造简单的图片加载框架——ImageLoader的实现

本篇主要做的事情，利用LruCache和DiskLruCache造一个ImageLoader。

对于OOM和LruCache和DiskLruCache还了解的猴子可以先看一下这两篇：

安卓OOM和Bitmap图片二级缓存机制（一）

安卓OOM和Bitmap图片二级缓存机制（二）

一、ImageLoader应该具备的功能：

图片的同步加载

图片的异步处理

图片压缩

内存缓存

磁盘缓存

网络拉取图片

同步加载： 就是以二级缓存的方式同步获取图片

异步加载： 异步，多线程，二级缓存获取图片

图片压缩： 压缩图片，有效防止OOM

内存缓存： 利用LruCache实现

磁盘缓存： 利用DiskLruCache实现

网络拉取： 没有缓存时，网络拉取

其中，LruCahce和DiskLruCache是这个图片框架的核心，除此之外，我们还需要注意的在ListView或者GridView上的一个复用问题。

（前提准备：网上拿到DiskLruCache的类源码，放入我们的工程中）

二、实现分析：

1、首先在ImageLoader这个方法里面初始化Lruchche和DiskLruCache这两个类

private ImageLoader(Context context) {
mContext = context.getApplicationContext();
int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
int cacheSize = maxMemory / 8;
mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
@Override
protected int sizeOf(String key, Bitmap bitmap) {
return bitmap.getRowBytes() * bitmap.getHeight() / 1024;
}
};
File diskCacheDir = getDiskCacheDir(mContext, "bitmap");
if (!diskCacheDir.exists()) {
diskCacheDir.mkdirs();
}
if (getUsableSpace(diskCacheDir) > DISK_CACHE_SIZE) {
try {
mDiskLruCache = DiskLruCache.open(diskCacheDir, 1, 1,
DISK_CACHE_SIZE);
mIsDiskLruCacheCreated = true;
} catch (IOException e) {
e.printStackTrace();
}
}
}

2、我们图片需要压缩，为了解耦，我们把图片压缩单独设置建立一个压缩类

public class ImageResizer {
private static final String TAG = "ImageResizer";

public ImageResizer() {
}

public Bitmap decodeSampledBitmapFromResource(Resources res,
int resId, int reqWidth, int reqHeight) {
// First decode with inJustDecodeBounds=true to check dimensions
final BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeResource(res, resId, options);

// Calculate inSampleSize
options.inSampleSize = calculateInSampleSize(options, reqWidth,
reqHeight);

// Decode bitmap with inSampleSize set
options.inJustDecodeBounds = false;
return BitmapFactory.decodeResource(res, resId, options);
}

public Bitmap decodeSampledBitmapFromFileDescriptor(FileDescriptor fd, int reqWidth, int reqHeight) {
// First decode with inJustDecodeBounds=true to check dimensions
final BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeFileDescriptor(fd, null, options);

// Calculate inSampleSize
options.inSampleSize = calculateInSampleSize(options, reqWidth,
reqHeight);

// Decode bitmap with inSampleSize set
options.inJustDecodeBounds = false;
return BitmapFactory.decodeFileDescriptor(fd, null, options);
}

public int calculateInSampleSize(BitmapFactory.Options options,
int reqWidth, int reqHeight) {
if (reqWidth == 0 || reqHeight == 0) {
return 1;
}

// Raw height and width of image
final int height = options.outHeight;
final int width = options.outWidth;
Log.d(TAG, "origin, w= " + width + " h=" + height);
int inSampleSize = 1;

if (height > reqHeight || width > reqWidth) {
final int halfHeight = height / 2;
final int halfWidth = width / 2;

// Calculate the largest inSampleSize value that is a power of 2 and
// keeps both
// height and width larger than the requested height and width.
while ((halfHeight / inSampleSize) >= reqHeight
&& (halfWidth / inSampleSize) >= reqWidth) {
inSampleSize *= 2;
}
}

Log.d(TAG, "sampleSize:" + inSampleSize);
return inSampleSize;
}
}

3、LruCache的取存

取和存

private void addBitmapToMemoryCache(String key, Bitmap bitmap) {
if (getBitmapFromMemCache(key) == null) {
mMemoryCache.put(key, bitmap);
}
}

private Bitmap getBitmapFromMemCache(String key) {
return mMemoryCache.get(key);
}

