文章目錄
除了Thread,Android中扮演線程的角色還有:AsyncTask、HandlerThread、IntentService。
- AsyncTask:內部封裝線程池、handler,便於在子線程中更新UI。
- HandlerThread:可以使用消息循環的線程,在它內部可以使用Handler。
- IntentService:內部使用HandlerThread執行任務,完畢後會自動退出。(相比後臺線程)因是組件,優先級高,不易被殺死。
線程是操作系統調度的最小單元,是一種受限的資源,不可能無限制的產生。且線程的創建和銷燬需要相應的開銷。且存在大量線程時,系統會通過時間片輪轉的方式調度線程,因此線程不可能做到並行,除非線程數小於等於cpu數。所以需要 線程池,它可以緩存一定數量的線程,避免頻繁地線程創建和銷燬帶來的系統開銷。
一、Android中的線程形態
1.1 AsyncTask
AsyncTask是用來在線程池中處理異步任務,並可以把處理進度和結果發送到UI線程。
1.1.1 使用方法
AsyncTask的基本使用方法,示例如下:
private void testAsyncTask() {
//一般要在主線程實例化。(實際在9.0上 子線程創建實例然後主線程execute沒問題)
//三個泛型參數依次表示參數類型、進度類型、結果類型。
//覆寫的這幾個方法不可以直接調用
AsyncTask<Integer, Integer, String> task = new AsyncTask<Integer, Integer, String>() {
@Override
protected void onPreExecute() {
super.onPreExecute();
//主線程執行,在異步任務之前
Log.i(TAG, "testAsyncTask onPreExecute: ");
}
@Override
protected String doInBackground(Integer... integers) {
Log.i(TAG, "testAsyncTask doInBackground: ");
//任務在 線程池中執行 耗時操作
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
//發出進度
publishProgress(50);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
//再發出進度
publishProgress(100);
return "我是結果。參數是" + integers[0];
}
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
//在主線程執行,在異步任務執行完之後
Log.i(TAG, "testAsyncTask onPostExecute: "+s);
}
@Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
//執行在主線程,調用publishProgress()後就會執行
Log.i(TAG, "testAsyncTask onProgressUpdate: 進度:"+values[0]+"%");
}
@Override
protected void onCancelled() {
super.onCancelled();
//取消任務
}
};
//必須要在主線程執行execute,且只能執行一次
task.execute(100);
}
執行結果日誌如下:
2020-01-14 11:29:03.510 13209-13209/com.hfy.demo01 I/hfy: testAsyncTask onPreExecute:
2020-01-14 11:29:03.511 13209-13282/com.hfy.demo01 I/hfy: testAsyncTask doInBackground:
2020-01-14 11:29:04.558 13209-13209/com.hfy.demo01 I/hfy: testAsyncTask onProgressUpdate: 進度:50%
2020-01-14 11:29:05.589 13209-13209/com.hfy.demo01 I/hfy: testAsyncTask onProgressUpdate: 進度:100%
2020-01-14 11:29:05.590 13209-13209/com.hfy.demo01 I/hfy: testAsyncTask onPostExecute: 我是結果。參數是100
1.1.2 原理分析:
先看構造方法
public AsyncTask() {
this((Looper) null);
}
public AsyncTask(@Nullable Handler handler) {
this(handler != null ? handler.getLooper() : null);
}
/**
* Creates a new asynchronous task. This constructor must be invoked on the UI thread.
