Bitmap的高效加载以及缓存机制

在实际的开发过程中，经常会对图片进行获取，并为用户展示在界面。由于android系统对每个应用有一定的内存限制，如果不合理的利用就会造成内存泄漏的情况。因此，在加载bitmap时进行恰当的优化，可以节省系统资源。另外，在进行网络、文件系统获取图片资源时，采用缓存机制，减少网络获取次数，会让应用更流畅，用户体验更友好。下面就从bitmap的高效加载、LruCache、DiskLruCache这三方面进行阐述：

Bitmap的加载

首先，先了解如何加载bitmap。bitmap是一张png、jpg等多种格式的图片，通过BitmapFactory的decodeFile、decodeResource、decodeStream、decodeByteArray四个方法分别从文件系统，资源，输入流以及字节数组中加载一个bitmap对象。这四类方法最终都在android的底层实现，对应着BitmapFactory的类的几个native方法。

那么如何高效的加载bitmap呢？其核心思想是通过BitmapFactory.Options来加载所需尺寸的图片。例如，当我们通过ImageView来加载图片时，若ImageView的尺寸要小于图片的尺寸，此时将图片按原比例放入imageView中，并不会完全显示。通过BitmapFactory.Options按一定的采样率来缩小图片，将缩小后的图片放入ImageView中显示，这样减少了内存的开销，一定程度上避免了OOM，提高了bitmap加载性能。

通过BitmapFactory.Options来缩放图片，主要用到了inSampleSize参数。当参数的值为1时，采样的大小为原始图片大小；当inSampleSize值大于1时，图片将被缩小，假设inSampleSize = 2，图片的长宽均被缩小为原始比例的1/2，像素并为原来的1/4，因此所在内存同样变为原来的1/4。值得注意的是，只有当inSampleSize的值大于1时，图片才会被缩小，由于长、宽被同时作用，图片总是被缩小为inSampleSize的2次方倍，即inSampleSize = 4，则图片将会被缩小为原比例的1/16。当inSampleSize的值小于1时，无效。实际情况中，假如ImageView的大小为100*100像素，图片大小为200*200，此时只要将inSampleSize值设为2即可。如果图片为200*300呢？此时inSampleSize的值还是设置为2比较合适，如果设置成3，那么图片尺寸将远小于ImageView，导致图片将被拉升，从而变得模糊。

通过采样率可以高效的加载bitmap，那么如何获取图片的采样率呢？

首先将BitmapFactory.Options的inJustDecodeBounds参数设置为true；

从BitmapFactory.Options中取出原始图片的宽高，对应于onWidth、onHeight；

根据目标view所需大小结合采样率规则，计算出inSampleSize 值；

将BitmapFactory.Options的inJustDecodeBounds参数设置为false，重新加载图片。

private Bitmap setBitmapImage(Resources res, int id, int width, int height){
BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeResource(res,id,options);

//获取inSampleSize值
options.inSampleSize = getinSampleSize(options,width,height);

options.inJustDecodeBounds = false;
return BitmapFactory.decodeResource(res,id,options);
}

private int getinSampleSize(BitmapFactory.Options options, int width, int height) {
int w = options.outWidth;
int h = options.outHeight;
int inSampleSize = 1;
if (w > width || h > height){
int halfw = w / 2;
int halfh = h / 2;
while ((halfw / inSampleSize) >= width && (halfh / inSampleSize) >= height){
inSampleSize *= 2;
}
}
return inSampleSize;
}

经过上面的步骤，加载出来的图片就是缩放后的图片，当然也有可能不需要缩放。值得一提的是，当inJustDecodeBounds参数为true时，BitmapFactory只会去解析原始图片的宽高，并不会真正的去加载图片，并且该操作是轻量级的。在使用的时候，假如ImageView的大小为100*100，则：

mImageView.setImageBitmap(setBitmapImage(getResources(),R.id.image,100,100));

上面介绍的BitmapFactory.decodeResource方法，其他三种加载Bitmap方法同样支持采样率的使用。

Android缓存策略

在应用的开发过程中，经常会涉及到网络请求，以获取图片、视频等资源。但是对于移动设备来说，数据流量的消耗对用户来说是很关心的，如何为用户减少网络请求，节省流量消耗，显得很有必要。因此，就引入了缓存的概念。以图片为例，当用户第一次网络获取图片后，通过缓存机制将图片存储到存储设备上，下次需要再次用到该图片时，直接从存储设备获取，不需要网络获取。多数情况下，为了提高应用的交互性，通常会将部分图片存储到内存中，再次使用时直接从内存获取，这样速度快于从存储设备、网络中获取。

