前言：
上篇我们总结了Bitmap的处理，同时对比了各种处理的效率以及对内存占用大小，点击查看。我们得知一个应用如果使用大量图片就会导致OOM（out of memory）,那该如何处理才能近可能的降低oom发生的概率呢？之前我们一直在使用SoftReference软引用，SoftReference是一种现在已经不再推荐使用的方式，因为从 Android 2.3 (API Level 9)开始，垃圾回收器会更倾向于回收持有软引用或弱引用的对象，这让软引用变得不再可靠，所以今天我们来认识一种新的缓存处理算法Lru，然后学习一下基于Lru的Lrucache、DiskLruCache 实现我们的图片缓存。
Lru：
LRU是Least Recently Used 的缩写，翻译过来就是“最近最少使用”，LRU缓存就是使用这种原理实现，简单的说就是缓存一定量的数据，当超过设定的阈值时就把一些过期的数据删除掉，比如我们缓存10000条数据，当数据小于10000时可以随意添加，当超过10000时就需要把新的数据添加进来，同时要把过期数据删除，以确保我们最大缓存10000条，那怎么确定删除哪条过期数据呢，采用LRU算法实现的话就是将最老的数据删掉。
基于LruCache实现内存缓存：
1.）初始化MemoryCache
这里内存缓存的是Drawable 而不是Bitmap 理由是Drawable相对Bitmap来说有很大的内存优势
int maxMemory = (int) Runtime.getRuntime().maxMemory();//获取系统分配给应用的总内存大小
int mCacheSize = maxMemory / 8;//设置图片内存缓存占用八分之一
mMemoryCache = new LruCache<String, Drawable>(mCacheSize) {
//必须重写此方法，来测量Bitmap的大小
@Override
protected int sizeOf(String key, Drawable value) {
if (value instanceof BitmapDrawable) {
Bitmap bitmap = ((BitmapDrawable) value).getBitmap();
return bitmap == null ? 0 : bitmap.getByteCount();
}
return super.sizeOf(key, value);
}
};2.）添加一个Drawable到内存缓存
private void addDrawableToMemoryCache(String key, Drawable drawable) {
if (getDrawableFromMemCache(key) == null && drawable != null) {
mMemoryCache.put(key, drawable);
}
}
3.）从内存缓存中获取一个Drawable
public Drawable getDrawableFromMemCache(String key) {
return mMemoryCache.get(key);
}
4.）从内存缓存中移除一个Drawable
public void removeCacheFromMemory(String key) {
mMemoryCache.remove(key);
}
5.）清空内存缓存
public void cleanMemoryCCache() {
mMemoryCache.evictAll();
}
其实Lru缓存机制本质上就是存储在一个LinkedHashMap存储，为了保障插入的数据顺序，方便清理。
基于DiskLruCache实现磁盘缓存：
DiskLruCache类并不是谷歌官方实现，需要自行下载，下载地址：https://github.com/JakeWharton/DiskLruCache
1.）初始化DiskLruCache
File cacheDir = context.getCacheDir();//指定的是数据的缓存地址
long diskCacheSize = 1024 * 1024 * 30;//最多可以缓存多少字节的数据
int appVersion = DiskLruUtils.getAppVersion(context);//指定当前应用程序的版本号
int valueCount = 1;//指定同一个key可以对应多少个缓存文件
try {
mDiskCache = DiskLruCache.open(cacheDir, appVersion, valueCount, diskCacheSize);
} catch (Exception ex) {
}
2.）写入一个文件到磁盘缓存
private void addBitmapToDiskCache(String key, byte[] value) {
OutputStream out = null;
try {
DiskLruCache.Editor editor = mDiskCache.edit(key);
if (editor != null) {
out = editor.newOutputStream(0);
if (value != null && value.length > 0) {
out.write(value);
out.flush();
editor.commit();
} else {
editor.abort();
}
}
mDiskCache.flush();
} catch (IOException e) {
e.printStackTrace();
} finally {
DiskLruUtils.closeQuietly(out);
}
}3.）从磁盘缓存中读取Drawable
public Drawable getDrawableFromDiskCache(String key) {
try {
DiskLruCache.Snapshot snapShot = mDiskCache.get(key);
if (snapShot != null) {
InputStream is = snapShot.getInputStream(0);
Bitmap bitmap = BitmapFactory.decodeStream(is);
Drawable drawable = DiskLruUtils.bitmap2Drawable(bitmap);
//从磁盘中读取到之后 加入内存缓存
addDrawableToMemoryCache(key, drawable);
return drawable;
}
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
4.）从磁盘缓存中移除
public void removeCacheFromDisk(String key) {
try {
