Android的线程和线程池
lbbk4462
7年前
<p>线程在Android中是一个很重要的概念，从用途上来说，线程分为主线程和子线程，主线程主要处理和界面相关的事情，而子线程则往往用于执行耗时操作。在Android中扮演线程角色的还有很多，比如AsyncTask和IntentService，同时HandlerThread也是一种特殊的线程，但他们本质都是传统的线程。AsyncTask底层用到了线程池，对于IntentService和HandlerThread来说，它们的底层则直接使用了线程。</p> <p>不同形式的线程虽然都是线程，但是它们具有不同的特性和使用场景。AsyncTask封装了线程池和Handler，它主要是为了方便开发者在子线程中更新UI，HandlerThread是一中消息循环的线程，在它的内部可以使用Handler。IntentService是一个服务，系统对其进行了封装使其可以更方便地执行后台任务，IntentService内部采用HandlerThread来执行任务，当任务执行完毕后IntentService会自动退出。</p> <p>在操作系统中，线程是操作系统的调度的最小单元，同时线程又是一种受限的系统资源，即线程不可能无限制地产生，并且线程的创建和销毁都会相应的开销。如果一个进程中频繁地创建和销毁线程，这显然不是高效的做法，正确的做法是采用线程池，在这个线程池中会缓存一定数量的线程，通过线程池就可以避免因为频繁创建和销毁线程所带来的系统开销。</p> <h3>Android中的线程形态</h3> <p>AsyncTask</p> <p>AsyncTask是一种轻量级的异步任务类，它可以在线程池中执行后台任务，然后把执行的进度和最终结果传递给主线程并在主线程中更新UI。从实现上来说，AsyncTask封装了Thread和Handler，通过AsyncTask可以更加方便地执行后台任务以及在主线程中访问UI，但是AsyncTask并不适合进行特别耗时的后台任务，对于特别耗时的任务来说，用线程池比较好点。</p> <p>AsyncTask提供了4个核心方法：</p> <ul> <li>onPreExecute()，在主线程中执行，在异步任务执行之前，次方法会被调用，做一些准备工作。</li> <li>doInBackground(Params…params)，在线程池中执行，次方法用于执行异步任务，params参数表示异步任务的输入参数。</li> <li>onProgressUpdate(Progress…values)，在主线程中执行，当后台任务的执行进度发生改变时此方法会被调用。</li> <li>onPostExecute(Result result)，在主线程中执行，在异步任务执行之后，此方法会被调用。</li> </ul> <p>看下源码:</p> <pre> <code class="language-java">public abstract class AsyncTask<Params, Progress, Result> { private static final String LOG_TAG = "AsyncTask"; private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors(); // We want at least 2 threads and at most 4 threads in the core pool, // preferring to have 1 less than the CPU count to avoid saturating // the CPU with background work private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4)); private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1; private static final int KEEP_ALIVE_SECONDS = 30; private static final ThreadFactory sThreadFactory = new ThreadFactory() { private final AtomicInteger mCount = new AtomicInteger(1); public Thread newThread(Runnable r) { return new Thread(r, "AsyncTask #" + mCount.getAndIncrement()); } }; private static final BlockingQueue<Runnable> sPoolWorkQueue = new LinkedBlockingQueue<Runnable>(128); /** * An {@link Executor} that can be used to execute tasks in parallel. */ public static final Executor THREAD_POOL_EXECUTOR; static { ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor( CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory); threadPoolExecutor.allowCoreThreadTimeOut(true); THREAD_POOL_EXECUTOR = threadPoolExecutor; } /** * An {@link Executor} that executes tasks one at a time in serial * order. This serialization is global to a particular process. */ public static final Executor SERIAL_EXECUTOR = new SerialExecutor(); private static final int MESSAGE_POST_RESULT = 0x1; private static final int MESSAGE_POST_PROGRESS = 0x2; private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR; private static InternalHandler sHandler; private final WorkerRunnable<Params, Result> mWorker; private final FutureTask<Result> mFuture; private volatile Status mStatus = Status.PENDING; private final AtomicBoolean mCancelled = new AtomicBoolean(); private final AtomicBoolean mTaskInvoked = new AtomicBoolean(); private static class SerialExecutor implements Executor { final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>(); Runnable mActive; public synchronized void execute(final Runnable r) { mTasks.offer(new Runnable() { public void run() { try { r.run(); } finally { scheduleNext(); } } }); if (mActive == null) { scheduleNext(); } } protected synchronized void scheduleNext() { if ((mActive = mTasks.poll()) != null) { THREAD_POOL_EXECUTOR.execute(mActive); } } } /** * Indicates the current status of the task. Each status will be set only once * during the lifetime of a task. */ public enum Status { /** * Indicates that the task has not been executed yet. */ PENDING, /** * Indicates that the task is running. */ RUNNING, /** * Indicates that {@link AsyncTask#onPostExecute} has finished. */ FINISHED, } private static Handler getHandler() { synchronized (AsyncTask.class) { if (sHandler == null) { sHandler = new InternalHandler(); } return sHandler; } } /** @hide */ public static void setDefaultExecutor(Executor exec) { sDefaultExecutor = exec; } /** * Creates a new asynchronous task. This constructor must be invoked on the UI thread. */ public AsyncTask() { mWorker = new WorkerRunnable<Params, Result>() { public Result call() throws Exception { mTaskInvoked.set(true); Result result = null; try { Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked result = doInBackground(mParams); Binder.flushPendingCommands(); } catch (Throwable tr) { mCancelled.set(true); throw tr; } finally { postResult(result); } return result; } }; mFuture = new FutureTask<Result>(mWorker) { @Override protected void done() { try { postResultIfNotInvoked(get()); } catch (InterruptedException e) { android.util.Log.w(LOG_TAG, e); } catch (ExecutionException e) { throw new RuntimeException("An error occurred while executing doInBackground()", e.getCause()); } catch (CancellationException e) { postResultIfNotInvoked(null); } } }; } private void postResultIfNotInvoked(Result result) { final boolean wasTaskInvoked = mTaskInvoked.get(); if (!wasTaskInvoked) { postResult(result); } } private Result postResult(Result result) { @SuppressWarnings("unchecked") Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult<Result>(this, result)); message.sendToTarget(); return result; } /** * Returns the current status of this task. * * @return The current status. */ public final Status getStatus() { return mStatus; } /** * Override this method to perform a computation on a background thread. The * specified parameters are the parameters passed to {@link #execute} * by the caller of this task. * * This method can call {@link #publishProgress} to publish updates * on the UI thread. * * @param params The parameters of the task. * * @return A result, defined by the subclass of this task. * * @see #onPreExecute() * @see #onPostExecute * @see #publishProgress */ @WorkerThread protected abstract Result doInBackground(Params... params); /** * Runs on the UI thread before {@link #doInBackground}. * * @see #onPostExecute * @see #doInBackground */ @MainThread protected void onPreExecute() { } /** * <p>Runs on the UI thread after {@link #doInBackground}. The * specified result is the value returned by {@link #doInBackground}.</p> * * <p>This method won't be invoked if the task was cancelled.</p> * * @param result The result of the operation computed by {@link #doInBackground}. * * @see #onPreExecute * @see #doInBackground * @see #onCancelled(Object) */ @SuppressWarnings({"UnusedDeclaration"}) @MainThread protected void onPostExecute(Result result) { } /** * Runs on the UI thread after {@link #publishProgress} is invoked. * The specified values are the values passed to {@link #publishProgress}. * * @param values The values indicating progress. * * @see #publishProgress * @see #doInBackground */ @SuppressWarnings({"UnusedDeclaration"}) @MainThread protected void onProgressUpdate(Progress... values) { } /** * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and * {@link #doInBackground(Object[])} has finished.</p> * * <p>The default implementation simply invokes {@link #onCancelled()} and * ignores the result. If you write your own implementation, do not call * <code>super.onCancelled(result)</code>.</p> * * @param result The result, if any, computed in * {@link #doInBackground(Object[])}, can be null * * @see #cancel(boolean) * @see #isCancelled() */ @SuppressWarnings({"UnusedParameters"}) @MainThread protected void onCancelled(Result result) { onCancelled(); } /** * <p>Applications should preferably override {@link #onCancelled(Object)}. * This method is invoked by the default implementation of * {@link #onCancelled(Object)}.</p> * * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and * {@link #doInBackground(Object[])} has finished.