删除的话满了就自动删除

4、DiskLruCache的取存删

取和删

/**
* 磁盘缓存的添加
* @param url
* @param reqWidth
* @param reqHeight
* @return
* @throws IOException
*
*
* 磁盘缓存的添加：添加需要通过Editor来完成，利用commit和abort方法来 提交 和 撤销 操作
*/
private Bitmap loadBitmapFromHttp(String url, int reqWidth, int reqHeight)
throws IOException {
if (Looper.myLooper() == Looper.getMainLooper()) {
throw new RuntimeException("can not visit network from UI Thread.");
}
if (mDiskLruCache == null) {
return null;
}

String key = hashKeyFormUrl(url);
DiskLruCache.Editor editor = mDiskLruCache.edit(key);
if (editor != null) {
OutputStream outputStream = editor.newOutputStream(DISK_CACHE_INDEX);
if (downloadUrlToStream(url, outputStream)) {
editor.commit();
} else {
editor.abort();
}
mDiskLruCache.flush();
}
return loadBitmapFromDiskCache(url, reqWidth, reqHeight);
}

/**
* 磁盘缓存的读取
* @param url
* @param reqWidth
* @param reqHeight
* @return
* @throws IOException
* 读取：需要通过Snapshot来完成，通过Snapshot可以得到磁盘缓存对象对应的FileInputStream
* 但是FileInputStream无法很好地进行压缩
* 所以我们通过 FileDescripot 来加载压缩后的图片，得到加载后的Bitmap添加到内存缓存中
*/
private Bitmap loadBitmapFromDiskCache(String url, int reqWidth,
int reqHeight) throws IOException {
if (Looper.myLooper() == Looper.getMainLooper()) {
Log.w(TAG, "load bitmap from UI Thread, it's not recommended!");
}
if (mDiskLruCache == null) {
return null;
}

Bitmap bitmap = null;
String key = hashKeyFormUrl(url);
DiskLruCache.Snapshot snapShot = mDiskLruCache.get(key);
if (snapShot != null) {
FileInputStream fileInputStream = (FileInputStream)snapShot.getInputStream(DISK_CACHE_INDEX);
FileDescriptor fileDescriptor = fileInputStream.getFD();
bitmap = mImageResizer.decodeSampledBitmapFromFileDescriptor(fileDescriptor,
reqWidth, reqHeight);
if (bitmap != null) {
addBitmapToMemoryCache(key, bitmap);
}
}

return bitmap;
}

5、同步接口的实现

以同步接口的方式去加载图片，走的是内存，磁盘接着网络的获取渠道

同步接口的不能再主线程执行，需要在外部的线程调用

/**
*
* 同步加载  （从内存缓存、磁盘缓存、网络）
* @param uri http url
* @param reqWidth the width ImageView desired
* @param reqHeight the height ImageView desired
* @return bitmap, maybe null.
*
* 同步加载的设计步骤：
* 先从内存缓存尝试加载图片，找不到就去磁盘缓存拿，磁盘缓存拿不到就去网络拿
* 这个方法不能再线程执行，在主线程执行就抛异常（有一个检查当前线程的Looper是否为主线程的Looper的判断）
*/
public Bitmap loadBitmap(String uri, int reqWidth, int reqHeight) {
Bitmap bitmap = loadBitmapFromMemCache(uri);
if (bitmap != null) {
Log.d(TAG, "loadBitmapFromMemCache,url:" + uri);
return bitmap;
}

try {
bitmap = loadBitmapFromDiskCache(uri, reqWidth, reqHeight);
if (bitmap != null) {
Log.d(TAG, "loadBitmapFromDisk,url:" + uri);
return bitmap;
}
bitmap = loadBitmapFromHttp(uri, reqWidth, reqHeight);
Log.d(TAG, "loadBitmapFromHttp,url:" + uri);
} catch (IOException e) {
e.printStackTrace();
}

if (bitmap == null && !mIsDiskLruCacheCreated) {
Log.w(TAG, "encounter error, DiskLruCache is not created.");
bitmap = downloadBitmapFromUrl(uri);
}

return bitmap;
}

6、异步接口的设计

/**
* 异步接口
* load bitmap from memory cache or disk cache or network async, then bind imageView and bitmap.
* NOTE THAT: should run in UI Thread
* @param uri http url
* @param imageView bitmap's bind object
*/
public void bindBitmap(final String uri, final ImageView imageView) {
bindBitmap(uri, imageView, 0, 0);
}

