前言
其实很幸运，入职一周之后就能跟着两个师兄做android开发，师兄都是大神，身为小白的我只能多多学习，多多努力。最近一段时间都忙的没机会总结，今天刚完成了android客户端图片异步加载的类，这里记录一下（ps：其实我这里都是参考网上开源实现）
原理
在ListView或者GridView中加载图片的原理基本都是一样的：
先从内存缓存中获取，取到则返回，取不到进行下一步
从文件缓存中获取，取到则返回并更新到内存缓存，取不到则进行进行下一步
从网络上下载图片，并更新内存缓存和文件缓存
流程图如下：
同时，要注意线程的数量。一般在listview中加载图片，大家都是开启新的线程去加载，但是当快速滑动时，很容易造成OOM，因此需要控制线程数量。我们可以通过线程池控制线程的数量，具体线程池的大小还需要根据处理器的情况和业务情况自行判断
建立线程池的方法如下：
ExecutorService executorService = Executors.newFixedThreadPool(5); // 5是可变的
文件缓存类
import java.io.File; import android.content.Context; public class FileCache { private static final String DIR_NAME = "your_dir"; private File cacheDir; public FileCache(Context context) { // Find the directory to save cached images if (android.os.Environment.getExternalStorageState().equals( android.os.Environment.MEDIA_MOUNTED)) { cacheDir = new File( android.os.Environment.getExternalStorageDirectory(), DIR_NAME); } else { cacheDir = context.getCacheDir(); } if (!cacheDir.exists()) { cacheDir.mkdirs(); } } public File getFile(String url) { // Identify images by url's hash code String filename = String.valueOf(url.hashCode()); File f = new File(cacheDir, filename); return f; } public void clear() { File[] files = cacheDir.listFiles(); if (files == null) { return; } else { for (File f : files) { f.delete(); } } } } 内存缓存类
这里使用了软引用，Map<String, SoftReference<Bitmap>> cache，可以google一下软引用的机制，简单的说：实现了map，同时当内存紧张时可以被回收，不会造成内存泄露
import java.lang.ref.SoftReference; import java.util.Collections; import java.util.LinkedHashMap; import java.util.Map; import android.graphics.Bitmap; public class MemoryCache { private Map<String, SoftReference<Bitmap>> cache = Collections .synchronizedMap(new LinkedHashMap<String, SoftReference<Bitmap>>( 10, 1.5f, true)); public Bitmap get(String id) { if (!cache.containsKey(id)) { return null; } SoftReference<Bitmap> ref = cache.get(id); return ref.get(); } public void put(String id, Bitmap bitmap) { cache.put(id, new SoftReference<Bitmap>(bitmap)); } public void clear() { cache.clear(); } } 图片加载类
import java.io.File; import java.io.FileInputStream; import java.io.FileNotFoundException; import java.io.FileOutputStream; import java.io.InputStream; import java.io.OutputStream; import java.net.HttpURLConnection; import java.net.URL; import java.util.Collections; import java.util.Map; import java.util.WeakHashMap; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import android.content.Context; import android.graphics.Bitmap; import android.graphics.BitmapFactory; import android.os.Handler; import android.widget.ImageView; public class ImageLoader { /** * Network time out */ private static final int TIME_OUT = 30000; /** * Default picture resource */ private static final int DEFAULT_BG = R.drawable.plate_list_head_bg; /** * Thread pool number */ private static final int THREAD_NUM = 5; /** * Memory image cache */ MemoryCache memoryCache = new MemoryCache(); /** * File image cache */ FileCache fileCache; /** * Judge image view if it is reuse */ private Map<ImageView, String> imageViews = Collections .synchronizedMap(new WeakHashMap<ImageView, String>()); /** * Thread pool */ ExecutorService executorService; /** * Handler to display images in UI thread */ Handler handler = new Handler(); public ImageLoader(Context context) { fileCache = new FileCache(context); executorService = Executors.newFixedThreadPool(THREAD_NUM); } public void disPlayImage(String url, ImageView imageView) { imageViews.put(imageView, url); Bitmap bitmap = memoryCache.get(url); if (bitmap != null) { // Display image from Memory cache imageView.setImageBitmap(bitmap); } else { // Display image from File cache or Network queuePhoto(url, imageView); } } private void queuePhoto(String url, ImageView imageView) { PhotoToLoad photoToLoad = new PhotoToLoad(url, imageView); executorService.submit(new PhotosLoader(photoToLoad)); } private Bitmap getBitmap(String url) { File f = fileCache.getFile(url); // From File cache Bitmap bmp = decodeFile(f); if (bmp != null) { return bmp; } // From Network try { Bitmap bitmap = null; URL imageUrl = new URL(url); HttpURLConnection conn = (HttpURLConnection) imageUrl .openConnection(); conn.setConnectTimeout(TIME_OUT); conn.setReadTimeout(TIME_OUT); conn.setInstanceFollowRedirects(true); InputStream is = conn.getInputStream(); OutputStream os = new FileOutputStream(f); copyStream(is, os); os.close(); conn.disconnect(); bitmap = decodeFile(f); return bitmap; } catch (Throwable ex) { if (ex instanceof OutOfMemoryError) { clearCache(); } return null; } } private void copyStream(InputStream is, OutputStream os) { int buffer_size = 1024; try { byte[] bytes = new byte[buffer_size]; while (true) { int count = is.read(bytes, 0, buffer_size); if (count == -1) { break; } os.write(bytes, 0, count); } } catch (Exception e) { } } private Bitmap decodeFile(File f) { try { // TODO:Compress image size FileInputStream fileInputStream = new FileInputStream(f); Bitmap bitmap = BitmapFactory.decodeStream(fileInputStream); return bitmap; } catch (FileNotFoundException e) { return null; } } private void clearCache() { memoryCache.clear(); fileCache.clear(); } /** * Task for the queue * * @author zhengyi.wzy * */ private class PhotoToLoad { public String url; public ImageView imageView; public PhotoToLoad(String url, ImageView imageView) { this.url = url; this.imageView = imageView; } } /** * Asynchronous to load picture * * @author zhengyi.wzy * */ class PhotosLoader implements Runnable { PhotoToLoad photoToLoad; public PhotosLoader(PhotoToLoad photoToLoad) { this.photoToLoad = photoToLoad; } private boolean imageViewReused(PhotoToLoad photoToLoad) { String tag = imageViews.get(photoToLoad.imageView); if (tag == null || !tag.equals(photoToLoad.url)) { return true; } return false; } @Override public void run() { // Abort current thread if Image View reused if (imageViewReused(photoToLoad)) { return; } Bitmap bitmap = getBitmap(photoToLoad.url); // Update Memory memoryCache.put(photoToLoad.url, bitmap); if (imageViewReused(photoToLoad)) { return; } // Don't change UI in children thread BitmapDisplayer bd = new BitmapDisplayer(bitmap, photoToLoad); handler.post(bd); } class BitmapDisplayer implements Runnable { Bitmap bitmap; PhotoToLoad photoToLoad; public BitmapDisplayer(Bitmap bitmap, PhotoToLoad photoToLoad) { this.bitmap = bitmap; this.photoToLoad = photoToLoad; } @Override public void run() { if (imageViewReused(photoToLoad)) { return; } if (bitmap != null) { photoToLoad.imageView.setImageBitmap(bitmap); } else { photoToLoad.imageView.setImageResource(DEFAULT_BG); } } } } } 调用方法
ImageLoader imageLoader = new ImageLoader(context); imageLoader.disPlayImage(imageUrl, imageView);