执行。示意图如下：
创建一个包含2个worker线程的pool，main线程提交2个任务（task-1、task-2），触发worker线程工作，task-1任务fork出4个子任务。main线程负责同步两个worker线程的工作进度。demo小程序演示了worker-2窃取worker-1的子任务：
package com.focuse.jdkdemo.concurrent;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.locks.LockSupport;
/**
* @date ：Created in 2020/2/16 上午11:10
* @description：
* @modified By：
*/
public class ForkJoinDemo {
private static Thread worker1 = null;
private static Thread worker2 = null;
private static AtomicBoolean worker1Park = new AtomicBoolean();
private static AtomicBoolean worker2Park = new AtomicBoolean();
static {
worker1Park.set(false);
worker2Park.set(false);
}
public abstract static class ForkJoinTaskDemo extends ForkJoinTask {
@Override
public Object getRawResult() {
return null;
}
@Override
protected void setRawResult(Object value) {
}
}
public static void setParkFlag() {
if (Thread.currentThread().getName().equals("worker-1")) {
//自旋设置, 保证与main线程同步
while (!worker1Park.compareAndSet(false, true)) {
}
} else if (Thread.currentThread().getName().equals("worker-2")) {
//自旋设置, 保证与main线程同步
while (!worker2Park.compareAndSet(false, true)) {
}
}
}
private static Runnable task1 = new Runnable() {
@Override
public void run() {
worker1 = Thread.currentThread();
worker1.setName("worker-1");
System.out.println(Thread.currentThread().getName() + " execute task1");
//暂停并且设置暂停flag以便main结束自旋
setParkFlag();
LockSupport.park();
ForkJoinTask task11 = new ForkJoinTaskDemo() {
@Override
protected boolean exec() {
System.out.println(Thread.currentThread().getName() + " execute task1-1");
//暂停并且设置暂停flag以便main结束自旋
setParkFlag();
LockSupport.park();
return true;
}
};
task11.fork();
ForkJoinTask task12 = new ForkJoinTaskDemo() {
@Override
protected boolean exec() {
System.out.println(Thread.currentThread().getName() + " execute task1-2");
//暂停并且设置暂停flag以便main结束自旋
setParkFlag();
LockSupport.park();
return true;
}
};
task12.fork();
ForkJoinTask task13 = new ForkJoinTaskDemo() {
@Override
protected boolean exec() {
System.out.println(Thread.currentThread().getName() + " execute task1-3");
//暂停并且设置暂停flag以便main结束自旋
setParkFlag();
LockSupport.park();
return true;
}
};
task13.fork();
ForkJoinTask task14 = new ForkJoinTaskDemo() {
@Override
protected boolean exec() {
System.out.println(Thread.currentThread().getName() + " execute task1-4");
//暂停并且设置暂停flag以便main结束自旋
setParkFlag();
LockSupport.park();
return true;
}
};
task14.fork();
//提交4个子任务后暂停
setParkFlag();
LockSupport.park();
}
};
private static Runnable task2 = new Runnable() {
@Override
public void run() {
worker2 = Thread.currentThread();
worker2.setName("worker-2");
System.out.println(Thread.currentThread().getName() + " execute task2");
//暂停并且设置暂停flag以便main结束自旋
setParkFlag();
LockSupport.park();
}
};
public static void main(String[] args) {
//只用两个线程，方便测试
ForkJoinPool pool = new ForkJoinPool(2);
//step-1、step-2 提交2各任务，期望worker-1 worker-2各执行一个任务
System.out.println("step-1 step-2: 提交2个任务，触发2个worker线程");
pool.submit(task1);
pool.submit(task2);
