android線程與線程池-----線程池（二）《android開發藝術與探索》

標籤：

android 中的線程池

 

線程池的優點：

1 重用線程池中的線程，避免了線程的建立和銷毀帶來的效能開銷
2 能有效控制最大並發數，避免大量線程之間因為喜歡搶資源而導致阻塞
3 能夠對線程進行簡單的管理,提供定時執行以及指定間隔時間迴圈執行等

android 中的線程池源自java 中的Executor，Executor是一個介面，正真的實現是ThreadPoolExecutor。
ThreadPoolExecutor 提供參數配置線程池。

下面是一個常用的構造方法：

    public ThreadPoolExecutor(int corePoolSize,                                   int maximumPoolSize,                                   long keepAliveTime,                                   TimeUnit unit,                                   BlockingQueue<Runnable> workQueue,                                   ThreadFactory threadFactory) {             this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,                  threadFactory, defaultHandler);  

corePoolSize線程池的核心線程數  
maximumPoolSize線程池中能容納的最大線程數，當活動線程達到這個數之後，後續的任務會被阻塞
keepAliveTime 非核心線程閑置時的逾時時間長度，超過這個時間長度，非核心線程就會被回收
unit 用於指定keepAliveTime參數的時間單位。
workQueue  線程池中的任務隊列
threadFactory 線程工廠

ThreadPoolExecutor執行任務時大致遵循的規則：
1 如果線程池中的線程數量未達到核心線程的數量，那麼會直接啟動一個核心線程來執行任務。
2 如果線程池中的線程數量已達到或者超過核心線程的數量，那麼任務會被插入到任務隊列中排隊等待執行
3 如果2中無法將任務插入到任務隊列，這往往是由於任務隊列已滿，這個時候如果線程數量未達到線程池規定的最大值，那麼會立刻啟動一個非核心線程執行任務
4 如果3中線程數量達到線程池規定的最大值，那麼就拒絕執行此任務ThreadPoolExecutor會調用rejectedExecution的rejectedExecution 來通知調用者

這個是AsyncTask的線程池的配置：

     private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();          private static final int CORE_POOL_SIZE = CPU_COUNT + 1;//核心線程數等於CPU核心數+1          private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;//線程池的最大線程數為CPU核心數的2倍+1          private static final int KEEP_ALIVE = 1;//核心線程無逾時機制，非核心線程逾時1秒             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());              }          };      //任務任務隊列容量128       private static final BlockingQueue<Runnable> sPoolWorkQueue =                  new LinkedBlockingQueue<Runnable>(128);                   public static final Executor THREAD_POOL_EXECUTOR                  = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,                          TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);  

下面來看線程池的分類
Executors類建立線程池：

FixedThreadPool   Executors類newFixedThreadPool建立：   public static ExecutorService newFixedThreadPool(int nThreads) {          return new ThreadPoolExecutor(nThreads, nThreads,                                        0L, TimeUnit.MILLISECONDS,                                        new LinkedBlockingQueue<Runnable>());      }  public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {          return new ThreadPoolExecutor(nThreads, nThreads,                                        0L, TimeUnit.MILLISECONDS,                                        new LinkedBlockingQueue<Runnable>(),                                        threadFactory);      }  

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

ScheduledThreadPool   public static ScheduledExecutorService newSingleThreadScheduledExecutor() {          return new DelegatedScheduledExecutorService              (new ScheduledThreadPoolExecutor(1));      }      public static ScheduledExecutorService newScheduledThreadPool(              int corePoolSize, ThreadFactory threadFactory) {          return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);      }  

SingleThreadExecutor     public static ExecutorService newSingleThreadExecutor() {          return new FinalizableDelegatedExecutorService              (new ThreadPoolExecutor(1, 1,                                      0L, TimeUnit.MILLISECONDS,                                      new LinkedBlockingQueue<Runnable>()));      }      public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) {          return new DelegatedScheduledExecutorService              (new ScheduledThreadPoolExecutor(1, threadFactory));      }  

 

android線程與線程池-----線程池（二）《android開發藝術與探索》