为什么使用线程池及线程池讲解

曾经终败给现在 2022-06-13 08:10 354阅读 0赞

当我们需要频繁的创建多个线程进行耗时操作时，每次通过new Thread实现并不是一个好的方式，每次new Thred新建和销毁对象性能较差，线程缺乏管理，并不能充分利用每一个线程，可能无限制新建线程，相互之间竞争，可能占用过多系统资源导致死锁，并且缺乏定时执行、定期执行、线程中断等功能。

java提供了四种线程池，他能够有效的管理、调度线程，避免过多的资源消耗。线程池的有点有以下几点：

1）重用存在的线程，减少对象创建销毁的开销。

2）可有效的控制最大并发线程数，提高系统资源的使用率，同时避免过多资源竞争，避免堵塞。

3）提供定时执行、定期执行、单线程、并发数控制等功能。

线程池都实现了ExecutorService接口，该接口定义了线程池需要实现的接口，如submit、execute、shutdown等。它的实现由ThreadPoolExecutor和ScheduledThreadPoolExecutor,ThreadPoolExecutor也就是我们运用最多的线程池实现，ScheduledThreadPoolExecutor则用于周期性的执行任务。同常我们并不会直接new的形式创建线程池，因为创建参数过程相对复杂，我们通常通过Executors工厂类来简化这个过程。

public class Executors {
 
 /**
 * Creates a thread pool that reuses a fixed number of threads
 * operating off a shared unbounded queue. At any point, at most
 * <tt>nThreads</tt> threads will be active processing tasks.
 * If additional tasks are submitted when all threads are active,
 * they will wait in the queue until a thread is available.
 * If any thread terminates due to a failure during execution
 * prior to shutdown, a new one will take its place if needed to
 * execute subsequent tasks. The threads in the pool will exist
 * until it is explicitly {@link ExecutorService#shutdown shutdown}.
 *
 * @param nThreads the number of threads in the pool
 * @return the newly created thread pool
 * @throws IllegalArgumentException if <tt>nThreads <= 0</tt>
 */
 public static ExecutorService newFixedThreadPool(int nThreads) {
 return new ThreadPoolExecutor(nThreads, nThreads,
 0L, TimeUnit.MILLISECONDS,
 new LinkedBlockingQueue<Runnable>());
 }
 
 /**
 * Creates a thread pool that reuses a fixed number of threads
 * operating off a shared unbounded queue, using the provided
 * ThreadFactory to create new threads when needed. At any point,
 * at most <tt>nThreads</tt> threads will be active processing
 * tasks. If additional tasks are submitted when all threads are
 * active, they will wait in the queue until a thread is
 * available. If any thread terminates due to a failure during
 * execution prior to shutdown, a new one will take its place if
 * needed to execute subsequent tasks. The threads in the pool will
 * exist until it is explicitly {@link ExecutorService#shutdown
 * shutdown}.
 *
 * @param nThreads the number of threads in the pool
 * @param threadFactory the factory to use when creating new threads
 * @return the newly created thread pool
 * @throws NullPointerException if threadFactory is null
 * @throws IllegalArgumentException if <tt>nThreads <= 0</tt>
 */
 public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
 return new ThreadPoolExecutor(nThreads, nThreads,
 0L, TimeUnit.MILLISECONDS,
 new LinkedBlockingQueue<Runnable>(),
 threadFactory);
 }
 
 /**
 * Creates an Executor that uses a single worker thread operating
 * off an unbounded queue. (Note however that if this single
 * thread terminates due to a failure during execution prior to
 * shutdown, a new one will take its place if needed to execute
 * subsequent tasks.) Tasks are guaranteed to execute
 * sequentially, and no more than one task will be active at any
 * given time. Unlike the otherwise equivalent
 * <tt>newFixedThreadPool(1)</tt> the returned executor is
 * guaranteed not to be reconfigurable to use additional threads.
 *
 * @return the newly created single-threaded Executor
 */
 public static ExecutorService newSingleThreadExecutor() {
 return new FinalizableDelegatedExecutorService
 (new ThreadPoolExecutor(1, 1,
 0L, TimeUnit.MILLISECONDS,
 new LinkedBlockingQueue<Runnable>()));
 }
 
