基於Android 7.1的AsyncTask原理分析
阿新 • • 發佈:2019-01-04
對比一下AsyncTask在子執行緒和在主執行緒初始化的區別
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, "建立AsyncTask的子執行緒, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
new AsyncTask<Void, Void, String>() {
@Override
protected void onPreExecute() {
android.util.Log.e(TAG, "子執行緒內初始化AsyncTask #onPreExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
super .onPreExecute();
}
@Override
protected String doInBackground(Void... params) {
android.util.Log.e(TAG, "子執行緒內初始化AsyncTask #doInBackground, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
return null;
}
@Override
protected void onPostExecute(String result) {
android.util.Log.e(TAG, "子執行緒內初始化AsyncTask #onPostExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
super.onPostExecute(result);
}
}.execute((Void) null);
}
}).start();
// Caused by: java.util.concurrent.RejectedExecutionException:
// Task [email protected] rejected from
// [email protected]
// [Running, pool size = 17, active threads = 17, queued tasks = 128, completed tasks = 0]
// for(int i = 0;i<200;i++) {
// new AsyncTask<Void, Void, Void>() {
// @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<Void, Void, String>() {
@Override
protected void onPreExecute() {
android.util.Log.e(TAG, "主執行緒內初始化AsyncTask #onPreExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
super.onPreExecute();
}
@Override
protected String doInBackground(Void... params) {
android.util.Log.e(TAG, "主執行緒內初始化AsyncTask #doInBackground, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
return null;
}
@Override
protected void onPostExecute(String result) {
android.util.Log.e(TAG, "主執行緒內初始化AsyncTask #onPostExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
super.onPostExecute(result);
}
}.execute((Void) null);
}
}
在子執行緒初始化的列印日誌:
在主執行緒初始化的列印日誌:
總結:
- AsyncTask裡的Handler為InternalHandler,這裡直接使用的主執行緒的Looper( super(Looper.getMainLooper());),如果去看API 22以下的程式碼,會發現它沒有這個建構函式,而是使用預設的;預設情況下,Handler會使用當前執行緒的Looper,如果你的AsyncTask是在子執行緒建立的,那麼很不幸,你的onPreExecute和onPostExecute並非在UI執行緒執行,而是被Handler post到建立它的那個執行緒執行;如果你在這兩個地方更新了UI,那麼直接導致崩潰,API 25的程式碼其實稍微更近一步(通過上面案例日誌分析),即使你在子執行緒中建立AsyncTask,onPostExecute也會在UI執行緒執行,但是onPreExecute還是在建立AsyncTask的執行緒中執行
- 所以 歸結為一點:為了避免不必要的麻煩,AsyncTask必須在主執行緒初始化,原始碼註釋也是這麼提示我們的
- 在Android 1.5剛引入的時候,AsyncTask的execute是序列執行的;到了Android 1.6直到Android 2.3.2,又被修改為並行執行了。Android 3.0以上,AsyncTask預設序列執行的
- 預設序列的設計是為了避免相同資源的同步訪問問題
- 如果希望AsyncTask可以並行處理的話,可以使用executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR)
- 如果任務過多,超過了工作佇列以及執行緒數目的限制導致這個執行緒池發生阻塞,預設的處理方式會直接丟擲一個異常導致程序掛掉 如上述案例模擬(當然可以重寫AsyncTask,在executeOnExecutor中對exec.execute(mFuture)做保護,捕獲RejectedExecutionException異常,這裡可以同時做一個統計,如果出現問題,說明AsyncTask不適合這種場景,需要考慮重構,既然知道有此問題,最好主動避免涉及)
AsyncTask原始碼
這裡是以API 25為例進行分析的:
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個執行緒,最多需要4個執行緒在核心池中
// 可以比CPU數量少一個,避免讓所有的CPU都在處理後臺工作
// 這個計算方式不同版本表現不一樣,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;
// 執行緒建立工廠
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());
}
};
// 工作佇列長度為128
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
// 可以用於並行執行任務的 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);
// 核心執行緒空閒允許超時處理 待遇同非核心執行緒
threadPoolExecutor.allowCoreThreadTimeOut(true);
THREAD_POOL_EXECUTOR = threadPoolExecutor;
}
// 序列執行任務的Executor(按順序每次同時只能執行一個任務)
// 指定程序全域性有效
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
//預設序列執行 而非並行執行
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();
// SERIAL_EXECUTOR也使用THREAD_POOL_EXECUTOR實現的
// 只不過通過佇列mTasks排程實現序列的控制
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);
}
}
}
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;
}
// 建立一個非同步任務,建構函式必須在UI執行緒呼叫
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;
}
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);
}
// 基於指定引數執行任務 預設序列執行
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
//基於指定引數執行任務,可以自定義Executor,也可以直接使用THREAD_POOL_EXECUTOR
@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;
}
//簡易版 可以直接執行一個Runnable物件
@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<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;
}
}
}