安卓开发笔记——关于图片的三级缓存策略（内存LruCache+磁盘DiskLruCache+网络Volley）

　　在开发安卓应用中避免不了要使用到网络图片，获取网络图片很简单，但是需要付出一定的代价——流量。对于少数的图片而言问题不大，但如果手机应用中包含大量的图片，这势必会耗费用户的一定流量，如果我们不加以处理，每次打开应用都去网络获取图片，那么用户可就不乐意了，这里的处理就是指今天要讲的缓存策略（缓存层分为三层：内存层，磁盘层，网络层）。

　　关于缓存层的工作，当我们第一次打开应用获取图片时，先到网络去下载图片，然后依次存入内存缓存，磁盘缓存，当我们再一次需要用到刚才下载的这张图片时，就不需要再重复的到网络上去下载，直接可以从内存缓存和磁盘缓存中找，由于内存缓存速度较快，我们优先到内存缓存中寻找该图片，如果找到则运用，如果没有找到（内存缓存大小有限），那么我们再到磁盘缓存中去找。只要我们合理的去协调这三层缓存运用，便可以提升应用性能和用户体验。

1、内存层：（手机内存）

内存缓存相对于磁盘缓存而言，速度要来的快很多，但缺点容量较小且会被系统回收，这里的实现我用到了LruCache。

LruCache这个类是Android3.1版本中提供的，如果你是在更早的Android版本中开发，则需要导入android-support-v4的jar包。

磁盘层：（SD卡）

相比内存缓存而言速度要来得慢很多，但容量很大，这里的实现我用到了DiskLruCache类。

DiskLruCache是非Google官方编写，但获得官方认证的硬盘缓存类，该类没有限定在Android内，所以理论上java应用也可以使用DiskLreCache来缓存。

这是DiskLruCache类的下载地址：http://pan.baidu.com/s/1hq0D53m

网络层：（移动网络，无线网络）

这个就没什么解释的了，就是我们上网用的流量。这里的网络访问实现我用到了开源框架Volley。

开源框架Volley是2013年Google I/O大会发布的，Volley是Android平台上的网络通信库，能使网络通信更快，更简单，更健壮。它的设计目标就是非常适合去进行数据量不大，但通信频繁的网络操作，而对于大数据量的网络操作，比如说下载文件等，Volley的表现就会非常糟糕。

这是Volley的下载地址：http://pan.baidu.com/s/1hq1t2yo

先来看下效果图：

正常网络下：                                    　　　　断开网络，飞行模式下：

  　　　　 　　

Log日志打印：

来看下代码实现：

1、由于应用中很多地方需要用到上下文对象，这里我自定义了一个全局的Application，用来提供上下文对象

 package com.lcw.rabbit.image.utils;

 import android.app.Application;
 /**
  * Application类，提供全局上下文对象
  * @author Rabbit_Lee
  *
  */
 public class MyApplication extends Application {

     public static String TAG;
     public static MyApplication myApplication;

     public static MyApplication newInstance() {
         return myApplication;
     }

     @Override
     public void onCreate() {
         super.onCreate();
         TAG = this.getClass().getSimpleName();
         myApplication = this;

     }
 }

2、Volley请求队列处理类，用来管理Rquest请求对象操作

 package com.lcw.rabbit.image;

 import com.android.volley.Request;
 import com.android.volley.RequestQueue;
 import com.android.volley.toolbox.Volley;
 import com.lcw.rabbit.image.utils.MyApplication;

 /**
  * 请求队列处理类
  * 获取RequestQueue对象
  */
 public class VolleyRequestQueueManager {
     // 获取请求队列类
     public static RequestQueue mRequestQueue = Volley.newRequestQueue(MyApplication.newInstance());

     //添加任务进任务队列
     public static void addRequest(Request<?> request, Object tag) {
         if (tag != null) {
             request.setTag(tag);
         }
         mRequestQueue.add(request);
     }

     //取消任务
     public static void cancelRequest(Object tag){
         mRequestQueue.cancelAll(tag);
     }

 }

3、这里附上2个工具类（生成MD5序列帮助类，DiskLruCache磁盘缓存类）

 package com.lcw.rabbit.image.utils;

 import java.math.BigInteger;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;

 public class MD5Utils {
     /**
      * 使用md5的算法进行加密
      */
     public static String md5(String plainText) {
         byte[] secretBytes = null;
         try {
             secretBytes = MessageDigest.getInstance("md5").digest(
                     plainText.getBytes());
         } catch (NoSuchAlgorithmException e) {
             throw new RuntimeException("没有md5这个算法！");
         }
         String md5code = new BigInteger(1, secretBytes).toString(16);// 16进制数字
         // 如果生成数字未满32位，需要前面补0
         for (int i = 0; i < 32 - md5code.length(); i++) {
             md5code = "0" + md5code;
         }
         return md5code;
     }

