線程池框架.png
線程池源碼解析
核心的構(gòu)造方法
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
核心參數(shù)說明
int corePoolSize, 池中保持的最大線程數(shù)，包括空線程
int maximumPoolSize, 池中允許最大線程數(shù)
long keepAliveTime, 當(dāng)線程大于核心線程時，此為終止前多余的線程等待新任務(wù)的最長時間
TimeUnit unit, keepAliveTime 參數(shù)的時長單位
BlockingQueue<Runnable> workQueue, 執(zhí)行用于保持任務(wù)的隊列。此隊列僅由execute方法提交的Runnable任務(wù)
ThreadFactory threadFactory, 執(zhí)行程序創(chuàng)建線程時使用的工廠
RejectedExecutionHandler handler 由于超出線程范圍和隊列容量而執(zhí)行的拒絕策略。
runnableTaskQueue（任務(wù)隊列）：用于保存等待執(zhí)行的任務(wù)的阻塞隊列。可以選擇以下幾
個阻塞隊列。
ArrayBlockingQueue:是一個基于數(shù)組結(jié)構(gòu)的有界阻塞隊列，此隊列按FIFO（先進(jìn)先出）原則對元素進(jìn)行排序。
LinkedBlockingQueue:一個基于鏈表結(jié)構(gòu)的阻塞隊列，此隊列按FIFO排序元素，吞吐量通常要高于ArrayBlockingQueue。靜態(tài)工廠方法Executors.newFixedThreadPool()使用了這個隊列。
SynchronousQueue:一個不存儲元素的阻塞隊列。每個插入操作必須等到另一個線程調(diào)用移除操作，否則插入操作一直處于阻塞狀態(tài)，吞吐量通常要高于Linked-BlockingQueue，靜態(tài)工廠方法Executors.newCachedThreadPool使用了這個隊列。
PriorityBlockingQueue:一個具有優(yōu)先級的無限阻塞隊列。
RejectedExecutionHandler（飽和策略）：當(dāng)隊列和線程池都滿了，說明線程池處于飽和狀態(tài)，那么必須采取一種策略處理提交的新任務(wù)。這個策略默認(rèn)情況下是AbortPolic
線程池運行核心代碼
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
int c = ctl.get();
//判斷是否小于核心數(shù)量，是直接新增work成功后直接退出
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
// 增加失敗后繼續(xù)獲取標(biāo)記
c = ctl.get();
}
//判斷是運行狀態(tài)并且扔到workQueue里成功后
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//再次check判斷運行狀態(tài)如果是非運行狀態(tài)就移除出去&reject掉
if (! isRunning(recheck) && remove(command))
reject(command);
//否則發(fā)現(xiàn)可能運行線程數(shù)是0那么增加一個null的worker。
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//直接增加worker如果不成功直接reject
else if (!addWorker(command, false))
reject(command);
}
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;// 兩種情況1.如果非運行狀態(tài) 2.不是這種情況（停止?fàn)顟B(tài)并且是null對象并且workQueue不等于null）
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false; // 判斷是否飽和容量了
if (compareAndIncrementWorkerCount(c)) //增加一個work數(shù)量 然后跳出去
break retry;
c = ctl.get(); // 增加work失敗后繼續(xù)遞歸
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask);//增加一個worker
final Thread t = w.thread;
if (t != null) {//判斷是否 為null
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
//鎖定后并重新檢查下 是否存在線程工廠的失敗或者鎖定前的關(guān)閉
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {{ //本次要是新增加work成功就調(diào)用start運行
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();//1.取到當(dāng)前線程
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) {//獲取任務(wù) 看看是否能拿到
w.lock();
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();// 確保線程是能中斷的
try {
beforeExecute(wt, task);//開始任務(wù)前的鉤子
Throwable thrown = null;
try {
task.run();//執(zhí)行任務(wù)
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown); //任務(wù)后的鉤子
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
/**
* Performs cleanup and bookkeeping for a dying worker. Called
* only from worker threads. Unless completedAbruptly is set,
* assumes that workerCount has already been adjusted to account
* for exit. This method removes thread from worker set, and
* possibly terminates the pool or replaces the worker if either
* it exited due to user task exception or if fewer than
* corePoolSize workers are running or queue is non-empty but
* there are no workers.