*/
public AsyncTask(@Nullable Looper callbackLooper) {
mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
? getMainHandler()
: new Handler(callbackLooper);
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 {
//執行完,發出結果
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 static Handler getMainHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
//這裏傳入的是主線程的looper,所以用來把消息切到主線程
sHandler = new InternalHandler(Looper.getMainLooper());
}
return sHandler;
}
}
看到,首先使用主線程的Looper創建了InternalHandler實例。然後創建了WorkerRunnable的實例mWorker,call方法中看到調用了 doInBackground(mParams),可以猜想call方法是執行在線程池的。然後創建了FutureTask的實例mFuture並傳入了mWorker,mFuture怎麼使用的呢?後面會分析道。我們可以先看下Handler的實現InternalHandler:
private static class InternalHandler extends Handler {
public InternalHandler(Looper looper) {
super(looper);
}
@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;
}
}
}
看到有處理髮送結果、處理髮送進度的消息。消息從哪發來的呢?先留個疑問。繼續看AsyncTask的execute方法:
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
/**
* 串行 執行器
*/
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
@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;
}
execute方法走到了executeOnExecutor方法,先進行當前任務狀態的判斷,默認是準備執行任務的PENDING狀態,然後變爲RUNNING。但如果正在執行的RUNNING、執行完的FINISHED都會拋出異常。這也是一個任務實例只能執行一次的原因。然後又走到了onPreExecute(),因爲execute執行在UI線程 所以也解釋了其是執行在UI線程的原因。接着把參數賦值給mWorker,mFuture作爲參數執行 靜態的sDefaultExecutor的execute()方法。注意到sDefaultExecutor是SerialExecutor實例,去瞅瞅:
//線程池
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;
}
private static class SerialExecutor implements Executor {
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive;
//execute方法加了鎖
public synchronized void execute(final Runnable r) {
//把r存入到任務隊列的隊尾
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
//任務執行完,就執行下一個
scheduleNext();
}
}
});
//把r存入任務隊列後,然後當前沒有取出的任務,就 取 隊列頭部 的任務執行
if (mActive == null) {
scheduleNext();
}
}
protected synchronized void scheduleNext() {
//取 隊列頭部 的任務執行
if ((mActive = mTasks.poll()) != null) {
//THREAD_POOL_EXECUTOR是線程池
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
上面都有註釋,可見SerialExecutor就是串行執行器,最終執行在THREAD_POOL_EXECUTOR的線程池中。r.run()實際走的是FutureTask的run方法:
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
public void run() {
if (state != NEW ||
!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
//callable的call方法
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
FutureTask的run方法中調用的傳入的callable的call()方法,再結合上面AsyncTask的構造方法,mWorker就是實現callable的call()方法。所以裏面的doInBackground方法就會串行的執行在線程池中。因爲串行,那使用execute方法不能執行特別耗時的任務,否則會阻塞後面等待的任務。若想要並行,可採用AsyncTask的executeOnExecutor方法,傳入線程池THREAD_POOL_EXECUTOR即可。
還注意到,doInBackground執行完後調用了postResult(result),result就是doInBackground返回值:
private Handler getHandler() {
return mHandler;
}
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
看到,使用handler發送消息,消息類型就是MESSAGE_POST_RESULT,前面看到InternalHandler內部handleMessage是有處理的,就是調用task的finish方法:
private void finish(Result result) {
if (isCancelled()) {
//如果任務取消了,回調onCancelled
onCancelled(result);
} else {
//沒有取消
onPostExecute(result);
}
//修改任務狀態爲完成
mStatus = Status.FINISHED;
}
可見如果沒有調用cancel(),就會走onPostExecute,所以onPostExecute也是執行在UI線程的。
最後看下publishProgress方法:
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}
如果沒有取消任務,也是用handler,消息類型就是MESSAGE_POST_PROGRESS,前面看到InternalHandler內部handleMessage是有處理的,最後在UI線程執行onProgressUpdate方法。
舉兩個例子🌰
例子1,默認的串行執行:
new AsyncTask<Void, Void, Void>() {
@Override
protected Void doInBackground(Void... voids) {
Log.i(TAG, "task1 SERIAL_EXECUTOR doInBackground: ");
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return null;
}
}.execute();
new AsyncTask<Void, Void, Void>() {
@Override
protected Void doInBackground(Void... voids) {
Log.i(TAG, "task2 SERIAL_EXECUTOR doInBackground: ");
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return null;
}
}.execute();
new AsyncTask<Void, Void, Void>() {
@Override
protected Void doInBackground(Void... voids) {
Log.i(TAG, "task3 SERIAL_EXECUTOR doInBackground: ");
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return null;
}
}.execute();
執行結果如下,每隔兩秒打印一次,可見是串行執行。
2020-01-16 14:51:40.836 13346-13599/com.hfy.demo01 I/hfy: task1 SERIAL_EXECUTOR doInBackground:
2020-01-16 14:51:42.876 13346-13598/com.hfy.demo01 I/hfy: task2 SERIAL_EXECUTOR doInBackground:
2020-01-16 14:51:44.915 13346-13599/com.hfy.demo01 I/hfy: task3 SERIAL_EXECUTOR doInBackground:
例子2,並行執行-直接使用線程池:
new AsyncTask<Void, Void, Void>() {
