Java并发编程之线程池ThreadPoolExecutor解析
线程池存在的意义
平常使用线程即new Thread()然后调用start()方法去启动这个线程，但是在频繁的业务情况下如果在生产环境大量的创建Thread对象是则会浪费资源，不仅增加GC回收压力，并且还浪费了时间，创建线程是需要花时间的；
线程池的存在就是降低频繁的创建线程，降低资源的消耗以及创建时间的浪费，并且可以同一管理。
ThreadPoolExecutor
在JDK中所有的线程池的父类就是ThreadPoolExecutor，以下是它的构造方法
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:
* {@code corePoolSize < 0}
* {@code keepAliveTime < 0}
* {@code maximumPoolSize <= 0}
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue 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;
} int corePoolSize ：线程池中核心线程数，小于corePoolSize ，就会创建新线程，等于corePoolSize ，这个任务就会保存到BlockingQueue，如果调用prestartAllCoreThreads()方法就会一次性的启动corePoolSize个数的线程。
int maximumPoolSize： 允许的最大线程数，BlockingQueue也满了，小于maximumPoolSize时候就会再次创建新的线程
long keepAliveTime：线程空闲下来后，存活的时间，这个参数只在大于corePoolSize才有用
TimeUnit unit：存活时间的单位值
BlockingQueue
ThreadFactory threadFactory：创建线程的工厂，给新建的线程赋予名字
RejectedExecutionHandler handler：饱和策略
AbortPolicy ：直接抛出异常，默认；
CallerRunsPolicy：用调用者所在的线程来执行任务
DiscardOldestPolicy：丢弃阻塞队列里最老的任务，队列里最靠前的任务
DiscardPolicy ：当前任务直接丢弃
也可以实现自己的饱和策略，实现RejectedExecutionHandler接口即可
实现基本原理
主要是依赖BlockingQueue
在提交Runnable可执行的线程时，
当前线程数小于corePoolSize 的时候，仅仅是将Runnable添加到HashSet
当前线程数大于或等于corePoolSize 的时候，会将Runnable添加到workerQueue队列中等待并且会添加一个null的Runnable到addWorker()方法当中。如果队列满了offer失败就会执相应的reject拒绝策略。
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);
}
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;
}
在addWorker()方法当中，如果Runnable为空的话，会直接返回false，否则将创建一个Worker对象并且启动它，在runWorker中，首先执行完后传输过来的Runnable对象中的run()，然后循环去workerQueue队列使用take方法拿等待队列中的Runnable对象，并且执行相应的run()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);
}
}关闭线程池的方法：
shutdownNow():设置线程池的状态，还会尝试停止正在运行或者暂停任务的线程
shutdown()设置线程池的状态，只会中断所有没有执行任务的线程
工作机制
合理配置线程池
根据任务的性质来：计算密集型（CPU），IO密集型，混合型
计算密集型：加密，大数分解，正则…….， 线程数适当小一点，最大推荐：机器的Cpu核心数+1，为什么+1，防止页缺失，(机器的Cpu核心=Runtime.getRuntime().availableProcessors();)
IO密集型：读取文件，数据库连接，网络通讯, 线程数适当大一点，机器的Cpu核心数*2,
混合型：尽量拆分，IO密集型>>计算密集型，拆分意义不大，IO密集型~计算密集型
队列的选择上，应该使用有界，无界队列可能会导致内存溢出
Executors预定义的线程池
FixedThreadPool:创建固定线程数量的，适用于负载较重的服务器，使用了无界队列
SingleThreadPoolExecutor:创建单个线程，需要顺序保证执行任务，不会有多个线程活动，使用了无界队列
CachedThreadPool:会根据需要来创建新线程的，执行很多短期异步任务的程序，使用了SynchronousQueue
WorkStealingPool（JDK7以后）: 基于ForkJoinPool实现
Executor框架
还有一个是定时器，待会儿再说吧