ThreadPoolExecutor類實現了ExecutorService接口和Executor接口，可以設置線程池corePoolSize，最大線程池大小，AliveTime，拒絕策略等。常用構造方法：
ThreadPoolExecutor(int corePoolSize, int maximumPoolSize,
long keepAliveTime, TimeUnit unit,
BlockingQueue workQueue,
RejectedExecutionHandler handler)
corePoolSize： 線程池維護線程的最少數量
maximumPoolSize：線程池維護線程的最大數量
keepAliveTime： 線程池維護線程所允許的空閒時間
unit： 線程池維護線程所允許的空閒時間的單位
workQueue： 線程池所使用的緩衝隊列
handler： 線程池對拒絕任務的處理策略
當一個任務通過execute(Runnable)方法欲添加到線程池時：
l 如果此時線程池中的數量小於corePoolSize，即使線程池中的線程都處於空閒狀態，也要創建新的線程來處理被添加的任務。
2 如果此時線程池中的數量等於 corePoolSize，但是緩衝隊列 workQueue未滿，那麼任務被放入緩衝隊列。
3 如果此時線程池中的數量大於corePoolSize，緩衝隊列workQueue滿，並且線程池中的數量小於maximumPoolSize，建新的線程來處理被添加的任務。
4 如果此時線程池中的數量大於corePoolSize，緩衝隊列workQueue滿，並且線程池中的數量等於maximumPoolSize，那麼通過 handler所指定的策略來處理此任務。也就是：處理任務的優先級爲：核心線程corePoolSize、任務隊列workQueue、最大線程maximumPoolSize，如果三者都滿了，使用handler處理被拒絕的任務。
當線程池中的線程數量大於 corePoolSize時，如果某線程空閒時間超過keepAliveTime，線程將被終止。這樣，線程池可以動態的調整池中的線程數。
handler有四個選擇：
拋出java.util.concurrent.RejectedExecutionException異常 ，示例如下：
private static class Worker implements Runnable {
public void run() {
System.out.println(Thread.currentThread().getName() + " is running");
}
}
public static void main(String[] args) {
int corePoolSize = 5;
int maxPoolSize = 10;
long keepAliveTime = 5;
BlockingQueue<Runnable> queue = new LinkedBlockingQueue<Runnable>(10);
//拒絕策略1：將拋出 RejectedExecutionException.
RejectedExecutionHandler handler =
new ThreadPoolExecutor.AbortPolicy();
ThreadPoolExecutor executor = new ThreadPoolExecutor
(corePoolSize, maxPoolSize,
keepAliveTime, TimeUnit.SECONDS,
queue, handler);
for(int i=0; i<100; i++) {
executor.execute(new Worker());
}
executor.shutdown();
}
運行結果如下：
pool-1-thread-2 is running
pool-1-thread-3 is running
Exception in thread "main" java.util.concurrent.RejectedExecutionException
pool-1-thread-1 is running
pool-1-thread-7 is running
pool-1-thread-6 is running
pool-1-thread-4 is running
pool-1-thread-9 is running
pool-1-thread-8 is running
pool-1-thread-5 is running
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:1760)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:767)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:658)
at concurrent.ThreadPoolDemo.main(ThreadPoolDemo.java:33)
pool-1-thread-10 is running
處理源碼如下：
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
throw new RejectedExecutionException();
}
策略2：ThreadPoolExecutor.CallerRunsPolicy
用於被拒絕任務的處理程序，它直接在 execute 方法的調用線程中運行被拒絕的任務；如果執行程序已關閉，則會丟棄該任務。如下：
RejectedExecutionHandler handler =
new ThreadPoolExecutor.CallerRunsPolicy();
運行如下：
pool-1-thread-7 is running
pool-1-thread-7 is running
pool-1-thread-7 is running
pool-1-thread-7 is running
pool-1-thread-7 is running
pool-1-thread-7 is running
pool-1-thread-2 is running
pool-1-thread-3 is running
pool-1-thread-1 is running
pool-1-thread-8 is running
main is running
main is running
main is running
pool-1-thread-4 is running
pool-1-thread-7 is running
pool-1-thread-7 is running
pool-1-thread-7 is running
處理源碼如下：
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
r.run();
}
}
策略3：
RejectedExecutionHandler handler =
new ThreadPoolExecutor.DiscardOldestPolicy();
這樣運行結果就不會有100個線程全部被執行。處理源碼如下：
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
if (!e.isShutdown()) {
e.getQueue().poll();
e.execute(r);
}
}
策略4：ThreadPoolExecutor.DiscardPolicy
用於被拒絕任務的處理程序，默認情況下它將丟棄被拒絕的任務。
運行結果也不會全部執行100個線程。
源碼如下，實際就是對線程不執行操作：
public static class DiscardPolicy implements RejectedExecutionHandler {
/**
* Creates a <tt>DiscardPolicy</tt>.
*/
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) {
}
}