上面引入了缓存机制，那么具体怎么实现呢？针对缓存机制，有不同的缓存策略，各种策略间并没有统一的标准，但基本包括添加、获取以及删除操作。添加和获取比较好理解，但是为什么还要删除呢？我们都知道，对于手机等移动设备来说，硬件是有限的，特别是存储能力，不可能达到无穷大。因此，在进行缓存时，当缓存容量满时，需要进行旧缓存的删除，以便进行新缓存。如何进行新旧文件的替换，这里就需要缓存算法的支持。目前最常用的缓存算法为近期最少使用算法LRU，核心思想为当缓存满时，会优先淘汰那些近期最少使用的缓存对象。采用Lru缓存算法的缓存机制有两种：LruCache和DiskLruCache，LruCache实现内存缓存，DiskLruCache实现储存设备缓存。

LruCache

LruCache是一个泛型类，内部采用LinkedHashMap以强引用的方式存储外部缓存对象，提供get和put方法来实现缓存对象的获取和添加，当缓存满时，LruCache会移除较早使用的缓存对象，加入新的缓存对象。介绍下几个概念：

1、强引用：直接的对象引用

2、软引用：当一个对象只有软引用存在时，内存资源不足时，此对象会被gc回收；

3、 弱引用：当一个对象只有弱引用存在时，此对象随时会被gc回收；

public class LruCache<K, V> {
private final LinkedHashMap<K, V> map;

LruCache是线程安全的，源码也比较简单。下面介绍其基本使用：

int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
int memory = maxMemory / 8;
mImageCache = new LruCache<String, Bitmap>(memory){
@Override
protected int sizeOf(String key, Bitmap value) {
return value.getRowBytes()*value.getHeight() / 1024;
}
};

上面的代码，先计算出当前进程的可用内存，设置缓存大小为可用内存大小的八分之一，通过sizeOf方法完成对bitmap对象大小的判断。除了LruCache的创建外，LruCache还包括缓存对象的添加，获取，删除等。

mImageCache.put(key,value);
mImageCache.get(key);
mImageCache.remove(key);

DisLruCache

DisLruCache用于实现磁盘缓存，通过将缓存对象写入文件系统而实现缓存的效果。值得注意的是，虽然DisLruCache得到了Android官方文档的推荐，但其并不包括在SDK中。当我们需要使用DisLruCache是，需要手动下载源文件，并引入项目中，下载地址：

android.googlesource.com/platform/libcore/+/jb-mr2-release/luni/src/main/java/libcore/io/DiskLruCache.java

如果不能访问可以点击这里进行下载。下面分别从DiskLruCache的创建、添加以及查找来描述其使用。

1、DiskLruCache的创建

DiskLruCache的创建并不能通过构造方法实现，

public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)

四个参数中，directory表示磁盘缓存对象所存储的文件系统路径，通常都会存放在 /sdcard/Android/data/ 包名 /cache 这个路径下面，当然，我们还可以选择支持的其他路径。当应用被卸载后，缓存文件也将被删除。

第二个参数appVersion表示应用版本号，一般设为1即可。当应用版本号发生变化时，系统内关于该应用的缓存将被清除。

第三个参数表示单个节点所对应数据的个数，一般设为1即可。第四个参数表示缓存的总大小，当缓存大小超过这个值后，DiskLruCache会清除一些缓存对象，以不大于这个值。

File dir = new File("/sdcard/Android/data/com.example.huangzheng.bitmaptest/cache");
int maxSize = 1024 * 1024 * 50;
if (!dir.exists()){
dir.mkdir();
}
try {
mDiskCache = DiskLruCache.open(dir,1,1,maxSize);
} catch (IOException e) {
e.printStackTrace();
}

2、DiskLruCache的缓存添加

还是一样的，我们以网络获取一张图片，并写入磁盘缓存为例。首先通过下面的代码获取网络图片，并通过OutputStream写入本地。

//网络获取图片，并写入输出流
private boolean downloadUrlToStream(String urlstring, OutputStream outputstream){
HttpURLConnection httpconnection = null;
BufferedOutputStream out = null;
BufferedInputStream in = null;
try {
final URL url = new URL(urlstring);
httpconnection = (HttpURLConnection) url.openConnection();
in = new BufferedInputStream(httpconnection.getInputStream(),8*1024);
out = new BufferedOutputStream(outputstream,8*1024);
int b;
if ((b = in.read()) != -1){
out.write(b);
}
return true;
}catch (IOException e) {
e.printStackTrace();
} finally {
if (httpconnection != null){
httpconnection.disconnect();
}
try {
if (out != null){
out.close();
}
if (in != null){
in.close();
}
} catch (IOException e) {
e.printStackTrace();
}
}
return false;
}

DiskLruCache的写入操作是通过其Editor 类实现，Editor 不能new，需要调用DiskLruCache的edit方法来创建。

public Editor edit(String key) throws IOException {
return edit(key, ANY_SEQUENCE_NUMBER);
}