*
* @param w the worker
* @param completedAbruptly if the worker died due to user exception
*/
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w); //移除work
} finally {
mainLock.unlock();
}
tryTerminate();
int c = ctl.get();
if (runStateLessThan(c, STOP)) { //判斷是否還有任務(wù)
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
addWorker(null, false);
}
}
1、如果當(dāng)前池大小 poolSize 小于 corePoolSize ，則創(chuàng)建新線程執(zhí)行任務(wù)
2、如果當(dāng)前池大小poolSize 大于cprePoolSize 且等待隊列未滿，則進(jìn)入等待隊列
3、如果當(dāng)前池大小 poolSize 大于 corePoolSize 且小于 maximumPoolSize ，且等待隊列已滿，則創(chuàng)建新線程執(zhí)行任務(wù)
4、如果當(dāng)前池大小 poolSize 大于 corePoolSize 且大于 maximumPoolSize ，且等待隊列已滿，則調(diào)用拒絕策
略來處理該任務(wù)
拒絕策略
AbortPolicy：拋出異常， 默認(rèn)
CallerRunsPolicy：不使用線程池執(zhí)行
DiscardPolicy：直接丟棄任務(wù)
DiscardOldestPolicy：丟棄隊列中最舊的任務(wù)對 于 線 程 池 選 擇 的 拒 絕 策 略 可 以 通 過 RejectedExecutionHandler handler = new
ThreadPoolExecutor.CallerRunsPolicy();來設(shè)置。
線程池執(zhí)行流程.png
調(diào)度線程池原理
public static void main(String[] args) {
ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(10);
scheduledExecutorService.schedule(new Runnable() {
public void run() {
System.out.println("5秒之后執(zhí)行");
}
}, 5, TimeUnit.SECONDS);
}
public ScheduledFuture<?> schedule(Runnable command,
long delay,
TimeUnit unit) {
if (command == null || unit == null)
throw new NullPointerException();
RunnableScheduledFuture<?> t = decorateTask(command,
new ScheduledFutureTask<Void>(command, null,
triggerTime(delay, unit)));
delayedExecute(t);
return t;
}
/**
* Main execution method for delayed or periodic tasks. If pool
* is shut down, rejects the task. Otherwise adds task to queue
* and starts a thread, if necessary, to run it. (We cannot
* prestart the thread to run the task because the task (probably)
* shouldn't be run yet.) If the pool is shut down while the task
* is being added, cancel and remove it if required by state and
* run-after-shutdown parameters.
*
* @param task the task
*/
private void delayedExecute(RunnableScheduledFuture<?> task) {
if (isShutdown())
reject(task);
else {
super.getQueue().add(task); //獲取延遲隊列（DelayedWorkQueue）添加任務(wù)，通過DelayedWorkQueue 延遲隊列實現(xiàn) offer獲取對象的延遲
if (isShutdown() &&
!canRunInCurrentRunState(task.isPeriodic()) &&
remove(task))
task.cancel(false);
else
ensurePrestart();
}
}
/**
確保有work執(zhí)行
* Same as prestartCoreThread except arranges that at least one
* thread is started even if corePoolSize is 0.
