Android的线程和线程池--《Android开发艺术探索》阅读笔记——第十一章

文章目录

  • 一、Android中的线程形态
    • 1.1 AsyncTask
      • 1.1.1 使用方法
      • 1.1.2 原理分析:
    • 1.2 HandlerThread
    • 1.3 IntentService
  • 二、Android中的线程池
    • 2.1 ThreadPoolExecutor
    • 2.2 线程池的分类
      • 2.2.1 FixedThreadPool
      • 2.2.2 CachedThreadPool
      • 2.2.3 ScheduledThreadPool
      • 2.2.4 SingleThreadExecutor

除了Thread,Android中扮演线程的角色还有:AsyncTask、HandlerThread、IntentService。

  • AsyncTask:内部封装线程池、handler,便于在子线程中更新UI。
  • HandlerThread:可以使用消息循环的线程,在它内部可以使用Handler。
  • IntentService:内部使用HandlerThread执行任务,完毕后会自动退出。(相比后台线程)因是组件,优先级高,不易被杀死。

线程是操作系统调度的最小单元,是一种受限的资源,不可能无限制的产生。且线程的创建和销毁需要相应的开销。且存在大量线程时,系统会通过时间片轮转的方式调度线程,因此线程不可能做到并行,除非线程数小于等于cpu数。所以需要 线程池,它可以缓存一定数量的线程,避免频繁地线程创建和销毁带来的系统开销。

一、Android中的线程形态

1.1 AsyncTask

AsyncTask是用来在线程池中处理异步任务,并可以把处理进度和结果发送到UI线程。

1.1.1 使用方法

AsyncTask的基本使用方法,示例如下:

    private void testAsyncTask() {
        //一般要在主线程实例化。(实际在9.0上 子线程创建实例然后主线程execute没问题)
        //三个泛型参数依次表示参数类型、进度类型、结果类型。
        //覆写的这几个方法不可以直接调用
        AsyncTask<Integer, Integer, String> task = new AsyncTask<Integer, Integer, String>() {

            @Override
            protected void onPreExecute() {
                super.onPreExecute();
                //主线程执行,在异步任务之前
                Log.i(TAG, "testAsyncTask onPreExecute: ");
            }

            @Override
            protected String doInBackground(Integer... integers) {
                Log.i(TAG, "testAsyncTask doInBackground: ");
                //任务在 线程池中执行 耗时操作
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                //发出进度
                publishProgress(50);
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                //再发出进度
                publishProgress(100);

                return "我是结果。参数是" + integers[0];
            }

            @Override
            protected void onPostExecute(String s) {
                super.onPostExecute(s);
                //在主线程执行,在异步任务执行完之后
                Log.i(TAG, "testAsyncTask onPostExecute: "+s);
            }

            @Override
            protected void onProgressUpdate(Integer... values) {
                super.onProgressUpdate(values);
                //执行在主线程,调用publishProgress()后就会执行
                Log.i(TAG, "testAsyncTask onProgressUpdate: 进度:"+values[0]+"%");
            }

            @Override
            protected void onCancelled() {
                super.onCancelled();
                //取消任务
            }
        };
        //必须要在主线程执行execute,且只能执行一次
        task.execute(100);
    }

执行结果日志如下:

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: 进度:50%
2020-01-14 11:29:05.589 13209-13209/com.hfy.demo01 I/hfy: testAsyncTask onProgressUpdate: 进度:100%
2020-01-14 11:29:05.590 13209-13209/com.hfy.demo01 I/hfy: testAsyncTask onPostExecute: 我是结果。参数是100

1.1.2 原理分析:

先看构造方法

    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 {
                	//执行完,发出结果
                    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) {
            	//这里传入的是主线程的looper,所以用来把消息切到主线程
                sHandler = new InternalHandler(Looper.getMainLooper());
            }
            return sHandler;
        }
    }

看到,首先使用主线程的Looper创建了InternalHandler实例。然后创建了WorkerRunnable的实例mWorker,call方法中看到调用了 doInBackground(mParams),可以猜想call方法是执行在线程池的。然后创建了FutureTask的实例mFuture并传入了mWorker,mFuture怎么使用的呢?后面会分析道。我们可以先看下Handler的实现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;
            }
        }
    }

看到有处理发送结果、处理发送进度的消息。消息从哪发来的呢?先留个疑问。继续看AsyncTask的execute方法:

