在android应用开发的时候，加载网络图片是一个非常重要的部分，很多图片不可能放在本地，所以就必须要从服务器或者网络读取图片。

软引用是一个现在非常流行的方法，用户体验比较好，不用每次都需要从网络下载图片，如果下载后就存到本地，下次读取时首先查看本地有没有，如果没有再从网络读取。

下面就分享一下异步加载网络图片的方法吧。

FileCache.java

import java.io.File; 
import android.content.Context; 
 
public class FileCache { 
 
  private File cacheDir; 
 
  public FileCache(Context context) { 
    // 找一个用来缓存图片的路径 
    if (android.os.Environment.getExternalStorageState().equals( 
        android.os.Environment.MEDIA_MOUNTED)) 
      cacheDir = new File(android.os.Environment.getExternalStorageDirectory(), 
          "文件夹名称"); 
    else 
      cacheDir = context.getCacheDir(); 
    if (!cacheDir.exists()) 
      cacheDir.mkdirs(); 
  } 
 
  public File getFile(String url) { 
    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; 
    for (File f : files) 
      f.delete(); 
  } 
 
}

 HttpUtil.java

import java.io.ByteArrayOutputStream; 
import java.io.File; 
import java.io.FileNotFoundException; 
import java.io.FileOutputStream; 
import java.io.IOException; 
import java.io.InputStream; 
import java.io.OutputStream; 
import java.io.UnsupportedEncodingException; 
import java.net.HttpURLConnection; 
import java.net.MalformedURLException; 
import java.net.ProtocolException; 
import java.net.URL; 
import java.net.URLEncoder; 
import java.util.Map; 
 
/**
* Http 请求工具类
* 
* @author Scorpio.Liu
* 
*/ 
public class HttpUtil { 
 
  /**
  * 获取响应字符串
  * 
  * @param path
  *      路径
  * @param parameters
  *      参数
  * @return 响应字符串
  */ 
  public static String getResponseStr(String path, Map<String, String> parameters) { 
    StringBuffer buffer = new StringBuffer(); 
    URL url; 
    try { 
      if (parameters != null && !parameters.isEmpty()) { 
        for (Map.Entry<String, String> entry : parameters.entrySet()) { 
          // 完成转码操作 
          buffer.append(entry.getKey()).append("=") 
              .append(URLEncoder.encode(entry.getValue(), "UTF-8")).append("&"); 
        } 
        buffer.deleteCharAt(buffer.length() - 1); 
      } 
      url = new URL(path); 
      HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection(); 
      urlConnection.setConnectTimeout(3000); 
      urlConnection.setRequestMethod("POST"); 
      urlConnection.setDoInput(true);// 表示从服务器获取数据 
      urlConnection.setDoOutput(true);// 表示向服务器写数据 
      // 获得上传信息的字节大小以及长度 
      byte[] mydata = buffer.toString().getBytes(); 
      // 表示设置请求体的类型是文本类型 
      urlConnection.setRequestProperty("Content-Type", 
          "application/x-www-form-urlencoded"); 
      urlConnection.setRequestProperty("Content-Length", String.valueOf(mydata.length)); 
      // 获得输出流,向服务器输出数据 
      OutputStream outputStream = urlConnection.getOutputStream(); 
      outputStream.write(mydata, 0, mydata.length); 
      outputStream.close(); 
      int responseCode = urlConnection.getResponseCode(); 
      if (responseCode == 200) { 
        return changeInputStream(urlConnection.getInputStream()); 
      } 
    } catch (UnsupportedEncodingException e) { 
      e.printStackTrace(); 
    } catch (MalformedURLException e) { 
      e.printStackTrace(); 
    } catch (ProtocolException e) { 
      e.printStackTrace(); 
    } catch (IOException e) { 
      e.printStackTrace(); 
    } 
    return null; 
  } 
 
  private static String changeInputStream(InputStream inputStream) { 
    ByteArrayOutputStream outputStream = new ByteArrayOutputStream(); 
    byte[] data = new byte[1024]; 
    int len = 0; 
    String result = ""; 
    if (inputStream != null) { 
      try { 
        while ((len = inputStream.read(data)) != -1) { 
          outputStream.write(data, 0, len); 
        } 
        result = new String(outputStream.toByteArray(), "UTF-8"); 
      } catch (IOException e) { 
        e.printStackTrace(); 
      } 
    } 
    return result; 
  } 
 
