使用Executors创建线程池
Executor是一个工厂类，可以直接创建线程池，从最简单的定长线程池开始学习
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
可以看到真正创建线程池的方法是调用ThreadPollExecutor的构造方法
public ThreadPoolExecutor(int corePoolSize, //核心线程数
int maximumPoolSize, //最大线程数
long keepAliveTime, //非核心线程的空闲存货时间
TimeUnit unit, //时间单位
BlockingQueue<Runnable> workQueue //等待队列
) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory() //创建线程池的工厂, defaultHandler //拒绝策略);
}
当创建一个线程池后就可以，想线程池中提交任务了,先看ThreadPollExecutor顶层接口的java.util.concurrent.Executor#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.
* 如过还有空闲的核心线程，尝试开启一个新线程执行任务，addWorker时还会检测runState和线程数，防止添加不应该添加的线程。
*
* 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.
* 入队失败
*/
int c = ctl.get(); // 获取正在运行的线程数
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return; //添加成功
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) { //添加到队列
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command)) //
reject(command); //拒绝
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
接下来看下java.util.concurrent.ThreadPoolExecutor#addWorker方法
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)) //cas修改运行线程数
break retry;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs) //检查运行状态
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); //添加的Set集合
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start(); //调用start方法
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
worker的定义
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
execute没有返回值，如果想获取返回值，可以使用java.util.concurrent.AbstractExecutorService#submit(java.lang.Runnable)
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
执行线程池中的所有任务可以使用java.util.concurrent.AbstractExecutorService#invokeAll(java.util.Collection<? extends java.util.concurrent.Callable>)
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
throws InterruptedException {
if (tasks == null)
throw new NullPointerException();
ArrayList<Future<T>> futures = new ArrayList<Future<T>>(tasks.size());
boolean done = false;
try {
for (Callable<T> t : tasks) { //遍历tasks 执行execute()
RunnableFuture<T> f = newTaskFor(t);
futures.add(f);
execute(f);
}
for (int i = 0, size = futures.size(); i < size; i++) {
Future<T> f = futures.get(i);
if (!f.isDone()) {
try {
f.get();
} catch (CancellationException ignore) {
} catch (ExecutionException ignore) {
}
}
}
done = true;
return futures;
} finally {
if (!done)
for (int i = 0, size = futures.size(); i < size; i++)
futures.get(i).cancel(true);
}
}
那么线程池在哪里实现了复用呢？上面可以看到addWorker方法中执行的是Worker的run()方法，真正执行的是java.util.concurrent.ThreadPoolExecutor#runWorker，然后循环执行任务
while (task != null || (task = getTask()) != null) {
}
在java.util.concurrent.ThreadPoolExecutor#getTask中，当活动线程数不大于最大线程数时，就会从队列中获取任务，如果获取不到就阻塞，这样核心线程就不会被回收，每当任务来时也能直接执行，不用再创建线程
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
//..
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
待续。。