 /**
 * Creates an Executor that uses a single worker thread operating
 * off an unbounded queue, and uses the provided ThreadFactory to
 * create a new thread when needed. Unlike the otherwise
 * equivalent <tt>newFixedThreadPool(1, threadFactory)</tt> the
 * returned executor is guaranteed not to be reconfigurable to use
 * additional threads.
 *
 * @param threadFactory the factory to use when creating new
 * threads
 *
 * @return the newly created single-threaded Executor
 * @throws NullPointerException if threadFactory is null
 */
 public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {
 return new FinalizableDelegatedExecutorService
 (new ThreadPoolExecutor(1, 1,
 0L, TimeUnit.MILLISECONDS,
 new LinkedBlockingQueue<Runnable>(),
 threadFactory));
 }
 
 /**
 * Creates a thread pool that creates new threads as needed, but
 * will reuse previously constructed threads when they are
 * available. These pools will typically improve the performance
 * of programs that execute many short-lived asynchronous tasks.
 * Calls to <tt>execute</tt> will reuse previously constructed
 * threads if available. If no existing thread is available, a new
 * thread will be created and added to the pool. Threads that have
 * not been used for sixty seconds are terminated and removed from
 * the cache. Thus, a pool that remains idle for long enough will
 * not consume any resources. Note that pools with similar
 * properties but different details (for example, timeout parameters)
 * may be created using {@link ThreadPoolExecutor} constructors.
 *
 * @return the newly created thread pool
 */
 public static ExecutorService newCachedThreadPool() {
 return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
 60L, TimeUnit.SECONDS,
 new SynchronousQueue<Runnable>());
 }
 
 /**
 * Creates a thread pool that creates new threads as needed, but
 * will reuse previously constructed threads when they are
 * available, and uses the provided
 * ThreadFactory to create new threads when needed.
 * @param threadFactory the factory to use when creating new threads
 * @return the newly created thread pool
 * @throws NullPointerException if threadFactory is null
 */
 public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
 return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
 60L, TimeUnit.SECONDS,
 new SynchronousQueue<Runnable>(),
 threadFactory);
 }
 
 /**
 * Creates a single-threaded executor that can schedule commands
 * to run after a given delay, or to execute periodically.
 * (Note however that if this single
 * thread terminates due to a failure during execution prior to
 * shutdown, a new one will take its place if needed to execute
 * subsequent tasks.) Tasks are guaranteed to execute
 * sequentially, and no more than one task will be active at any
 * given time. Unlike the otherwise equivalent
 * <tt>newScheduledThreadPool(1)</tt> the returned executor is
 * guaranteed not to be reconfigurable to use additional threads.
 * @return the newly created scheduled executor
 */
 public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
 return new DelegatedScheduledExecutorService
 (new ScheduledThreadPoolExecutor(1));
 }
 
 /**
 * Creates a single-threaded executor that can schedule commands
 * to run after a given delay, or to execute periodically. (Note
 * however that if this single thread terminates due to a failure
 * during execution prior to shutdown, a new one will take its
 * place if needed to execute subsequent tasks.) Tasks are
 * guaranteed to execute sequentially, and no more than one task
 * will be active at any given time. Unlike the otherwise
 * equivalent <tt>newScheduledThreadPool(1, threadFactory)</tt>
 * the returned executor is guaranteed not to be reconfigurable to
 * use additional threads.
 * @param threadFactory the factory to use when creating new
 * threads
 * @return a newly created scheduled executor
 * @throws NullPointerException if threadFactory is null
 */
 public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) {
 return new DelegatedScheduledExecutorService
 (new ScheduledThreadPoolExecutor(1, threadFactory));
 }
 
 /**
 * Creates a thread pool that can schedule commands to run after a
 * given delay, or to execute periodically.
 * @param corePoolSize the number of threads to keep in the pool,
 * even if they are idle.
 * @return a newly created scheduled thread pool
 * @throws IllegalArgumentException if <tt>corePoolSize < 0</tt>
 */
 public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
 return new ScheduledThreadPoolExecutor(corePoolSize);
 }
 
 /**
 * Creates a thread pool that can schedule commands to run after a
 * given delay, or to execute periodically.
 * @param corePoolSize the number of threads to keep in the pool,
 * even if they are idle.
 * @param threadFactory the factory to use when the executor
 * creates a new thread.
 * @return a newly created scheduled thread pool
 * @throws IllegalArgumentException if <tt>corePoolSize < 0</tt>
 * @throws NullPointerException if threadFactory is null
 */
 public static ScheduledExecutorService newScheduledThreadPool(
 int corePoolSize, ThreadFactory threadFactory) {
 return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
 }
 