mDiskCache.remove(key);
} catch (Exception e) {
}
}
5.）清空磁盘缓存
public void cleanDiskCache() {
try {
mDiskCache.delete();
} catch (Exception e) {
}
}
图片下载过程：
接下来实例中用到了一点RxJava的知识有不了解RxJava的请自行了解一下。
1.）采用异步方式操作磁盘缓存和网络下载， 内存缓存可以在主线程中操作
public void disPlay(final ImageView imageView, String imageUrl) {
//生成唯一key
final String key = DiskLruUtils.hashKeyForDisk(imageUrl);
//先从内存中读取
Drawable drawableFromMemCache = getDrawableFromMemCache(key);
if (drawableFromMemCache != null) {
imageView.setImageDrawable(drawableFromMemCache);
return;
}
Observable.just(imageUrl)
.map(new Func1<String, Drawable>() {
@Override
public Drawable call(String imageUrl) { // 参数类型 String
//从磁盘中读取
Drawable drawableFromDiskCache = getDrawableFromDiskCache(key);
if (drawableFromDiskCache != null) {
return drawableFromDiskCache;
}
//网络下载
return download(imageUrl); // 返回类型 Drawable
}
})
.subscribeOn(Schedulers.io()) // 指定 subscribe() 发生在 IO 线程
.observeOn(AndroidSchedulers.mainThread()) // 指定 Subscriber 的回调发生在主线程
.subscribe(new Action1<Drawable>() {
@Override
public void call(Drawable drawable) { // 参数类型 Drawable
imageView.setImageDrawable(drawable);
}
});
}
2.）下载图片过程以及处理
private Drawable download(String imageUrl) {
HttpURLConnection urlConnection = null;
ByteArrayOutputStream bos = null;
InputStream ins = null;
try {
final URL url = new URL(imageUrl);
urlConnection = (HttpURLConnection) url.openConnection();
ins = urlConnection.getInputStream();
bos = new ByteArrayOutputStream();
int b;
while ((b = ins.read()) != -1) {
bos.write(b);
}
bos.flush();
byte[] bytes = bos.toByteArray();
Bitmap bitmap = DiskLruUtils.bytes2Bitmap(bytes);
String key = DiskLruUtils.hashKeyForDisk(imageUrl);
Drawable drawable = DiskLruUtils.bitmap2Drawable(bitmap);
//加入内存缓存
addDrawableToMemoryCache(key, drawable);
//加入磁盘缓存
addBitmapToDiskCache(key, bytes);
return drawable;
} catch (IOException e) {
e.printStackTrace();
} finally {
if (urlConnection != null) {
urlConnection.disconnect();
}
DiskLruUtils.closeQuietly(bos);
DiskLruUtils.closeQuietly(ins);
}
return null;
}
附上最终图片缓存单例简单实现全部代码以及DiskLruUtils工具类代码
ImageLoadManager.java
public class ImageLoadManager {
private LruCache<String, Drawable> mMemoryCache;//内存缓存
private DiskLruCache mDiskCache;//磁盘缓存
private static ImageLoadManager mInstance;//获取图片下载单例引用
private ImageLoadManager(Context context) {
int maxMemory = (int) Runtime.getRuntime().maxMemory();//获取系统分配给应用的总内存大小
int mCacheSize = maxMemory / 8;//设置图片内存缓存占用八分之一
mMemoryCache = new LruCache<String, Drawable>(mCacheSize) {
//必须重写此方法，来测量Bitmap的大小
@Override
protected int sizeOf(String key, Drawable value) {
if (value instanceof BitmapDrawable) {
Bitmap bitmap = ((BitmapDrawable) value).getBitmap();
return bitmap == null ? 0 : bitmap.getByteCount();
}
return super.sizeOf(key, value);
}
};
File cacheDir = context.getCacheDir();//指定的是数据的缓存地址
long diskCacheSize = 1024 * 1024 * 30;//最多可以缓存多少字节的数据
int appVersion = DiskLruUtils.getAppVersion(context);//指定当前应用程序的版本号
int valueCount = 1;//指定同一个key可以对应多少个缓存文件
try {
