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

 1 package com.lcw.rabbit.image.utils;
 2 
 3 import android.app.Application;
 4 /**
 5  * Application类，提供全局上下文对象
 6  * @author Rabbit_Lee
 7  *
 8  */
 9 public class MyApplication extends Application {
10 
11     public static String TAG;
12     public static MyApplication myApplication;
13 
14     public static MyApplication newInstance() {
15         return myApplication;
16     }
17 
18     @Override
19     public void onCreate() {
20         super.onCreate();
21         TAG = this.getClass().getSimpleName();
22         myApplication = this;
23 
24     }
25 }

 

2、Volley请求队列处理类，用来管理Rquest请求对象操作

 1 package com.lcw.rabbit.image;
 2 
 3 import com.android.volley.Request;
 4 import com.android.volley.RequestQueue;
 5 import com.android.volley.toolbox.Volley;
 6 import com.lcw.rabbit.image.utils.MyApplication;
 7 
 8 /**
 9  * 请求队列处理类
10  * 获取RequestQueue对象
11  */
12 public class VolleyRequestQueueManager {
13     // 获取请求队列类
14     public static RequestQueue mRequestQueue = Volley.newRequestQueue(MyApplication.newInstance());
15 
16     //添加任务进任务队列
17     public static void addRequest(Request request, Object tag) {
18         if (tag != null) {
19             request.setTag(tag);
20         }
21         mRequestQueue.add(request);
22     }
23     
24     //取消任务
25     public static void cancelRequest(Object tag){
26         mRequestQueue.cancelAll(tag);
27     }
28     
29     
30 
31 }

 

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

 1 package com.lcw.rabbit.image.utils;
 2 
 3 import java.math.BigInteger;
 4 import java.security.MessageDigest;
 5 import java.security.NoSuchAlgorithmException;
 6 
 7 public class MD5Utils {
 8     /**
 9      * 使用md5的算法进行加密
10      */
11     public static String md5(String plainText) {
12         byte[] secretBytes = null;
13         try {
14             secretBytes = MessageDigest.getInstance("md5").digest(
15                     plainText.getBytes());
16         } catch (NoSuchAlgorithmException e) {
17             throw new RuntimeException("没有md5这个算法！");
18         }
19         String md5code = new BigInteger(1, secretBytes).toString(16);// 16进制数字
20         // 如果生成数字未满32位，需要前面补0
21         for (int i = 0; i < 32 - md5code.length(); i++) {
22             md5code = "0" + md5code;
23         }
24         return md5code;
25     }
26 
27 }

MD5转换类

 

  1 /*
  2  * Copyright (C) 2011 The Android Open Source Project
  3  *
  4  * Licensed under the Apache License, Version 2.0 (the "License");
  5  * you may not use this file except in compliance with the License.
  6  * You may obtain a copy of the License at
  7  *
  8  *      http://www.apache.org/licenses/LICENSE-2.0
  9  *
 10  * Unless required by applicable law or agreed to in writing, software
 11  * distributed under the License is distributed on an "AS IS" BASIS,
 12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 13  * See the License for the specific language governing permissions and
 14  * limitations under the License.