 
 /**
 * Returns an object that delegates all defined {@link
 * ExecutorService} methods to the given executor, but not any
 * other methods that might otherwise be accessible using
 * casts. This provides a way to safely "freeze" configuration and
 * disallow tuning of a given concrete implementation.
 * @param executor the underlying implementation
 * @return an <tt>ExecutorService</tt> instance
 * @throws NullPointerException if executor null
 */
 public static ExecutorService unconfigurableExecutorService(ExecutorService executor) {
 if (executor == null)
 throw new NullPointerException();
 return new DelegatedExecutorService(executor);
 }
 
 /**
 * Returns an object that delegates all defined {@link
 * ScheduledExecutorService} methods to the given executor, but
 * not any other methods that might otherwise be accessible using
 * casts. This provides a way to safely "freeze" configuration and
 * disallow tuning of a given concrete implementation.
 * @param executor the underlying implementation
 * @return a <tt>ScheduledExecutorService</tt> instance
 * @throws NullPointerException if executor null
 */
 public static ScheduledExecutorService unconfigurableScheduledExecutorService(ScheduledExecutorService executor) {
 if (executor == null)
 throw new NullPointerException();
 return new DelegatedScheduledExecutorService(executor);
 }
 
 /**
 * Returns a default thread factory used to create new threads.
 * This factory creates all new threads used by an Executor in the
 * same {@link ThreadGroup}. If there is a {@link
 * java.lang.SecurityManager}, it uses the group of {@link
 * System#getSecurityManager}, else the group of the thread
 * invoking this <tt>defaultThreadFactory</tt> method. Each new
 * thread is created as a non-daemon thread with priority set to
 * the smaller of <tt>Thread.NORM_PRIORITY</tt> and the maximum
 * priority permitted in the thread group. New threads have names
 * accessible via {@link Thread#getName} of
 * pool-N-thread-M, where N is the sequence
 * number of this factory, and M is the sequence number
 * of the thread created by this factory.
 * @return a thread factory
 */
 public static ThreadFactory defaultThreadFactory() {
 return new DefaultThreadFactory();
 }
 
 /**
 * Returns a thread factory used to create new threads that
 * have the same permissions as the current thread.
 * This factory creates threads with the same settings as {@link
 * Executors#defaultThreadFactory}, additionally setting the
 * AccessControlContext and contextClassLoader of new threads to
 * be the same as the thread invoking this
 * <tt>privilegedThreadFactory</tt> method. A new
 * <tt>privilegedThreadFactory</tt> can be created within an
 * {@link AccessController#doPrivileged} action setting the
 * current thread's access control context to create threads with
 * the selected permission settings holding within that action.
 *
 * Note that while tasks running within such threads will have
 * the same access control and class loader settings as the
 * current thread, they need not have the same {@link
 * java.lang.ThreadLocal} or {@link
 * java.lang.InheritableThreadLocal} values. If necessary,
 * particular values of thread locals can be set or reset before
 * any task runs in {@link ThreadPoolExecutor} subclasses using
 * {@link ThreadPoolExecutor#beforeExecute}. Also, if it is
 * necessary to initialize worker threads to have the same
 * InheritableThreadLocal settings as some other designated
 * thread, you can create a custom ThreadFactory in which that
 * thread waits for and services requests to create others that
 * will inherit its values.
 *
 * @return a thread factory
 * @throws AccessControlException if the current access control
 * context does not have permission to both get and set context
 * class loader.
 */
 public static ThreadFactory privilegedThreadFactory() {
 return new PrivilegedThreadFactory();
 }
 
 /**
 * Returns a {@link Callable} object that, when
 * called, runs the given task and returns the given result. This
 * can be useful when applying methods requiring a
 * <tt>Callable</tt> to an otherwise resultless action.
 * @param task the task to run
 * @param result the result to return
 * @return a callable object
 * @throws NullPointerException if task null
 */
 public static <T> Callable<T> callable(Runnable task, T result) {
 if (task == null)
 throw new NullPointerException();
 return new RunnableAdapter<T>(task, result);
 }
 
 /**
 * Returns a {@link Callable} object that, when
 * called, runs the given task and returns <tt>null</tt>.
 * @param task the task to run
 * @return a callable object
 * @throws NullPointerException if task null
 */
 public static Callable<Object> callable(Runnable task) {
 if (task == null)
 throw new NullPointerException();
 return new RunnableAdapter<Object>(task, null);
 }
 