*/
void ensurePrestart() {
int wc = workerCountOf(ctl.get());
if (wc < corePoolSize)
addWorker(null, true);
else if (wc == 0)
addWorker(n ull, false);
}
//add 方法內(nèi)部
public boolean offer(Runnable x) {
if (x == null)
throw new NullPointerException();
RunnableScheduledFuture<?> e = (RunnableScheduledFuture<?>)x;
final ReentrantLock lock = this.lock;
lock.lock(); //加鎖。此處會阻塞
try {
int i = size;
if (i >= queue.length) //擴(kuò)容
grow();
size = i + 1;
if (i == 0) {
queue[0] = e;
setIndex(e, 0);//第一個直接設(shè)置索引和下標(biāo)0
} else {
siftUp(i, e); //排序算法對任務(wù)進(jìn)行排序
}
if (queue[0] == e) {
leader = null;
available.signal(); //喚醒所有的被擠壓的wait線程
}
} finally {
lock.unlock();
}
return true;
}
/**
* Sifts element added at bottom up to its heap-ordered spot.
* Call only when holding lock.
二叉樹的堆排序算法
*/
private void siftUp(int k, RunnableScheduledFuture<?> key) {
while (k > 0) {
int parent = (k - 1) >>> 1;
RunnableScheduledFuture<?> e = queue[parent];
if (key.compareTo(e) >= 0)
break;
queue[k] = e;
setIndex(e, k);
k = parent;
}
queue[k] = key;
setIndex(key, k);
}
//work運行的時候調(diào)用queue的take方法
public RunnableScheduledFuture<?> take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
for (;;) {
RunnableScheduledFuture<?> first = queue[0]; //獲取第一個對象
if (first == null)
available.await();
else {
long delay = first.getDelay(NANOSECONDS); //延遲時間
if (delay <= 0)
return finishPoll(first);
first = null; // don't retain ref while waiting
if (leader != null)
available.await(); ///因為沒有執(zhí)行線程初始化，所以等等什么時候有了自己被他人喚醒
else {
Thread thisThread = Thread.currentThread();
leader = thisThread;
try {
available.awaitNanos(delay);
} finally {
if (leader == thisThread)
leader = null;
}
}
}
}
} finally {
if (leader == null && queue[0] != null)
available.signal();
lock.unlock();
}
}
/**
* Performs common bookkeeping for poll and take: Replaces
* first element with last and sifts it down. Call only when
* holding lock.
* @param f the task to remove and return
*/
private RunnableScheduledFuture<?> finishPoll(RunnableScheduledFuture<?> f) {
int s = --size;
RunnableScheduledFuture<?> x = queue[s]; //重排序隊列
queue[s] = null;
if (s != 0)
siftDown(0, x);
setIndex(f, -1);
return f;
}
scheduleAtFixedRate 與 scheduleWithFixedDelay 的區(qū)別
scheduleAtFixedRate ，是以上一個任務(wù)開始的時間計時，period時間過去后，檢測上一個任務(wù)是否執(zhí)行完畢，如果上一個任務(wù)執(zhí)行完畢，則當(dāng)前任務(wù)立即執(zhí)行，如果上一個任務(wù)沒有執(zhí)行完畢，則需要等上一個任務(wù)執(zhí)行完畢后立即執(zhí)行。
scheduleWithFixedDelay，是以上一個任務(wù)結(jié)束時開始計時，period時間過去后，立即執(zhí)行。
合理配置線程池
要想合理地配置線程池，就必須首先分析任務(wù)特性，可以從以下幾個角度來分析。
任務(wù)的性質(zhì)：CPU密集型任務(wù)、IO密集型任務(wù)和混合型任務(wù)
任務(wù)的優(yōu)先級：高中低
任務(wù)的執(zhí)行時間：長中短
任務(wù) 的依賴性;是否依賴其他系統(tǒng)資源
CPU密集型任務(wù)應(yīng)配置盡可能小的線程，如配置NCPU+1個線程的線程池，
IO密集型任務(wù)線程并不是一直在執(zhí)行任務(wù) ，則應(yīng)配置盡可能多的線程，如2*NCPU 。
可以通過Runtime.getRuntime().availableProcessors()方法獲得當(dāng)前設(shè)備的CPU個數(shù)
優(yōu)先級不同的任務(wù)可以使用優(yōu)先級隊列PriorityBlockingQueue來處理，它可以讓優(yōu)先級高的任務(wù)先執(zhí)行。