while (!worker1Park.get()) {
}
//step-3 唤醒worker-1 产生4个任务 然后worker-1继续暂停
System.out.println("\n*******************************************");
System.out.println("step-3 唤醒worker-1 产生4个子任务");
LockSupport.unpark(worker1);
//自旋设置, 保证与worker-1线程同步
while (!worker1Park.compareAndSet(true, false)) {
}
while (!worker1Park.get() || !worker2Park.get()) {
}
//step-4 唤醒worker-1 worker-2
System.out.println("\n*******************************************");
System.out.println("step-4 唤醒worker-1、worker-2：worker-1弹出task1-4执行，worker-2窃取task1-1执行");
LockSupport.unpark(worker1);
LockSupport.unpark(worker2);
//自旋设置, 保证与worker-1线程同步
while (!worker1Park.compareAndSet(true, false)) {
}
//自旋设置, 保证与worker-2线程同步
while (!worker2Park.compareAndSet(true, false)) {
}
while (!worker1Park.get() || !worker2Park.get()) {
}
//step-5 唤醒worker-1 worker-2
System.out.println("\n*******************************************");
System.out.println("step-5 唤醒worker-1、worker-2：worker-1弹出task1-3执行，worker-2窃取task1-2执行");
LockSupport.unpark(worker1);
LockSupport.unpark(worker2);
//自旋设置, 保证与worker-1线程同步
while (!worker1Park.compareAndSet(true, false)) {
}
//自旋设置, 保证与worker-2线程同步
while (!worker2Park.compareAndSet(true, false)) {
}
while (!worker1Park.get() || !worker2Park.get()) {
}
//唤醒worker-1 worker-2 结束
LockSupport.unpark(worker1);
LockSupport.unpark(worker2);
//自旋设置, 保证与worker-1线程同步
while (!worker1Park.compareAndSet(true, false)) {
}
//自旋设置, 保证与worker-2线程同步
while (!worker2Park.compareAndSet(true, false)) {
}
}
}运行结果如下：
这一part围绕线程池(ForkJoinPool)结构已经线程池的关键动作讲一下过程，线程池的关键动作无非是提交任务、运行任务、获取任务结果。不过，对于ForkJoinPool有还有任务fork动作。
ForkJoinPool里面大量用到bit运算。做一下简短说明：计算机的运算以补码计算。补码怎么来？正数的补码就是原码，最高位是符号位为0；负数的补码是其正数的补码按位取反（包括符号位），最后加1。
ForkJoinPool几个主要的成员变量说明如下：
源码如下（笔者添加了注释）：
public class ForkJoinPool extends AbstractExecutorService {
......
//笔者注：runState的bit位：SHUTDOWN是负数(int的最高bit位为1)，其余的是2的整数次方
private static final int RSLOCK = 1;
private static final int RSIGNAL = 1 << 1;
private static final int STARTED = 1 << 2;
private static final int STOP = 1 << 29;
private static final int TERMINATED = 1 << 30;
private static final int SHUTDOWN = 1 << 31;
// Instance fields
//笔者注:ctl是ForkJoinPool的控制字段。long是64位bit，
//最高16位(63~48)表示活跃的线程，值为活跃的线程-parallelism（补码表示），
//第47~32位表示当前所有工作线程(包括未激活的)，值为所有工作线程数-parallelism（补码表示），
//第31～16位表示waiters线程链中top线程的版本计数和线程状态
//第15～0位表示waiters线程链中top线程的本地WorkQueue在ForkJoinPool.workQueues的下标索引
volatile long ctl; // main pool control
volatile int runState; // lockable status
//笔者注：config高16位是mode(FIFO或者LIFO),低16位是parallelism
final int config; // parallelism, mode
//笔者注：防止index冲突的随机数种子
int indexSeed; // to generate worker index
//笔者注：WorkQueue数组，奇数WorkQueue有worker线程，偶数WorkQueue接收外部提交任务
volatile WorkQueue[] workQueues; // main registry
//工作线程的创建工厂
final ForkJoinWorkerThreadFactory factory;
final UncaughtExceptionHandler ueh; // per-worker UEH
final String workerNamePrefix; // to create worker name string
//工作窃取的计数
volatile AtomicLong stealCounter; // also used as sync monitor
......