 /**
 * Returns a {@link Callable} object that, when
 * called, runs the given privileged action and returns its result.
 * @param action the privileged action to run
 * @return a callable object
 * @throws NullPointerException if action null
 */
 public static Callable<Object> callable(final PrivilegedAction<?> action) {
 if (action == null)
 throw new NullPointerException();
 return new Callable<Object>() {
 	 public Object call() { return action.run(); }};
 }
 
 /**
 * Returns a {@link Callable} object that, when
 * called, runs the given privileged exception action and returns
 * its result.
 * @param action the privileged exception action to run
 * @return a callable object
 * @throws NullPointerException if action null
 */
 public static Callable<Object> callable(final PrivilegedExceptionAction<?> action) {
 if (action == null)
 throw new NullPointerException();
 	return new Callable<Object>() {
 	 public Object call() throws Exception { return action.run(); }};
 }
 
 /**
 * Returns a {@link Callable} object that will, when
 * called, execute the given <tt>callable</tt> under the current
 * access control context. This method should normally be
 * invoked within an {@link AccessController#doPrivileged} action
 * to create callables that will, if possible, execute under the
 * selected permission settings holding within that action; or if
 * not possible, throw an associated {@link
 * AccessControlException}.
 * @param callable the underlying task
 * @return a callable object
 * @throws NullPointerException if callable null
 *
 */
 public static <T> Callable<T> privilegedCallable(Callable<T> callable) {
 if (callable == null)
 throw new NullPointerException();
 return new PrivilegedCallable<T>(callable);
 }
 
 /**
 * Returns a {@link Callable} object that will, when
 * called, execute the given <tt>callable</tt> under the current
 * access control context, with the current context class loader
 * as the context class loader. This method should normally be
 * invoked within an {@link AccessController#doPrivileged} action
 * to create callables that will, if possible, execute under the
 * selected permission settings holding within that action; or if
 * not possible, throw an associated {@link
 * AccessControlException}.
 * @param callable the underlying task
 *
 * @return a callable object
 * @throws NullPointerException if callable null
 * @throws AccessControlException if the current access control
 * context does not have permission to both set and get context
 * class loader.
 */
 public static <T> Callable<T> privilegedCallableUsingCurrentClassLoader(Callable<T> callable) {
 if (callable == null)
 throw new NullPointerException();
 return new PrivilegedCallableUsingCurrentClassLoader<T>(callable);
 }
 
 // Non-public classes supporting the public methods
 
 /**
 * A callable that runs given task and returns given result
 */
 static final class RunnableAdapter<T> implements Callable<T> {
 final Runnable task;
 final T result;
 RunnableAdapter(Runnable task, T result) {
 this.task = task;
 this.result = result;
 }
 public T call() {
 task.run();
 return result;
 }
 }
 
 /**
 * A callable that runs under established access control settings
 */
 static final class PrivilegedCallable<T> implements Callable<T> {
 private final AccessControlContext acc;
 private final Callable<T> task;
 private T result;
 private Exception exception;
 PrivilegedCallable(Callable<T> task) {
 this.task = task;
 this.acc = AccessController.getContext();
 }
 
 public T call() throws Exception {
 AccessController.doPrivileged(new PrivilegedAction<T>() {
 public T run() {
 try {
 result = task.call();
 } catch (Exception ex) {
 exception = ex;
 }
 return null;
 }
 }, acc);
 if (exception != null)
 throw exception;
 else
 return result;
 }
 }
 
 /**
 * A callable that runs under established access control settings and
 * current ClassLoader
 */
 static final class PrivilegedCallableUsingCurrentClassLoader<T> implements Callable<T> {
 private final ClassLoader ccl;
 private final AccessControlContext acc;
 private final Callable<T> task;
 private T result;
 private Exception exception;
 PrivilegedCallableUsingCurrentClassLoader(Callable<T> task) {
 this.task = task;
 this.ccl = Thread.currentThread().getContextClassLoader();
 this.acc = AccessController.getContext();
 acc.checkPermission(new RuntimePermission("getContextClassLoader"));
 acc.checkPermission(new RuntimePermission("setContextClassLoader"));
 }
 
 public T call() throws Exception {
 AccessController.doPrivileged(new PrivilegedAction<T>() {
 public T run() {
 ClassLoader savedcl = null;
 Thread t = Thread.currentThread();
 try {
 ClassLoader cl = t.getContextClassLoader();
 if (ccl != cl) {
 t.setContextClassLoader(ccl);
 savedcl = cl;
 }
 result = task.call();
 } catch (Exception ex) {
 exception = ex;
 } finally {
 if (savedcl != null)
 t.setContextClassLoader(savedcl);
 }
 return null;
 }
 }, acc);
 if (exception != null)
 throw exception;
 else
 return result;
 }
 }
 
