Android 源码分析01_AsyncTask

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            /*
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             * Copyright (C) 2008 The Android Open Source Project

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             *

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             * Licensed under the Apache License, Version 2.0 (the "License");

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             * you may not use this file except in compliance with the License.

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             * You may obtain a copy of the License at

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             *

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             *      
            http://www.apache.org/licenses/LICENSE-2.0
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             *

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             * Unless required by applicable law or agreed to in writing, software

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             * distributed under the License is distributed on an "AS IS" BASIS,

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             * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

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             * See the License for the specific language governing permissions and

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             * limitations under the License.

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            */
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            package
             android.os;

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             19
            import
             java.util.ArrayDeque;

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            import
             java.util.concurrent.BlockingQueue;

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            import
             java.util.concurrent.Callable;

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            import
             java.util.concurrent.CancellationException;

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            import
             java.util.concurrent.Executor;

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            import
             java.util.concurrent.ExecutionException;

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            import
             java.util.concurrent.FutureTask;

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            import
             java.util.concurrent.LinkedBlockingQueue;

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            import
             java.util.concurrent.ThreadFactory;

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            import
             java.util.concurrent.ThreadPoolExecutor;

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            import
             java.util.concurrent.TimeUnit;

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            import
             java.util.concurrent.TimeoutException;

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            import
             java.util.concurrent.atomic.AtomicBoolean;

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            import
             java.util.concurrent.atomic.AtomicInteger;