</p> * * @see #onCancelled(Object) * @see #cancel(boolean) * @see #isCancelled() */ @MainThread protected void onCancelled() { } /** * Returns <tt>true</tt> if this task was cancelled before it completed * normally. If you are calling {@link #cancel(boolean)} on the task, * the value returned by this method should be checked periodically from * {@link #doInBackground(Object[])} to end the task as soon as possible. * * @return <tt>true</tt> if task was cancelled before it completed * * @see #cancel(boolean) */ public final boolean isCancelled() { return mCancelled.get(); } /** * <p>Attempts to cancel execution of this task. This attempt will * fail if the task has already completed, already been cancelled, * or could not be cancelled for some other reason. If successful, * and this task has not started when <tt>cancel</tt> is called, * this task should never run. If the task has already started, * then the <tt>mayInterruptIfRunning</tt> parameter determines * whether the thread executing this task should be interrupted in * an attempt to stop the task.</p> * * <p>Calling this method will result in {@link #onCancelled(Object)} being * invoked on the UI thread after {@link #doInBackground(Object[])} * returns. Calling this method guarantees that {@link #onPostExecute(Object)} * is never invoked. After invoking this method, you should check the * value returned by {@link #isCancelled()} periodically from * {@link #doInBackground(Object[])} to finish the task as early as * possible.</p> * * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this * task should be interrupted; otherwise, in-progress tasks are allowed * to complete. * * @return <tt>false</tt> if the task could not be cancelled, * typically because it has already completed normally; * <tt>true</tt> otherwise * * @see #isCancelled() * @see #onCancelled(Object) */ public final boolean cancel(boolean mayInterruptIfRunning) { mCancelled.set(true); return mFuture.cancel(mayInterruptIfRunning); } /** * Waits if necessary for the computation to complete, and then * retrieves its result. * * @return The computed result. * * @throws CancellationException If the computation was cancelled. * @throws ExecutionException If the computation threw an exception. * @throws InterruptedException If the current thread was interrupted * while waiting. */ public final Result get() throws InterruptedException, ExecutionException { return mFuture.get(); } /** * Waits if necessary for at most the given time for the computation * to complete, and then retrieves its result. * * @param timeout Time to wait before cancelling the operation. * @param unit The time unit for the timeout. * * @return The computed result. * * @throws CancellationException If the computation was cancelled. * @throws ExecutionException If the computation threw an exception. * @throws InterruptedException If the current thread was interrupted * while waiting. * @throws TimeoutException If the wait timed out. */ public final Result get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException { return mFuture.get(timeout, unit); } /** * Executes the task with the specified parameters. The task returns * itself (this) so that the caller can keep a reference to it. * * <p>Note: this function schedules the task on a queue for a single background * thread or pool of threads depending on the platform version. When first * introduced, AsyncTasks were executed serially on a single background thread. * Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed * to a pool of threads allowing multiple tasks to operate in parallel. Starting * {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are back to being * executed on a single thread to avoid common application errors caused * by parallel execution. If you truly want parallel execution, you can use * the {@link #executeOnExecutor} version of this method * with {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings * on its use. * * <p>This method must be invoked on the UI thread. * * @param params The parameters of the task. * * @return This instance of AsyncTask. * * @throws IllegalStateException If {@link #getStatus()} returns either * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}. * * @see #executeOnExecutor(java.util.concurrent.Executor, Object[]) * @see #execute(Runnable) */ @MainThread public final AsyncTask<Params, Progress, Result> execute(Params... params) { return executeOnExecutor(sDefaultExecutor, params); } /** * Executes the task with the specified parameters. The task returns * itself (this) so that the caller can keep a reference to it. * * <p>This method is typically used with {@link #THREAD_POOL_EXECUTOR} to * allow multiple tasks to run in parallel on a pool of threads managed by * AsyncTask, however you can also use your own {@link Executor} for custom * behavior. * * <p><em>Warning:</em> Allowing multiple tasks to run in parallel from * a thread pool is generally <em>not</em> what one wants, because the order * of their operation is not defined. For example, if these tasks are used * to modify any state in common (such as writing a file due to a button click), * there are no guarantees on the order of the modifications. * Without careful work it is possible in rare cases for the newer version * of the data to be over-written by an older one, leading to obscure data * loss and stability issues. Such changes are best * executed in serial; to guarantee such work is serialized regardless of * platform version you can use this function with {@link #SERIAL_EXECUTOR}. * * <p>This method must be invoked on the UI thread. * * @param exec The executor to use. {@link #THREAD_POOL_EXECUTOR} is available as a * convenient process-wide thread pool for tasks that are loosely coupled. * @param params The parameters of the task. * * @return This instance of AsyncTask. * * @throws IllegalStateException If {@link #getStatus()} returns either * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}. * * @see #execute(Object[]) */ @MainThread public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec, Params... params) { if (mStatus != Status.PENDING) { switch (mStatus) { case RUNNING: throw new IllegalStateException("Cannot execute task:" + " the task is already running."); case FINISHED: throw new IllegalStateException("Cannot execute task:" + " the task has already been executed " + "(a task can be executed only once)"); } } mStatus = Status.RUNNING; onPreExecute(); mWorker.mParams = params; exec.execute(mFuture); return this; } /** * Convenience version of {@link #execute(Object...)} for use with * a simple Runnable object. See {@link #execute(Object[])} for more * information on the order of execution. * * @see #execute(Object[]) * @see #executeOnExecutor(java.util.concurrent.Executor, Object[]) */ @MainThread public static void execute(Runnable runnable) { sDefaultExecutor.execute(runnable); } /** * This method can be invoked from {@link #doInBackground} to * publish updates on the UI thread while the background computation is * still running. Each call to this method will trigger the execution of * {@link #onProgressUpdate} on the UI thread. * * {@link #onProgressUpdate} will not be called if the task has been * canceled. * * @param values The progress values to update the UI with. * * @see #onProgressUpdate * @see #doInBackground */ @WorkerThread protected final void publishProgress(Progress... values) { if (!isCancelled()) { getHandler().obtainMessage(MESSAGE_POST_PROGRESS, new AsyncTaskResult<Progress>(this, values)).sendToTarget(); } } private void finish(Result result) { if (isCancelled()) { onCancelled(result); } else { onPostExecute(result); } mStatus = Status.FINISHED; } private static class InternalHandler extends Handler { public InternalHandler() { super(Looper.getMainLooper()); } @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"}) @Override public void handleMessage(Message msg) { AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj; switch (msg.what) { case MESSAGE_POST_RESULT: // There is only one result result.mTask.finish(result.mData[0]); break; case MESSAGE_POST_PROGRESS: result.mTask.onProgressUpdate(result.mData); break; } } } private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { Params[] mParams; } @SuppressWarnings({"RawUseOfParameterizedType"}) private static class AsyncTaskResult<Data> { final AsyncTask mTask; final Data[] mData; AsyncTaskResult(AsyncTask task, Data... data) { mTask = task; mData = data; } } }</code></pre> <p>从中我们知道了，线程池中线程的数量跟CPU内核多少有关，在一个处理队列中最多只有128个，这个并发数超过就会报异常，同时源码里也看到，是通过sHandler发送一个MESSAGE_POST_RESULT的消息进行最终处理的。</p> <p>sHandler是一个静态的Handler对象，为了能够将执行环境切换到主线程，这就要求sHandler这个对象必须在主线程中创建。由于静态成员会在加载类的时候进行初始化，因此这就变相要求AsyncTask的类必须在主线程中加载，否则同一个进程中的AsyncTask都无法正常工作。</p> <p>还有一点要注意下，从Android 3.0开始，默认情况下AsyncTask是串行执行的。但在Android 3.0之前是并行执行的。