 15  */
 16 
 17 package com.lcw.rabbit.image.utils;
 18 
 19 import java.io.BufferedInputStream;
 20 import java.io.BufferedWriter;
 21 import java.io.Closeable;
 22 import java.io.EOFException;
 23 import java.io.File;
 24 import java.io.FileInputStream;
 25 import java.io.FileNotFoundException;
 26 import java.io.FileOutputStream;
 27 import java.io.FileWriter;
 28 import java.io.FilterOutputStream;
 29 import java.io.IOException;
 30 import java.io.InputStream;
 31 import java.io.InputStreamReader;
 32 import java.io.OutputStream;
 33 import java.io.OutputStreamWriter;
 34 import java.io.Reader;
 35 import java.io.StringWriter;
 36 import java.io.Writer;
 37 import java.lang.reflect.Array;
 38 import java.nio.charset.Charset;
 39 import java.util.ArrayList;
 40 import java.util.Arrays;
 41 import java.util.Iterator;
 42 import java.util.LinkedHashMap;
 43 import java.util.Map;
 44 import java.util.concurrent.Callable;
 45 import java.util.concurrent.ExecutorService;
 46 import java.util.concurrent.LinkedBlockingQueue;
 47 import java.util.concurrent.ThreadPoolExecutor;
 48 import java.util.concurrent.TimeUnit;
 49 
 50 /**
 51  ******************************************************************************
 52  * Taken from the JB source code, can be found in:
 53  * libcore/luni/src/main/java/libcore/io/DiskLruCache.java
 54  * or direct link:
 55  * https://android.googlesource.com/platform/libcore/+/android-4.1.1_r1/luni/src/main/java/libcore/io/DiskLruCache.java
 56  ******************************************************************************
 57  *
 58  * A cache that uses a bounded amount of space on a filesystem. Each cache
 59  * entry has a string key and a fixed number of values. Values are byte
 60  * sequences, accessible as streams or files. Each value must be between {@code
 61  * 0} and {@code Integer.MAX_VALUE} bytes in length.
 62  *
 63  * 
The cache stores its data in a directory on the filesystem. This
 64  * directory must be exclusive to the cache; the cache may delete or overwrite
 65  * files from its directory. It is an error for multiple processes to use the
 66  * same cache directory at the same time.
 67  *
 68  * 
This cache limits the number of bytes that it will store on the
 69  * filesystem. When the number of stored bytes exceeds the limit, the cache will
 70  * remove entries in the background until the limit is satisfied. The limit is
 71  * not strict: the cache may temporarily exceed it while waiting for files to be
 72  * deleted. The limit does not include filesystem overhead or the cache
 73  * journal so space-sensitive applications should set a conservative limit.
 74  *
 75  * 
Clients call {@link #edit} to create or update the values of an entry. An
 76  * entry may have only one editor at one time; if a value is not available to be
 77  * edited then {@link #edit} will return null.
 78  * 

 79  *     
When an entry is being created it is necessary to
 80  *         supply a full set of values; the empty value should be used as a
 81  *         placeholder if necessary.
 82  *     
When an entry is being edited, it is not necessary
 83  *         to supply data for every value; values default to their previous
 84  *         value.
 85  * 
 86  * Every {@link #edit} call must be matched by a call to {@link Editor#commit}
 87  * or {@link Editor#abort}. Committing is atomic: a read observes the full set
 88  * of values as they were before or after the commit, but never a mix of values.
 89  *
 90  * 
Clients call {@link #get} to read a snapshot of an entry. The read will
 91  * observe the value at the time that {@link #get} was called. Updates and
 92  * removals after the call do not impact ongoing reads.
 93  *
 94  * 
This class is tolerant of some I/O errors. If files are missing from the
 95  * filesystem, the corresponding entries will be dropped from the cache. If
 96  * an error occurs while writing a cache value, the edit will fail silently.
 97  * Callers should handle other problems by catching {@code IOException} and
 98  * responding appropriately.