@Override
protected Void doInBackground(Void... voids) {
Log.i(TAG, "task1 THREAD_POOL_EXECUTOR doInBackground: ");
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return null;
}
}.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
new AsyncTask<Void, Void, Void>() {
@Override
protected Void doInBackground(Void... voids) {
Log.i(TAG, "task2 THREAD_POOL_EXECUTOR doInBackground: ");
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return null;
}
}.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
new AsyncTask<Void, Void, Void>() {
@Override
protected Void doInBackground(Void... voids) {
Log.i(TAG, "task3 THREAD_POOL_EXECUTOR doInBackground: ");
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return null;
}
}.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
執行結果如下,同時打印出來,可見是並行執行。
2020-01-16 14:51:38.772 13346-13599/com.hfy.demo01 I/hfy: task1 THREAD_POOL_EXECUTOR doInBackground:
2020-01-16 14:51:38.773 13346-13600/com.hfy.demo01 I/hfy: task2 THREAD_POOL_EXECUTOR doInBackground:
2020-01-16 14:51:38.774 13346-13601/com.hfy.demo01 I/hfy: task3 THREAD_POOL_EXECUTOR doInBackground:
總結一下,AsyncTask內部默認 使用串行執行器 串行地 在線程池 執行任務,我們也可以使用executeOnExecutor直接使用線程池並行執行。內部使用handler把進度和結果從線程池切換到UI線程。
1.2 HandlerThread
HandlerThread繼承自Thread,內部已準備了Looper並已開啓循環。所以可以在UI線程使用handler發送任務到HandlerThread中執行,且可以隨意多次發送任務。(而普通thread執行完run方法中的耗時操作就結束了。)當不使用時 如onDestroy中,使用quit()或quitSafely()退出即可。
public class HandlerThread extends Thread {
int mPriority;
int mTid = -1;
Looper mLooper;
private @Nullable Handler mHandler;
public HandlerThread(String name) {
super(name);
mPriority = Process.THREAD_PRIORITY_DEFAULT;
}
public HandlerThread(String name, int priority) {
super(name);
mPriority = priority;
}
//looper開啓之前可以做一些事情
protected void onLooperPrepared() {
}
@Override
public void run() {
mTid = Process.myTid();
//給當前線程準備Looper實例
Looper.prepare();
//加鎖,保證能獲取到looer實例
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
//開啓循環
Looper.loop();
mTid = -1;
}
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 {
//獲取不到就等 run中的notifyAll()被調用
wait();
} catch (InterruptedException e) {
}
}
}
return mLooper;
}
//直接退出
public boolean quit() {
Looper looper = getLooper();
if (looper != null) {
looper.quit();
return true;
}
return false;
}
//安全退出(執行完正在正在執行的任務再退出)
public boolean quitSafely() {
Looper looper = getLooper();
if (looper != null) {
looper.quitSafely();
return true;
}
return false;
}
public int getThreadId() {
return mTid;
}
}
舉個例子🌰
private void testHandlerThread() {
HandlerThread handlerThread = new HandlerThread("HandlerThreadName");
handlerThread.start();
Handler handler = new Handler(handlerThread.getLooper()) {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case 1000:
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
Log.i(TAG, "handleMessage: thread name="+Thread.currentThread().getName()+",what="+msg.what);
break;
case 1001:
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
Log.i(TAG, "handleMessage: thread name="+Thread.currentThread().getName()+",what="+msg.what);
break;
default:
break;
}
}
};
Log.i(TAG, "sendMessage thread name="+Thread.currentThread().getName());
handler.sendMessage(Message.obtain(handler, 1000));
handler.sendMessage(Message.obtain(handler, 1001));
}
可見在主線程發送了兩個任務,順序執行在HandlerThread了。
2020-01-16 17:12:46.832 16293-16293/com.hfy.demo01 I/hfy: sendMessage thread name=main
2020-01-16 17:12:48.833 16293-17187/com.hfy.demo01 I/hfy: handleMessage: thread name=HandlerThreadName,what=1000
2020-01-16 17:12:49.834 16293-17187/com.hfy.demo01 I/hfy: handleMessage: thread name=HandlerThreadName,what=1001
1.3 IntentService
IntentService是繼承自Service的抽象類,可執行後臺耗時任務,執行完後會自動停止。
因爲是Service,即是Android的組件,優先級比單純的線程高,不容易被系統殺死,所以可用來執行優先級高的後臺任務,
public abstract class IntentService extends Service {
private volatile Looper mServiceLooper;
private volatile ServiceHandler mServiceHandler;
private String mName;
private boolean mRedelivery;
//內部類handler
private final class ServiceHandler extends Handler {
public ServiceHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
//先回調出去,待IntentService子類覆寫自己的邏輯
onHandleIntent((Intent)msg.obj);
//結束service自己,msg.arg1是標記某次startService的Id。
//但如果此時外部又調用了startService,那麼最新的請求id就不是msg.arg1了,所以下面這句就不會結束service。
stopSelf(msg.arg1);
}
}
//name是線程名
public IntentService(String name) {
super();
mName = name;
}
...