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            public
            abstract
            class AsyncTask<Params, Progress, Result> {
 35privatestaticfinal String LOG_TAG = "AsyncTask";
 36 37//获取当前的cpu核心数 38privatestaticfinalint CPU_COUNT = Runtime.getRuntime().availableProcessors();
 39//线程池核心容量 40privatestaticfinalint CORE_POOL_SIZE = CPU_COUNT + 1;
 41//线程池最大容量 42privatestaticfinalint MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
 43//过剩的空闲线程的存活时间 44privatestaticfinalint KEEP_ALIVE = 1;
 45//ThreadFactory 线程工厂，通过工厂方法newThread来获取新线程 46privatestaticfinal ThreadFactory sThreadFactory = new ThreadFactory() {
 47//原子整数，可以在超高并发下正常工作 48privatefinal AtomicInteger mCount = new AtomicInteger(1);
 49 50public Thread newThread(Runnable r) {
 51returnnew Thread(r, "AsyncTask #" + mCount.getAndIncrement());
 52        }
 53    };
 54//静态阻塞式队列，用来存放待执行的任务，初始容量：128个 55privatestaticfinal BlockingQueue<Runnable> sPoolWorkQueue =
 56new LinkedBlockingQueue<Runnable>(128);
 57 58/** 59     * 静态并发线程池，可以用来并行执行任务，尽管从3.0开始，AsyncTask默认是串行执行任务
 60     * 但是我们仍然能构造出并行的AsyncTask
 61*/ 62publicstaticfinal Executor THREAD_POOL_EXECUTOR
 63             = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
 64                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
 65 66/** 67     * 静态串行任务执行器，其内部实现了串行控制，
 68     * 循环的取出一个个任务交给上述的并发线程池去执行
 69*/ 70publicstaticfinal Executor SERIAL_EXECUTOR = new SerialExecutor();
 71//消息类型：发送结果 72privatestaticfinalint MESSAGE_POST_RESULT = 0x1;
 73//消息类型：更新进度 74privatestaticfinalint MESSAGE_POST_PROGRESS = 0x2;
 75/**静态Handler，用来发送上述两种通知，采用UI线程的Looper来处理消息
 76     * 这就是为什么AsyncTask必须在UI线程调用，因为子线程
 77     * 默认没有Looper无法创建下面的Handler，程序会直接Crash
 78*/ 79privatestaticfinal InternalHandler sHandler = new InternalHandler();
 80//默认任务执行器，被赋值为串行任务执行器，就是它，AsyncTask变成串行的了 81privatestaticvolatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
 82//如下两个变量我们先不要深究，不影响我们对整体逻辑的理解 83privatefinal WorkerRunnable<Params, Result> mWorker;
 84privatefinal FutureTask<Result> mFuture;
 85//任务的状态 默认为挂起，即等待执行，其类型标识为易变的（volatile） 86privatevolatile Status mStatus = Status.PENDING;
 87//原子布尔型，支持高并发访问，标识任务是否被取消 88privatefinal AtomicBoolean mCancelled = new AtomicBoolean();
 89//原子布尔型，支持高并发访问，标识任务是否被执行过 90privatefinal AtomicBoolean mTaskInvoked = new AtomicBoolean();
 91 92/*串行执行器的实现，我们要好好看看，它是怎么把并行转为串行的
 93     *目前我们需要知道，asyncTask.execute(Params ...)实际上会调用
 94     *SerialExecutor的execute方法，这一点后面再说明。也就是说：当你的asyncTask执行的时候，
 95     *首先你的task会被加入到任务队列，然后排队，一个个执行
 96*/ 97privatestaticclass SerialExecutor implements Executor {
 98//线性双向队列，用来存储所有的AsyncTask任务 99final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
100//当前正在执行的AsyncTask任务101        Runnable mActive;
102103publicsynchronizedvoid execute(final Runnable r) {
104//将新的AsyncTask任务加入到双向队列中105             mTasks.offer(new Runnable() {
106publicvoid run() {
107try {
108//执行AsyncTask任务109                        r.run();
110                     } finally {
111//当前AsyncTask任务执行完毕后，进行下一轮执行，如果还有未执行任务的话
112//这一点很明显体现了AsyncTask是串行执行任务的，总是一个任务执行完毕才会执行下一个任务113                        scheduleNext();
114                    }
115                }
116            });
117//如果当前没有任务在执行，直接进入执行逻辑118if (mActive == null) {
119                scheduleNext();
120            }
121        }
122123protectedsynchronizedvoid scheduleNext() {
124//从任务队列中取出队列头部的任务，如果有就交给并发线程池去执行125if ((mActive = mTasks.poll()) != null) {
126                THREAD_POOL_EXECUTOR.execute(mActive);
127            }
128        }
129    }
130131/**132     * 任务的三种状态
133*/134publicenum Status {
135/**136         * 任务等待执行
137*/138        PENDING,
139/**140         * 任务正在执行
141*/142        RUNNING,
143/**144         * 任务已经执行结束
145*/146        FINISHED,
147    }
148149/** 隐藏API：在UI线程中调用，用来初始化Handler */150publicstaticvoid init() {
151        sHandler.getLooper();
152    }
153154/** 隐藏API：为AsyncTask设置默认执行器 */155publicstaticvoid setDefaultExecutor(Executor exec) {
156         sDefaultExecutor = exec;
157    }
158159/**160     * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
161*/162public AsyncTask() {
163         mWorker = new WorkerRunnable<Params, Result>() {
164public Result call() throws Exception {
165                 mTaskInvoked.set(true);
166167                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
168//noinspection unchecked169return postResult(doInBackground(mParams));
170            }
171        };
172173         mFuture = new FutureTask<Result>(mWorker) {
174            @Override
175protectedvoid done() {
176try {
177                    postResultIfNotInvoked(get());
178                 } catch (InterruptedException e) {
179                    android.util.Log.w(LOG_TAG, e);
180                 } catch (ExecutionException e) {
181thrownew RuntimeException("An error occured while executing doInBackground()",
182                            e.getCause());
183                 } catch (CancellationException e) {
184                     postResultIfNotInvoked(null);
185                }
186            }
187        };
188    }
189190privatevoid postResultIfNotInvoked(Result result) {
191finalboolean wasTaskInvoked = mTaskInvoked.get();