mDiskCache = DiskLruCache.open(cacheDir, appVersion, valueCount, diskCacheSize);
} catch (Exception ex) {
}
}
public static ImageLoadManager getInstance(Context context) {
ImageLoadManager inst = mInstance;
if (inst == null) {
synchronized (RequestManager.class) {
inst = mInstance;
if (inst == null) {
inst = new ImageLoadManager(context.getApplicationContext());
mInstance = inst;
}
}
}
return inst;
}
public void disPlay(final ImageView imageView, String imageUrl) {
//生成唯一key
final String key = DiskLruUtils.hashKeyForDisk(imageUrl);
//先从内存中读取
Drawable drawableFromMemCache = getDrawableFromMemCache(key);
if (drawableFromMemCache != null) {
imageView.setImageDrawable(drawableFromMemCache);
return;
}
Observable.just(imageUrl)
.map(new Func1<String, Drawable>() {
@Override
public Drawable call(String imageUrl) { // 参数类型 String
//从磁盘中读取
Drawable drawableFromDiskCache = getDrawableFromDiskCache(key);
if (drawableFromDiskCache != null) {
return drawableFromDiskCache;
}
//网络下载
return download(imageUrl); // 返回类型 Drawable
}
})
.subscribeOn(Schedulers.io()) // 指定 subscribe() 发生在 IO 线程
.observeOn(AndroidSchedulers.mainThread()) // 指定 Subscriber 的回调发生在主线程
.subscribe(new Action1<Drawable>() {
@Override
public void call(Drawable drawable) { // 参数类型 Drawable
imageView.setImageDrawable(drawable);
}
});
}
private void addDrawableToMemoryCache(String key, Drawable drawable) {
if (getDrawableFromMemCache(key) == null && drawable != null) {
mMemoryCache.put(key, drawable);
}
}
public Drawable getDrawableFromMemCache(String key) {
return mMemoryCache.get(key);
}
public Drawable getDrawableFromDiskCache(String key) {
try {
DiskLruCache.Snapshot snapShot = mDiskCache.get(key);
if (snapShot != null) {
InputStream is = snapShot.getInputStream(0);
Bitmap bitmap = BitmapFactory.decodeStream(is);
Drawable drawable = DiskLruUtils.bitmap2Drawable(bitmap);
//从磁盘中读取到之后 加入内存缓存
addDrawableToMemoryCache(key, drawable);
return drawable;
}
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
private void addBitmapToDiskCache(String key, byte[] value) {
OutputStream out = null;
try {
DiskLruCache.Editor editor = mDiskCache.edit(key);
if (editor != null) {
out = editor.newOutputStream(0);
if (value != null && value.length > 0) {
out.write(value);
out.flush();
editor.commit();
} else {
editor.abort();
}
}
mDiskCache.flush();
} catch (IOException e) {
e.printStackTrace();
} finally {
DiskLruUtils.closeQuietly(out);
}
}
private Drawable download(String imageUrl) {
HttpURLConnection urlConnection = null;
ByteArrayOutputStream bos = null;
InputStream ins = null;
try {
final URL url = new URL(imageUrl);
urlConnection = (HttpURLConnection) url.openConnection();
ins = urlConnection.getInputStream();
bos = new ByteArrayOutputStream();
int b;
while ((b = ins.read()) != -1) {
bos.write(b);
}
bos.flush();
byte[] bytes = bos.toByteArray();
Bitmap bitmap = DiskLruUtils.bytes2Bitmap(bytes);
String key = DiskLruUtils.hashKeyForDisk(imageUrl);
Drawable drawable = DiskLruUtils.bitmap2Drawable(bitmap);
//加入内存缓存