 /**
 * The default thread factory
 */
 static class DefaultThreadFactory implements ThreadFactory {
 static final AtomicInteger poolNumber = new AtomicInteger(1);
 final ThreadGroup group;
 final AtomicInteger threadNumber = new AtomicInteger(1);
 final String namePrefix;
 
 DefaultThreadFactory() {
 SecurityManager s = System.getSecurityManager();
 group = (s != null)? s.getThreadGroup() :
 Thread.currentThread().getThreadGroup();
 namePrefix = "pool-" +
 poolNumber.getAndIncrement() +
 "-thread-";
 }
 
 public Thread newThread(Runnable r) {
 Thread t = new Thread(group, r,
 namePrefix + threadNumber.getAndIncrement(),
 0);
 if (t.isDaemon())
 t.setDaemon(false);
 if (t.getPriority() != Thread.NORM_PRIORITY)
 t.setPriority(Thread.NORM_PRIORITY);
 return t;
 }
 }
 
 /**
 * Thread factory capturing access control and class loader
 */
 static class PrivilegedThreadFactory extends DefaultThreadFactory {
 private final ClassLoader ccl;
 private final AccessControlContext acc;
 
 PrivilegedThreadFactory() {
 super();
 this.ccl = Thread.currentThread().getContextClassLoader();
 this.acc = AccessController.getContext();
 acc.checkPermission(new RuntimePermission("setContextClassLoader"));
 }
 
 public Thread newThread(final Runnable r) {
 return super.newThread(new Runnable() {
 public void run() {
 AccessController.doPrivileged(new PrivilegedAction<Object>() {
 public Object run() {
 Thread.currentThread().setContextClassLoader(ccl);
 r.run();
 return null;
 }
 }, acc);
 }
 });
 }
 
 }
 
 /**
 * A wrapper class that exposes only the ExecutorService methods
 * of an ExecutorService implementation.
 */
 static class DelegatedExecutorService extends AbstractExecutorService {
 private final ExecutorService e;
 DelegatedExecutorService(ExecutorService executor) { e = executor; }
 public void execute(Runnable command) { e.execute(command); }
 public void shutdown() { e.shutdown(); }
 public List<Runnable> shutdownNow() { return e.shutdownNow(); }
 public boolean isShutdown() { return e.isShutdown(); }
 public boolean isTerminated() { return e.isTerminated(); }
 public boolean awaitTermination(long timeout, TimeUnit unit)
 throws InterruptedException {
 return e.awaitTermination(timeout, unit);
 }
 public Future<?> submit(Runnable task) {
 return e.submit(task);
 }
 public <T> Future<T> submit(Callable<T> task) {
 return e.submit(task);
 }
 public <T> Future<T> submit(Runnable task, T result) {
 return e.submit(task, result);
 }
 public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
 throws InterruptedException {
 return e.invokeAll(tasks);
 }
 public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
 long timeout, TimeUnit unit)
 throws InterruptedException {
 return e.invokeAll(tasks, timeout, unit);
 }
 public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
 throws InterruptedException, ExecutionException {
 return e.invokeAny(tasks);
 }
 public <T> T invokeAny(Collection<? extends Callable<T>> tasks,
 long timeout, TimeUnit unit)
 throws InterruptedException, ExecutionException, TimeoutException {
 return e.invokeAny(tasks, timeout, unit);
 }
 }
 
 static class FinalizableDelegatedExecutorService
 	extends DelegatedExecutorService {
 	FinalizableDelegatedExecutorService(ExecutorService executor) {
 	 super(executor);
 	}
 	protected void finalize() {
 	 super.shutdown();
 	}
 }
 
 /**
 * A wrapper class that exposes only the ScheduledExecutorService
 * methods of a ScheduledExecutorService implementation.
 */
 static class DelegatedScheduledExecutorService
 extends DelegatedExecutorService
 implements ScheduledExecutorService {
 private final ScheduledExecutorService e;
 DelegatedScheduledExecutorService(ScheduledExecutorService executor) {
 super(executor);
 e = executor;
 }
 public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) {
 return e.schedule(command, delay, unit);
 }
 public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {
 return e.schedule(callable, delay, unit);
 }
 public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit) {
 return e.scheduleAtFixedRate(command, initialDelay, period, unit);
 }
 public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit) {
 return e.scheduleWithFixedDelay(command, initialDelay, delay, unit);
 }
 }
 
