Android 中使用 ThreadPoolExecutor

Posted 2022-07-20 码农乐园

线程池执行器以及它们在 android 中的使用。我们将通过大量示例代码彻底介绍这些主题。

虽然现在我们都使用RxJava、Kotlin-Coroutines来执行后台任务，但了解 ThreadPoolExecutor 很重要，因为所有这些库都是在 ThreadPoolExecutor 之上构建的。

线程池

线程池管理一个工作线程池（具体数量取决于其实现）。

任务队列保存等待池中任何空闲线程执行的任务。任务由生产者添加到队列中，而工作线程充当消费者，只要有空闲线程准备好执行新的后台执行，就会从队列中消费任务。

线程池执行器

ThreadPoolExecutor使用线程池中的一个线程执行给定任务。

ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue
);

这些参数是什么？

corePoolSize：保留在池中的最小线程数。最初，池中有零个线程。但是随着任务被添加到队列中，新的线程被创建。如果有空闲线程——但线程数低于 corePoolSize——那么新线程将继续被创建。

maximumPoolSize：池中允许的最大线程数。如果这超过了 corePoolSize——并且当前线程数 >= corePoolSize——那么只有当队列已满时才会创建新的工作线程。

keepAliveTime：当线程数大于核心时，非核心线程（多余的空闲线程）会等待一个新的任务，如果在这个参数定义的时间内没有得到一个，就会终止。

unit：keepAliveTime的时间单位。

workQueue：任务队列，只会存放可运行的任务。它必须是一个 BlockingQueue。

为什么在 Android 或 JAVA 应用程序中使用线程池执行器？

它是一个强大的任务执行框架，因为它支持队列中的任务添加、任务取消和任务优先级。

它减少了与线程创建相关的开销，因为它在其线程池中管理所需数量的线程。

在 Android 中使用 ThreadPoolExecutor

首先，创建一个 PriorityThreadFactory：

public class PriorityThreadFactory implements ThreadFactory 




    private final int mThreadPriority;




    public PriorityThreadFactory(int threadPriority) 
        mThreadPriority = threadPriority;
    




    @Override
    public Thread newThread(final Runnable runnable) 
        Runnable wrapperRunnable = new Runnable() 
            @Override
            public void run() 
                try 
                    Process.setThreadPriority(mThreadPriority);
                 catch (Throwable t) 




                
                runnable.run();
            
        ;
        return new Thread(wrapperRunnable);
    

创建一个 MainThreadExecutor：

public class MainThreadExecutor implements Executor 




    private final Handler handler = new Handler(Looper.getMainLooper());




    @Override
    public void execute(Runnable runnable) 
        handler.post(runnable);
    

创建一个 DefaultExecutorSupplier：




/*
* Singleton class for default executor supplier
*/
public class DefaultExecutorSupplier
    /*
    * Number of cores to decide the number of threads
    */
    public static final int NUMBER_OF_CORES = Runtime.getRuntime().availableProcessors();
    
    /*
    * thread pool executor for background tasks
    */
    private final ThreadPoolExecutor mForBackgroundTasks;
    /*
    * thread pool executor for light weight background tasks
    */
    private final ThreadPoolExecutor mForLightWeightBackgroundTasks;
    /*
    * thread pool executor for main thread tasks
    */
    private final Executor mMainThreadExecutor;
    /*
    * an instance of DefaultExecutorSupplier
    */
    private static DefaultExecutorSupplier sInstance;




    /*
    * returns the instance of DefaultExecutorSupplier
    */
    public static DefaultExecutorSupplier getInstance() 
       if (sInstance == null) 
         synchronized(DefaultExecutorSupplier.class)                                                                  
             sInstance = new DefaultExecutorSupplier();      
        
        return sInstance;
    




    /*
    * constructor for  DefaultExecutorSupplier
    */ 
    private DefaultExecutorSupplier() 
        
        // setting the thread factory
        ThreadFactory backgroundPriorityThreadFactory = new 
                PriorityThreadFactory(Process.THREAD_PRIORITY_BACKGROUND);
        
        // setting the thread pool executor for mForBackgroundTasks;
        mForBackgroundTasks = new ThreadPoolExecutor(
                NUMBER_OF_CORES * 2,
                NUMBER_OF_CORES * 2,
                60L,
                TimeUnit.SECONDS,
                new LinkedBlockingQueue<Runnable>(),
                backgroundPriorityThreadFactory
        );
        
        // setting the thread pool executor for mForLightWeightBackgroundTasks;
        mForLightWeightBackgroundTasks = new ThreadPoolExecutor(
                NUMBER_OF_CORES * 2,
                NUMBER_OF_CORES * 2,
                60L,
                TimeUnit.SECONDS,
                new LinkedBlockingQueue<Runnable>(),
                backgroundPriorityThreadFactory
        );
        
        // setting the thread pool executor for mMainThreadExecutor;
        mMainThreadExecutor = new MainThreadExecutor();
    




    /*
    * returns the thread pool executor for background task
    */
    public ThreadPoolExecutor forBackgroundTasks() 
        return mForBackgroundTasks;
    




    /*
    * returns the thread pool executor for light weight background task
    */
    public ThreadPoolExecutor forLightWeightBackgroundTasks() 
        return mForLightWeightBackgroundTasks;
    




    /*
    * returns the thread pool executor for main thread task
    */
    public Executor forMainThreadTasks() 
        return mMainThreadExecutor;
    

注意：不同线程池可用的线程数取决于您的要求。




现在在您的代码中如下使用它




/*
* Using it for Background Tasks
*/
public void doSomeBackgroundWork()
  DefaultExecutorSupplier.getInstance().forBackgroundTasks()
    .execute(new Runnable() 
    @Override
    public void run() 
       // do some background work here.
    