    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
    /**
     *  串行 执行器
     */
    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,先进行当前任务状态的判断,默认是准备执行任务的PENDING状态,然后变为RUNNING。但如果正在执行的RUNNING、执行完的FINISHED都会抛出异常。这也是一个任务实例只能执行一次的原因。然后又走到了onPreExecute(),因为execute执行在UI线程 所以也解释了其是执行在UI线程的原因。接着把参数赋值给mWorker,mFuture作为参数执行 静态的sDefaultExecutor的execute()方法。注意到sDefaultExecutor是SerialExecutor实例,去瞅瞅:

	//线程池
    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方法加了锁
        public synchronized void execute(final Runnable r) {
        	//把r存入到任务队列的队尾
            mTasks.offer(new Runnable() {
                public void run() {
                    try {
                        r.run();
                    } finally {
                    	//任务执行完,就执行下一个
                        scheduleNext();
                    }
                }
            });
            //把r存入任务队列后,然后当前没有取出的任务,就 取 队列头部 的任务执行
            if (mActive == null) {
                scheduleNext();
            }
        }

        protected synchronized void scheduleNext() {
        	//取 队列头部 的任务执行
            if ((mActive = mTasks.poll()) != null) {
            	//THREAD_POOL_EXECUTOR是线程池
                THREAD_POOL_EXECUTOR.execute(mActive);
            }
        }
    }

上面都有注释,可见SerialExecutor就是串行执行器,最终执行在THREAD_POOL_EXECUTOR的线程池中。r.run()实际走的是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方法中调用的传入的callable的call()方法,再结合上面AsyncTask的构造方法,mWorker就是实现callable的call()方法。所以里面的doInBackground方法就会串行的执行在线程池中。因为串行,那使用execute方法不能执行特别耗时的任务,否则会阻塞后面等待的任务。若想要并行,可采用AsyncTask的executeOnExecutor方法,传入线程池THREAD_POOL_EXECUTOR即可

还注意到,doInBackground执行完后调用了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发送消息,消息类型就是MESSAGE_POST_RESULT,前面看到InternalHandler内部handleMessage是有处理的,就是调用task的finish方法:

    private void finish(Result result) {
        if (isCancelled()) {
        	//如果任务取消了,回调onCancelled
            onCancelled(result);
        } else {
        	//没有取消
            onPostExecute(result);
        }
        //修改任务状态为完成
        mStatus = Status.FINISHED;
    }

可见如果没有调用cancel(),就会走onPostExecute,所以onPostExecute也是执行在UI线程的

最后看下publishProgress方法:

    protected final void publishProgress(Progress... values) {
        if (!isCancelled()) {
            getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();
        }
    }

如果没有取消任务,也是用handler,消息类型就是MESSAGE_POST_PROGRESS,前面看到InternalHandler内部handleMessage是有处理的,最后在UI线程执行onProgressUpdate方法。

举两个例子
例子1,默认的串行执行:

        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();

执行结果如下,每隔两秒打印一次,可见是串行执行。

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,并行执行-直接使用线程池:

        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);

执行结果如下,同时打印出来,可见是并行执行。

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: 

总结一下,AsyncTask内部默认 使用串行执行器 串行地 在线程池 执行任务,我们也可以使用executeOnExecutor直接使用线程池并行执行。内部使用handler把进度和结果从线程池切换到UI线程。

1.2 HandlerThread

HandlerThread继承自Thread,内部已准备了Looper并已开启循环。所以可以在UI线程使用handler发送任务到HandlerThread中执行,且可以随意多次发送任务。(而普通thread执行完run方法中的耗时操作就结束了。)当不使用时 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开启之前可以做一些事情
    protected void onLooperPrepared() {
    }

    @Override
    public void run() {
        mTid = Process.myTid();
        //给当前线程准备Looper实例
        Looper.prepare();
        //加锁,保证能获取到looer实例
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        //开启循环
        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 {
                	//获取不到就等 run中的notifyAll()被调用
                    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(执行完正在正在执行的任务再退出)
    public boolean quitSafely() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quitSafely();
            return true;
        }
        return false;
    }

    public int getThreadId() {
        return mTid;
    }
}

举个例子

    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));
    }

可见在主线程发送了两个任务,顺序执行在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是继承自Service的抽象类,可执行后台耗时任务,执行完后会自动停止。
因为是Service,即是Android的组件,优先级比单纯的线程高,不容易被系统杀死,所以可用来执行优先级高的后台任务,

public abstract class IntentService extends Service {
    private volatile Looper mServiceLooper;
    private volatile ServiceHandler mServiceHandler;
    private String mName;
    private boolean mRedelivery;
	