 99  */
100 public final class DiskLruCache implements Closeable {
101     static final String JOURNAL_FILE = "journal";
102     static final String JOURNAL_FILE_TMP = "journal.tmp";
103     static final String MAGIC = "libcore.io.DiskLruCache";
104     static final String VERSION_1 = "1";
105     static final long ANY_SEQUENCE_NUMBER = -1;
106     private static final String CLEAN = "CLEAN";
107     private static final String DIRTY = "DIRTY";
108     private static final String REMOVE = "REMOVE";
109     private static final String READ = "READ";
110 
111     private static final Charset UTF_8 = Charset.forName("UTF-8");
112     private static final int IO_BUFFER_SIZE = 8 * 1024;
113 
114     /*
115      * This cache uses a journal file named "journal". A typical journal file
116      * looks like this:
117      *     libcore.io.DiskLruCache
118      *     1
119      *     100
120      *     2
121      *
122      *     CLEAN 3400330d1dfc7f3f7f4b8d4d803dfcf6 832 21054
123      *     DIRTY 335c4c6028171cfddfbaae1a9c313c52
124      *     CLEAN 335c4c6028171cfddfbaae1a9c313c52 3934 2342
125      *     REMOVE 335c4c6028171cfddfbaae1a9c313c52
126      *     DIRTY 1ab96a171faeeee38496d8b330771a7a
127      *     CLEAN 1ab96a171faeeee38496d8b330771a7a 1600 234
128      *     READ 335c4c6028171cfddfbaae1a9c313c52
129      *     READ 3400330d1dfc7f3f7f4b8d4d803dfcf6
130      *
131      * The first five lines of the journal form its header. They are the
132      * constant string "libcore.io.DiskLruCache", the disk cache's version,
133      * the application's version, the value count, and a blank line.
134      *
135      * Each of the subsequent lines in the file is a record of the state of a
136      * cache entry. Each line contains space-separated values: a state, a key,
137      * and optional state-specific values.
138      *   o DIRTY lines track that an entry is actively being created or updated.
139      *     Every successful DIRTY action should be followed by a CLEAN or REMOVE
140      *     action. DIRTY lines without a matching CLEAN or REMOVE indicate that
141      *     temporary files may need to be deleted.
142      *   o CLEAN lines track a cache entry that has been successfully published
143      *     and may be read. A publish line is followed by the lengths of each of
144      *     its values.
145      *   o READ lines track accesses for LRU.
146      *   o REMOVE lines track entries that have been deleted.
147      *
148      * The journal file is appended to as cache operations occur. The journal may
149      * occasionally be compacted by dropping redundant lines. A temporary file named
150      * "journal.tmp" will be used during compaction; that file should be deleted if
151      * it exists when the cache is opened.
152      */
153 
154     private final File directory;
155     private final File journalFile;
156     private final File journalFileTmp;
157     private final int appVersion;
158     private final long maxSize;
159     private final int valueCount;
160     private long size = 0;
161     private Writer journalWriter;
162     private final LinkedHashMap lruEntries
163             = new LinkedHashMap(0, 0.75f, true);
164     private int redundantOpCount;
165 
166     /**
167      * To differentiate between old and current snapshots, each entry is given
168      * a sequence number each time an edit is committed. A snapshot is stale if
169      * its sequence number is not equal to its entry's sequence number.
170      */
171     private long nextSequenceNumber = 0;
172 
173     /* From java.util.Arrays */
174     @SuppressWarnings("unchecked")
175     private static  T[] copyOfRange(T[] original, int start, int end) {
176         final int originalLength = original.length; // For exception priority compatibility.
177         if (start > end) {
178             throw new IllegalArgumentException();
179         }
180         if (start < 0 || start > originalLength) {
181             throw new ArrayIndexOutOfBoundsException();
182         }
183         final int resultLength = end - start;
184         final int copyLength = Math.min(resultLength, originalLength - start);
185         final T[] result = (T[]) Array
186                 .newInstance(original.getClass().getComponentType(), resultLength);
187         System.arraycopy(original, start, result, 0, copyLength);
188         return result;
189     }
190 
191     /**
192      * Returns the remainder of 'reader' as a string, closing it when done.
193      */
194     public static String readFully(Reader reader) throws IOException {
195         try {
196             StringWriter writer = new StringWriter();
197             char[] buffer = new char[1024];
198             int count;
199             while ((count = reader.read(buffer)) != -1) {
200                 writer.write(buffer, 0, count);
201             }
202             return writer.toString();
203         } finally {
204             reader.close();
205         }
206     }
207 
208     /**
209      * Returns the ASCII characters up to but not including the next "\r\n", or
210      * "\n".