  public static InputStream getInputStream(String path) { 
    URL url; 
    try { 
      url = new URL(path); 
      HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection(); 
      urlConnection.setConnectTimeout(3000); 
      urlConnection.setRequestMethod("GET"); 
      urlConnection.setDoInput(true);// 表示从服务器获取数据 
      urlConnection.connect(); 
      if (urlConnection.getResponseCode() == 200) 
        return urlConnection.getInputStream(); 
    } catch (MalformedURLException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } catch (IOException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } catch (Exception e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } 
    return null; 
  } 
 
  public static byte[] readStream(InputStream inStream) throws Exception { 
    ByteArrayOutputStream outSteam = new ByteArrayOutputStream(); 
    byte[] buffer = new byte[1024]; 
    int len = -1; 
    while ((len = inStream.read(buffer)) != -1) { 
      outSteam.write(buffer, 0, len); 
 
    } 
    outSteam.close(); 
    inStream.close(); 
    return outSteam.toByteArray(); 
  } 
 
  public static void CopyStream(String url, File f) { 
    FileOutputStream fileOutputStream = null; 
    InputStream inputStream = null; 
    try { 
      inputStream = getInputStream(url); 
      byte[] data = new byte[1024]; 
      int len = 0; 
      fileOutputStream = new FileOutputStream(f); 
      while ((len = inputStream.read(data)) != -1) { 
        fileOutputStream.write(data, 0, len); 
      } 
    } catch (FileNotFoundException e) { 
      e.printStackTrace(); 
    } catch (IOException e) { 
      e.printStackTrace(); 
    } finally { 
      if (inputStream != null) { 
        try { 
          inputStream.close(); 
        } catch (IOException e) { 
          e.printStackTrace(); 
        } 
      } 
      if (fileOutputStream != null) { 
        try { 
          fileOutputStream.close(); 
        } catch (IOException e) { 
          e.printStackTrace(); 
        } 
      } 
    } 
  } 
 
} 

MemoryCache.java

import java.lang.ref.SoftReference; 
import java.util.Collections; 
import java.util.HashMap; 
import java.util.Map; 
import android.graphics.Bitmap; 
 
public class MemoryCache { 
  private Map<String, SoftReference<Bitmap>> cache = Collections 
      .synchronizedMap(new HashMap<String, SoftReference<Bitmap>>());// 软引用 
 
  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(); 
  } 
} 

ImageLoader.java

import java.io.File; 
import java.io.FileInputStream; 
import java.io.FileNotFoundException; 
import java.io.UnsupportedEncodingException; 
import java.net.URLEncoder; 
import java.util.Collections; 
import java.util.Map; 
import java.util.WeakHashMap; 
import java.util.concurrent.ExecutorService; 
import java.util.concurrent.Executors; 
import android.app.Activity; 
import android.content.Context; 
import android.graphics.Bitmap; 
import android.graphics.BitmapFactory; 
import android.graphics.drawable.BitmapDrawable; 
import android.widget.ImageView; 
 
public class ImageLoader { 
 
  private MemoryCache memoryCache = new MemoryCache(); 
  private FileCache fileCache; 
  private Map<ImageView, String> imageViews = Collections 
      .synchronizedMap(new WeakHashMap<ImageView, String>()); 
  private ExecutorService executorService; 
  private boolean isSrc; 
 
  /**
  * @param context
  *      上下文对象
  * @param flag
  *      true为source资源，false为background资源
  */ 
  public ImageLoader(Context context, boolean flag) { 
    fileCache = new FileCache(context); 
    executorService = Executors.newFixedThreadPool(5); 
    isSrc = flag; 
  } 
 
  final int stub_id = R.drawable.ic_launcher; 
 
  public void DisplayImage(String url, ImageView imageView) { 
    String u1 = url.substring(0, url.lastIndexOf("/") + 1); 
    String u2 = url.substring(url.lastIndexOf("/") + 1); 
    try { 
      u2 = URLEncoder.encode(u2, "UTF-8"); 
    } catch (UnsupportedEncodingException e) { 
      e.printStackTrace(); 
    } 
    url = u1 + u2; 
    imageViews.put(imageView, url); 
    Bitmap bitmap = memoryCache.get(url); 
    if (bitmap != null) { 
      if (isSrc) 
        imageView.setImageBitmap(bitmap); 
      else 
        imageView.setBackgroundDrawable(new BitmapDrawable(bitmap)); 
    } else { 
      queuePhoto(url, imageView); 
      if (isSrc) 
        imageView.setImageResource(stub_id); 
      else 
        imageView.setBackgroundResource(stub_id); 
    } 
  } 
 