</p> <p>HandlerThread</p> <p>HandlerThread继承了Thread，它是一种可以使用Handler的Thread，它的实现很简单，就在run方法中通过Looper.prepare()来创建消息队列，并通过Looper.loop()来开启消息循环，这样在实际的使用中就允许在HandlerThread中创建Handler。看下源代码：</p> <pre> <code class="language-java">public class HandlerThread extends Thread { int mPriority; int mTid = -1; Looper mLooper; public HandlerThread(String name) { super(name); mPriority = Process.THREAD_PRIORITY_DEFAULT; } /** * Constructs a HandlerThread. * @param name * @param priority The priority to run the thread at. The value supplied must be from * {@link android.os.Process} and not from java.lang.Thread. */ public HandlerThread(String name, int priority) { super(name); mPriority = priority; } /** * Call back method that can be explicitly overridden if needed to execute some * setup before Looper loops. */ protected void onLooperPrepared() { } @Override public void run() { mTid = Process.myTid(); Looper.prepare(); synchronized (this) { mLooper = Looper.myLooper(); notifyAll(); } Process.setThreadPriority(mPriority); onLooperPrepared(); Looper.loop(); mTid = -1; } /** * This method returns the Looper associated with this thread. If this thread not been started * or for any reason is isAlive() returns false, this method will return null. If this thread * has been started, this method will block until the looper has been initialized. * @return The looper. */ public Looper getLooper() { if (!isAlive()) { return null; } // If the thread has been started, wait until the looper has been created. synchronized (this) { while (isAlive() && mLooper == null) { try { wait(); } catch (InterruptedException e) { } } } return mLooper; } /** * Quits the handler thread's looper. * <p> * Causes the handler thread's looper to terminate without processing any * more messages in the message queue. * </p><p> * Any attempt to post messages to the queue after the looper is asked to quit will fail. * For example, the {@link Handler#sendMessage(Message)} method will return false. * </p><p class="note"> * Using this method may be unsafe because some messages may not be delivered * before the looper terminates. Consider using {@link #quitSafely} instead to ensure * that all pending work is completed in an orderly manner. * </p> * * @return True if the looper looper has been asked to quit or false if the * thread had not yet started running. * * @see #quitSafely */ public boolean quit() { Looper looper = getLooper(); if (looper != null) { looper.quit(); return true; } return false; } /** * Quits the handler thread's looper safely. * <p> * Causes the handler thread's looper to terminate as soon as all remaining messages * in the message queue that are already due to be delivered have been handled. * Pending delayed messages with due times in the future will not be delivered. * </p><p> * Any attempt to post messages to the queue after the looper is asked to quit will fail. * For example, the {@link Handler#sendMessage(Message)} method will return false. * </p><p> * If the thread has not been started or has finished (that is if * {@link #getLooper} returns null), then false is returned. * Otherwise the looper is asked to quit and true is returned. * </p> * * @return True if the looper looper has been asked to quit or false if the * thread had not yet started running. */ public boolean quitSafely() { Looper looper = getLooper(); if (looper != null) { looper.quitSafely(); return true; } return false; } /** * Returns the identifier of this thread. See Process.myTid(). */ public int getThreadId() { return mTid; } }</code></pre> <p>IntentService</p> <p>IntentService是一种特殊的Service，它继承了Service并且它是一种抽象类，因此必须创建它的子类才能使用IntentService。IntentService可用于执行后台耗时的任务，当任务执行后它会自动停止，同时由于IntentService是服务的原因，这导致他的优先级比单纯的线程要高很多，所以IntentService比较适合执行一些高优先级的后台任务，因为它的优先级高不容易被系统杀死。