/**
* 异步接口
* @param uri
* @param imageView
* @param reqWidth
* @param reqHeight
*
* 实现过程：
* 首先先去内存缓存读取图片
* 如果读取就直接返回结果
*
* 如果读取不到就调用loadBitmap方法，当图片加载成功后再将图片，图片的地址以及需要绑定的imageView封装成
* 一个LoaderResult对象，然后再通过mHMainandler向主线程发送一条消息，这样就可以在imageView中设置图片了
* 之所以通过Handler中转是因为子线程无法直接更新UI
*
* bindBitmap中用到了线程池和Handler
*
*
*/
public void bindBitmap(final String uri, final ImageView imageView,
final int reqWidth, final int reqHeight) {
imageView.setTag(TAG_KEY_URI, uri);
Bitmap bitmap = loadBitmapFromMemCache(uri);
if (bitmap != null) {
imageView.setImageBitmap(bitmap);
return;
}

Runnable loadBitmapTask = new Runnable() {

@Override
public void run() {
Bitmap bitmap = loadBitmap(uri, reqWidth, reqHeight);
if (bitmap != null) {
LoaderResult result = new LoaderResult(imageView, uri, bitmap);
mMainHandler.obtainMessage(MESSAGE_POST_RESULT, result).sendToTarget();
}
}
};
THREAD_POOL_EXECUTOR.execute(loadBitmapTask);
}

7、解决View的复用错位

每次设置图片之前都检查他的url有没有改变，如果改变就不再给其设置图片，这样就解决了图片错误的问题

private Handler mMainHandler = new Handler(Looper.getMainLooper()) {
@Override
public void handleMessage(Message msg) {
LoaderResult result = (LoaderResult) msg.obj;
ImageView imageView = result.imageView;
imageView.setImageBitmap(result.bitmap);
String uri = (String) imageView.getTag(TAG_KEY_URI);
if (uri.equals(result.uri)) {
imageView.setImageBitmap(result.bitmap);
} else {
Log.w(TAG, "set image bitmap,but url has changed, ignored!");
}
};
};

完整代码

ImageLoader完整代码

public class ImageLoader {

private static final String TAG = "ImageLoader";

public static final int MESSAGE_POST_RESULT = 1;

private static final int CPU_COUNT = Runtime.getRuntime()
.availableProcessors();
private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final long KEEP_ALIVE = 10L;

private static final int TAG_KEY_URI = R.id.imageloader_uri;
private static final long DISK_CACHE_SIZE = 1024 * 1024 * 50;
private static final int IO_BUFFER_SIZE = 8 * 1024;
private static final int DISK_CACHE_INDEX = 0;
private boolean mIsDiskLruCacheCreated = false;

private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);

public Thread newThread(Runnable r) {
return new Thread(r, "ImageLoader#" + mCount.getAndIncrement());
}
};

public static final Executor THREAD_POOL_EXECUTOR = new ThreadPoolExecutor(
CORE_POOL_SIZE, MAXIMUM_POOL_SIZE,
KEEP_ALIVE, TimeUnit.SECONDS,
new LinkedBlockingQueue<Runnable>(), sThreadFactory);

private Handler mMainHandler = new Handler(Looper.getMainLooper()) {
@Override
public void handleMessage(Message msg) {
LoaderResult result = (LoaderResult) msg.obj;
ImageView imageView = result.imageView;
imageView.setImageBitmap(result.bitmap);
String uri = (String) imageView.getTag(TAG_KEY_URI);
if (uri.equals(result.uri)) {
imageView.setImageBitmap(result.bitmap);
} else {
Log.w(TAG, "set image bitmap,but url has changed, ignored!");
}
};
};

private Context mContext;
private ImageResizer mImageResizer = new ImageResizer();
private LruCache<String, Bitmap> mMemoryCache;
private DiskLruCache mDiskLruCache;

private ImageLoader(Context context) {
mContext = context.getApplicationContext();
int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
int cacheSize = maxMemory / 8;
mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {
@Override
protected int sizeOf(String key, Bitmap bitmap) {
return bitmap.getRowBytes() * bitmap.getHeight() / 1024;
}
};
File diskCacheDir = getDiskCacheDir(mContext, "bitmap");
if (!diskCacheDir.exists()) {
diskCacheDir.mkdirs();
}
if (getUsableSpace(diskCacheDir) > DISK_CACHE_SIZE) {
try {
mDiskLruCache = DiskLruCache.open(diskCacheDir, 1, 1,
DISK_CACHE_SIZE);
mIsDiskLruCacheCreated = true;
} catch (IOException e) {
e.printStackTrace();
}
}
}

/**
* build a new instance of ImageLoader
* @param context
* @return a new instance of ImageLoader
*/
public static ImageLoader build(Context context) {
return new ImageLoader(context);
}

// 内存缓存的添加
private void addBitmapToMemoryCache(String key, Bitmap bitmap) {
if (getBitmapFromMemCache(key) == null) {
mMemoryCache.put(key, bitmap);
}
}