211      *
212      * @throws java.io.EOFException if the stream is exhausted before the next newline
213      *     character.
214      */
215     public static String readAsciiLine(InputStream in) throws IOException {
216         // TODO: support UTF-8 here instead
217 
218         StringBuilder result = new StringBuilder(80);
219         while (true) {
220             int c = in.read();
221             if (c == -1) {
222                 throw new EOFException();
223             } else if (c == '\n') {
224                 break;
225             }
226 
227             result.append((char) c);
228         }
229         int length = result.length();
230         if (length > 0 && result.charAt(length - 1) == '\r') {
231             result.setLength(length - 1);
232         }
233         return result.toString();
234     }
235 
236     /**
237      * Closes 'closeable', ignoring any checked exceptions. Does nothing if 'closeable' is null.
238      */
239     public static void closeQuietly(Closeable closeable) {
240         if (closeable != null) {
241             try {
242                 closeable.close();
243             } catch (RuntimeException rethrown) {
244                 throw rethrown;
245             } catch (Exception ignored) {
246             }
247         }
248     }
249 
250     /**
251      * Recursively delete everything in {@code dir}.
252      */
253     // TODO: this should specify paths as Strings rather than as Files
254     public static void deleteContents(File dir) throws IOException {
255         File[] files = dir.listFiles();
256         if (files == null) {
257             throw new IllegalArgumentException("not a directory: " + dir);
258         }
259         for (File file : files) {
260             if (file.isDirectory()) {
261                 deleteContents(file);
262             }
263             if (!file.delete()) {
264                 throw new IOException("failed to delete file: " + file);
265             }
266         }
267     }
268 
269     /** This cache uses a single background thread to evict entries. */
270     private final ExecutorService executorService = new ThreadPoolExecutor(0, 1,
271             60L, TimeUnit.SECONDS, new LinkedBlockingQueue());
272     private final Callable cleanupCallable = new Callable() {
273         @Override public Void call() throws Exception {
274             synchronized (DiskLruCache.this) {
275                 if (journalWriter == null) {
276                     return null; // closed
277                 }
278                 trimToSize();
279                 if (journalRebuildRequired()) {
280                     rebuildJournal();
281                     redundantOpCount = 0;
282                 }
283             }
284             return null;
285         }
286     };
287 
288     private DiskLruCache(File directory, int appVersion, int valueCount, long maxSize) {
289         this.directory = directory;
290         this.appVersion = appVersion;
291         this.journalFile = new File(directory, JOURNAL_FILE);
292         this.journalFileTmp = new File(directory, JOURNAL_FILE_TMP);
293         this.valueCount = valueCount;
294         this.maxSize = maxSize;
295     }
296 
297     /**
298      * Opens the cache in {@code directory}, creating a cache if none exists
299      * there.
300      *
301      * @param directory a writable directory
302      * @param appVersion
303      * @param valueCount the number of values per cache entry. Must be positive.