从edit方法可以看到，需要传入key值，通常情况下图片的url中包括一些特殊字符，比较常规的做法是通过MD5进行编码，保证字符串的唯一性，并且所有字符都在0-F之间，再写入：

//MD5转换图片url key
private String hashKeyForDisk(String key){
String cacheKey = null;
try {
MessageDigest md = MessageDigest.getInstance("MD5");
md.update(key.getBytes());
cacheKey = bytesToHexString(md.digest());
} catch (NoSuchAlgorithmException e) {
e.printStackTrace();
}
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();
}

上面的准备做完，有了DiskLruCache.Editor实例后，通过它的newOutputStream方法获取OutputStream，并传入downloadUrlToStream方法实现下载和缓存，最后通过commit()进行提交，如果downloadUrlToStream返回false，则通过abort()方法进行中断。

new Thread(new Runnable() {
@Override
public void run() {
String imageUrl = "http://img.my.csdn.net/uploads/201309/01/1378037235_7476.jpg";
String key = hashKeyForDisk(imageUrl);
try {
DiskLruCache.Editor edit = mDiskCache.edit(key);
OutputStream out = edit.newOutputStream(0);
if (downloadUrlToStream(imageUrl,out)){
edit.commit();
} else {
edit.abort();
}
mDiskCache.flush();
} catch (IOException e) {
e.printStackTrace();
}
}
}).start();

在测试中，发现上面缓存地址有问题，无论怎样都找不到路径，因此通过下面方法获取到缓存路径：

public File getDiskCacheDir(Context context, String uniqueName) {
String cachePath;
if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())
|| !Environment.isExternalStorageRemovable()) {
cachePath = context.getExternalCacheDir().getPath();
} else {
cachePath = context.getCacheDir().getPath();
}
return new File(cachePath + File.separator + uniqueName);
}

执行程序，结果如下：



3、DiskLruCache的缓存读取

缓存成功后，从缓存文件里读取缓存相对就比较简单了。通过DiskLruCache.Snapshot读取缓存，同样的，Snapshot不可以直接new，需要DiskLruCache的get(key)方法获取实例，最后由Snapshot的getInputStream()方法获取缓存文件的输入流，BitmapFactory将输入流转换成bitmap。

try {
DiskLruCache.Snapshot snapshot = mDiskCache.get(key);
if (snapshot != null){
InputStream is = snapshot.getInputStream(0);
Bitmap img= BitmapFactory.decodeStream(is);
mImageView.setImageBitmap(img);
}
} catch (IOException e) {
e.printStackTrace();
}

这样，即使在移动设备没网的情况下，也能从缓存获取图片资源。上面介绍了磁盘缓存的创建，写入以及获取，相信对DiskLruCache的基本用法已有了了解，下面介绍其另外一些常用的API：

remove(key) 删除缓存中key所对应的缓存文件。在开发过程中，一般不需要我们手动去删除，当达到设定的缓存上限时，DiskLruCache会通过算法约定，自动删除部分近期最少使用的缓存文件。

size() 以byte为单位，返回当前缓存路径下所有缓存数据的总字节数；

flush() 该方法的作用是将内存中的操作记录到日志文件（journal文件）中，DiskLruCache正常工作将依赖该日志文件；

close() 和open方法相对应，用于关闭DiskLruCache缓存；

delete() 用于将所有缓存数据删除。比如我们在清理应用缓存时的“清除缓存”，就是调用的该方法。

综上，介绍了Bitmap的高效加载，LruCache、DiskLruCache的基本用法，相信在以后的开发过程中，涉及到图片的处理上会有所提升。最后，贴一下DiskLruCache的源码：

/*
* Copyright (C) 2011 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*      http://www.apache.org/licenses/LICENSE-2.0 *
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

import java.io.BufferedInputStream;
import java.io.BufferedWriter;
import java.io.Closeable;
import java.io.EOFException;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileWriter;
import java.io.FilterOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.Reader;
import java.io.StringWriter;
import java.io.Writer;
import java.lang.reflect.Array;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