192if (!wasTaskInvoked) {
193            postResult(result);
194        }
195    }
196//doInBackground执行完毕，发送消息197private Result postResult(Result result) {
198         @SuppressWarnings("unchecked")
199         Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
200new AsyncTaskResult<Result>(this, result));
201        message.sendToTarget();
202return result;
203    }
204205/**206     * 返回任务的状态
207*/208publicfinal Status getStatus() {
209return mStatus;
210    }
211212/**213     * 这个方法是我们必须要重写的，用来做后台计算
214     * 所在线程：后台线程
215*/216protectedabstract Result doInBackground(Params... params);
217218/**219     * 在doInBackground之前调用，用来做初始化工作
220     * 所在线程：UI线程
221*/222protectedvoid onPreExecute() {
223    }
224225/**226     * 在doInBackground之后调用，用来接受后台计算结果更新UI
227     * 所在线程：UI线程
228*/229protectedvoid onPostExecute(Result result) {
230    }
231232/**233     * Runs on the UI thread after {@link #publishProgress} is invoked.
234     /**
235     * 在publishProgress之后调用，用来更新计算进度
236     * 所在线程：UI线程
237*/238protectedvoid onProgressUpdate(Progress... values) {
239    }
240241/**242     * cancel被调用并且doInBackground执行结束，会调用onCancelled，表示任务被取消
243     * 这个时候onPostExecute不会再被调用，二者是互斥的，分别表示任务取消和任务执行完成
244     * 所在线程：UI线程
245*/246     @SuppressWarnings({"UnusedParameters"})
247protectedvoid onCancelled(Result result) {
248        onCancelled();
249    }    
250251protectedvoid onCancelled() {
252    }
253254publicfinalboolean isCancelled() {
255return mCancelled.get();
256    }
257258publicfinalboolean cancel(boolean mayInterruptIfRunning) {
259         mCancelled.set(true);
260return mFuture.cancel(mayInterruptIfRunning);
261    }
262263publicfinal Result get() throws InterruptedException, ExecutionException {
264return mFuture.get();
265    }
266267publicfinal Result get(long timeout, TimeUnit unit) throws InterruptedException,
268            ExecutionException, TimeoutException {
269return mFuture.get(timeout, unit);
270    }
271272/**273     * 这个方法如何执行和系统版本有关，在AsyncTask的使用规则里已经说明，如果你真的想使用并行AsyncTask，
274     * 也是可以的，只要稍作修改
275     * 必须在UI线程调用此方法
276*/277publicfinal AsyncTask<Params, Progress, Result> execute(Params... params) {
278//串行执行279return executeOnExecutor(sDefaultExecutor, params);
280//如果我们想并行执行，这样改就行了，当然这个方法我们没法改
281//return executeOnExecutor(THREAD_POOL_EXECUTOR, params);282    }
283284/**285     * 通过这个方法我们可以自定义AsyncTask的执行方式，串行or并行，甚至可以采用自己的Executor
286     * 为了实现并行，我们可以在外部这么用AsyncTask：
287     * asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, Params... params);
288     * 必须在UI线程调用此方法
289*/290publicfinal AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
291            Params... params) {
292if (mStatus != Status.PENDING) {
293switch (mStatus) {
294case RUNNING:
295thrownew IllegalStateException("Cannot execute task:"
296                             + " the task is already running.");
297case FINISHED:
298thrownew IllegalStateException("Cannot execute task:"
299                             + " the task has already been executed "
300                             + "(a task can be executed only once)");
301            }
302        }
303304         mStatus = Status.RUNNING;
305//这里#onPreExecute会最先执行306        onPreExecute();
307308         mWorker.mParams = params;
309//然后后台计算#doInBackground才真正开始310        exec.execute(mFuture);
311//接着会有#onProgressUpdate被调用，最后是#onPostExecute312313returnthis;
314    }
315316/**317     * 这是AsyncTask提供的一个静态方法，方便我们直接执行一个runnable
318*/319publicstaticvoid execute(Runnable runnable) {
320        sDefaultExecutor.execute(runnable);
321    }
322323/**324     * 打印后台计算进度，onProgressUpdate会被调用
325*/326protectedfinalvoid publishProgress(Progress... values) {
327if (!isCancelled()) {
328            sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
329new AsyncTaskResult<Progress>(this, values)).sendToTarget();
330        }
331    }
332333//任务结束的时候会进行判断，如果任务没有被取消，则onPostExecute会被调用334privatevoid finish(Result result) {
335if (isCancelled()) {
336            onCancelled(result);
337         } else {
338            onPostExecute(result);
339        }
340         mStatus = Status.FINISHED;
341    }
342343//AsyncTask内部Handler，用来发送后台计算进度更新消息和计算完成消息344privatestaticclass InternalHandler extends Handler {
345         @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
346        @Override
347publicvoid handleMessage(Message msg) {
348             AsyncTaskResult result = (AsyncTaskResult) msg.obj;
349switch (msg.what) {
350case MESSAGE_POST_RESULT:
351// There is only one result352                     result.mTask.finish(result.mData[0]);
353break;
354case MESSAGE_POST_PROGRESS:
355                    result.mTask.onProgressUpdate(result.mData);
356break;
357            }
358        }
359    }
360361privatestaticabstractclass WorkerRunnable<Params, Result> implements Callable<Result> {
362        Params[] mParams;
363    }
364365     @SuppressWarnings({"RawUseOfParameterizedType"})
366privatestaticclass AsyncTaskResult<Data> {
367final AsyncTask mTask;
368final Data[] mData;
369370        AsyncTaskResult(AsyncTask task, Data... data) {
371             mTask = task;
372             mData = data;
373        }
374    }
375 }