Java線程池ThreadPoolExecutor使用與解析
概述
JDK提供了一個工具類Executors來很是方便的建立線程池，下面主要經過一個示例來分析Java線程池的實現原理。java
使用
Runnable runnable = new Runnable() {
@Override
public void run() {
// do something
}
};
ExecutorService executorService = Executors.newFixedThreadPool(2);
executorService.submit(runnable);
executorService.shutdown();
例子裏面使用了Executors.newFixedThreadPool(2)建立了一個固定只有2個線程的線程池，返回了一個ExecutorService對象，而後調用executorService.submit()方法來啓動一個線程，最後調用executorService.shutdown()來關閉線程池。編程
使用起來很是的方便，接下來經過深刻源代碼看一下背後的原理。併發
源碼分析
ExecutorService
看一下ExecutorService的定義異步
public interface ExecutorService extends Executor {
void shutdown();
List<Runnable> shutdownNow();
boolean isShutdown();
boolean isTerminated();
boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException;
<T> Future<T> submit(Callable<T> task);
<T> Future<T> submit(Runnable task, T result);
Future<?> submit(Runnable task);
...
}
ExecutorService繼承自Executoride
public interface Executor {
void execute(Runnable command);
}
列出了一部分的接口，主要是提供了幾個啓動線程執行線程任務的方法，接收不一樣的參數，以及關閉線程池的方法。submit方法接收Runnable或者Callable方法，返回一個Future對象用於異步獲取執行結果。execute方法只接收一個Runnable參數，而且沒有返回值。工具
Executors
再看一下Executors工具類的定義oop
public class Executors {
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public static ExecutorService newWorkStealingPool() {
return new ForkJoinPool
(Runtime.getRuntime().availableProcessors(),
ForkJoinPool.defaultForkJoinWorkerThreadFactory,
null, true);
}
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
...
}
大體是這個樣子的，這裏列出了一部分，提供了建立固定線程數的線程池（newFixedThreadPool），工做竊取的線程池（newWorkStealingPool），單個線程的線程池（newSingleThreadExecutor），不知道怎麼稱呼的線程池（newCachedThreadPool）。源碼分析
ThreadPoolExecutor
聲明
以FixedThreadPool爲例一探究竟，看一下FixedThreadPool返回的 ThreadPoolExecutor到底是什麼東西學習
public class ThreadPoolExecutor extends AbstractExecutorService {...}
public abstract class AbstractExecutorService implements ExecutorService {
...
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
...
}
ThreadPoolExecutor繼承自AbstractExecutorService，AbstractExecutorService是一個實現了ExecutorService的抽象類。ui
抽象類中提供了submit方法的具體實現，將傳入的Runnable或者Callable方法經過newTaskFor方法轉換成一個FutureTask對象（它是RunnableFuture）的實現類，而後調用父類的execute方法執行任務，最終返回runnableFuture對象。從這能夠看出來ExecutorService.submit()方法內部仍是經過調用Executor.execute()方法來執行的，只是將參數轉換成一個Future對象，經過Future對象來獲取執行結果。
內部結構
/**
* The runState provides the main lifecycle control, taking on values:
*
* RUNNING: Accept new tasks and process queued tasks
* SHUTDOWN: Don't accept new tasks, but process queued tasks
* STOP: Don't accept new tasks, don't process queued tasks,
* and interrupt in-progress tasks
* TIDYING: All tasks have terminated, workerCount is zero,
* the thread transitioning to state TIDYING
* will run the terminated() hook method
* TERMINATED: terminated() has completed
*
* The numerical order among these values matters, to allow
* ordered comparisons. The runState monotonically increases over
* time, but need not hit each state. The transitions are:
*
* RUNNING -> SHUTDOWN
* On invocation of shutdown(), perhaps implicitly in finalize()
* (RUNNING or SHUTDOWN) -> STOP
* On invocation of shutdownNow()
* SHUTDOWN -> TIDYING
* When both queue and pool are empty
* STOP -> TIDYING
* When pool is empty
* TIDYING -> TERMINATED
* When the terminated() hook method has completed
*/
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
private static int runStateOf(int c) { return c & ~CAPACITY; }
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }
定義了5種線程池的狀態
- RUNNING 表示線程池能夠接受新的任務
- SHUTDOWN 表示不接受新的任務，可是繼續處理隊列中的任務
- STOP 表示不接受新的任務，中斷當前處理的任務和隊列種的任務
- TIDYING 表示全部任務都已經停止了，全部線程都中止了，將要執行 terminated()方法以前的狀態
- TERMINATED 表示 terminated()方法已經執行完了
有5種狀態變化的流程
- RUNNING -> SHUTDOWN 調用了 shutdown()
- (RUNNING or SHUTDOWN) -> STOP 調用了 shutdownNow()
- SHUTDOWN -> TIDYING 等待處理的隊列和線程池都爲空的時候
- STOP -> TIDYING 線程池爲空尚未執行terminated()以前的狀態
- TIDYING -> TERMINTED 已經執行完terminated()方法
AtomicInteger類型的ctl變量存着當前worker（Worker是一個內部類，下面會詳細解釋）的數量。
ThreadPoolExecutor用一個32位整型的高3位表示運行的狀態，剩下的29位表示能夠支持的線程數。
COUNT_BITS 爲32-3 = 29， 好比 RUNNING 是 -1 << COUNT_BITS，即-1帶符號位左移29位，就是101000...0，STOP爲001000...0，TIDYING爲010000...0。
CAPACITY 爲 (1 << COUNT_BITS) - 1，1左移29位以後-1，最後的結果位 0001111...1，最高3位是0 剩下的29位都是1。