 }

MD5转换类

 /*
  * 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.
  */

 package com.lcw.rabbit.image.utils;

 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");
         }
     }
 }

DiskLruCache磁盘缓存类

4、图片缓存类，包含（LruCache内存缓存，DiskLruCache磁盘缓存）

 package com.lcw.rabbit.image.utils;

 import java.io.File;
 import java.io.IOException;
 import java.io.OutputStream;

 import android.content.Context;
 import android.content.pm.PackageInfo;
 import android.content.pm.PackageManager.NameNotFoundException;
 import android.graphics.Bitmap;
 import android.graphics.Bitmap.CompressFormat;
 import android.graphics.BitmapFactory;
 import android.os.Environment;
 import android.support.v4.util.LruCache;
 import android.util.Log;

 import com.android.volley.toolbox.ImageLoader.ImageCache;
 import com.lcw.rabbit.image.utils.DiskLruCache.Snapshot;

 /**
  * 图片缓存帮助类
  *
  * 包含内存缓存LruCache和磁盘缓存DiskLruCache
  *
  * @author Rabbit_Lee
  *
  */
 public class ImageCacheUtil implements ImageCache {

     private String TAG=ImageCacheUtil.this.getClass().getSimpleName();

     //缓存类
     private static LruCache<String, Bitmap> mLruCache;
     private static DiskLruCache mDiskLruCache;

     //磁盘缓存大小
     private static final int DISKMAXSIZE = 10 * 1024 * 1024;

     public ImageCacheUtil() {
         // 获取应用可占内存的1/8作为缓存
         int maxSize = (int) (Runtime.getRuntime().maxMemory() / 8);
         // 实例化LruCaceh对象
         mLruCache = new LruCache<String, Bitmap>(maxSize) {
             @Override
             protected int sizeOf(String key, Bitmap bitmap) {
                 return bitmap.getRowBytes() * bitmap.getHeight();
             }
         };
         try {
             // 获取DiskLruCahce对象
             mDiskLruCache = DiskLruCache.open(getDiskCacheDir(MyApplication.newInstance(), "Rabbit"), getAppVersion(MyApplication.newInstance()), 1, DISKMAXSIZE);
         } catch (IOException e) {
             e.printStackTrace();
         }
     }

     /**
      * 从缓存（内存缓存，磁盘缓存）中获取Bitmap
      */
     @Override
     public Bitmap getBitmap(String url) {
         if (mLruCache.get(url) != null) {
             // 从LruCache缓存中取
             Log.i(TAG,"从LruCahce获取");
             return mLruCache.get(url);
         } else {
             String key = MD5Utils.md5(url);
             try {
                 if (mDiskLruCache.get(key) != null) {
                     // 从DiskLruCahce取
                     Snapshot snapshot = mDiskLruCache.get(key);
                     Bitmap bitmap = null;
                     if (snapshot != null) {
                         bitmap = BitmapFactory.decodeStream(snapshot.getInputStream(0));
                         // 存入LruCache缓存
                         mLruCache.put(url, bitmap);
                         Log.i(TAG,"从DiskLruCahce获取");
                     }
                     return bitmap;
                 }
             } catch (IOException e) {
                 e.printStackTrace();
             }
         }
         return null;
     }

     /**
      * 存入缓存（内存缓存，磁盘缓存）
      */
     @Override
     public void putBitmap(String url, Bitmap bitmap) {
         // 存入LruCache缓存
         mLruCache.put(url, bitmap);
         // 判断是否存在DiskLruCache缓存，若没有存入
         String key = MD5Utils.md5(url);
         try {
             if (mDiskLruCache.get(key) == null) {
                 DiskLruCache.Editor editor = mDiskLruCache.edit(key);
                 if (editor != null) {
                     OutputStream outputStream = editor.newOutputStream(0);
                     if (bitmap.compress(CompressFormat.JPEG, 100, outputStream)) {
                         editor.commit();
                     } else {
                         editor.abort();
                     }
                 }
                 mDiskLruCache.flush();
             }
         } catch (IOException e) {
             e.printStackTrace();
         }

     }

     /**
      * 该方法会判断当前sd卡是否存在，然后选择缓存地址
      *
      * @param context
      * @param uniqueName
      * @return
      */
     public static 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);
     }

     /**
      * 获取应用版本号
      *
      * @param context
      * @return
      */
     public int getAppVersion(Context context) {
         try {
             PackageInfo info = context.getPackageManager().getPackageInfo(context.getPackageName(), 0);
             return info.versionCode;
         } catch (NameNotFoundException e) {
             e.printStackTrace();
         }
         return 1;
     }