304      * @param maxSize the maximum number of bytes this cache should use to store
305      * @throws java.io.IOException if reading or writing the cache directory fails
306      */
307     public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)
308             throws IOException {
309         if (maxSize <= 0) {
310             throw new IllegalArgumentException("maxSize <= 0");
311         }
312         if (valueCount <= 0) {
313             throw new IllegalArgumentException("valueCount <= 0");
314         }
315 
316         // prefer to pick up where we left off
317         DiskLruCache cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
318         if (cache.journalFile.exists()) {
319             try {
320                 cache.readJournal();
321                 cache.processJournal();
322                 cache.journalWriter = new BufferedWriter(new FileWriter(cache.journalFile, true),
323                         IO_BUFFER_SIZE);
324                 return cache;
325             } catch (IOException journalIsCorrupt) {
326 //                System.logW("DiskLruCache " + directory + " is corrupt: "
327 //                        + journalIsCorrupt.getMessage() + ", removing");
328                 cache.delete();
329             }
330         }
331 
332         // create a new empty cache
333         directory.mkdirs();
334         cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
335         cache.rebuildJournal();
336         return cache;
337     }
338 
339     private void readJournal() throws IOException {
340         InputStream in = new BufferedInputStream(new FileInputStream(journalFile), IO_BUFFER_SIZE);
341         try {
342             String magic = readAsciiLine(in);
343             String version = readAsciiLine(in);
344             String appVersionString = readAsciiLine(in);
345             String valueCountString = readAsciiLine(in);
346             String blank = readAsciiLine(in);
347             if (!MAGIC.equals(magic)
348                     || !VERSION_1.equals(version)
349                     || !Integer.toString(appVersion).equals(appVersionString)
350                     || !Integer.toString(valueCount).equals(valueCountString)
351                     || !"".equals(blank)) {
352                 throw new IOException("unexpected journal header: ["
353                         + magic + ", " + version + ", " + valueCountString + ", " + blank + "]");
354             }
355 
356             while (true) {
357                 try {
358                     readJournalLine(readAsciiLine(in));
359                 } catch (EOFException endOfJournal) {
360                     break;
361                 }
362             }
363         } finally {
364             closeQuietly(in);
365         }
366     }
367 
368     private void readJournalLine(String line) throws IOException {
369         String[] parts = line.split(" ");
370         if (parts.length < 2) {
371             throw new IOException("unexpected journal line: " + line);
372         }
373 
374         String key = parts[1];
375         if (parts[0].equals(REMOVE) && parts.length == 2) {
376             lruEntries.remove(key);
377             return;
378         }
379 
380         Entry entry = lruEntries.get(key);
381         if (entry == null) {
382             entry = new Entry(key);
383             lruEntries.put(key, entry);
384         }
385 
386         if (parts[0].equals(CLEAN) && parts.length == 2 + valueCount) {
387             entry.readable = true;
388             entry.currentEditor = null;
389             entry.setLengths(copyOfRange(parts, 2, parts.length));
390         } else if (parts[0].equals(DIRTY) && parts.length == 2) {
391             entry.currentEditor = new Editor(entry);
392         } else if (parts[0].equals(READ) && parts.length == 2) {
393             // this work was already done by calling lruEntries.get()
394         } else {
395             throw new IOException("unexpected journal line: " + line);
396         }
397     }
398 
399     /**
400      * Computes the initial size and collects garbage as a part of opening the
401      * cache. Dirty entries are assumed to be inconsistent and will be deleted.
402      */
403     private void processJournal() throws IOException {
404         deleteIfExists(journalFileTmp);
405         for (Iterator i = lruEntries.values().iterator(); i.hasNext(); ) {
406             Entry entry = i.next();
407             if (entry.currentEditor == null) {
408                 for (int t = 0; t < valueCount; t++) {
409                     size += entry.lengths[t];
410                 }
411             } else {
412                 entry.currentEditor = null;
413                 for (int t = 0; t < valueCount; t++) {
414                     deleteIfExists(entry.getCleanFile(t));
415                     deleteIfExists(entry.getDirtyFile(t));
416                 }
417                 i.remove();
418             }
419         }
420     }
421 
422     /**
423      * Creates a new journal that omits redundant information. This replaces the
424      * current journal if it exists.