 
 /** Cannot instantiate. */
 private Executors() {}
 }

启动指定数量的线程---------ThreadPoolExecutor

ThreadPoolExecutor是线程池的实现之一，他的功能是启动指定数量的线程以及将任务添加到一个队列中，并且将任务分发给空闲线程。ExecutorService的生命周期包括三种状态：运行、关闭、终止。创建后便进入运行状态，当调用了shutdown（）方法时便进入关闭状态，此时意味着ExecutorService不再接受新的任务，但它还在执行已经提交了得任务。当所有已经提交了得任务完成后，就变成终止状态。

public ThreadPoolExecutor（int corePoolSize,

int maximumPoolSize

long keepAliveTime

TimeUnit unit

BlockingQueue<Runnable> workQueue

ThreadFactory threadFactory

RejectedExecutionHandler handler）

参数说明：

corePoolSize：线程池中所保存的核心线程数。线程池启动后默认是空的，只有任务来临时才会创建线程以处理请求。prestarAllCoreThreads方法可以在线程池启动后即启动所有核心线程以等待任务。

maximumPoolSize：线程池允许创建的最大线程数。当workQueue使用无界队列时（如LinkBlockingQueue），则此参数无效。它与corePoolSize的作用是调整“线程池中实际运行的线程的数量”。例如，当新任务提交给线程池时，如果线程池中运行的线程数量小于corePoolSize，则创建新线城来处理请求；如果此时线程池中运行的线程数量大于corePoolSize但是却小于maximumPoolSize，则仅当阻塞队列（workQueue）满时才创建新线程。如果设置的corePoolSize等于maximumPoolSize则创建了固定大小的线程池。如果将maximumPoolSize设置为基本的无界值（如Integer.MAX\_VALUE），则允许线程池适应任意数量的并发任务。

keepAliveTime：当前线程池线程总数大于核心线程时，终止多余的空闲线程的时间

Unit ：keepAliveTime的时间单位，可选分、毫秒、秒

workQueue：任务队列，如果当前线程达到核心线程数，且当前所有线程都处于活跃状态时，将新加入的任务放入此队列

threadFactory：线程工厂，让用户可以定制线程的创建过程，通常不需要设置

Handler：拒绝策略，当线程池与workQueue队列都满了的情况下，对新加任务采取的处理策略

其中的workQueue有下列几个常用实现。

1）ArrayBlockingQueue：基于数组结构的有界队列，此队列按FIFO(先进先出)原则对任务进行排序。如果此队列满了还有任务进来，则调用拒绝策略。

2）LinkedBlockingQueue：基于链表结构的无界队列，此队列按FIFO原则对任务进行排序。因为他是无界的，根本不会满，所以采用此队列后线程池将忽略拒绝策略（handler）参数，同时还将忽略最大线程数maximumPoolSize参数

3）SynchronousQueue：直接将任务提交給线程而不是将他加入到队列，实际上此队列是空的。每个插入的操作必须等到另一个调用移除的操作；如果新任务来了线程池没有任何可用线程处理的话则调用拒绝策略。其实要是把maximumPoolSize设置成无界的，加上此队列，就等同于Executors.newCachedThreadPool()。

4）PriorityBlockingQueue：具有优先级的有界队列，可以自定义优先级，默认是按自然排序。

当线程池和workQueue队列都满了的情况下，对新加任务采取的处理策略的默认实现。

1）AbortPolicy：拒绝任务，抛出RejectedExecutionException异常。线程池的默认策略。

2）CallRunsPolicy：拒绝新任务被加入，如果该线程池好没有被关闭，那么将这个新任务执行在调用线程中

3）DiscardOldestPolicy：如果执行程序尚未被关闭，则位于工作队列头部的任务将被删除，然后重试执行程序（如果失败，重复此过程）。这样的结果是最后加入的任务反而有可能被执行，先加入的都被删除了。

4）DiscardPolicy：加不进的任务都被抛弃了，同时没有异常抛出

定时执行一些任务——————ScheduledThreadPoolExecutor

在某些情况下，我们可能需要定时执行一些任务，此时可以通过ScheduledThreadPoolExecutor来实现。我们只需要通过Executoes的newScheduledThreadPool函数即可创建定时执行任务的线程池。

在具有N个处理器的机器上，线程池具有N或N+1个线程时一般会会获得最大cpu利用率