/**
******************************************************************************
* Taken from the JB source code, can be found in:
* libcore/luni/src/main/java/libcore/io/DiskLruCache.java
* or direct link:
* https://android.googlesource.com/platform/libcore/+/android-4.1.1_r1/luni/src/main/java/libcore/io/DiskLruCache.java ******************************************************************************
*
* A cache that uses a bounded amount of space on a filesystem. Each cache
* entry has a string key and a fixed number of values. Values are byte
* sequences, accessible as streams or files. Each value must be between {@code
* 0} and {@code Integer.MAX_VALUE} bytes in length.
*
* <p>The cache stores its data in a directory on the filesystem. This
* directory must be exclusive to the cache; the cache may delete or overwrite
* files from its directory. It is an error for multiple processes to use the
* same cache directory at the same time.
*
* <p>This cache limits the number of bytes that it will store on the
* filesystem. When the number of stored bytes exceeds the limit, the cache will
* remove entries in the background until the limit is satisfied. The limit is
* not strict: the cache may temporarily exceed it while waiting for files to be
* deleted. The limit does not include filesystem overhead or the cache
* journal so space-sensitive applications should set a conservative limit.
*
* <p>Clients call {@link #edit} to create or update the values of an entry. An
* entry may have only one editor at one time; if a value is not available to be
* edited then {@link #edit} will return null.
* <ul>
*     <li>When an entry is being <strong>created</strong> it is necessary to
*         supply a full set of values; the empty value should be used as a
*         placeholder if necessary.
*     <li>When an entry is being <strong>edited</strong>, it is not necessary
*         to supply data for every value; values default to their previous
*         value.
* </ul>
* Every {@link #edit} call must be matched by a call to {@link Editor#commit}
* or {@link Editor#abort}. Committing is atomic: a read observes the full set
* of values as they were before or after the commit, but never a mix of values.
*
* <p>Clients call {@link #get} to read a snapshot of an entry. The read will
* observe the value at the time that {@link #get} was called. Updates and
* removals after the call do not impact ongoing reads.
*
* <p>This class is tolerant of some I/O errors. If files are missing from the
* filesystem, the corresponding entries will be dropped from the cache. If
* an error occurs while writing a cache value, the edit will fail silently.
* Callers should handle other problems by catching {@code IOException} and
* responding appropriately.
*/
public final class DiskLruCache implements Closeable {
static final String JOURNAL_FILE = "journal";
static final String JOURNAL_FILE_TMP = "journal.tmp";
static final String MAGIC = "libcore.io.DiskLruCache";
static final String VERSION_1 = "1";
static final long ANY_SEQUENCE_NUMBER = -1;
private static final String CLEAN = "CLEAN";
private static final String DIRTY = "DIRTY";
private static final String REMOVE = "REMOVE";
private static final String READ = "READ";

private static final Charset UTF_8 = Charset.forName("UTF-8");
private static final int IO_BUFFER_SIZE = 8 * 1024;

/*
* This cache uses a journal file named "journal". A typical journal file
* looks like this:
*     libcore.io.DiskLruCache
*     1
*     100
*     2
*
*     CLEAN 3400330d1dfc7f3f7f4b8d4d803dfcf6 832 21054
*     DIRTY 335c4c6028171cfddfbaae1a9c313c52
*     CLEAN 335c4c6028171cfddfbaae1a9c313c52 3934 2342
*     REMOVE 335c4c6028171cfddfbaae1a9c313c52
*     DIRTY 1ab96a171faeeee38496d8b330771a7a
*     CLEAN 1ab96a171faeeee38496d8b330771a7a 1600 234
*     READ 335c4c6028171cfddfbaae1a9c313c52
*     READ 3400330d1dfc7f3f7f4b8d4d803dfcf6
*
* The first five lines of the journal form its header. They are the
* constant string "libcore.io.DiskLruCache", the disk cache's version,
* the application's version, the value count, and a blank line.
*
* Each of the subsequent lines in the file is a record of the state of a
* cache entry. Each line contains space-separated values: a state, a key,
* and optional state-specific values.
*   o DIRTY lines track that an entry is actively being created or updated.
*     Every successful DIRTY action should be followed by a CLEAN or REMOVE
*     action. DIRTY lines without a matching CLEAN or REMOVE indicate that
*     temporary files may need to be deleted.
*   o CLEAN lines track a cache entry that has been successfully published
*     and may be read. A publish line is followed by the lengths of each of
*     its values.
*   o READ lines track accesses for LRU.
*   o REMOVE lines track entries that have been deleted.
*
* The journal file is appended to as cache operations occur. The journal may
* occasionally be compacted by dropping redundant lines. A temporary file named
* "journal.tmp" will be used during compaction; that file should be deleted if
* it exists when the cache is opened.
*/

private final File directory;
private final File journalFile;
private final File journalFileTmp;
private final int appVersion;
private final long maxSize;
private final int valueCount;
private long size = 0;
private Writer journalWriter;
private final LinkedHashMap<String, Entry> lruEntries
= new LinkedHashMap<String, Entry>(0, 0.75f, true);
private int redundantOpCount;

/**
* To differentiate between old and current snapshots, each entry is given
* a sequence number each time an edit is committed. A snapshot is stale if
* its sequence number is not equal to its entry's sequence number.
*/
private long nextSequenceNumber = 0;

/* From java.util.Arrays */
@SuppressWarnings("unchecked")
private static <T> T[] copyOfRange(T[] original, int start, int end) {
final int originalLength = original.length; // For exception priority compatibility.
if (start > end) {
throw new IllegalArgumentException();
}
if (start < 0 || start > originalLength) {
throw new ArrayIndexOutOfBoundsException();
}
final int resultLength = end - start;
final int copyLength = Math.min(resultLength, originalLength - start);
final T[] result = (T[]) Array
.newInstance(original.getClass().getComponentType(), resultLength);
System.arraycopy(original, start, result, 0, copyLength);
return result;
}

