ThreadPoolExecutor原理及使用
大家先从ThreadPoolExecutor的总体流程入手：
针对ThreadPoolExecutor代码，我们来看下execute方法：
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
//poolSize大于等于corePoolSize时不增加线程，反之新初始化线程
if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {
//线程执行状态外为执行，同时可以添加到队列中
if (runState == RUNNING && workQueue.offer(command)) {
if (runState != RUNNING || poolSize == 0)
ensureQueuedTaskHandled(command);
}
//poolSize大于等于corePoolSize时，新初始化线程
else if (!addIfUnderMaximumPoolSize(command))
//无法添加初始化执行线程，怎么执行reject操作（调用RejectedExecutionHandler）
reject(command); // is shutdown or saturated
}
}
我们再看下真正的线程执行者（Worker）：
private final class Worker implements Runnable {
/**
* Runs a single task between before/after methods.
*/
private void runTask(Runnable task) {
final ReentrantLock runLock = this.runLock;
runLock.lock();
try {
/*
* If pool is stopping ensure thread is interrupted;
* if not, ensure thread is not interrupted. This requires
* a double-check of state in case the interrupt was
* cleared concurrently with a shutdownNow -- if so,
* the interrupt is re-enabled.
*/
//当线程池的执行状态为关闭等，则执行当前线程的interrupt()操作
if ((runState >= STOP ||
(Thread.interrupted() && runState >= STOP)) &&
hasRun)
thread.interrupt();
/*
* Track execution state to ensure that afterExecute
* is called only if task completed or threw
* exception. Otherwise, the caught runtime exception
* will have been thrown by afterExecute itself, in
* which case we don't want to call it again.
*/
boolean ran = false;
beforeExecute(thread, task);
try {
//任务执行
task.run();
ran = true;
afterExecute(task, null);
++completedTasks;
} catch (RuntimeException ex) {
if (!ran)
afterExecute(task, ex);
throw ex;
}
} finally {
runLock.unlock();
}
}
/**
* Main run loop
*/
public void run() {
try {
hasRun = true;
Runnable task = firstTask;
firstTask = null;
//判断是否存在需要执行的任务
while (task != null || (task = getTask()) != null) {
runTask(task);
task = null;
}
} finally {
//如果没有，则将工作线程移除，当poolSize为0是则尝试关闭线程池
workerDone(this);
}
}
}
/* Utilities for worker thread control */
/**
* Gets the next task for a worker thread to run. The general
* approach is similar to execute() in that worker threads trying
* to get a task to run do so on the basis of prevailing state
* accessed outside of locks. This may cause them to choose the
* "wrong" action, such as trying to exit because no tasks
* appear to be available, or entering a take when the pool is in
* the process of being shut down. These potential problems are
* countered by (1) rechecking pool state (in workerCanExit)
* before giving up, and (2) interrupting other workers upon
* shutdown, so they can recheck state. All other user-based state
* changes (to allowCoreThreadTimeOut etc) are OK even when
* performed asynchronously wrt getTask.
*
* @return the task
*/
Runnable getTask() {
for (;;) {
try {
int state = runState;
if (state > SHUTDOWN)
return null;
Runnable r;
if (state == SHUTDOWN) // Help drain queue
r = workQueue.poll();
//当线程池大于corePoolSize，同时，存在执行超时时间，则等待相应时间，拿出队列中的线程
else if (poolSize > corePoolSize || allowCoreThreadTimeOut)
r = workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS);
else
//阻塞等待队列中可以取到新线程
r = workQueue.take();
if (r != null)
return r;
//判断线程池运行状态，如果大于corePoolSize,或者线程队列为空，也或者线程池为终止的工作线程可以销毁
if (workerCanExit()) {
if (runState >= SHUTDOWN) // Wake up others
interruptIdleWorkers();
return null;
}
// Else retry
} catch (InterruptedException ie) {
// On interruption, re-check runState
}
}
}
/**
* Performs bookkeeping for an exiting worker thread.
* @param w the worker
*/
//记录执行任务数量，将工作线程移除，当poolSize为0是则尝试关闭线程池
void workerDone(Worker w) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w);
if (--poolSize == 0)
tryTerminate();
} finally {
mainLock.unlock();
}
}
通过上述代码，总结下四个关键字的用法
- corePoolSize 核心线程数量
线程保有量,线程池总永久保存执行线程的数量
- maximumPoolSize 最大线程数量
最大线程量，线程最多不能超过此属性设置的数量，当大于线程保有量后，会新启动线程来满足线程执行。
- 线程存活时间
获取队列中任务的超时时间，当阈值时间内无法获取线程，则会销毁处理线程，前提是线程数量在corePoolSize 以上
- 执行队列
执行队列是针对任务的缓存，任务在提交至线程池时，都会压入到执行队列中。所以这里大家最好设置下队列的上限，防止溢出
ThreadPoolExecuter的几种实现
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
- CachedThreadPool 执行线程不固定，
坏处：只能用在短时间完成的任务（占用时间较长的操作可以导致线程数无限增大，系统资源耗尽）
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
- 单线程线程池
好处：针对单cpu，单线程避免系统资源的抢夺
坏处：多cpu多线程时，不能完全利用cpu资源
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>(),
threadFactory);
}
- 固定长度线程池
好处：线程数量固定，不会存在线程重复初始化
坏处：没有对队列大小进行限制，线程初始化后，再也不能回收线程资源