}WorkQueue的几个重要成员变量说明如下：
源码如下（笔者添加了注释）：
static final class WorkQueue {
......
// Instance fields
//笔者注：最高位为1表示非激活，第30～16位版本计数，第0表示是否在运行任务(1-scanning，0-busy)
//
volatile int scanState; // versioned, <0: inactive; odd:scanning
//笔者注：实际是前一个非激活线程，这样就形成了一个waiters线程链
int stackPred; // pool stack (ctl) predecessor
int nsteals; // number of steals
int hint; // randomization and stealer index hint
//高16位是mode（FIFO或LIFO），低16位是ForkJoinPool.workQueues的下标
int config; // pool index and mode
volatile int qlock; // 1: locked, < 0: terminate; else 0
//笔者注：任务队列的队尾，工作窃取就是窃取base指向的任务
volatile int base; // index of next slot for poll
//笔者注：任务队列的队头（指向空），下一个push的位置
int top; // index of next slot for push
//笔者注：任务队列
ForkJoinTask<?>[] array; // the elements (initially unallocated)
//笔者注：所属的ForkJoinPool实例
final ForkJoinPool pool; // the containing pool (may be null)
//笔者注：所属的worker线程，如果在ForkJoinPool.workQueues数组中的下标是奇数则不为空
final ForkJoinWorkerThread owner; // owning thread or null if shared
volatile Thread parker; // == owner during call to park; else null
volatile ForkJoinTask<?> currentJoin; // task being joined in awaitJoin
volatile ForkJoinTask<?> currentSteal; // mainly used by helpStealer
......
}ForkJoinPool作为ExecutorService的一个实现类，有submit方法提交任务，直接贴源码出来如下：
public class ForkJoinPool extends AbstractExecutorService {
......
//提交任务
public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
if (task == null)
throw new NullPointerException();
externalPush(task);
return task;
}
......
}
submit方法判断了一下任务是否为null，然后直接调用externalPush，源码如下：
public class ForkJoinPool extends AbstractExecutorService {
......
/**
**/
final void externalPush(ForkJoinTask<?> task) {
WorkQueue[] ws; WorkQueue q; int m;
int r = ThreadLocalRandom.getProbe();
int rs = runState;
//笔者注：(1)计算一个偶数下标，如果该下标下WorkQueue不为空则尝试添加到该WorkQueue
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)) {// 笔者注：(2)加锁锁住改WorkQueue
ForkJoinTask<?>[] a; int am, n, s;
//笔者注：WorkQueue的任务队列不为null且未满 a.length - 1 > q.top - q.base
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);
U.putOrderedInt(q, QTOP, s + 1);
//笔者注：释放锁
U.putIntVolatile(q, QLOCK, 0);
//笔者注：唤醒工作线程
if (n <= 1)
signalWork(ws, q);
return;
}
//笔者注：释放锁
U.compareAndSwapInt(q, QLOCK, 1, 0);
}
externalSubmit(task);
}
......
}submit的任务是提交到偶数下标的workQueue中。函数externalPush先计算一个下标位置"m & r & SQMASK"，这个位置是偶数下标，如何保证是偶数？"m & r & SQMASK"又是什么含义？m是pool.workQueues的数组size - 1(即2的整数次方-1)，跟m做&运算能保证下标不超过workQueues.size; r是当前线程的局部变量，取这个值尽可能避免冲突；SQMASK是一个常量0x007e，第0bit位是0，这就保证了下标计算出来一定是偶数。
我们再看一下externalSubmit函数源码如下：
public class ForkJoinPool extends AbstractExecutorService {
......