/**
* Returns the remainder of 'reader' as a string, closing it when done.
*/
public static String readFully(Reader reader) throws IOException {
try {
StringWriter writer = new StringWriter();
char[] buffer = new char[1024];
int count;
while ((count = reader.read(buffer)) != -1) {
writer.write(buffer, 0, count);
}
return writer.toString();
} finally {
reader.close();
}
}

/**
* Returns the ASCII characters up to but not including the next "\r\n", or
* "\n".
*
* @throws java.io.EOFException if the stream is exhausted before the next newline
*     character.
*/
public static String readAsciiLine(InputStream in) throws IOException {
// TODO: support UTF-8 here instead

StringBuilder result = new StringBuilder(80);
while (true) {
int c = in.read();
if (c == -1) {
throw new EOFException();
} else if (c == '\n') {
break;
}

result.append((char) c);
}
int length = result.length();
if (length > 0 && result.charAt(length - 1) == '\r') {
result.setLength(length - 1);
}
return result.toString();
}

/**
* Closes 'closeable', ignoring any checked exceptions. Does nothing if 'closeable' is null.
*/
public static void closeQuietly(Closeable closeable) {
if (closeable != null) {
try {
closeable.close();
} catch (RuntimeException rethrown) {
throw rethrown;
} catch (Exception ignored) {
}
}
}

/**
* Recursively delete everything in {@code dir}.
*/
// TODO: this should specify paths as Strings rather than as Files
public static void deleteContents(File dir) throws IOException {
File[] files = dir.listFiles();
if (files == null) {
throw new IllegalArgumentException("not a directory: " + dir);
}
for (File file : files) {
if (file.isDirectory()) {
deleteContents(file);
}
if (!file.delete()) {
throw new IOException("failed to delete file: " + file);
}
}
}

/** This cache uses a single background thread to evict entries. */
private final ExecutorService executorService = new ThreadPoolExecutor(0, 1,
60L, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>());
private final Callable<Void> cleanupCallable = new Callable<Void>() {
@Override public Void call() throws Exception {
synchronized (DiskLruCache.this) {
if (journalWriter == null) {
return null; // closed
}
trimToSize();
if (journalRebuildRequired()) {
rebuildJournal();
redundantOpCount = 0;
}
}
return null;
}
};

private DiskLruCache(File directory, int appVersion, int valueCount, long maxSize) {
this.directory = directory;
this.appVersion = appVersion;
this.journalFile = new File(directory, JOURNAL_FILE);
this.journalFileTmp = new File(directory, JOURNAL_FILE_TMP);
this.valueCount = valueCount;
this.maxSize = maxSize;
}

/**
* Opens the cache in {@code directory}, creating a cache if none exists
* there.
*
* @param directory a writable directory
* @param appVersion
* @param valueCount the number of values per cache entry. Must be positive.
* @param maxSize the maximum number of bytes this cache should use to store
* @throws java.io.IOException if reading or writing the cache directory fails
*/
public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)
throws IOException {
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
if (valueCount <= 0) {
throw new IllegalArgumentException("valueCount <= 0");
}

// prefer to pick up where we left off
DiskLruCache cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
if (cache.journalFile.exists()) {
try {
cache.readJournal();
cache.processJournal();
cache.journalWriter = new BufferedWriter(new FileWriter(cache.journalFile, true),
IO_BUFFER_SIZE);
return cache;
} catch (IOException journalIsCorrupt) {
//                System.logW("DiskLruCache " + directory + " is corrupt: "
//                        + journalIsCorrupt.getMessage() + ", removing");
cache.delete();
}
}

// create a new empty cache
directory.mkdirs();
cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
cache.rebuildJournal();
return cache;
}

private void readJournal() throws IOException {
InputStream in = new BufferedInputStream(new FileInputStream(journalFile), IO_BUFFER_SIZE);
try {
String magic = readAsciiLine(in);
String version = readAsciiLine(in);
String appVersionString = readAsciiLine(in);
String valueCountString = readAsciiLine(in);
String blank = readAsciiLine(in);
if (!MAGIC.equals(magic)
|| !VERSION_1.equals(version)
|| !Integer.toString(appVersion).equals(appVersionString)
|| !Integer.toString(valueCount).equals(valueCountString)
|| !"".equals(blank)) {
throw new IOException("unexpected journal header: ["
+ magic + ", " + version + ", " + valueCountString + ", " + blank + "]");
}

while (true) {
try {
readJournalLine(readAsciiLine(in));
} catch (EOFException endOfJournal) {
break;
}
}
} finally {
closeQuietly(in);
}
}

private void readJournalLine(String line) throws IOException {
String[] parts = line.split(" ");
if (parts.length < 2) {
throw new IOException("unexpected journal line: " + line);
}

