Android Studio - ExecutorService（線程池）（五十二）

Android Studio - ExecutorService（線程池）（五十二）：

當要使用大量的非同步任務，不想每次都創建子線程，可以使用 ExecutorService（線程池），令到減輕每個非同步任務的調用開銷和提高性能。

Android ExecutorService（線程池）

new Thread() 的缺點：

• 每次 new Thread() 耗費性能
• 調用 new Thread() 創建的線程缺乏管理，被稱為野線程，而且可以無限制創建，之間相互競爭，會導致過多佔用系統資源導致系統癱瘓
• 不利於擴展，比如如定時執行、定期執行、線程中斷

ExecutorService 的優點：

• 重用存在的線程，減少物件創建、消亡的開銷，性能佳
• 可有效控制最大併發線程數，提高系統資源的使用率，同時避免過多資源競爭，避免堵塞，提供定時執行、定期執行、單線程、併發數控制等功能

通過 Executors 的靜態方法來創建，一般有三種：

• 單線程：Executors.newSingleThreadExecutor()
• 固定數量線程：Executors.newFixedThreadPool()
• 動態線程：Executors.newCachedThreadPool()

ThreadPoolExecutor 類的典型構造：

public ThreadPoolExecutor(int corePoolSize,                               int maximumPoolSize,                               long keepAliveTime,                               TimeUnit unit,                               BlockingQueue<Runnable> workQueue) {         this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,              Executors.defaultThreadFactory(), defaultHandler);     }

ThreadPoolExecutor 的配置參數：

public ThreadPoolExecutor(int corePoolSize,                       int maximumPoolSize,                       long keepAliveTime,                       TimeUnit unit,                       BlockingQueue workQueue,                       ThreadFactory threadFactory,                       RejectedExecutionHandler handler) {     this.corePoolSize = corePoolSize;     this.maximumPoolSize = maximumPoolSize;     this.workQueue = workQueue;     this.keepAliveTime = unit.toNanos(keepAliveTime);     this.threadFactory = threadFactory;     this.handler = handler; }

操作系統：Windows 7 64-bit 版本
開發環境：Android Studio 4.0.1 版本
原程式：C:\Development\Development_Android\Android_Project\DIY-Android-009-01D ExecutorService
程式：C:\Development\Development_Android\Android_Project\DIY-Android-009-01D ExecutorService

MainActivity.xml：

import androidx.appcompat.app.AppCompatActivity; import android.os.Bundle; import android.view.View; import android.widget.TextView; import java.util.concurrent.LinkedBlockingDeque; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit;   public class MainActivity extends AppCompatActivity {       // Gets the number of available cores (not always the same as the maximum number of cores)     private static int NUMBER_OF_CORES = Runtime.getRuntime().availableProcessors();     // Sets the amount of time an idle thread waits before terminating     private static final int KEEP_ALIVE_TIME = 1000;     // Sets the Time Unit to Milliseconds     private static final TimeUnit KEEP_ALIVE_TIME_UNIT = TimeUnit.MILLISECONDS;     // Used to update UI with work progress     private int count = 0;       @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);     }       // This is the runnable task that we will run 100 times     private Runnable runnable = new Runnable() {         @Override         public void run() {             // Do some work that takes 100 milliseconds             try {                 Thread.sleep(100);             } catch (InterruptedException e) {                 e.printStackTrace();             }               // Update the UI with progress             runOnUiThread(new Runnable() {                 @Override                 public void run() {                     count++;                     String msg = count < 100 ? "Thread Working " : "Finished ... ";                     updateStatus(msg + count);                 }             });         }     };       // Button click and performs work using a thread pool     public void buttonClickThreadPool(View view) {         count = 0;         ThreadPoolExecutor mThreadPoolExecutor = new ThreadPoolExecutor(                 NUMBER_OF_CORES + 2,        // Initial pool size                 NUMBER_OF_CORES + 2,   // Max pool size                 KEEP_ALIVE_TIME,       // Time idle thread waits before terminating                 KEEP_ALIVE_TIME_UNIT,  // Sets the Time Unit for KEEP_ALIVE_TIME                 new LinkedBlockingDeque<Runnable>());  // Work Queue         for (int i = 0; i < 100; i++) {             mThreadPoolExecutor.execute(runnable);         }     }       private void updateStatus(String msg) {         ((TextView) findViewById(R.id.text)).setText(msg);     } }

active_main.xml：

<?xml version="1.0" encoding="utf-8"?> <ScrollView xmlns:android="http://schemas.android.com/apk/res/android"     android:layout_width="match_parent"     android:layout_height="wrap_content"     android:fillViewport="false">     <LinearLayout         android:orientation="vertical"         android:layout_width="match_parent"         android:layout_height="match_parent">           <TextView             android:id="@+id/text"             android:layout_width="match_parent"             android:layout_height="53dp"             android:text="TextView" />           <Button             android:id="@+id/poolButton"             android:layout_width="match_parent"             android:layout_height="wrap_content"             android:onClick="buttonClickThreadPool"             android:text="Thread Pool" />     </LinearLayout> </ScrollView>

Android Studio ExecutorService（線程池）程式

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