425      */
426     private synchronized void rebuildJournal() throws IOException {
427         if (journalWriter != null) {
428             journalWriter.close();
429         }
430 
431         Writer writer = new BufferedWriter(new FileWriter(journalFileTmp), IO_BUFFER_SIZE);
432         writer.write(MAGIC);
433         writer.write("\n");
434         writer.write(VERSION_1);
435         writer.write("\n");
436         writer.write(Integer.toString(appVersion));
437         writer.write("\n");
438         writer.write(Integer.toString(valueCount));
439         writer.write("\n");
440         writer.write("\n");
441 
442         for (Entry entry : lruEntries.values()) {
443             if (entry.currentEditor != null) {
444                 writer.write(DIRTY + ' ' + entry.key + '\n');
445             } else {
446                 writer.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\n');
447             }
448         }
449 
450         writer.close();
451         journalFileTmp.renameTo(journalFile);
452         journalWriter = new BufferedWriter(new FileWriter(journalFile, true), IO_BUFFER_SIZE);
453     }
454 
455     private static void deleteIfExists(File file) throws IOException {
456 //        try {
457 //            Libcore.os.remove(file.getPath());
458 //        } catch (ErrnoException errnoException) {
459 //            if (errnoException.errno != OsConstants.ENOENT) {
460 //                throw errnoException.rethrowAsIOException();
461 //            }
462 //        }
463         if (file.exists() && !file.delete()) {
464             throw new IOException();
465         }
466     }
467 
468     /**
469      * Returns a snapshot of the entry named {@code key}, or null if it doesn't
470      * exist is not currently readable. If a value is returned, it is moved to
471      * the head of the LRU queue.
472      */
473     public synchronized Snapshot get(String key) throws IOException {
474         checkNotClosed();
475         validateKey(key);
476         Entry entry = lruEntries.get(key);
477         if (entry == null) {
478             return null;
479         }
480 
481         if (!entry.readable) {
482             return null;
483         }
484 
485         /*
486          * Open all streams eagerly to guarantee that we see a single published
487          * snapshot. If we opened streams lazily then the streams could come
488          * from different edits.
489          */
490         InputStream[] ins = new InputStream[valueCount];
491         try {
492             for (int i = 0; i < valueCount; i++) {
493                 ins[i] = new FileInputStream(entry.getCleanFile(i));
494             }
495         } catch (FileNotFoundException e) {
496             // a file must have been deleted manually!
497             return null;
498         }
499 
500         redundantOpCount++;
501         journalWriter.append(READ + ' ' + key + '\n');
502         if (journalRebuildRequired()) {
503             executorService.submit(cleanupCallable);
504         }
505 
506         return new Snapshot(key, entry.sequenceNumber, ins);
507     }
508 
509     /**
510      * Returns an editor for the entry named {@code key}, or null if another
511      * edit is in progress.
512      */
513     public Editor edit(String key) throws IOException {
514         return edit(key, ANY_SEQUENCE_NUMBER);
515     }
516 
517     private synchronized Editor edit(String key, long expectedSequenceNumber) throws IOException {
518         checkNotClosed();
519         validateKey(key);
520         Entry entry = lruEntries.get(key);
521         if (expectedSequenceNumber != ANY_SEQUENCE_NUMBER
522                 && (entry == null || entry.sequenceNumber != expectedSequenceNumber)) {
523             return null; // snapshot is stale
524         }
525         if (entry == null) {
526             entry = new Entry(key);
527             lruEntries.put(key, entry);
528         } else if (entry.currentEditor != null) {
529             return null; // another edit is in progress
530         }
531 
532         Editor editor = new Editor(entry);
533         entry.currentEditor = editor;
534 
535         // flush the journal before creating files to prevent file leaks
536         journalWriter.write(DIRTY + ' ' + key + '\n');
537         journalWriter.flush();
538         return editor;
539     }
540 
541     /**
542      * Returns the directory where this cache stores its data.
543      */
544     public File getDirectory() {
545         return directory;
546     }
547 
548     /**
549      * Returns the maximum number of bytes that this cache should use to store
550      * its data.
551      */
552     public long maxSize() {
553         return maxSize;
554     }
555 
556     /**
557      * Returns the number of bytes currently being used to store the values in
558      * this cache. This may be greater than the max size if a background
559      * deletion is pending.