@Override
public void onCreate() {
super.onCreate();
//創建HandlerThread實例 並啓動
HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
thread.start();
//創建對應looper的handler,所以mServiceHandler的handleMessage執行在 線程中
mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper);
}
@Override
public void onStart(@Nullable Intent intent, int startId) {
//發送消息,參數是startId、intent
//每次startService() 都會走這裏
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.obj = intent;
mServiceHandler.sendMessage(msg);
}
@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() {
//退出looper循環
mServiceLooper.quit();
}
...
//執行子在線程,同時只存在一個intent(因爲looper的隊列),所以如果此方法執行時間過長,會阻塞其他請求,所有請求執行完,service會自動停止,所以不能手動調用stopSelf。
@WorkerThread
protected abstract void onHandleIntent(@Nullable Intent intent);
}
onCreate中創建了HandlerThread實例,對應的Handler實例mServiceHandler,所以mServiceHandler發送的任務都會在線程中執行。
onStartCommand中調用的是onStart,onStart中確實使用mServiceHandler發送消息,攜帶的參數是startId、intent,startId是每次啓動service的標記,intent就是啓動service的intent。
onDestroy中退出looper循環。
發送的消息在哪處理的呢?
那就看ServiceHandler,其繼承自Handler,handleMessage方法中先調用了抽象方法onHandleIntent((Intent)msg.obj),參數就是啓動service的intent。所以IntentService 的子類必須要重寫onHandleIntent,並處理這個intent。因爲mServiceHandler拿到的HandlerThread的looper,所以這個onHandleIntent()就是執行在子線程中的。
接着調用了stopSelf(msg.arg1),msg.arg1)就是前面說的啓動service的標記。對stopSelf(int startId)說明如下:
startId:代表啓動服務的次數,由系統生成。
stopSelf(int startId):在其參數startId跟 最後啓動該service時生成的ID相等時纔會執行停止服務。stopSelf():直接停止服務。
使用場景: 如果同時有多個服務啓動請求發送到onStartCommand(),不應該在處理完一個請求後調用stopSelf();因爲在調用此函數銷燬service之前,可能service又接收到新的啓動請求,如果此時service被銷燬,新的請求將得不到處理。此情況應該調用stopSelf(int startId)。
所以,當多次啓動service,就會多次調用 onStart,那麼會有多個任務發出,當每次任務執行完onHandleIntent時,stopSelf(int startId)中會判斷,若又啓動service那麼就不會停止。那麼looper繼續取下個消息繼續處理。直到stopSelf中的startId和最新啓動的startId相同,就會停止。因爲是looper,所以這些任務都是按啓動service的順序執行的。
舉個例子🌰
private void testIntentService() {
Intent intent= new Intent(this, MyIntentService.class);
intent.putExtra("task_name","task1");
startService(intent);
intent.putExtra("task_name","task2");
startService(intent);
intent.putExtra("task_name","task3");
startService(intent);
}
public static class MyIntentService extends IntentService {
public MyIntentService() {
super("MyIntentServiceThread");
}
@Override
protected void onHandleIntent(Intent intent) {
Log.i(TAG, "MyIntentService onHandleIntent: begin."+intent.getStringExtra("task_name"));
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
Log.i(TAG, "MyIntentService onHandleIntent: done."+intent.getStringExtra("task_name"));
}
@Override
public void onDestroy() {
Log.i(TAG, "MyIntentService onDestroy: ");
super.onDestroy();
}
}
2020-01-17 09:58:44.639 11117-11236/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: begin.task1
2020-01-17 09:58:46.640 11117-11236/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: done.task1
2020-01-17 09:58:46.641 11117-11236/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: begin.task2
2020-01-17 09:58:48.642 11117-11236/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: done.task2