	//内部类handler
    private final class ServiceHandler extends Handler {
        public ServiceHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message msg) {
        	//先回调出去,待IntentService子类覆写自己的逻辑
            onHandleIntent((Intent)msg.obj);
            //结束service自己,msg.arg1是标记某次startService的Id。
            //但如果此时外部又调用了startService,那么最新的请求id就不是msg.arg1了,所以下面这句就不会结束service。
            stopSelf(msg.arg1);
        }
    }
	//name是线程名
    public IntentService(String name) {
        super();
        mName = name;
    }
    ...
    @Override
    public void onCreate() {
        super.onCreate();
        //创建HandlerThread实例 并启动
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();
		//创建对应looper的handler,所以mServiceHandler的handleMessage执行在 线程中
        mServiceLooper = thread.getLooper();
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }

    @Override
    public void onStart(@Nullable Intent intent, int startId) {
    	//发送消息,参数是startId、intent
    	//每次startService() 都会走这里
        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循环
        mServiceLooper.quit();
    }

    ...

     //执行子在线程,同时只存在一个intent(因为looper的队列),所以如果此方法执行时间过长,会阻塞其他请求,所有请求执行完,service会自动停止,所以不能手动调用stopSelf。
    @WorkerThread
    protected abstract void onHandleIntent(@Nullable Intent intent);
}

onCreate中创建了HandlerThread实例,对应的Handler实例mServiceHandler,所以mServiceHandler发送的任务都会在线程中执行。
onStartCommand中调用的是onStart,onStart中确实使用mServiceHandler发送消息,携带的参数是startId、intent,startId是每次启动service的标记,intent就是启动service的intent。
onDestroy中退出looper循环。

发送的消息在哪处理的呢?
那就看ServiceHandler,其继承自Handler,handleMessage方法中先调用了抽象方法onHandleIntent((Intent)msg.obj),参数就是启动service的intent。所以IntentService 的子类必须要重写onHandleIntent,并处理这个intent。因为mServiceHandler拿到的HandlerThread的looper,所以这个onHandleIntent()就是执行在子线程中的。
接着调用了stopSelf(msg.arg1),msg.arg1)就是前面说的启动service的标记。对stopSelf(int startId)说明如下:

startId:代表启动服务的次数,由系统生成。
stopSelf(int startId):在其参数startId跟 最后启动该service时生成的ID相等时才会执行停止服务。

stopSelf():直接停止服务。

使用场景: 如果同时有多个服务启动请求发送到onStartCommand(),不应该在处理完一个请求后调用stopSelf();因为在调用此函数销毁service之前,可能service又接收到新的启动请求,如果此时service被销毁,新的请求将得不到处理。此情况应该调用stopSelf(int startId)。

所以,当多次启动service,就会多次调用 onStart,那么会有多个任务发出,当每次任务执行完onHandleIntent时,stopSelf(int startId)中会判断,若又启动service那么就不会停止。那么looper继续取下个消息继续处理。直到stopSelf中的startId和最新启动的startId相同,就会停止。因为是looper,所以这些任务都是按启动service的顺序执行的。

举个例子

    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:

静态内部类MyIntentService继承IntentService并重写了onHandleIntent,就是睡两秒。然后不间断连续启动3次,由日志可见是顺序执行的,最后都执行完才走到onDestroy。

再看,如果是间隔三秒发送呢:

        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:

可见每个任务执行完 就走onDestroy了。这是因为当一个任务执行完,走到stopSelf(int startId)时,后面还没有再次开启service,所以此时的stopSelf中的startId就是最新的,所以就会停止服务了。

二、Android中的线程池

线程池优点如下:

  • 能够重用线程池中的线程,避免线程的创建、销毁带来的性能开销。
  • 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互相抢占系统资源而导致的阻塞现象。
  • 能对线程进行简单管理,并提供定时执行、指定间隔循环执行的功能。
    Android中的线程池来源于Java的Executor,正在的实现是ThreadPoolExecutor。

2.1 ThreadPoolExecutor

ThreadPoolExecutor是线程池的真正实现,看下其构造方法里的参数,参数会影响线程池的功能特性。

    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             threadFactory, defaultHandler);
    }
  • corePoolSize,核心线程数,默认一般核心线程会在线程池中一直存活,即使处理空闲状态。但当allowCoreThreadTimeOut设置为true,那么核心线程就会有闲置超时时间,闲置超过超时时间就会终止。超时时间由keepAliveTime和unit指定。
  • maximumPoolSize,最大线程数,当活动线程到达这个数,后续的新任务会被阻塞。
  • keepAliveTime,非核心线程闲置时 的超时时长。非核心线程闲置时间超过此时间就会被回收。当allowCoreThreadTimeOut设置为true,keepAliveTime也会作用于核心线程。
  • unit,是keepAliveTime的时间单位,取值是枚举,有TimeUnit.MINUTES、TimeUnit.SECONDS、TimeUnit.MILLISECONDS等。
  • workQueue,线程池中的任务队列,通过线程池的execute方法提交的Runnable会存在这个队列中。
  • threadFactory,线程工厂,为线程池提供创建新线程的能力。