// 内存缓存的读取
private Bitmap getBitmapFromMemCache(String key) {
return mMemoryCache.get(key);
}

/**
* 异步接口
* load bitmap from memory cache or disk cache or network async, then bind imageView and bitmap.
* NOTE THAT: should run in UI Thread
* @param uri http url
* @param imageView bitmap's bind object
*/
public void bindBitmap(final String uri, final ImageView imageView) {
bindBitmap(uri, imageView, 0, 0);
}

/**
* 异步接口
* @param uri
* @param imageView
* @param reqWidth
* @param reqHeight
*
* 实现过程：
* 首先先去内存缓存读取图片
* 如果读取就直接返回结果
*
* 如果读取不到就调用loadBitmap方法，当图片加载成功后再将图片，图片的地址以及需要绑定的imageView封装成
* 一个LoaderResult对象，然后再通过mHMainandler向主线程发送一条消息，这样就可以在imageView中设置图片了
* 之所以通过Handler中转是因为子线程无法直接更新UI
*
* bindBitmap中用到了线程池和Handler
*
*
*/
public void bindBitmap(final String uri, final ImageView imageView,
final int reqWidth, final int reqHeight) {
imageView.setTag(TAG_KEY_URI, uri);
Bitmap bitmap = loadBitmapFromMemCache(uri);
if (bitmap != null) {
imageView.setImageBitmap(bitmap);
return;
}

Runnable loadBitmapTask = new Runnable() {

@Override
public void run() {
Bitmap bitmap = loadBitmap(uri, reqWidth, reqHeight);
if (bitmap != null) {
LoaderResult result = new LoaderResult(imageView, uri, bitmap);
mMainHandler.obtainMessage(MESSAGE_POST_RESULT, result).sendToTarget();
}
}
};
THREAD_POOL_EXECUTOR.execute(loadBitmapTask);
}
/**
*
* 同步加载  （从内存缓存、磁盘缓存、网络）
* @param uri http url
* @param reqWidth the width ImageView desired
* @param reqHeight the height ImageView desired
* @return bitmap, maybe null.
*
* 同步加载的设计步骤：
* 先从内存缓存尝试加载图片，找不到就去磁盘缓存拿，磁盘缓存拿不到就去网络拿
* 这个方法不能再线程执行，在主线程执行就抛异常（有一个检查当前线程的Looper是否为主线程的Looper的判断）
*/
public Bitmap loadBitmap(String uri, int reqWidth, int reqHeight) {
Bitmap bitmap = loadBitmapFromMemCache(uri);
if (bitmap != null) {
Log.d(TAG, "loadBitmapFromMemCache,url:" + uri);
return bitmap;
}

try {
bitmap = loadBitmapFromDiskCache(uri, reqWidth, reqHeight);
if (bitmap != null) {
Log.d(TAG, "loadBitmapFromDisk,url:" + uri);
return bitmap;
}
bitmap = loadBitmapFromHttp(uri, reqWidth, reqHeight);
Log.d(TAG, "loadBitmapFromHttp,url:" + uri);
} catch (IOException e) {
e.printStackTrace();
}

if (bitmap == null && !mIsDiskLruCacheCreated) {
Log.w(TAG, "encounter error, DiskLruCache is not created.");
bitmap = downloadBitmapFromUrl(uri);
}

return bitmap;
}private Bitmap loadBitmapFromMemCache(String url) {
final String key = hashKeyFormUrl(url);
Bitmap bitmap = getBitmapFromMemCache(key);
return bitmap;
}
/**
* 磁盘缓存的添加
* @param url
* @param reqWidth
* @param reqHeight
* @return
* @throws IOException
*
*
* 磁盘缓存的添加：添加需要通过Editor来完成，利用commit和abort方法来 提交 和 撤销 操作
*/
private Bitmap loadBitmapFromHttp(String url, int reqWidth, int reqHeight)
throws IOException {
if (Looper.myLooper() == Looper.getMainLooper()) {
throw new RuntimeException("can not visit network from UI Thread.");
}
if (mDiskLruCache == null) {
return null;
}

String key = hashKeyFormUrl(url);
DiskLruCache.Editor editor = mDiskLruCache.edit(key);
if (editor != null) {
OutputStream outputStream = editor.newOutputStream(DISK_CACHE_INDEX);
if (downloadUrlToStream(url, outputStream)) {
editor.commit();
} else {
editor.abort();
}
mDiskLruCache.flush();
}
return loadBitmapFromDiskCache(url, reqWidth, reqHeight);
}