String key = parts[1];
if (parts[0].equals(REMOVE) && parts.length == 2) {
lruEntries.remove(key);
return;
}

Entry entry = lruEntries.get(key);
if (entry == null) {
entry = new Entry(key);
lruEntries.put(key, entry);
}

if (parts[0].equals(CLEAN) && parts.length == 2 + valueCount) {
entry.readable = true;
entry.currentEditor = null;
entry.setLengths(copyOfRange(parts, 2, parts.length));
} else if (parts[0].equals(DIRTY) && parts.length == 2) {
entry.currentEditor = new Editor(entry);
} else if (parts[0].equals(READ) && parts.length == 2) {
// this work was already done by calling lruEntries.get()
} else {
throw new IOException("unexpected journal line: " + line);
}
}

/**
* Computes the initial size and collects garbage as a part of opening the
* cache. Dirty entries are assumed to be inconsistent and will be deleted.
*/
private void processJournal() throws IOException {
deleteIfExists(journalFileTmp);
for (Iterator<Entry> i = lruEntries.values().iterator(); i.hasNext(); ) {
Entry entry = i.next();
if (entry.currentEditor == null) {
for (int t = 0; t < valueCount; t++) {
size += entry.lengths[t];
}
} else {
entry.currentEditor = null;
for (int t = 0; t < valueCount; t++) {
deleteIfExists(entry.getCleanFile(t));
deleteIfExists(entry.getDirtyFile(t));
}
i.remove();
}
}
}

/**
* Creates a new journal that omits redundant information. This replaces the
* current journal if it exists.
*/
private synchronized void rebuildJournal() throws IOException {
if (journalWriter != null) {
journalWriter.close();
}

Writer writer = new BufferedWriter(new FileWriter(journalFileTmp), IO_BUFFER_SIZE);
writer.write(MAGIC);
writer.write("\n");
writer.write(VERSION_1);
writer.write("\n");
writer.write(Integer.toString(appVersion));
writer.write("\n");
writer.write(Integer.toString(valueCount));
writer.write("\n");
writer.write("\n");

for (Entry entry : lruEntries.values()) {
if (entry.currentEditor != null) {
writer.write(DIRTY + ' ' + entry.key + '\n');
} else {
writer.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\n');
}
}

writer.close();
journalFileTmp.renameTo(journalFile);
journalWriter = new BufferedWriter(new FileWriter(journalFile, true), IO_BUFFER_SIZE);
}

private static void deleteIfExists(File file) throws IOException {
//        try {
//            Libcore.os.remove(file.getPath());
//        } catch (ErrnoException errnoException) {
//            if (errnoException.errno != OsConstants.ENOENT) {
//                throw errnoException.rethrowAsIOException();
//            }
//        }
if (file.exists() && !file.delete()) {
throw new IOException();
}
}

/**
* Returns a snapshot of the entry named {@code key}, or null if it doesn't
* exist is not currently readable. If a value is returned, it is moved to
* the head of the LRU queue.
*/
public synchronized Snapshot get(String key) throws IOException {
checkNotClosed();
validateKey(key);
Entry entry = lruEntries.get(key);
if (entry == null) {
return null;
}

if (!entry.readable) {
return null;
}

/*
* Open all streams eagerly to guarantee that we see a single published
* snapshot. If we opened streams lazily then the streams could come
* from different edits.
*/
InputStream[] ins = new InputStream[valueCount];
try {
for (int i = 0; i < valueCount; i++) {
ins[i] = new FileInputStream(entry.getCleanFile(i));
}
} catch (FileNotFoundException e) {
// a file must have been deleted manually!
return null;
}

redundantOpCount++;
journalWriter.append(READ + ' ' + key + '\n');
if (journalRebuildRequired()) {
executorService.submit(cleanupCallable);
}

return new Snapshot(key, entry.sequenceNumber, ins);
}

/**
* Returns an editor for the entry named {@code key}, or null if another
* edit is in progress.
*/
public Editor edit(String key) throws IOException {
return edit(key, ANY_SEQUENCE_NUMBER);
}

private synchronized Editor edit(String key, long expectedSequenceNumber) throws IOException {
checkNotClosed();
validateKey(key);
Entry entry = lruEntries.get(key);
if (expectedSequenceNumber != ANY_SEQUENCE_NUMBER
&& (entry == null || entry.sequenceNumber != expectedSequenceNumber)) {
return null; // snapshot is stale
}
if (entry == null) {
entry = new Entry(key);
lruEntries.put(key, entry);
} else if (entry.currentEditor != null) {
return null; // another edit is in progress
}

