線程池的幾個重要的參數：
1、corePoolSize：線程池的核心線程數（也是默認線程數）
2、maximumPoolSize：最大線程數
3、keepAliveTime：允許的線程最大空閑時間（單位/秒）
線程池內部是通過隊列+線程實現的，當我們利用線程池執行任務時：
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如果此時線程池中的線程數量小于corePoolSize，即使線程池中的線程都處于空閑狀態，也要創建新的線程來處理被添加的任務，
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如里此時線程池中的線程數量等于corePoolSize，但是緩沖隊列workQueue未滿，那么任務被放入緩沖隊列。
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如果此時線程池中的線程數量大于等于corePoolSize，緩中隊列workQueue滿，并且線程池中的數量小于maximumPooISize，建新的線程來外理被添加的任務。
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如果此時線程池中的線程數量大于corePoosize，緩沖隊列workQueue滿，并目線程池中的數量等于maximumPoosize，那么通過 handler所指定的策略來處理此任務。
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當線程池中的線程數量大于 corePoolsize時，如果某線程空閑時間超過keepAliveTime，線程將被終止。這樣，線程可以動態的調整池中的線程數。
線程池的拒絕策略：指的是RejectedExecutionHandler接口的實現類。
在線程池接口的源代碼中，可以發現ThreadPoolExecutor接口內置了4種拒絕策略。
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);}}}}四種拒絕策略的相關說明：
1、AbortPolicy：
默認。拒絕這個任務，并且拋出RejectedExecutionException異常。
2、DiscardPolicy：
隊列滿了，丟掉任務，不會拋出異常！
3、DiscardOldestPolicy
隊列滿了，嘗試去和最早的競爭，也不會拋出異常！
拋棄最老任務策略，也就是說如果隊列滿了，就會將最早進入隊列的任務拋棄，從隊列中騰出空間，再嘗試加入隊列。因為隊列是隊尾進、隊頭出，隊頭元素是最老的，所以每次都是移除隊頭元素后再嘗試入隊。4、CallerRunsPolicy
調用者執行策略。在新任務被添加到線程池時，如果添加失敗，那么提交任務的線程會自己去執行該任務，不會使用線程池中的線程去執行新任務。
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