workerCountOf(int c) 用來計算當前線程數，用的方法是 c & CAPACITY 即 c & 0001111...1，取除了高3位的剩下29位來判斷。
runStateOf(int c) 用來查看當前的線程狀態， c & ~CAPACITY 即 c & 1110000...0，取高3位來判斷。
private final BlockingQueue<Runnable> workQueue;
private final ReentrantLock mainLock = new ReentrantLock();
private final HashSet<Worker> workers = new HashSet<Worker>();
private final Condition termination = mainLock.newCondition();
private int largestPoolSize;
private long completedTaskCount;
private volatile ThreadFactory threadFactory;
private volatile RejectedExecutionHandler handler;
private volatile long keepAliveTime;
private volatile boolean allowCoreThreadTimeOut;
private volatile int corePoolSize;
private volatile int maximumPoolSize;
private static final RejectedExecutionHandler defaultHandler = new AbortPolicy();
private static final RuntimePermission shutdownPerm = new RuntimePermission("modifyThread");
在來看一些其餘的全局屬性。workerQueue 一個BlockingQueue存放Runnable對象，workers 一個HashSet存放Worker對象，還有一些corePoolSize maximumPoolSize等就是平時配置鏈接池的參數。
構造方法
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
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.acc = System.getSecurityManager() == null ?
null :
AccessController.getContext();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
Executors提供的newFIxedThreadPool方法其實建立的是一個ThreadPoolExecutor對象，以 newFixedPoolSize(2) 爲例，經過將corePoolSize maximumPoolSize都是設置爲2來實現固定數量的線程池。keepAliveTime設置爲0微秒。workerQueue傳入了一個LinkedBlockingQueue對象。
execute()方法
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();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
通讀幾遍代碼加上上面的註釋，基本能夠理解整個方法的意思。主要的思想是
- 當前線程數小於corePoolSize的時候調用addWorker來建立Worker類
- 當前線程數量大於corePoolSize的時候執行 workQueue.offer() 將任務加到等待隊列裏面
- 若是加不進等待隊列而且建立Worker失敗，就使用reject策略來拒絕當前任務
註釋中的第二點作了不少檢查，將任務加到等待隊列以後還要作一次檢查看看是否須要建立Worker，防止以前建立的Worker已經出現異常中止了。不理解不要緊，不影響對線程池原理的學習。
addWorker
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;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
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);
final Thread t = w.thread;
if (t != 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.
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) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
第一個for循環檢查線程數有沒有超過corePoolSize或者maximunPoolSize。過了這個for循環以後就是建立Worker的地方了
private final class Worker extends AbstractQueuedSynchronizer implements Runnable {
/** Thread this worker is running in. Null if factory fails. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
/** Per-thread task counter */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
public void run() {
runWorker(this);
}
...
}
Worker 類繼承自 AbstractQueuedSynchronizer 實現了 Runnable接口， AbstractQueuedSynchronizer 這個玩意特別厲害，是併發編程的核心類，因爲內容很是多本文不做解析。Worker類中維護了一個Thread對象，存了當前運行的線程，還維護了一個Runnable對象（firstTask），存了當前線程須要執行的對象。再回顧addWorker方法，其實就是用傳入的firstTask參數建立一個Worker對象，並使worker對象啓動一個線程去執行firstTask。重點在Worker對象的run方法，調用了一個runWorker(this)方法。
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) {
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();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} 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);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
runWorker方法接受一個Worker參數，將參數裏面的firstTask拿出來，而後調用 task.run() 方法直接運行這個task，運行完將task變量設置爲null。而後這裏有一個while循環 while (task != null || (task = getTask()) != null)，當task等於null的時候調用getTask()獲取任務。
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
getTask()方法裏面有一個死循環，boolean timed = allowCoreThreadTimeOut || wc > corePoolSize; timed變量判斷wc變量是否大於corePoolSize (allowCoreThreadTimeOut 默認爲 false)。而後下面有一行代碼判斷timed時候爲ture，若是爲true，調用 workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS)，不然調用workQueue.take()，從等待隊列中獲取等待被處理的線程，而後返回出去。poll和take的區別是 當隊列裏面沒有數據的時候poll立刻返回false，而take會堵塞當前線程直到隊列裏面有數據。這裏解釋了爲何線程池可以維持線程不釋放。
總結
當設置了corePoolSize的時候，這個參數表明了可以運行的線程數，當用戶執行submit方法的時候首先會去判斷當前線程數有沒有達到corePoolSize，若是沒有達到，就建立Worker對象並啓動線程執行任務，一個對象內維護一個線程，當線程數超過corePoolSize的時候，用戶執行submit方法的時候只是將任務放到等待隊列裏面，核心線程不斷從等待隊列裏面取出任務執行，沒有任務的時候一直被堵塞住，當有任務來的時候直接取出執行，避免了不斷建立線程帶來的開銷，以及增長了系統資源的利用率。
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