ForkJoinPool
ForkJoinPool 主要用于实现“分而治之”的算法，特别是分治之后递归调用的函数，例如 quick sort 等。
ForkJoinPool 最适合的是计算密集型的任务，如果存在 I/O，线程间同步，sleep() 等会造成线程长时间阻塞的情况时，最好配合使用 ManagedBlocker。
commonPool
static {
// initialize field offsets for CAS etc
try {
U = sun.misc.Unsafe.getUnsafe();
Class k = ForkJoinPool.class;
CTL = U.objectFieldOffset
(k.getDeclaredField("ctl"));
RUNSTATE = U.objectFieldOffset
(k.getDeclaredField("runState"));
STEALCOUNTER = U.objectFieldOffset
(k.getDeclaredField("stealCounter"));
Class tk = Thread.class;
PARKBLOCKER = U.objectFieldOffset
(tk.getDeclaredField("parkBlocker"));
Class wk = WorkQueue.class;
QTOP = U.objectFieldOffset
(wk.getDeclaredField("top"));
QLOCK = U.objectFieldOffset
(wk.getDeclaredField("qlock"));
QSCANSTATE = U.objectFieldOffset
(wk.getDeclaredField("scanState"));
QPARKER = U.objectFieldOffset
(wk.getDeclaredField("parker"));
QCURRENTSTEAL = U.objectFieldOffset
(wk.getDeclaredField("currentSteal"));
QCURRENTJOIN = U.objectFieldOffset
(wk.getDeclaredField("currentJoin"));
Class ak = ForkJoinTask[].class;
ABASE = U.arrayBaseOffset(ak);
int scale = U.arrayIndexScale(ak);
if ((scale & (scale - 1)) != 0)
throw new Error("data type scale not a power of two");
ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
} catch (Exception e) {
throw new Error(e);
}
commonMaxSpares = DEFAULT_COMMON_MAX_SPARES;
defaultForkJoinWorkerThreadFactory =
new DefaultForkJoinWorkerThreadFactory();
modifyThreadPermission = new RuntimePermission("modifyThread");
common = java.security.AccessController.doPrivileged
(new java.security.PrivilegedAction() {
public ForkJoinPool run() { return makeCommonPool(); }});
int par = common.config & SMASK; // report 1 even if threads disabled
commonParallelism = par > 0 ? par : 1;
} makeCommonPool
/**
* Creates and returns the common pool, respecting user settings
* specified via system properties.
*/
private static ForkJoinPool makeCommonPool() {
int parallelism = -1;
ForkJoinWorkerThreadFactory factory = null;
UncaughtExceptionHandler handler = null;
try { // ignore exceptions in accessing/parsing properties
String pp = System.getProperty
("java.util.concurrent.ForkJoinPool.common.parallelism");
String fp = System.getProperty
("java.util.concurrent.ForkJoinPool.common.threadFactory");
String hp = System.getProperty
("java.util.concurrent.ForkJoinPool.common.exceptionHandler");
if (pp != null)
parallelism = Integer.parseInt(pp);
if (fp != null)
factory = ((ForkJoinWorkerThreadFactory)ClassLoader.
getSystemClassLoader().loadClass(fp).newInstance());
if (hp != null)
handler = ((UncaughtExceptionHandler)ClassLoader.
getSystemClassLoader().loadClass(hp).newInstance());
} catch (Exception ignore) {
}
if (factory == null) {
if (System.getSecurityManager() == null)
factory = defaultForkJoinWorkerThreadFactory;
else // use security-managed default
factory = new InnocuousForkJoinWorkerThreadFactory();
}
if (parallelism < 0 && // default 1 less than #cores
(parallelism = Runtime.getRuntime().availableProcessors() - 1) <= 0)
parallelism = 1;
if (parallelism > MAX_CAP)
parallelism = MAX_CAP;
return new ForkJoinPool(parallelism, factory, handler, LIFO_QUEUE,
"ForkJoinPool.commonPool-worker-");
}配置参数
通过代码指定，必须得在commonPool初始化之前（parallel的stream被调用之前，一般可在系统启动后设置）注入进去，否则无法生效。
通过启动参数指定无此限制，较为安全
parallelism(
即配置线程池个数)
可以通过java.util.concurrent.ForkJoinPool.common.parallelism进行配置，最大值不能超过MAX_CAP,即32767.
static final int MAX_CAP = 0x7fff; //32767如果没有指定，则默认为Runtime.getRuntime().availableProcessors() - 1.