看下源码：</p> <pre> <code class="language-java">public abstract class IntentService extends Service { private volatile Looper mServiceLooper; private volatile ServiceHandler mServiceHandler; private String mName; private boolean mRedelivery; private final class ServiceHandler extends Handler { public ServiceHandler(Looper looper) { super(looper); } @Override public void handleMessage(Message msg) { onHandleIntent((Intent)msg.obj); stopSelf(msg.arg1); } } /** * Creates an IntentService. Invoked by your subclass's constructor. * * @param name Used to name the worker thread, important only for debugging. */ public IntentService(String name) { super(); mName = name; } /** * Sets intent redelivery preferences. Usually called from the constructor * with your preferred semantics. * * <p>If enabled is true, * {@link #onStartCommand(Intent, int, int)} will return * {@link Service#START_REDELIVER_INTENT}, so if this process dies before * {@link #onHandleIntent(Intent)} returns, the process will be restarted * and the intent redelivered. If multiple Intents have been sent, only * the most recent one is guaranteed to be redelivered. * * <p>If enabled is false (the default), * {@link #onStartCommand(Intent, int, int)} will return * {@link Service#START_NOT_STICKY}, and if the process dies, the Intent * dies along with it. */ public void setIntentRedelivery(boolean enabled) { mRedelivery = enabled; } @Override public void onCreate() { // TODO: It would be nice to have an option to hold a partial wakelock // during processing, and to have a static startService(Context, Intent) // method that would launch the service & hand off a wakelock. super.onCreate(); HandlerThread thread = new HandlerThread("IntentService[" + mName + "]"); thread.start(); mServiceLooper = thread.getLooper(); mServiceHandler = new ServiceHandler(mServiceLooper); } @Override public void onStart(@Nullable Intent intent, int startId) { Message msg = mServiceHandler.obtainMessage(); msg.arg1 = startId; msg.obj = intent; mServiceHandler.sendMessage(msg); } /** * You should not override this method for your IntentService. Instead, * override {@link #onHandleIntent}, which the system calls when the IntentService * receives a start request. * @see android.app.Service#onStartCommand */ @Override public int onStartCommand(@Nullable Intent intent, int flags, int startId) { onStart(intent, startId); return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY; } @Override public void onDestroy() { mServiceLooper.quit(); } /** * Unless you provide binding for your service, you don't need to implement this * method, because the default implementation returns null. * @see android.app.Service#onBind */ @Override @Nullable public IBinder onBind(Intent intent) { return null; } /** * This method is invoked on the worker thread with a request to process. * Only one Intent is processed at a time, but the processing happens on a * worker thread that runs independently from other application logic. * So, if this code takes a long time, it will hold up other requests to * the same IntentService, but it will not hold up anything else. * When all requests have been handled, the IntentService stops itself, * so you should not call {@link #stopSelf}. * * @param intent The value passed to {@link * android.content.Context#startService(Intent)}. * This may be null if the service is being restarted after * its process has gone away; see * {@link android.app.Service#onStartCommand} * for details. */ @WorkerThread protected abstract void onHandleIntent(@Nullable Intent intent); }</code></pre> <h3>Android中的线程池</h3> <p>线程池的优点：</p> <ul> <li>重用线程池中的线程，避免因为线程的创建和销毁所带来的性能开销。</li> <li>能有效控制线程池中的最大并发数，避免大量的线程之间因为互相抢占系统资源而导致的阻塞现象。</li> <li>能够对线程进行简单的管理，并提供定时执行以及指定间隔循环执行等功能。</li> </ul> <p>Android中的线程池的概念来源于Java中的Executor，Executor是一个接口，真正的线程池的实现为ThreadPoolExecutor。ThreadPoolExecutor提供一系列参数来配置线程池，通过不同的参数可以创建不同的线程池，从线程池的功能特性来说，线程池主要分为4类。</p> <p>ThreadPoolExecutor执行任务时大致遵循以下规则：</p> <ol> <li>如果线程池中的线程数量未达到核心线程的数量，那么会直接启动一个核心线程来执行任务。</li> <li>如果线程中的线程数量已经达到或者超过核心线程的数量，那么任务会被插入到任务队列中排队等待执行。</li> <li>如果在步骤2中无法将任务插入到任务队列中，这往往是由于任务队列已经满了， 这个时候如果线程数量未达到线程池规定的最大值，那么会立刻启动一个非核心线程来执行任务。</li> <li>如果步骤3的中线程数量已经达到线程池规定的最大值，那么就拒绝执行此任务，ThreadPoolExecutor会调用RejectedExecutionHandler的rejectedExecution方法来通知调用者。</li> </ol> <p>线程池主要有4类：</p> <ul> <li>FixThreadPool：这是一种线程数量固定的线程池，当线程处于空闲的时候，并不会被回收，除非线程池被关闭了。</li> <li>CachedThreadPool：这是一种线程数量不定的线程池，它只有非核心线程，并且最大线程数为Integer.MAX_VALUE。</li> <li>ScheduledThreadPool：它的核心线程数量是固定的，而非核心线程数是没有限制的，并且当非核心线程闲置时会被立即回收。</li> <li>SingleThreadExecutor：这类线程池内部只有一个核心线程，它确保所有的任务都在同一个线程中按顺序执行。</li> </ul> <p> </p> <p>来自：http://www.jianshu.com/p/24e6bf173082</p> <p> </p>