// addDrawableToMemoryCache(key, drawable);
//加入磁盘缓存
addBitmapToDiskCache(key, bytes);
return drawable;
} catch (IOException e) {
e.printStackTrace();
} finally {
if (urlConnection != null) {
urlConnection.disconnect();
}
DiskLruUtils.closeQuietly(bos);
DiskLruUtils.closeQuietly(ins);
}
return null;
}
public void removeCache(String key) {
removeCacheFromMemory(key);
removeCacheFromDisk(key);
}
public void removeCacheFromMemory(String key) {
mMemoryCache.remove(key);
}
public void removeCacheFromDisk(String key) {
try {
mDiskCache.remove(key);
} catch (Exception e) {
}
}
public long diskCacheSize() {
return mDiskCache.size();
}
public long memoryCacheSize() {
return mMemoryCache.size();
}
public void closeDiskCache() {
try {
mDiskCache.close();
} catch (Exception e) {
}
}
public void cleanCache() {
cleanMemoryCCache();
cleanDiskCache();
}
public void cleanDiskCache() {
try {
mDiskCache.delete();
} catch (Exception e) {
}
}
public void cleanMemoryCCache() {
mMemoryCache.evictAll();
}
}
DiskLruUtils.java
final class DiskLruUtils {
public static void closeQuietly(Closeable closeable) {
if (closeable != null) {
try {
closeable.close();
} catch (RuntimeException rethrown) {
throw rethrown;
} catch (Exception ignored) {
}
}
}
public static int getAppVersion(Context context) {
try {
PackageInfo info = context.getPackageManager().getPackageInfo(context.getPackageName(), 0);
return info.versionCode;
} catch (PackageManager.NameNotFoundException e) {
e.printStackTrace();
}
return 1;
}
public static String hashKeyForDisk(String key) {
String cacheKey;
try {
final MessageDigest mDigest = MessageDigest.getInstance("MD5");
mDigest.update(key.getBytes());
cacheKey = bytesToHexString(mDigest.digest());
} catch (NoSuchAlgorithmException e) {
cacheKey = String.valueOf(key.hashCode());
}
return cacheKey;
}
public static 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 static byte[] bitmap2Bytes(Bitmap bm) {
if (bm == null) {
return null;
}
ByteArrayOutputStream baos = new ByteArrayOutputStream();
bm.compress(Bitmap.CompressFormat.PNG, 100, baos);
return baos.toByteArray();
}
public static Bitmap bytes2Bitmap(byte[] bytes) {
return BitmapFactory.decodeByteArray(bytes, 0, bytes.length);
}
public static Bitmap drawable2Bitmap(Drawable drawable) {
if (drawable == null) {
return null;
}
// 取 drawable 的长宽
int w = drawable.getIntrinsicWidth();
int h = drawable.getIntrinsicHeight();
// 取 drawable 的颜色格式
Bitmap.Config config = drawable.getOpacity() != PixelFormat.OPAQUE ? Bitmap.Config.ARGB_8888 : Bitmap.Config.RGB_565;
// 建立对应 bitmap
Bitmap bitmap = Bitmap.createBitmap(w, h, config);
// 建立对应 bitmap 的画布
Canvas canvas = new Canvas(bitmap);
drawable.setBounds(0, 0, w, h);
// 把 drawable 内容画到画布中
drawable.draw(canvas);
return bitmap;
}
public static Drawable bitmap2Drawable(Bitmap bm) {
if (bm == null) {
return null;
}
BitmapDrawable bd = new BitmapDrawable(bm);
bd.setTargetDensity(bm.getDensity());
return new BitmapDrawable(bm);
}
}
以上就是基于Lru图片缓存简单实现，希望对大家的学习有所帮助，也希望大家多多支持编程网。
