基于Android 7.1的AsyncTask原理分析
-
- 对比一下AsyncTask在子线程和在主线程初始化的区别
- 在子线程初始化的打印日志
- 在主线程初始化的打印日志
- 总结
- AsyncTask源码
- 对比一下AsyncTask在子线程和在主线程初始化的区别
对比一下AsyncTask在子线程和在主线程初始化的区别
public class MainActivity extends AppCompatActivity {
private static final String TAG = MainActivity.class.getCanonicalName();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
android.util.Log.e(TAG, "#onCreate, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
new Thread(new Runnable() {
@Override
public void run() {
android.util.Log.e(TAG, "创建AsyncTask的子线程, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
new AsyncTask() {
@Override
protected void onPreExecute() {
android.util.Log.e(TAG, "子线程内初始化AsyncTask #onPreExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
super.onPreExecute();
}
@Override
protected String doInBackground(Void... params) {
android.util.Log.e(TAG, "子线程内初始化AsyncTask #doInBackground, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
return null;
}
@Override
protected void onPostExecute(String result) {
android.util.Log.e(TAG, "子线程内初始化AsyncTask #onPostExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
super.onPostExecute(result);
}
}.execute((Void) null);
}
}).start();
// Caused by: java.util.concurrent.RejectedExecutionException:
// Task android.os.AsyncTask$3@7c4920 rejected from
// java.util.concurrent.ThreadPoolExecutor@1f312d9
// [Running, pool size = 17, active threads = 17, queued tasks = 128, completed tasks = 0]
// for(int i = 0;i<200;i++) {
// new AsyncTask() {
// @Override
// protected Void doInBackground(Void... params) {
// // do something
// try {
// Thread.sleep(3000);
// } catch (InterruptedException e) {
// e.printStackTrace();
// }
// return null;
// }
// }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
// }
new AsyncTask() {
@Override
protected void onPreExecute() {
android.util.Log.e(TAG, "主线程内初始化AsyncTask #onPreExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
super.onPreExecute();
}
@Override
protected String doInBackground(Void... params) {
android.util.Log.e(TAG, "主线程内初始化AsyncTask #doInBackground, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
return null;
}
@Override
protected void onPostExecute(String result) {
android.util.Log.e(TAG, "主线程内初始化AsyncTask #onPostExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
super.onPostExecute(result);
}
}.execute((Void) null);
}
}
在子线程初始化的打印日志：
在主线程初始化的打印日志：
总结：
- AsyncTask里的Handler为InternalHandler，这里直接使用的主线程的Looper( super(Looper.getMainLooper());)，如果去看API 22以下的代码，会发现它没有这个构造函数，而是使用默认的；默认情况下，Handler会使用当前线程的Looper，如果你的AsyncTask是在子线程创建的，那么很不幸，你的onPreExecute和onPostExecute并非在UI线程执行，而是被Handler post到创建它的那个线程执行；如果你在这两个地方更新了UI，那么直接导致崩溃，API 25的代码其实稍微更近一步（通过上面案例日志分析），即使你在子线程中创建AsyncTask，onPostExecute也会在UI线程执行，但是onPreExecute还是在创建AsyncTask的线程中执行
- 所以 归结为一点：为了避免不必要的麻烦，AsyncTask必须在主线程初始化，源码注释也是这么提示我们的
- 在Android 1.5刚引入的时候，AsyncTask的execute是串行执行的；到了Android 1.6直到Android 2.3.2，又被修改为并行执行了。Android 3.0以上，AsyncTask默认串行执行的
- 默认串行的设计是为了避免相同资源的同步访问问题
- 如果希望AsyncTask可以并行处理的话，可以使用executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR)
- 如果任务过多，超过了工作队列以及线程数目的限制导致这个线程池发生阻塞，默认的处理方式会直接抛出一个异常导致进程挂掉 如上述案例模拟（当然可以重写AsyncTask，在executeOnExecutor中对exec.execute(mFuture)做保护，捕获RejectedExecutionException异常，这里可以同时做一个统计，如果出现问题，说明AsyncTask不适合这种场景，需要考虑重构，既然知道有此问题，最好主动避免涉及）
AsyncTask源码
这里是以API 25为例进行分析的：
public abstract class AsyncTask<Params, Progress, Result> {
private static final String LOG_TAG = "AsyncTask";
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
// 至少需要2个线程，最多需要4个线程在核心池中
// 可以比CPU数量少一个，避免让所有的CPU都在处理后台工作
// 这个计算方式不同版本表现不一样，早期是CPU+1
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;
// 线程创建工厂
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());
}
};
// 工作队列长度为128
private static final BlockingQueue sPoolWorkQueue =
new LinkedBlockingQueue(128);
// 可以用于并行执行任务的 Executor
public static final Executor THREAD_POOL_EXECUTOR;
// THREAD_POOL_EXECUTOR初始化
static {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
sPoolWorkQueue, sThreadFactory);
// 核心线程空闲允许超时处理 待遇同非核心线程
threadPoolExecutor.allowCoreThreadTimeOut(true);
THREAD_POOL_EXECUTOR = threadPoolExecutor;
}
// 串行执行任务的Executor（按顺序每次同时只能执行一个任务）
// 指定进程全局有效
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
//默认串行执行 而非并行执行
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
private static InternalHandler sHandler;
private final WorkerRunnable mWorker;
private final FutureTask mFuture;
private volatile Status mStatus = Status.PENDING;
private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
// SERIAL_EXECUTOR也使用THREAD_POOL_EXECUTOR实现的
// 只不过通过队列mTasks调度实现串行的控制
private static class SerialExecutor implements Executor {
final ArrayDeque mTasks = new ArrayDeque();
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);
}
}
}
public enum Status {
PENDING,
RUNNING,
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;
}
// 创建一个异步任务，构造函数必须在UI线程调用
public AsyncTask() {
mWorker = new WorkerRunnable() {
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(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(this, result));
message.sendToTarget();
return result;
}
public final Status getStatus() {
return mStatus;
}
@WorkerThread
protected abstract Result doInBackground(Params... params);
@MainThread
protected void onPreExecute() {
}
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onPostExecute(Result result) {
}
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onProgressUpdate(Progress... values) {
}
@SuppressWarnings({"UnusedParameters"})
@MainThread
protected void onCancelled(Result result) {
onCancelled();
}
@MainThread
protected void onCancelled() {
}
public final boolean isCancelled() {
return mCancelled.get();
}
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
public final Result get() throws InterruptedException, ExecutionException {
return mFuture.get();
}
public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
ExecutionException, TimeoutException {
return mFuture.get(timeout, unit);
}
// 基于指定参数执行任务 默认串行执行
@MainThread
public final AsyncTask execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
//基于指定参数执行任务，可以自定义Executor，也可以直接使用THREAD_POOL_EXECUTOR
@MainThread
public final AsyncTask 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;
}
//简易版 可以直接执行一个Runnable对象
@MainThread
public static void execute(Runnable runnable) {
sDefaultExecutor.execute(runnable);
}
@WorkerThread
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult 点击一下 有惊喜