本节学习Executors类源码
首先看一下常用的线程池的几种创建方式
/**
* 创建一个线程池，该线程池重用固定数量的线程，在共享的无界队列中运行
*/
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
/**
* 创建一个线程池，该线程池只有一个工作者线程运行在无界队列中
*/
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
/**
* 创建一个线程池，如果需要就创建线程, 但是会重用之前已创建的可用线程
*/
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
/**
* 创建一个可定时执行任务的线程池
*/
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
下面看一下ThreadPoolExecutor类
有几个核心的参数：
/**
* 阻塞队列，保存任务并交给工作者线程
*/
private final BlockingQueue<Runnable> workQueue;
/**
* 重入锁
*/
private final ReentrantLock mainLock = new ReentrantLock();
/**
* 工作者线程集合
*/
private final HashSet<Worker> workers = new HashSet<Worker>();
/**
* Wait condition to support awaitTermination
*/
private final Condition termination = mainLock.newCondition();
/**
* 跟踪最大线程池大小
*/
private int largestPoolSize;
/**
* 已执行完成的线程个数
*/
private long completedTaskCount;
/**
* 创建线程的工厂
*/
private volatile ThreadFactory threadFactory;
/**
* 饱和策略执行者
*/
private volatile RejectedExecutionHandler handler;
/**
* 空闲工作线程的空闲时间；超过corePoolSize的线程或者allowCoreThreadTimeOut为true的主线程使用这个作为超时时间；
否则线程一直等待任务或关闭
*/
private volatile long keepAliveTime;
/**
* 如果为false，核心线程会一直存活
* 如果为true，核心线程使用keepAliveTime等待空闲工作者线程超时
*/
private volatile boolean allowCoreThreadTimeOut;
/**
* 核心线程个数
*/
private volatile int corePoolSize;
/**
* 最大线程个数
*/
private volatile int maximumPoolSize;
/**
* 默认的拒绝策略
*/
private static final RejectedExecutionHandler defaultHandler =
new AbortPolicy();
/**
* 构造函数，创建线程池
*/
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();//非法校验
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);//毫秒
this.threadFactory = threadFactory;
this.handler = handler;
}
下面看一下线程池执行任务的过程：
/**
* 执行任务
*/
public void execute(Runnable command) {
if (command == null)//如果任务为空，抛异常
throw new NullPointerException();
/*
* 1. 如果工作者线程个数少于corePoolSize，添加一个工作者线程
* 2. 否则，如果线程池处于运行状态则尝试插入工作队列中，如果插入成功，再次检查线程池状态，如果非运行，则队列中移除任务，且拒绝该任务，如果工作者线程为0，则添加工作者线程
* 3. 如果不能插入工作队列中，则尝试创建新的线程，如果失败，则拒绝任务。
*/
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();//重新检查，如果线程数超过最大，则拒绝任务，从队列中移除任务
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
/**
* 创建新的线程
*/
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);//获取工作线程个数
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))//如果超过最大或者核心线程数，则返回false
return false;
if (compareAndIncrementWorkerCount(c))//线程个数+1
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);//创建新的线程
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();//获取锁
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);//添加到工作集合中
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;//设置最大线程数
workerAdded = true;
}
} finally {
mainLock.unlock();//释放锁
}
if (workerAdded) {
t.start();//执行任务
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);//任务启动失败，则从任务队列中移除该任务，ctl减一
}
return workerStarted;
}
/**
*拒绝任务
*/
final void reject(Runnable command) {
handler.rejectedExecution(command, this);
}
默认使用的是AbortPolicy，直接抛出RejectedExecutionException异常
还有三种策略：
1.CallerRunsPolicy：如果线程池非中断状态，则直接在调用拒绝的线程上执行该任务，否则丢弃任务。
2.DiscardOldestPolicy：如果线程池在非中断状态，则从工作队列中移除最近的任务，且执行新的任务，否则丢弃任务
3. DiscardPolicy ： 丢弃任务，什么也不做。
下面看一下任务的执行
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) {//如果task为空，则从队列中获取
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();//任务执行
} 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);//从workers移除
}
}