/**
* 磁盘缓存的读取
* @param url
* @param reqWidth
* @param reqHeight
* @return
* @throws IOException
* 读取：需要通过Snapshot来完成，通过Snapshot可以得到磁盘缓存对象对应的FileInputStream
* 但是FileInputStream无法很好地进行压缩
* 所以我们通过 FileDescripot 来加载压缩后的图片，得到加载后的Bitmap添加到内存缓存中
*/
private Bitmap loadBitmapFromDiskCache(String url, int reqWidth,
int reqHeight) throws IOException {
if (Looper.myLooper() == Looper.getMainLooper()) {
Log.w(TAG, "load bitmap from UI Thread, it's not recommended!");
}
if (mDiskLruCache == null) {
return null;
}

Bitmap bitmap = null;
String key = hashKeyFormUrl(url);
DiskLruCache.Snapshot snapShot = mDiskLruCache.get(key);
if (snapShot != null) {
FileInputStream fileInputStream = (FileInputStream)snapShot.getInputStream(DISK_CACHE_INDEX);
FileDescriptor fileDescriptor = fileInputStream.getFD();
bitmap = mImageResizer.decodeSampledBitmapFromFileDescriptor(fileDescriptor,
reqWidth, reqHeight);
if (bitmap != null) {
addBitmapToMemoryCache(key, bitmap);
}
}

return bitmap;
}

public boolean downloadUrlToStream(String urlString,
OutputStream outputStream) {
HttpURLConnection urlConnection = null;
BufferedOutputStream out = null;
BufferedInputStream in = null;

try {
final URL url = new URL(urlString);
urlConnection = (HttpURLConnection) url.openConnection();
in = new BufferedInputStream(urlConnection.getInputStream(),
IO_BUFFER_SIZE);
out = new BufferedOutputStream(outputStream, IO_BUFFER_SIZE);

int b;
while ((b = in.read()) != -1) {
out.write(b);
}
return true;
} catch (IOException e) {
Log.e(TAG, "downloadBitmap failed." + e);
} finally {
if (urlConnection != null) {
urlConnection.disconnect();
}
MyUtils.close(out);
MyUtils.close(in);
}
return false;
}

private Bitmap downloadBitmapFromUrl(String urlString) {
Bitmap bitmap = null;
HttpURLConnection urlConnection = null;
BufferedInputStream in = null;

try {
final URL url = new URL(urlString);
urlConnection = (HttpURLConnection) url.openConnection();
in = new BufferedInputStream(urlConnection.getInputStream(),
IO_BUFFER_SIZE);
bitmap = BitmapFactory.decodeStream(in);
} catch (final IOException e) {
Log.e(TAG, "Error in downloadBitmap: " + e);
} finally {
if (urlConnection != null) {
urlConnection.disconnect();
}
MyUtils.close(in);
}
return bitmap;
}

private String hashKeyFormUrl(String url) {
String cacheKey;
try {
final MessageDigest mDigest = MessageDigest.getInstance("MD5");
mDigest.update(url.getBytes());
cacheKey = bytesToHexString(mDigest.digest());
} catch (NoSuchAlgorithmException e) {
cacheKey = String.valueOf(url.hashCode());
}
return cacheKey;
}

private String bytesToHexString(byte[] bytes) {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < bytes.length; i++) {
String hex = Integer.toHexString(0xFF & bytes[i]);
if (hex.length() == 1) {
sb.append('0');
}
sb.append(hex);
}
return sb.toString();
}

public File getDiskCacheDir(Context context, String uniqueName) {
boolean externalStorageAvailable = Environment
.getExternalStorageState().equals(Environment.MEDIA_MOUNTED);
final String cachePath;
if (externalStorageAvailable) {
cachePath = context.getExternalCacheDir().getPath();
} else {
cachePath = context.getCacheDir().getPath();
}

return new File(cachePath + File.separator + uniqueName);
}

@TargetApi(VERSION_CODES.GINGERBREAD)
private long getUsableSpace(File path) {
if (Build.VERSION.SDK_INT >= VERSION_CODES.GINGERBREAD) {
return path.getUsableSpace();
}
final StatFs stats = new StatFs(path.getPath());
return (long) stats.getBlockSize() * (long) stats.getAvailableBlocks();
}

private static class LoaderResult {
public ImageView imageView;
public String uri;
public Bitmap bitmap;

public LoaderResult(ImageView imageView, String uri, Bitmap bitmap) {
this.imageView = imageView;
this.uri = uri;
this.bitmap = bitmap;
}
}
}

附上DiskLruCache源码

DiskLruCache源码参考地址

说明，本文主要参考《Android开发艺术探索》