  );





/*
* Using it for Light-Weight Background Tasks
*/
public void doSomeLightWeightBackgroundWork()
  DefaultExecutorSupplier.getInstance().forLightWeightBackgroundTasks()
    .execute(new Runnable() 
    @Override
    public void run() 
       // do some light-weight background work here.
    
  );





/*
* Using it for MainThread Tasks
*/
public void doSomeMainThreadWork()
  DefaultExecutorSupplier.getInstance().forMainThreadTasks()
    .execute(new Runnable() 
    @Override
    public void run() 
       // do some Main Thread work here.
    
  );

这样，我们可以为网络任务、I/O任务、繁重的后台任务和其他任务创建不同的线程池。

如何取消任务？

要取消任务，您必须获得该任务的未来。因此，您需要使用提交，而不是使用执行，这将返回未来。现在，这个未来可以用来取消任务。

/*
* Get the future of the task by submitting it to the pool
*/
Future future = DefaultExecutorSupplier.getInstance().forBackgroundTasks()
    .submit(new Runnable() 
    @Override
    public void run() 
      // do some background work here.
    
);




/*
* cancelling the task
*/
future.cancel(true);

如何设置任务的优先级？

假设一个队列中有 20 个任务，而线程池仅容纳 4 个线程。我们根据优先级执行新任务，因为线程池一次只能执行 4 个。

但是假设我们需要首先执行我们推入队列的最后一个任务。我们需要为该任务设置 IMMEDIATE 优先级，以便当线程从队列中获取新任务时，它首先执行该任务（因为它具有最高优先级）。

要设置任务的优先级，我们需要创建一个线程池执行器。

为优先级创建一个 ENUM：

/**
 * Priority levels
 */
public enum Priority 
    /**
     * NOTE: DO NOT CHANGE ORDERING OF THOSE CONSTANTS UNDER ANY CIRCUMSTANCES.
     * Doing so will make ordering incorrect.
     */




    /**
     * Lowest priority level. Used for prefetches of data.
     */
    LOW,




    /**
     * Medium priority level. Used for warming of data that might soon get visible.
     */
    MEDIUM,




    /**
     * Highest priority level. Used for data that are currently visible on screen.
     */
    HIGH,




    /**
     * Highest priority level. Used for data that are required instantly(mainly for emergency).
     */
    IMMEDIATE;

创建一个 PriorityRunnable：

public class PriorityRunnable implements Runnable 




    private final Priority priority;




    public PriorityRunnable(Priority priority) 
        this.priority = priority;
    




    @Override
    public void run() 
      // nothing to do here.
    




    public Priority getPriority() 
        return priority;
    





创建一个 PriorityThreadPoolExecutor，它扩展了 ThreadPoolExecutor。我们必须创建 PriorityFutureTask，它将实现 Comparable<PriorityFutureTask>




public class PriorityThreadPoolExecutor extends ThreadPoolExecutor 




   public PriorityThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime,
         TimeUnit unit, ThreadFactory threadFactory) 
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit,new PriorityBlockingQueue<Runnable>(), threadFactory);
    




    @Override
    public Future<?> submit(Runnable task) 
        PriorityFutureTask futureTask = new PriorityFutureTask((PriorityRunnable) task);
        execute(futureTask);
        return futureTask;
    




    private static final class PriorityFutureTask extends FutureTask<PriorityRunnable>
            implements Comparable<PriorityFutureTask> 
        private final PriorityRunnable priorityRunnable;




        public PriorityFutureTask(PriorityRunnable priorityRunnable) 
            super(priorityRunnable, null);
            this.priorityRunnable = priorityRunnable;
        
        
        /*
         * compareTo() method is defined in interface java.lang.Comparable and it is used
         * to implement natural sorting on java classes. natural sorting means the the sort 
         * order which naturally applies on object e.g. lexical order for String, numeric 
         * order for Integer or Sorting employee by there ID etc. most of the java core 
         * classes including String and Integer implements CompareTo() method and provide
         * natural sorting.
         */
        @Override
        public int compareTo(PriorityFutureTask other) 
            Priority p1 = priorityRunnable.getPriority();
            Priority p2 = other.priorityRunnable.getPriority();
            return p2.ordinal() - p1.ordinal();
        
    

首先，在 DefaultExecutorSupplier 中，使用 PriorityThreadPoolExecutor 而不是 ThreadPoolExecutor，如下所示：

public class DefaultExecutorSupplier




private final PriorityThreadPoolExecutor mForBackgroundTasks;




private DefaultExecutorSupplier() 
  
        mForBackgroundTasks = new PriorityThreadPoolExecutor(
                NUMBER_OF_CORES * 2,
                NUMBER_OF_CORES * 2,
                60L,
                TimeUnit.SECONDS,
                backgroundPriorityThreadFactory
        );




    

这是我们如何为任务设置高优先级的示例：




/*
* do some task at high priority
*/
public void doSomeTaskAtHighPriority()
  DefaultExecutorSupplier.getInstance().forBackgroundTasks()
    .submit(new PriorityRunnable(Priority.HIGH) 
    @Override
    public void run() 
      // do some background work here at high priority.
    
);

通过这种方式，可以确定任务的优先级