2020-01-17 09:58:48.644 11117-11236/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: begin.task3
2020-01-17 09:58:50.645 11117-11236/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: done.task3
2020-01-17 09:58:50.650 11117-11117/com.hfy.demo01 I/hfy: MyIntentService onDestroy:
靜態內部類MyIntentService繼承IntentService並重寫了onHandleIntent,就是睡兩秒。然後不間斷連續啓動3次,由日誌可見是順序執行的,最後都執行完才走到onDestroy。
再看,如果是間隔三秒發送呢:
private void testIntentService() {
Log.i(TAG, "testIntentService: task1");
Intent intent= new Intent(this, MyIntentService.class);
intent.putExtra("task_name","task1");
startService(intent);
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
Log.i(TAG, "testIntentService: task2");
intent.putExtra("task_name","task2");
startService(intent);
}
}, 3000);
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
Log.i(TAG, "testIntentService: task3");
intent.putExtra("task_name","task3");
startService(intent);
}
}, 3000);
}
2020-01-17 10:16:29.335 14739-14739/com.hfy.demo01 I/hfy: testIntentService: task1
2020-01-17 10:16:29.698 14739-14843/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: begin.task1
2020-01-17 10:16:31.698 14739-14843/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: done.task1
2020-01-17 10:16:31.701 14739-14739/com.hfy.demo01 I/hfy: MyIntentService onDestroy:
2020-01-17 10:16:32.371 14739-14739/com.hfy.demo01 I/hfy: testIntentService: task2
2020-01-17 10:16:32.390 14739-14862/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: begin.task2
2020-01-17 10:16:34.391 14739-14862/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: done.task2
2020-01-17 10:16:34.451 14739-14739/com.hfy.demo01 I/hfy: MyIntentService onDestroy:
2020-01-17 10:16:35.339 14739-14739/com.hfy.demo01 I/hfy: testIntentService: task3
2020-01-17 10:16:35.364 14739-14873/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: begin.task3
2020-01-17 10:16:37.364 14739-14873/com.hfy.demo01 I/hfy: MyIntentService onHandleIntent: done.task3
2020-01-17 10:16:37.367 14739-14739/com.hfy.demo01 I/hfy: MyIntentService onDestroy:
可見每個任務執行完 就走onDestroy了。這是因爲當一個任務執行完,走到stopSelf(int startId)時,後面還沒有再次開啓service,所以此時的stopSelf中的startId就是最新的,所以就會停止服務了。
二、Android中的線程池
線程池優點如下:
- 能夠重用線程池中的線程,避免線程的創建、銷燬帶來的性能開銷。
- 能有效控制線程池中的最大併發數,避免大量的線程之間因互相搶佔系統資源而導致的阻塞現象。
- 能對線程進行簡單管理,並提供定時執行、指定間隔循環執行的功能。
Android中的線程池來源於Java的Executor,正在的實現是ThreadPoolExecutor。
2.1 ThreadPoolExecutor
ThreadPoolExecutor是線程池的真正實現,看下其構造方法裏的參數,參數會影響線程池的功能特性。
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
threadFactory, defaultHandler);
}
- corePoolSize,核心線程數,默認一般核心線程會在線程池中一直存活,即使處理空閒狀態。但當allowCoreThreadTimeOut設置爲true,那麼核心線程就會有閒置超時時間,閒置超過超時時間就會終止。超時時間由keepAliveTime和unit指定。
- maximumPoolSize,最大線程數,當活動線程到達這個數,後續的新任務會被阻塞。
- keepAliveTime,非核心線程閒置時 的超時時長。非核心線程閒置時間超過此時間就會被回收。當allowCoreThreadTimeOut設置爲true,keepAliveTime也會作用於核心線程。
- unit,是keepAliveTime的時間單位,取值是枚舉,有TimeUnit.MINUTES、TimeUnit.SECONDS、TimeUnit.MILLISECONDS等。
- workQueue,線程池中的任務隊列,通過線程池的execute方法提交的Runnable會存在這個隊列中。
- threadFactory,線程工廠,爲線程池提供創建新線程的能力。