还有个不常用参数RejectedExecutionHandler handler,调用其rejectedExecution()方法来处理 当任务队列已满 时 不能执行新任务的情况。handler的默认实现是AbortPolicy,rejectedExecution()中会直接抛出异常RejectedExecutionException。其他实现如DiscardPolicy的rejectedExecution()中是不做任何事。还有CallerRunsPolicy、DiscardOldestPolicy。

ThreadPoolExecutor执行任务的执行规则 如下:

  • 如果线程池中的线程数未到达核心线程数,那么会直接启动一个核心线程来执行这个任务。(不管已启动的核心线程是否空闲)
  • 如果线程池中的线程数已到达或超过核心线程数,那么任务会插入到任务队列中排队等待执行。
  • 如果2中 任务无法插入到队列中,一般是对队列已满,若此时未达到最大线程数,就会启动非核心线程执行这个任务。
  • 如果3中线程数达到最大线程数,那么会拒绝执行任务,即会调用RejectedExecutionHandler的rejectedExecution()通知调用者。

我们看下AsyncTask中线程池的配置:

    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;
    }

可见THREAD_POOL_EXECUTOR这个线程池的配置如下:

  • 核心线程数,2-4个
  • 最大线程数,CPU核心数量 * 2 + 1
  • 超时时间30s,允许核心线程超时
  • 队列容量128

2.2 线程池的分类

Android中常见的4类线程池,都是直接或间接配置ThreadPoolExecutor实现自己特性的,它们是FixedThreadPool、CachedThreadPool、ScheduledThreadPool、SingleThreadExecutor。它们都可以通过工具类Executors获取。

2.2.1 FixedThreadPool

通过Executors的newFixedThreadPool方法获得。固定的核心线程数量,没有非核心线程,空闲时不会被回收,队列长度无限制。因为不会被回收,所以能快速执行外界请求

    public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }

2.2.2 CachedThreadPool

通过Executors的newCachedThreadPool方法获得。核心线程数0,非核心线程数无限制,空闲回收超时时间60s,队列不能插入任务。当所有线程都处于活动状态,就会创建新线程处理任务,否则利用空闲线程处理任务。当整个线程池空闲时 所有线程都会被回收,不占用系统资源。因此,适合执行大量耗时较少的任务

    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

2.2.3 ScheduledThreadPool

通过Executors的newScheduledThreadPool方法获得。固定核心线程数,不限制非核心线程数,非核心线程闲置回收超时时间是10ms。一般用于执行定时任务、固定周期的重复任务

    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

通过Executors的newSingleThreadExecutor方法获得。仅有1个核心线程,不会回收。可确保所有任务按顺序执行,不用处理线程同步的问题。

    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);
        //延迟2秒执行
        scheduledThreadPool.schedule(runnable, 2, TimeUnit.SECONDS);
        //延迟2秒执行,然后以每次 任务开始的时间计时, 1秒后,如果任务是结束的 就立刻执行下一次;如果没有结束,就等它结束后立即执行下一次。
        scheduledThreadPool.scheduleAtFixedRate(runnable, 0, 1, TimeUnit.SECONDS);
        //延迟3秒执行,然后以每次任务执行完后的时间计时, 2秒后,执行下一次~
        scheduledThreadPool.scheduleWithFixedDelay(runnable,1,2,TimeUnit.SECONDS);

        ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor();
        singleThreadExecutor.execute(runnable);
    }

注意点:scheduledThreadPool使用时注意区分scheduleAtFixedRate、scheduleWithFixedDelay的循环任务的逻辑区别

  • scheduleAtFixedRate,以每次 任务开始的时间计时, period时间后,如果任务是结束的就立刻执行下一次;如果没有结束,就等它结束后立即执行下一次。
  • scheduleWithFixedDelay,以每次任务执行完后的时间计时,period时间后,执行下一次。

并且,这两个方法都是在任务结束后才执行下一次,那么如果某个任务发生异无法执行完,那么整个循环任务就会失效。所以需要给任务添加超时机制(比如给任务加上try-catch-finally,catch住超时异常) 保证任务即使发生异常也可以结束,就可保证循环正常执行了。

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