代码指定(必须得在commonPool初始化之前注入进去，否则无法生效)
System.setProperty("java.util.concurrent.ForkJoinPool.common.parallelism", "8");或者参数指定
-Djava.util.concurrent.ForkJoinPool.common.parallelism=8threadFactory
默认为defaultForkJoinWorkerThreadFactory，没有securityManager的话。
/**
* Default ForkJoinWorkerThreadFactory implementation; creates a
* new ForkJoinWorkerThread.
*/
static final class DefaultForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
return new ForkJoinWorkerThread(pool);
}
}代码指定(必须得在commonPool初始化之前注入进去，否则无法生效)
System.setProperty("java.util.concurrent.ForkJoinPool.common.threadFactory",YourForkJoinWorkerThreadFactory.class.getName());参数指定
-Djava.util.concurrent.ForkJoinPool.common.threadFactory=com.xxx.xxx.YourForkJoinWorkerThreadFactoryexceptionHandler
如果没有设置，默认为null
/**
* 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;
}代码指定(必须得在commonPool初始化之前注入进去，否则无法生效)
System.setProperty("java.util.concurrent.ForkJoinPool.common.exceptionHandler",YourUncaughtExceptionHandler.class.getName());参数指定
-Djava.util.concurrent.ForkJoinPool.common.exceptionHandler=com.xxx.xxx.YourUncaughtExceptionHandlerWorkQueue
// Mode bits for ForkJoinPool.config and WorkQueue.config
static final int MODE_MASK = 0xffff << 16; // top half of int
static final int LIFO_QUEUE = 0;
static final int FIFO_QUEUE = 1 << 16;
static final int SHARED_QUEUE = 1 << 31; // must be negative
控制是FIFO还是LIFO
/**
* Takes next task, if one exists, in order specified by mode.
*/
final ForkJoinTask nextLocalTask() {
return (config & FIFO_QUEUE) == 0 ? pop() : poll();
}ForkJoinPool 的每个工作线程都维护着一个工作队列（WorkQueue），这是一个双端队列（Deque），里面存放的对象是任务（ForkJoinTask）。
每个工作线程在运行中产生新的任务（通常是因为调用了 fork()）时，会放入工作队列的队尾，并且工作线程在处理自己的工作队列时，使用的是 LIFO 方式，也就是说每次从队尾取出任务来执行。
每个工作线程在处理自己的工作队列同时，会尝试窃取一个任务（或是来自于刚刚提交到 pool的任务，或是来自于其他工作线程的工作队列），窃取的任务位于其他线程的工作队列的队首，也就是说工作线程在窃取其他工作线程的任务时，使用的是 FIFO 方式。
queue capacity
/**
* Capacity of work-stealing queue array upon initialization.
* Must be a power of two; at least 4, but should be larger to
* reduce or eliminate cacheline sharing among queues.
* Currently, it is much larger, as a partial workaround for
* the fact that JVMs often place arrays in locations that
* share GC bookkeeping (especially cardmarks) such that
* per-write accesses encounter serious memory contention.
*/
static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
/**
* Maximum size for queue arrays. Must be a power of two less
* than or equal to 1 << (31 - width of array entry) to ensure
* lack of wraparound of index calculations, but defined to a
* value a bit less than this to help users trap runaway
* programs before saturating systems.
*/
static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M超出报异常
/**
* Initializes or doubles the capacity of array. Call either
* by owner or with lock held -- it is OK for base, but not
* top, to move while resizings are in progress.
*/
final ForkJoinTask[] growArray() {
ForkJoinTask[] oldA = array;
int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
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
ForkJoinTask x;
int oldj = ((b & oldMask) << ASHIFT) + ABASE;
int j = ((b & mask) << ASHIFT) + ABASE;
x = (ForkJoinTask)U.getObjectVolatile(oldA, oldj);
if (x != null &&
U.compareAndSwapObject(oldA, oldj, x, null))
U.putObjectVolatile(a, j, x);
} while (++b != t);
}
return a;
}doc
Java 并发编程笔记：如何使用 ForkJoinPool 以及原理