560      */
561     public synchronized long size() {
562         return size;
563     }
564 
565     private synchronized void completeEdit(Editor editor, boolean success) throws IOException {
566         Entry entry = editor.entry;
567         if (entry.currentEditor != editor) {
568             throw new IllegalStateException();
569         }
570 
571         // if this edit is creating the entry for the first time, every index must have a value
572         if (success && !entry.readable) {
573             for (int i = 0; i < valueCount; i++) {
574                 if (!entry.getDirtyFile(i).exists()) {
575                     editor.abort();
576                     throw new IllegalStateException("edit didn't create file " + i);
577                 }
578             }
579         }
580 
581         for (int i = 0; i < valueCount; i++) {
582             File dirty = entry.getDirtyFile(i);
583             if (success) {
584                 if (dirty.exists()) {
585                     File clean = entry.getCleanFile(i);
586                     dirty.renameTo(clean);
587                     long oldLength = entry.lengths[i];
588                     long newLength = clean.length();
589                     entry.lengths[i] = newLength;
590                     size = size - oldLength + newLength;
591                 }
592             } else {
593                 deleteIfExists(dirty);
594             }
595         }
596 
597         redundantOpCount++;
598         entry.currentEditor = null;
599         if (entry.readable | success) {
600             entry.readable = true;
601             journalWriter.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\n');
602             if (success) {
603                 entry.sequenceNumber = nextSequenceNumber++;
604             }
605         } else {
606             lruEntries.remove(entry.key);
607             journalWriter.write(REMOVE + ' ' + entry.key + '\n');
608         }
609 
610         if (size > maxSize || journalRebuildRequired()) {
611             executorService.submit(cleanupCallable);
612         }
613     }
614 
615     /**
616      * We only rebuild the journal when it will halve the size of the journal
617      * and eliminate at least 2000 ops.
618      */
619     private boolean journalRebuildRequired() {
620         final int REDUNDANT_OP_COMPACT_THRESHOLD = 2000;
621         return redundantOpCount >= REDUNDANT_OP_COMPACT_THRESHOLD
622                 && redundantOpCount >= lruEntries.size();
623     }
624 
625     /**
626      * Drops the entry for {@code key} if it exists and can be removed. Entries
627      * actively being edited cannot be removed.
628      *
629      * @return true if an entry was removed.
630      */
631     public synchronized boolean remove(String key) throws IOException {
632         checkNotClosed();
633         validateKey(key);
634         Entry entry = lruEntries.get(key);
635         if (entry == null || entry.currentEditor != null) {
636             return false;
637         }
638 
639         for (int i = 0; i < valueCount; i++) {
640             File file = entry.getCleanFile(i);
641             if (!file.delete()) {
642                 throw new IOException("failed to delete " + file);
643             }
644             size -= entry.lengths[i];
645             entry.lengths[i] = 0;
646         }
647 
648         redundantOpCount++;
649         journalWriter.append(REMOVE + ' ' + key + '\n');
650         lruEntries.remove(key);
651 
652         if (journalRebuildRequired()) {
653             executorService.submit(cleanupCallable);
654         }
655 
656         return true;
657     }
658 
659     /**
660      * Returns true if this cache has been closed.
661      */
662     public boolean isClosed() {
663         return journalWriter == null;
664     }
665 
666     private void checkNotClosed() {
667         if (journalWriter == null) {
668             throw new IllegalStateException("cache is closed");
669         }
670     }
671 
672     /**
673      * Force buffered operations to the filesystem.
674      */
675     public synchronized void flush() throws IOException {
676         checkNotClosed();
677         trimToSize();
678         journalWriter.flush();
679     }
680 
681     /**
682      * Closes this cache. Stored values will remain on the filesystem.