private void externalSubmit(ForkJoinTask<?> task) {
int r; // initialize caller's probe
if ((r = ThreadLocalRandom.getProbe()) == 0) {
ThreadLocalRandom.localInit();
r = ThreadLocalRandom.getProbe();
}
//笔者注：循环操作，每次循环的动作都非常小，符合条件就跳出来
for (;;) {
WorkQueue[] ws; WorkQueue q; int rs, m, k;
boolean move = false;
//笔者注： runState为负数，则表示shutdown，终止线程池并跳出循环
if ((rs = runState) < 0) {
tryTerminate(false, false); // help terminate
throw new RejectedExecutionException();
}
//笔者注：pool未初始化，则初始化然后继续循环
else if ((rs & STARTED) == 0 || // initialize
((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());
//笔者注：这里保证workQueues是2的整数次方，方法巧妙，n经过一系列位运算
//变成了一个从第0位开始出现连续1的直到出现一个bit位是0，然后就一直是0，
//这样最后再加1就变成了2的整数次方。
// 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;
workQueues = new WorkQueue[n];
ns = STARTED;
}
} finally {
unlockRunState(rs, (rs & ~RSLOCK) | ns);
}
}
//笔者注：如果workQueue不为空，则加锁尝试添加。添加成功唤醒工作线程并跳出，否则继续循环
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
}
// 笔者注：如果pool未被锁，则创建一个WorkQueue，注意k在前面一个elseif赋值了一个偶数
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
//笔者注：重新获取下标
if (move)
r = ThreadLocalRandom.advanceProbe(r);
}
}
......
}这里是一个循环，每次循环依次判断条件，符合条件就跳出来：
submit任务添加完成，又1个疑问没揭开：为什么判断任务队列(workQueue.array)未满用a.length - 1 > q.top - q.base，感觉不需要-1？ 不过这不影响阅读，也不影响整体对ForkJoin的理解。先记录下来，未来可能整明白。
此外，ForkJoin框架的还可以通过ForkJoinTask.fork来添加任务，源码如下：
public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
......
//如果是work线程调用fork则添加到work线程的本地队列里面，否则添加到commonPool池子里面
public final ForkJoinTask<V> fork() {
Thread t;
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
((ForkJoinWorkerThread)t).workQueue.push(this);
else
ForkJoinPool.common.externalPush(this);
return this;
}
......
}这一节主要说明工作线程是怎么启动的？主要是添加任务时调用的signalWork：
public class ForkJoinPool extends AbstractExecutorService {
......
final void signalWork(WorkQueue[] ws, WorkQueue q) {
long c; int sp, i; WorkQueue v; Thread p;
//笔者注：ctl的高16位是激活线程数-parallelism的反码，最高位为1表示激活线程还可以增加
while ((c = ctl) < 0L) { // too few active
//笔者注：ctl的第32位是非激活线程链的top线程，为0表示没有非激活线程
if ((sp = (int)c) == 0) { // no idle workers
//笔者注：ctl的47~32位是线程总数-parallelism的反码，最高为1表示总的线程还可以加
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;
//笔者注：以下逻辑是获取非激活线程链的top线程然后激活，再激活之前要将下一个非激活线程(stackPred表示)放到ctl里面
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;
}
}
......
}创建worker则是调用pool中的factory(ForkJoinWorkerThreadFactory)创建一个工作线程，并将其注册到pool中，注册过程如下：
public class ForkJoinPool extends AbstractExecutorService {
......
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;
//保证i是奇数 | 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;
//笔者注：下标赋值scanState，因为worker线程最大值0x7fff+1，workQueues的size是其2倍，所以下标最大值0xffff(16bits)
w.scanState = i; // publication fence
ws[i] = w;
}
} finally {
unlockRunState(rs, rs & ~RSLOCK);
}
wt.setName(workerNamePrefix.concat(Integer.toString(i >>> 1)));
return w;
}
......
