线程池的几个重要的参数：
1、corePoolSize：线程池的核心线程数（也是默认线程数）
2、maximumPoolSize：最大线程数
3、keepAliveTime：允许的线程最大空闲时间（单位/秒）
线程池内部是通过队列+线程实现的，当我们利用线程池执行任务时：
-
如果此时线程池中的线程数量小于corePoolSize，即使线程池中的线程都处于空闲状态，也要创建新的线程来处理被添加的任务，
-
如里此时线程池中的线程数量等于corePoolSize，但是缓冲队列workQueue未满，那么任务被放入缓冲队列。
-
如果此时线程池中的线程数量大于等于corePoolSize，缓中队列workQueue满，并且线程池中的数量小于maximumPooISize，建新的线程来外理被添加的任务。
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如果此时线程池中的线程数量大于corePoosize，缓冲队列workQueue满，并目线程池中的数量等于maximumPoosize，那么通过 handler所指定的策略来处理此任务。
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当线程池中的线程数量大于 corePoolsize时，如果某线程空闲时间超过keepAliveTime，线程将被终止。这样，线程可以动态的调整池中的线程数。
线程池的拒绝策略：指的是RejectedExecutionHandler接口的实现类。
在线程池接口的源代码中，可以发现ThreadPoolExecutor接口内置了4种拒绝策略。
java
package java.util.concurrent;
public class ThreadPoolExecutor extends AbstractExecutorService {
/**
* A handler for rejected tasks that runs the rejected task
* directly in the calling thread of the {@code execute} method,
* unless the executor has been shut down, in which case the task
* is discarded.
*/
public static class CallerRunsPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code CallerRunsPolicy}.
*/
public CallerRunsPolicy() { }
/**
* Executes task r in the caller's thread, unless the executor
* has been shut down, in which case the task is discarded.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
r.run();
}
}
}
/**
* A handler for rejected tasks that throws a
* {@code RejectedExecutionException}.
*/
public static class AbortPolicy implements RejectedExecutionHandler {
/**
* Creates an {@code AbortPolicy}.
*/
public AbortPolicy() { }
/**
* Always throws RejectedExecutionException.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
* @throws RejectedExecutionException always
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
throw new RejectedExecutionException("Task " + r.toString() +
" rejected from " +
e.toString());
}
}
/**
* A handler for rejected tasks that silently discards the
* rejected task.
*/
public static class DiscardPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code DiscardPolicy}.
*/
public DiscardPolicy() { }
/**
* Does nothing, which has the effect of discarding task r.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
}
}
/**
* A handler for rejected tasks that discards the oldest unhandled
* request and then retries {@code execute}, unless the executor
* is shut down, in which case the task is discarded.
*/
public static class DiscardOldestPolicy implements RejectedExecutionHandler {
/**
* Creates a {@code DiscardOldestPolicy} for the given executor.
*/
public DiscardOldestPolicy() { }
/**
* Obtains and ignores the next task that the executor
* would otherwise execute, if one is immediately available,
* and then retries execution of task r, unless the executor
* is shut down, in which case task r is instead discarded.
*
* @param r the runnable task requested to be executed
* @param e the executor attempting to execute this task
*/
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
e.getQueue().poll();
e.execute(r);
}
}
}
}四种拒绝策略的相关说明：
1、AbortPolicy：
默认。拒绝这个任务，并且抛出RejectedExecutionException异常。
2、DiscardPolicy：
队列满了，丢掉任务，不会抛出异常！
3、DiscardOldestPolicy
队列满了，尝试去和最早的竞争，也不会抛出异常！
抛弃最老任务策略，也就是说如果队列满了，就会将最早进入队列的任务抛弃，从队列中腾出空间，再尝试加入队列。因为队列是队尾进、队头出，队头元素是最老的，所以每次都是移除队头元素后再尝试入队。
4、CallerRunsPolicy
调用者执行策略。在新任务被添加到线程池时，如果添加失败，那么提交任务的线程会自己去执行该任务，不会使用线程池中的线程去执行新任务。