 }

5、图片缓存管理类

这里的图片加载运用到了Volley自带的ImageLoader，在它的构造方法中需要一个ImageCache对象，在上面的图片缓存类已实现了该接口。

这里向外部提供了一个loadImage的重载方法，一个传入加载图片的宽高，一个默认加载原图，使得外部不再需要关注任何关于缓存的操作。

 package com.lcw.rabbit.image;

 import android.graphics.Bitmap;
 import android.widget.ImageView;

 import com.android.volley.VolleyError;
 import com.android.volley.toolbox.ImageLoader;
 import com.android.volley.toolbox.ImageLoader.ImageCache;
 import com.android.volley.toolbox.ImageLoader.ImageContainer;
 import com.android.volley.toolbox.ImageLoader.ImageListener;
 import com.lcw.rabbit.image.utils.ImageCacheUtil;

 /**
  * 图片缓存管理类 获取ImageLoader对象
  *
  * @author Rabbit_Lee
  *
  */
 public class ImageCacheManager {

     private static String TAG = ImageCacheManager.class.getSimpleName();

     // 获取图片缓存类对象
     private static ImageCache mImageCache = new ImageCacheUtil();
     // 获取ImageLoader对象
     public static ImageLoader mImageLoader = new ImageLoader(VolleyRequestQueueManager.mRequestQueue, mImageCache);

     /**
      * 获取ImageListener
      *
      * @param view
      * @param defaultImage
      * @param errorImage
      * @return
      */
     public static ImageListener getImageListener(final ImageView view, final Bitmap defaultImage, final Bitmap errorImage) {

         return new ImageListener() {

             @Override
             public void onErrorResponse(VolleyError error) {
                 // 回调失败
                 if (errorImage != null) {
                     view.setImageBitmap(errorImage);
                 }
             }

             @Override
             public void onResponse(ImageContainer response, boolean isImmediate) {
                 // 回调成功
                 if (response.getBitmap() != null) {
                     view.setImageBitmap(response.getBitmap());
                 } else if (defaultImage != null) {
                     view.setImageBitmap(defaultImage);
                 }
             }
         };

     }

     /**
      * 提供给外部调用方法
      *
      * @param url
      * @param view
      * @param defaultImage
      * @param errorImage
      */
     public static void loadImage(String url, ImageView view, Bitmap defaultImage, Bitmap errorImage) {
         mImageLoader.get(url, ImageCacheManager.getImageListener(view, defaultImage, errorImage), 0, 0);
     }

     /**
      * 提供给外部调用方法
      *
      * @param url
      * @param view
      * @param defaultImage
      * @param errorImage
      */
     public static void loadImage(String url, ImageView view, Bitmap defaultImage, Bitmap errorImage, int maxWidth, int maxHeight) {
         mImageLoader.get(url, ImageCacheManager.getImageListener(view, defaultImage, errorImage), maxWidth, maxHeight);
     }
 }

6、MainActivity类

 package com.lcw.rabbit.image;

 import android.app.Activity;
 import android.content.res.Resources;
 import android.graphics.Bitmap;
 import android.graphics.BitmapFactory;
 import android.os.Bundle;
 import android.view.View;
 import android.view.View.OnClickListener;
 import android.widget.Button;
 import android.widget.ImageView;

 public class MainActivity extends Activity {

     private Button mButton;
     private ImageView mImageView;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         mButton = (Button) findViewById(R.id.button);
         mImageView= (ImageView) findViewById(R.id.image);

         mButton.setOnClickListener(new OnClickListener() {

             @Override
             public void onClick(View v) {
                 String url = "http://img.blog.csdn.net/20130702124537953?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvdDEyeDM0NTY=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast";
                 ImageCacheManager.loadImage(url, mImageView, getBitmapFromRes(R.drawable.ic_launcher), getBitmapFromRes(R.drawable.ic_launcher));

             }
         });
     }

     public Bitmap getBitmapFromRes(int resId) {
         Resources res = this.getResources();
         return BitmapFactory.decodeResource(res, resId);

     }

 }

到这里代码就结束了，由于主要是讲关于缓存层的运用，关于Volley，LruCache，DiskCache的介绍使用，这里就不再阐述了，网上资料很多，大家查阅下便是。

有任何疑问或者建议，大家可以在文章评论给我留言，一起交流！

作者：李晨玮
出处：http://www.cnblogs.com/lichenwei/
本文版权归作者和博客园共有,欢迎转载,但未经作者同意必须保留此段声明,且在文章页面明显位置给出原文链接。
正在看本人博客的这位童鞋，我看你气度不凡，谈吐间隐隐有王者之气，日后必有一番作为！旁边有“推荐”二字，你就顺手把它点了吧，相得准，我分文不收；相不准，你也好回来找我！