Editor editor = new Editor(entry);
entry.currentEditor = editor;

// flush the journal before creating files to prevent file leaks
journalWriter.write(DIRTY + ' ' + key + '\n');
journalWriter.flush();
return editor;
}

/**
* Returns the directory where this cache stores its data.
*/
public File getDirectory() {
return directory;
}

/**
* Returns the maximum number of bytes that this cache should use to store
* its data.
*/
public long maxSize() {
return maxSize;
}

/**
* Returns the number of bytes currently being used to store the values in
* this cache. This may be greater than the max size if a background
* deletion is pending.
*/
public synchronized long size() {
return size;
}

private synchronized void completeEdit(Editor editor, boolean success) throws IOException {
Entry entry = editor.entry;
if (entry.currentEditor != editor) {
throw new IllegalStateException();
}

// if this edit is creating the entry for the first time, every index must have a value
if (success && !entry.readable) {
for (int i = 0; i < valueCount; i++) {
if (!entry.getDirtyFile(i).exists()) {
editor.abort();
throw new IllegalStateException("edit didn't create file " + i);
}
}
}

for (int i = 0; i < valueCount; i++) {
File dirty = entry.getDirtyFile(i);
if (success) {
if (dirty.exists()) {
File clean = entry.getCleanFile(i);
dirty.renameTo(clean);
long oldLength = entry.lengths[i];
long newLength = clean.length();
entry.lengths[i] = newLength;
size = size - oldLength + newLength;
}
} else {
deleteIfExists(dirty);
}
}

redundantOpCount++;
entry.currentEditor = null;
if (entry.readable | success) {
entry.readable = true;
journalWriter.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\n');
if (success) {
entry.sequenceNumber = nextSequenceNumber++;
}
} else {
lruEntries.remove(entry.key);
journalWriter.write(REMOVE + ' ' + entry.key + '\n');
}

if (size > maxSize || journalRebuildRequired()) {
executorService.submit(cleanupCallable);
}
}

/**
* We only rebuild the journal when it will halve the size of the journal
* and eliminate at least 2000 ops.
*/
private boolean journalRebuildRequired() {
final int REDUNDANT_OP_COMPACT_THRESHOLD = 2000;
return redundantOpCount >= REDUNDANT_OP_COMPACT_THRESHOLD
&& redundantOpCount >= lruEntries.size();
}

/**
* Drops the entry for {@code key} if it exists and can be removed. Entries
* actively being edited cannot be removed.
*
* @return true if an entry was removed.
*/
public synchronized boolean remove(String key) throws IOException {
checkNotClosed();
validateKey(key);
Entry entry = lruEntries.get(key);
if (entry == null || entry.currentEditor != null) {
return false;
}

for (int i = 0; i < valueCount; i++) {
File file = entry.getCleanFile(i);
if (!file.delete()) {
throw new IOException("failed to delete " + file);
}
size -= entry.lengths[i];
entry.lengths[i] = 0;
}

redundantOpCount++;
journalWriter.append(REMOVE + ' ' + key + '\n');
lruEntries.remove(key);

if (journalRebuildRequired()) {
executorService.submit(cleanupCallable);
}

return true;
}

/**
* Returns true if this cache has been closed.
*/
public boolean isClosed() {
return journalWriter == null;
}

private void checkNotClosed() {
if (journalWriter == null) {
throw new IllegalStateException("cache is closed");
}
}

/**
* Force buffered operations to the filesystem.
*/
public synchronized void flush() throws IOException {
checkNotClosed();
trimToSize();
journalWriter.flush();
}

/**
* Closes this cache. Stored values will remain on the filesystem.
*/
public synchronized void close() throws IOException {
if (journalWriter == null) {
return; // already closed
}
for (Entry entry : new ArrayList<Entry>(lruEntries.values())) {
if (entry.currentEditor != null) {
entry.currentEditor.abort();
}
}
trimToSize();
journalWriter.close();
journalWriter = null;
}

private void trimToSize() throws IOException {
while (size > maxSize) {
//            Map.Entry<String, Entry> toEvict = lruEntries.eldest();
final Map.Entry<String, Entry> toEvict = lruEntries.entrySet().iterator().next();
remove(toEvict.getKey());
}
}

/**
* Closes the cache and deletes all of its stored values. This will delete
* all files in the cache directory including files that weren't created by
* the cache.
*/
public void delete() throws IOException {
close();
deleteContents(directory);
}

private void validateKey(String key) {
if (key.contains(" ") || key.contains("\n") || key.contains("\r")) {
throw new IllegalArgumentException(
"keys must not contain spaces or newlines: \"" + key + "\"");
}
}

private static String inputStreamToString(InputStream in) throws IOException {
return readFully(new InputStreamReader(in, UTF_8));
}