  private void queuePhoto(String url, ImageView imageView) { 
    PhotoToLoad p = new PhotoToLoad(url, imageView); 
    executorService.submit(new PhotosLoader(p)); 
  } 
 
  private Bitmap getBitmap(String url) { 
    try { 
      File f = fileCache.getFile(url); 
      // 从sd卡 
      Bitmap b = onDecodeFile(f); 
      if (b != null) 
        return b; 
      // 从网络 
      Bitmap bitmap = null; 
      System.out.println("ImageLoader-->download"); 
      HttpUtil.CopyStream(url, f); 
      bitmap = onDecodeFile(f); 
 
      return bitmap; 
    } catch (Exception ex) { 
      ex.printStackTrace(); 
      return null; 
    } 
  } 
 
  public Bitmap onDecodeFile(File f) { 
    try { 
      return BitmapFactory.decodeStream(new FileInputStream(f)); 
    } catch (FileNotFoundException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
    } 
    return null; 
  } 
 
  /**
  * 解码图像用来减少内存消耗
  * 
  * @param f
  * @return
  */ 
  public Bitmap decodeFile(File f) { 
    try { 
      // 解码图像大小 
      BitmapFactory.Options o = new BitmapFactory.Options(); 
      o.inJustDecodeBounds = true; 
      BitmapFactory.decodeStream(new FileInputStream(f), null, o); 
      // 找到正确的刻度值，它应该是2的幂。 
      final int REQUIRED_SIZE = 70; 
      int width_tmp = o.outWidth, height_tmp = o.outHeight; 
      int scale = 1; 
      while (true) { 
        if (width_tmp / 2 < REQUIRED_SIZE || height_tmp / 2 < REQUIRED_SIZE) 
          break; 
        width_tmp /= 2; 
        height_tmp /= 2; 
        scale *= 2; 
      } 
      BitmapFactory.Options o2 = new BitmapFactory.Options(); 
      o2.inSampleSize = scale; 
      return BitmapFactory.decodeStream(new FileInputStream(f), null, o2); 
    } catch (FileNotFoundException e) { 
    } 
    return null; 
  } 
 
  /**
  * 任务队列
  * 
  * @author Scorpio.Liu
  * 
  */ 
  private class PhotoToLoad { 
    public String url; 
    public ImageView imageView; 
 
    public PhotoToLoad(String u, ImageView i) { 
      url = u; 
      imageView = i; 
    } 
  } 
 
  class PhotosLoader implements Runnable { 
    PhotoToLoad photoToLoad; 
 
    PhotosLoader(PhotoToLoad photoToLoad) { 
      this.photoToLoad = photoToLoad; 
    } 
 
    @Override 
    public void run() { 
      if (imageViewReused(photoToLoad)) 
        return; 
      Bitmap bmp = getBitmap(photoToLoad.url); 
      memoryCache.put(photoToLoad.url, bmp); 
      if (imageViewReused(photoToLoad)) 
        return; 
      BitmapDisplayer bd = new BitmapDisplayer(bmp, photoToLoad); 
      Activity a = (Activity) photoToLoad.imageView.getContext(); 
      a.runOnUiThread(bd); 
    } 
  } 
 
  boolean imageViewReused(PhotoToLoad photoToLoad) { 
    String tag = imageViews.get(photoToLoad.imageView); 
    if (tag == null || !tag.equals(photoToLoad.url)) 
      return true; 
    return false; 
  } 
 
  /**
  * 显示位图在UI线程
  * 
  * @author Scorpio.Liu
  * 
  */ 
  class BitmapDisplayer implements Runnable { 
    Bitmap bitmap; 
    PhotoToLoad photoToLoad; 
 
    public BitmapDisplayer(Bitmap b, PhotoToLoad p) { 
      bitmap = b; 
      photoToLoad = p; 
    } 
 
    public void run() { 
      if (imageViewReused(photoToLoad)) 
        return; 
      if (bitmap != null) { 
        if (isSrc) 
          photoToLoad.imageView.setImageBitmap(bitmap); 
        else 
          photoToLoad.imageView.setBackgroundDrawable(new BitmapDrawable(bitmap)); 
      } else { 
        if (isSrc) 
          photoToLoad.imageView.setImageResource(stub_id); 
        else 
          photoToLoad.imageView.setBackgroundResource(stub_id); 
      } 
    } 
  } 
 
  public void clearCache() { 
    memoryCache.clear(); 
    fileCache.clear(); 
  } 
 
} 

使用的时候用ImageLoader这个类就ok了，很方便~

希望本文所述对大家学习Android软件编程有所帮助。