阻塞式ThreadPoolExecutor
最近在做一个大文件批量数据导入数据库的时候遇到个问题，在使用ThreadPoolExecutor提交任务的时候，发现在线程池满的时候，不能达到阻塞线程的作用。导致的后果就是文件被不断读取到内存，然后丢给ThreadPoolExecutor执行，由于消费速度跟不上生产速度，导致内存不断增长，最后OOM。
于是开始研究ThreadPoolExecutor如何实现在任务满的情况下阻塞线程。
ThreadPoolExecutor类提供了多个参数用于定制化自己的线程池，常用的有corePoolSize，maximumPoolSize，workQueue等几个，如下面构造函数：
/**
* 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:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@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<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler)
为了实现阻塞式线程池，workQueue参数需要传一个有界的BlockQueue,默认Executors.newFixedThreadPool()传入的无参数LinkedBlockingQueue边界为Integer.MAX_VALUE，不能起到Block的效果。
传了有界BlockQueue之后，ThreadPoolExecutor在线程队列Blcok的时候不会阻塞线程提交，而是调用RejectedExecutionHandler，抛出RejectedExecutionException异常。
JDK默认提供了4种失败策略：
AbortPolicy(中止)、CallersRunPolicy(调用者运行)、DiscardPolicy(丢弃)、DiscardOldestPolicy(丢弃最旧的)
JDK默认使用了AbortPolicy(中止)策略，这个可以通过handler参数来设置。
这里收集了几种阻塞线程池提交的方法：
一、通过CallersRunPolicy调用策略实现
其中CallersRunPolicy(调用者运行)方法，在线程池队列满了后会调用主线程来执行任务，同样可以达到阻塞线程提交的目的。这样做有两个缺点：
1、执行任务的线程会是size+1个（主线程），这在有些资源敏感的场景是不被允许的
2、由于主线程被用于执行任务，如果这个任务比较大，会长时间阻塞主线程的执行，导致其他线程空闲时候也不能接受新的任务，形成资源浪费
实例代码如下：
new ThreadPoolExecutor(size, size, 0L, TimeUnit.MILLISECONDS, new SynchronousQueue<Runnable>(), new ThreadPoolExecutor.CallerRunsPolicy())
二、通过自定义RejectedExecutionHandler实现
通过自定义RejectedExecutionHandler，显示调用queue.put()阻塞方法来实现线程池阻塞。这种方法能够避免CallersRunPolicy方法的两个缺点。
示例代码如下：
new ThreadPoolExecutor(size, size, 0L, TimeUnit.MILLISECONDS, new SynchronousQueue<Runnable>(),
new RejectedExecutionHandler() {
@Override
public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
try {
executor.getQueue().put(r);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
});
三、通过其他多线程工具辅助控制
比如常用的可以通过信号量来控制，在提交任务的时候acquire，任务执行完后release。
这种方法的缺点是会侵入任务的执行过程
示例代码如下：
public static void main(String[] args) throws InterruptedException, ExecutionException {
// ExecutorService executorService = Executors.newFixedThreadPool(5);
ExecutorService executorService = new ThreadPoolExecutor(5, 5, 0L, TimeUnit.MILLISECONDS,
new ArrayBlockingQueue<Runnable>(10));
final Semaphore semaphore = new Semaphore(5);
final AtomicInteger counter = new AtomicInteger(0);
int i = 0;
while (true) {
semaphore.acquire();
executorService.submit(new Runnable() {
@Override
public void run() {
try {
int count = counter.addAndGet(1);
System.out.println(Thread.currentThread() + "start, counter: " + count);
try {
Thread.sleep(1000 * 2);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread() + "end, counter: " + count);
} finally {
semaphore.release();
}
}
});
if (++i > 20) {
break;
}
System.out.println("now it is " + i);
}
System.out.println("shotdown...");
ExecutorUtils.shutdownAndWait(executorService, executorService.toString());
System.out.println("Test ends.");
}
这里注意线程池的shutdown过程，没有使用简单的shutdown，因为这样会导致部分task没有执行完成
ExecutorUtils.shutdownAndWait方法代码如下：
public static void shutdownAndWait(ExecutorService executor, String name) {
log.info("Shutting down " + name);
executor.shutdown();
awaitTermination(executor, name);
}
private static void awaitTermination(ExecutorService executor, String name) {
try {
while (!executor.awaitTermination(EXECUTOR_TERMINATION_TIMEOUT, TimeUnit.SECONDS)) {
log.info("Waiting for all tasks complete execution in " + name);
}
log.info(name + " is shut down.");
} catch (InterruptedException e) {
log.error("Shutting down " + name + " failed.", e);
Thread.currentThread().interrupt();
}
}