@(Executors)[newFixedThreadPool]
[TOC]
java線程池
在面向對象編程中，創建和銷毀對象是很費時間的，因為創建一個對象要獲取內存資源或者其它更多資源。在Java中更是如此，虛擬機將試圖跟蹤每一個對象，以便能夠在對象銷毀后進行垃圾回收。所以提高服務程序效率的一個手段就是盡可能減少創建和銷毀對象的次數，特別是一些很耗資源的對象創建和銷毀。如何利用已有對象來服務就是一個需要解決的關鍵問題，其實這就是一些"池化資源"技術產生的原因。
newFixedPool作用
創建一個固定線程數的線程池，在任何時候最多只有nThreads個線程被創建。如果在所有線程都處于活動狀態時，有其他任務提交，他們將等待隊列中直到線程可用。如果任何線程由于執行過程中的故障而終止，將會有一個新線程將取代這個線程執行后續任務。
構造方法
newFixedPool擁有兩個構造方法：
參數為nThreads的構造方法:
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
構造方法中的nThreads代表固定線程池中的線程數，當使用此構造方法創建線程池后，就會創建nThreads個線程在線程池內。
參數為： nThreads,ThreadFactory的構造方法:
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>(),
threadFactory);
}
ThreadPoolExecutor構造方法
newFixedThreadPool(int nThreads) 使用的構造方法:
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
此ThreadPoolExecutor構造方法的參數有5個：
corePoolSize：線程池中所保存的線程數，包括空閑線程，newFixedThreadPool中傳入nThreads
maximumPoolSize：池中允許的最大線程數，newFixedThreadPool中傳入nThreads，使線程池的最大線程數與線程池中保存的線程數一致，使保證線程池的線程數是固定的
TimeUnit：參數的時間單位
keepAliveTime：當線程數大于corePoolSize時，此為終止前多余的空閑線程等待新任務的最長時間
LinkedBlockingQueue workQueuqe：當任務被執行前，放到此阻塞隊列中，任務將被放在阻塞隊列中直到使用execute方法提交Runnable任務,不同的線程池主要是這個參數的不通，比如scheduledThreadPoolExecutor這邊使用的就是DelayedWorkQueue這個隊列
**注意：
* The queue used for holding tasks and handing off to worker
* threads. We do not require that workQueue.poll() returning
* null necessarily means that workQueue.isEmpty(), so rely
* solely on isEmpty to see if the queue is empty (which we must
* do for example when deciding whether to transition from
* SHUTDOWN to TIDYING). This accommodates special-purpose
* queues such as DelayQueues for which poll() is allowed to
* return null even if it may later return non-null when delays
* expire.
*/
private final BlockingQueue<Runnable> workQueue;
newFixedThreadPool(in nThreads,ThreadFactory threadFactory)使用的構造方法:
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>(),
threadFactory);
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler); //創建默認線程工廠，并設置駁回的異常處理
}
創建ThreadPoolExecutor時傳入Executors.defaultThreadFactory()這個默認線程工廠實例。
在代碼中調用ThreadPoolExecutor中的內部類DefaultThreadFactory工廠，在包中的注釋是這么寫的：threadFactory the factory to use when the executor creates a new thread 。這是用來使用executor創建新線程的工廠。
static class DefaultThreadFactory implements ThreadFactory {
private static final AtomicInteger poolNumber = new AtomicInteger(1);
private final ThreadGroup group;
private final AtomicInteger threadNumber = new AtomicInteger(1);
private final String namePrefix;
DefaultThreadFactory() {
SecurityManager s = System.getSecurityManager();
group = (s != null) ? s.getThreadGroup() :
Thread.currentThread().getThreadGroup();
namePrefix = "pool-" +
poolNumber.getAndIncrement() +
"-thread-";
}
public Thread newThread(Runnable r) {
Thread t = new Thread(group, r,
namePrefix + threadNumber.getAndIncrement(),
0);
if (t.isDaemon())
t.setDaemon(false);
if (t.getPriority() != Thread.NORM_PRIORITY)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
}
調用DefaultThreadFactory的構造方法初始化線程組和生成線程前綴。在創建完線程工廠后，需要使用線程工廠來創建線程并啟動。
使用線程池中的線程執行Runnable任務
當之前的初始化（corePoolSize等于maxiumPoolSize等于nThreads，并創建defaultThreadFactory）就可以運行定義好的線程了。
執行線程的代碼如下：
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);
}
其中最重要的方法是addWorker，其方法目的是創建Worker對象，并將Worker對象加入到HashSet<Worker> workers中。在加入workers之前，先創建ReentrantLock排它鎖，將worker同步的加入到workers中。當worker成功被加入后，啟動本次放入set中的線程。
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;
}
Worker對象的構造方法：
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
通過線程工廠 來創建線程。
public Thread newThread(Runnable r) {
Thread t = new Thread(group, r,
namePrefix + threadNumber.getAndIncrement(),
0);
if (t.isDaemon())
t.setDaemon(false);
if (t.getPriority() != Thread.NORM_PRIORITY)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