/**
* A snapshot of the values for an entry.
*/
public final class Snapshot implements Closeable {
private final String key;
private final long sequenceNumber;
private final InputStream[] ins;

private Snapshot(String key, long sequenceNumber, InputStream[] ins) {
this.key = key;
this.sequenceNumber = sequenceNumber;
this.ins = ins;
}

/**
* Returns an editor for this snapshot's entry, or null if either the
* entry has changed since this snapshot was created or if another edit
* is in progress.
*/
public Editor edit() throws IOException {
return DiskLruCache.this.edit(key, sequenceNumber);
}

/**
* Returns the unbuffered stream with the value for {@code index}.
*/
public InputStream getInputStream(int index) {
return ins[index];
}

/**
* Returns the string value for {@code index}.
*/
public String getString(int index) throws IOException {
return inputStreamToString(getInputStream(index));
}

@Override public void close() {
for (InputStream in : ins) {
closeQuietly(in);
}
}
}

/**
* Edits the values for an entry.
*/
public final class Editor {
private final Entry entry;
private boolean hasErrors;

private Editor(Entry entry) {
this.entry = entry;
}

/**
* Returns an unbuffered input stream to read the last committed value,
* or null if no value has been committed.
*/
public InputStream newInputStream(int index) throws IOException {
synchronized (DiskLruCache.this) {
if (entry.currentEditor != this) {
throw new IllegalStateException();
}
if (!entry.readable) {
return null;
}
return new FileInputStream(entry.getCleanFile(index));
}
}

/**
* Returns the last committed value as a string, or null if no value
* has been committed.
*/
public String getString(int index) throws IOException {
InputStream in = newInputStream(index);
return in != null ? inputStreamToString(in) : null;
}

/**
* Returns a new unbuffered output stream to write the value at
* {@code index}. If the underlying output stream encounters errors
* when writing to the filesystem, this edit will be aborted when
* {@link #commit} is called. The returned output stream does not throw
* IOExceptions.
*/
public OutputStream newOutputStream(int index) throws IOException {
synchronized (DiskLruCache.this) {
if (entry.currentEditor != this) {
throw new IllegalStateException();
}
return new FaultHidingOutputStream(new FileOutputStream(entry.getDirtyFile(index)));
}
}

/**
* Sets the value at {@code index} to {@code value}.
*/
public void set(int index, String value) throws IOException {
Writer writer = null;
try {
writer = new OutputStreamWriter(newOutputStream(index), UTF_8);
writer.write(value);
} finally {
closeQuietly(writer);
}
}

/**
* Commits this edit so it is visible to readers.  This releases the
* edit lock so another edit may be started on the same key.
*/
public void commit() throws IOException {
if (hasErrors) {
completeEdit(this, false);
remove(entry.key); // the previous entry is stale
} else {
completeEdit(this, true);
}
}

/**
* Aborts this edit. This releases the edit lock so another edit may be
* started on the same key.
*/
public void abort() throws IOException {
completeEdit(this, false);
}

private class FaultHidingOutputStream extends FilterOutputStream {
private FaultHidingOutputStream(OutputStream out) {
super(out);
}

@Override public void write(int oneByte) {
try {
out.write(oneByte);
} catch (IOException e) {
hasErrors = true;
}
}

@Override public void write(byte[] buffer, int offset, int length) {
try {
out.write(buffer, offset, length);
} catch (IOException e) {
hasErrors = true;
}
}

@Override public void close() {
try {
out.close();
} catch (IOException e) {
hasErrors = true;
}
}

@Override public void flush() {
try {
out.flush();
} catch (IOException e) {
hasErrors = true;
}
}
}
}

private final class Entry {
private final String key;

/** Lengths of this entry's files. */
private final long[] lengths;

/** True if this entry has ever been published */
private boolean readable;

/** The ongoing edit or null if this entry is not being edited. */
private Editor currentEditor;

/** The sequence number of the most recently committed edit to this entry. */
private long sequenceNumber;

private Entry(String key) {
this.key = key;
this.lengths = new long[valueCount];
}

public String getLengths() throws IOException {
StringBuilder result = new StringBuilder();
for (long size : lengths) {
result.append(' ').append(size);
}
return result.toString();
}

/**
* Set lengths using decimal numbers like "10123".
*/
private void setLengths(String[] strings) throws IOException {
if (strings.length != valueCount) {
throw invalidLengths(strings);
}

try {
for (int i = 0; i < strings.length; i++) {
lengths[i] = Long.parseLong(strings[i]);
}
} catch (NumberFormatException e) {
throw invalidLengths(strings);
}
}

private IOException invalidLengths(String[] strings) throws IOException {
throw new IOException("unexpected journal line: " + Arrays.toString(strings));
}

public File getCleanFile(int i) {
return new File(directory, key + "." + i);
}

public File getDirtyFile(int i) {
return new File(directory, key + "." + i + ".tmp");
}
}
}