還有個不常用參數RejectedExecutionHandler handler,調用其rejectedExecution()方法來處理 當任務隊列已滿 時 不能執行新任務的情況。handler的默認實現是AbortPolicy,rejectedExecution()中會直接拋出異常RejectedExecutionException。其他實現如DiscardPolicy的rejectedExecution()中是不做任何事。還有CallerRunsPolicy、DiscardOldestPolicy。
ThreadPoolExecutor執行任務的執行規則 如下:
- 如果線程池中的線程數未到達核心線程數,那麼會直接啓動一個核心線程來執行這個任務。(不管已啓動的核心線程是否空閒)
- 如果線程池中的線程數已到達或超過核心線程數,那麼任務會插入到任務隊列中排隊等待執行。
- 如果2中 任務無法插入到隊列中,一般是對隊列已滿,若此時未達到最大線程數,就會啓動非核心線程執行這個任務。
- 如果3中線程數達到最大線程數,那麼會拒絕執行任務,即會調用RejectedExecutionHandler的rejectedExecution()通知調用者。
我們看下AsyncTask中線程池的配置:
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;
}
可見THREAD_POOL_EXECUTOR這個線程池的配置如下:
- 核心線程數,2-4個
- 最大線程數,CPU核心數量 * 2 + 1
- 超時時間30s,允許核心線程超時
- 隊列容量128
2.2 線程池的分類
Android中常見的4類線程池,都是直接或間接配置ThreadPoolExecutor實現自己特性的,它們是FixedThreadPool、CachedThreadPool、ScheduledThreadPool、SingleThreadExecutor。它們都可以通過工具類Executors獲取。
2.2.1 FixedThreadPool
通過Executors的newFixedThreadPool方法獲得。固定的核心線程數量,沒有非核心線程,空閒時不會被回收,隊列長度無限制。因爲不會被回收,所以能快速執行外界請求。
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
2.2.2 CachedThreadPool
通過Executors的newCachedThreadPool方法獲得。核心線程數0,非核心線程數無限制,空閒回收超時時間60s,隊列不能插入任務。當所有線程都處於活動狀態,就會創建新線程處理任務,否則利用空閒線程處理任務。當整個線程池空閒時 所有線程都會被回收,不佔用系統資源。因此,適合執行大量耗時較少的任務
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
2.2.3 ScheduledThreadPool
通過Executors的newScheduledThreadPool方法獲得。固定核心線程數,不限制非核心線程數,非核心線程閒置回收超時時間是10ms。一般用於執行定時任務、固定週期的重複任務。
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
private static final long DEFAULT_KEEPALIVE_MILLIS = 10L;
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE,
DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
new DelayedWorkQueue());
}
2.2.4 SingleThreadExecutor
通過Executors的newSingleThreadExecutor方法獲得。僅有1個核心線程,不會回收。可確保所有任務按順序執行,不用處理線程同步的問題。
private void testThreadPoolExecutor() {
Runnable runnable = new Runnable() {
@Override
public void run() {
try {
Log.i(TAG, "testThreadPoolExecutor: run begin");
Thread.sleep(4000);
Log.i(TAG, "testThreadPoolExecutor: run end");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
ExecutorService fixedThreadPool = Executors.newFixedThreadPool(4);
fixedThreadPool.execute(runnable);
ExecutorService cachedThreadPool = Executors.newCachedThreadPool();
cachedThreadPool.execute(runnable);
ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(4);
scheduledThreadPool.execute(runnable);
//延遲2秒執行
scheduledThreadPool.schedule(runnable, 2, TimeUnit.SECONDS);
//延遲2秒執行,然後以每次 任務開始的時間計時, 1秒後,如果任務是結束的 就立刻執行下一次;如果沒有結束,就等它結束後立即執行下一次。
scheduledThreadPool.scheduleAtFixedRate(runnable, 0, 1, TimeUnit.SECONDS);
//延遲3秒執行,然後以每次任務執行完後的時間計時, 2秒後,執行下一次~
scheduledThreadPool.scheduleWithFixedDelay(runnable,1,2,TimeUnit.SECONDS);
ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor();
singleThreadExecutor.execute(runnable);
}
注意點:scheduledThreadPool使用時注意區分scheduleAtFixedRate、scheduleWithFixedDelay的循環任務的邏輯區別:
- scheduleAtFixedRate,以每次 任務開始的時間計時, period時間後,如果任務是結束的就立刻執行下一次;如果沒有結束,就等它結束後立即執行下一次。
- scheduleWithFixedDelay,以每次任務執行完後的時間計時,period時間後,執行下一次。
並且,這兩個方法都是在任務結束後才執行下一次,那麼如果某個任務發生異無法執行完,那麼整個循環任務就會失效。所以需要給任務添加超時機制(比如給任務加上try-catch-finally,catch住超時異常) 保證任務即使發生異常也可以結束,就可保證循環正常執行了。