Fork/Join中的调用原理
错误程序
public class Calculator extends RecursiveTask {
private static final int THRESHOLD = 100;
private int start;
private int end;
public Calculator(int start, int end) {
this.start = start;
this.end = end;
}
@Override
protected Integer compute() {
int sum = 0;
if((start - end) < THRESHOLD){
for(int i = start; i< end;i++){
sum += i;
}
}else{
int middle = (start + end) /2;
Calculator left = new Calculator(start, middle);
Calculator right = new Calculator(middle + 1, end);
left.fork();
right.fork();
sum = left.join() + right.join();
}
return sum;
}
}（1）fork
1.基本作用
将任务放入任务队列队尾
2.源码
public final ForkJoinTask fork() {
((ForkJoinWorkerThread) Thread.currentThread())
.pushTask(this);
return this;
} 因为运行ForkJoinTask的线程就是ForkJoinWorkerThread，所以可以调用其中的pushTask方法
/**
* Pushes a task. Call only from this thread.
*
* @param t the task. Caller must ensure non-null.
*/
final void pushTask(ForkJoinTask t) {
ForkJoinTask[] q; int s, m;
if ((q = queue) != null) { // ignore if queue removed
long u = (((s = queueTop) & (m = q.length - 1)) << ASHIFT) + ABASE;
UNSAFE.putOrderedObject(q, u, t);
queueTop = s + 1; // or use putOrderedInt
if ((s -= queueBase) <= 2)
pool.signalWork();
else if (s == m) //扩容用
growQueue();
}
}定义：
ForkJoinTask[] queue;初始化：
protected void onStart() {
queue = new ForkJoinTask[INITIAL_QUEUE_CAPACITY];
....
} if ((s -= queueBase) <= 2)
pool.signalWork();（2）join
1.基本作用
阻塞当前线程，直到本任务执行完毕（相当于ForkJoinTask版的Thread.join）
2.源码
public final V join() {
if (doJoin() != NORMAL)
return reportResult();
else
return getRawResult();
} private int doJoin() {
Thread t; ForkJoinWorkerThread w; int s; boolean completed;
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
if ((s = status) < 0)
return s;
if ((w = (ForkJoinWorkerThread)t).unpushTask(this)) {
try {
completed = exec();
} catch (Throwable rex) {
return setExceptionalCompletion(rex);
}
if (completed)
return setCompletion(NORMAL);
}
return w.joinTask(this);
}
else
return externalAwaitDone();
} volatile int status; // accessed directly by pool and workers
private static final int NORMAL = -1;
private static final int CANCELLED = -2;
private static final int EXCEPTIONAL = -3;
private static final int SIGNAL = 1;然后通过unpushTask取任务然后执行，看下unpushTask
final boolean unpushTask(ForkJoinTask t) {
ForkJoinTask[] q;
int s;
if ((q = queue) != null && (s = queueTop) != queueBase &&
UNSAFE.compareAndSwapObject
(q, (((q.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
queueTop = s; // or putOrderedInt
return true;
}
return false;
}（3）分析
left.fork();
right.fork();
sum = left.join() + right.join();1.把left放入任务队列，right放入任务队列。此时right在队尾，left在倒数第二个
2.left.join()，因为当前right在队尾，所以说取不出来！！！因而线程就会阻塞
正确的是调用invokeAll，看下invokeAll
public static void invokeAll(ForkJoinTask t1, ForkJoinTask t2) {
t2.fork();
t1.invoke();
t2.join();
}对应上例就是：把right放入队列尾部，然后left开始执行，最后对right进行join，直到left执行完毕才能执行（当然right可能被其他线程窃取，这样通过状态判断就直接返回了）
更一般的
public static void invokeAll(ForkJoinTask... tasks) {
Throwable ex = null;
int last = tasks.length - 1;
for (int i = last; i >= 0; --i) {
ForkJoinTask t = tasks[i];
if (t == null) {
if (ex == null)
ex = new NullPointerException();
}
else if (i != 0)
t.fork();
else if (t.doInvoke() < NORMAL && ex == null)
ex = t.getException();
}
for (int i = 1; i <= last; ++i) {
ForkJoinTask t = tasks[i];
if (t != null) {
if (ex != null)
t.cancel(false);
else if (t.doJoin() < NORMAL && ex == null)
ex = t.getException();
}
}
if (ex != null)
UNSAFE.throwException(ex);
}（4）invoke
1.基本作用
直接执行任务
2.源码
public final V invoke() {
if (doInvoke() != NORMAL)
return reportResult();
else
return getRawResult();
} private int doInvoke() {
int s; boolean completed;
if ((s = status) < 0)
return s;
try {
completed = exec();
} catch (Throwable rex) {
return setExceptionalCompletion(rex);
}
if (completed)
return setCompletion(NORMAL);
else
return doJoin();
} protected final boolean exec() {
result = compute();
return true;
} protected final boolean exec() {
compute();
return true;
}