}注册实际上就是创建一个WorkQueue对象维护起来，这里不多介绍，就注意一点worker线程本地队列一定是ForkJoinPool.workQueues的奇数下标的元素。“i = ((s << 1) | 1) & m” 中位运算"| 1"就保证i是奇数，后面递增的step是2的整数次方，所以i = i + step还是奇数。同时，下标索引赋值给WorkQueue的scanState变量，因为worker线程最大值0x7fff+1，workQueues的size是其2倍，所以下标最大值0xffff(16bits)。
work线程创建完成直接start：
public class ForkJoinPool extends AbstractExecutorService {
......
private boolean createWorker() {
ForkJoinWorkerThreadFactory fac = factory;
Throwable ex = null;
ForkJoinWorkerThread wt = null;
try {
if (fac != null && (wt = fac.newThread(this)) != null) {
//笔者注：启动工作线程
wt.start();
return true;
}
} catch (Throwable rex) {
ex = rex;
}
deregisterWorker(wt, ex);
return false;
}
......
}线程启动执行ForkJoinWorkerThread.run，然后又会调用ForkJoinPool.runWorker
public class ForkJoinWorkerThread extends Thread {
......
public void run() {
if (workQueue.array == null) { // only run once
Throwable exception = null;
try {
onStart();
pool.runWorker(workQueue);
} catch (Throwable ex) {
exception = ex;
} finally {
......
}
}
}
......
}
public class ForkJoinPool extends AbstractExecutorService {
......
final void runWorker(WorkQueue w) {
//笔者注：扩容w的任务队列，此处第一次调用相当于初始化
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;;) {
//笔者注：扫描窃取任务，如果窃取到则执行任务
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
}
}
......
}runWorker是最顶层的循环，线程一直循环直到终止。runWorker调用scan方法，scan就是工作窃取的核心实现，源码如下：
public class ForkJoinPool extends AbstractExecutorService {
......
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) {
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) {
//笔者注：任务存在，竞争窃取任务，竞争成功返回窃取成果，如果队列中任务还有剩，唤醒非激活线程
if (U.compareAndSwapObject(a, i, t, null)) {
q.base = b + 1;
if (n < -1) // signal others
signalWork(ws, q);
return t;
}
}
//笔者注：如果当前线程 未激活，尝试激活非激活链的top线程（如果oldSum不为0则再扫描一次并且会将oldSum设置为0）
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;
}
//笔者注：又扫描了一圈
if ((k = (k + 1) & m) == origin) { // continue until stable
//笔者注：如果checSum==oldSum 说明连续2圈扫描过程中各队列的base都没变(即没有任务)，
//设置线程未激活。然后继续扫描，如果第3圈扫描到任务，则尝试激活线程，如果第3圈还
//没扫到且checkSum又没变，那就返回null，尝试让线程park。
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;
}
......
}ForkJoinTask是Future的实现类，所以get方法可以获取结果。不过，作为ForkJoin框架的任务，有自己独有的方法-join()
public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
......
public final V join() {
int s;
if ((s = doJoin() & DONE_MASK) != NORMAL)
reportException(s);
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) :
externalAwaitDone();
}
private int externalAwaitDone() {
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) {
boolean interrupted = false;
do {
//笔者注：设置当前任务的信号bit位，当任务结束时就会调用notifyAll
if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
synchronized (this) {
if (status >= 0) {
try {
//等待该任务对象notify
wait(0L);
} catch (InterruptedException ie) {
interrupted = true;
}
}
else
notifyAll();
}
}
} while ((s = status) >= 0);
if (interrupted)
Thread.currentThread().interrupt();
}
return s;
}
private int setCompletion(int completion) {
for (int s;;) {
if ((s = status) < 0)
return s;
if (U.compareAndSwapInt(this, STATUS, s, s | completion)) {
//笔者注：如果状态的信号位(第17位)是1，则notifyAll唤醒所有等待结果的线程
if ((s >>> 16) != 0)
synchronized (this) { notifyAll(); }
return completion;
}
}
}
......
}join方法最终调用externalAwaitDone方法，externalAwaitDone设置任务的信号位为1，然后调用wait等待task对象唤醒。当任务完成时调用setCompletion方法。setCompletion判断如果信号位为1(说明有线程等待结果)，则唤醒线程(notifyAll)。get方法和join方法差别不大，主要是对异常的处理和线程中断的处理不一样，此处不做赘述。
好记性不如烂笔头，随时记录当下的理解！