683      */
684     public synchronized void close() throws IOException {
685         if (journalWriter == null) {
686             return; // already closed
687         }
688         for (Entry entry : new ArrayList(lruEntries.values())) {
689             if (entry.currentEditor != null) {
690                 entry.currentEditor.abort();
691             }
692         }
693         trimToSize();
694         journalWriter.close();
695         journalWriter = null;
696     }
697 
698     private void trimToSize() throws IOException {
699         while (size > maxSize) {
700 //            Map.Entry toEvict = lruEntries.eldest();
701             final Map.Entry toEvict = lruEntries.entrySet().iterator().next();
702             remove(toEvict.getKey());
703         }
704     }
705 
706     /**
707      * Closes the cache and deletes all of its stored values. This will delete
708      * all files in the cache directory including files that weren't created by
709      * the cache.
710      */
711     public void delete() throws IOException {
712         close();
713         deleteContents(directory);
714     }
715 
716     private void validateKey(String key) {
717         if (key.contains(" ") || key.contains("\n") || key.contains("\r")) {
718             throw new IllegalArgumentException(
719                     "keys must not contain spaces or newlines: \"" + key + "\"");
720         }
721     }
722 
723     private static String inputStreamToString(InputStream in) throws IOException {
724         return readFully(new InputStreamReader(in, UTF_8));
725     }
726 
727     /**
728      * A snapshot of the values for an entry.
729      */
730     public final class Snapshot implements Closeable {
731         private final String key;
732         private final long sequenceNumber;
733         private final InputStream[] ins;
734 
735         private Snapshot(String key, long sequenceNumber, InputStream[] ins) {
736             this.key = key;
737             this.sequenceNumber = sequenceNumber;
738             this.ins = ins;
739         }
740 
741         /**
742          * Returns an editor for this snapshot's entry, or null if either the
743          * entry has changed since this snapshot was created or if another edit
744          * is in progress.
745          */
746         public Editor edit() throws IOException {
747             return DiskLruCache.this.edit(key, sequenceNumber);
748         }
749 
750         /**
751          * Returns the unbuffered stream with the value for {@code index}.
752          */
753         public InputStream getInputStream(int index) {
754             return ins[index];
755         }
756 
757         /**
758          * Returns the string value for {@code index}.
759          */
760         public String getString(int index) throws IOException {
761             return inputStreamToString(getInputStream(index));
762         }
763 
764         @Override public void close() {
765             for (InputStream in : ins) {
766                 closeQuietly(in);
767             }
768         }
769     }
770 
771     /**
772      * Edits the values for an entry.
773      */
774     public final class Editor {
775         private final Entry entry;
776         private boolean hasErrors;
777 
778         private Editor(Entry entry) {
779             this.entry = entry;
780         }
781 
782         /**
783          * Returns an unbuffered input stream to read the last committed value,
784          * or null if no value has been committed.
785          */
786         public InputStream newInputStream(int index) throws IOException {
787             synchronized (DiskLruCache.this) {
788                 if (entry.currentEditor != this) {
789                     throw new IllegalStateException();
790                 }
791                 if (!entry.readable) {
792                     return null;
793                 }
794                 return new FileInputStream(entry.getCleanFile(index));
795             }
796         }
797 
798         /**
799          * Returns the last committed value as a string, or null if no value
800          * has been committed.
801          */
802         public String getString(int index) throws IOException {
803             InputStream in = newInputStream(index);
804             return in != null ? inputStreamToString(in) : null;
805         }
806 
807         /**
808          * Returns a new unbuffered output stream to write the value at
809          * {@code index}. If the underlying output stream encounters errors
810          * when writing to the filesystem, this edit will be aborted when
811          * {@link #commit} is called. The returned output stream does not throw
812          * IOExceptions.
813          */
814         public OutputStream newOutputStream(int index) throws IOException {
815             synchronized (DiskLruCache.this) {
816                 if (entry.currentEditor != this) {
817                     throw new IllegalStateException();
818                 }
819                 return new FaultHidingOutputStream(new FileOutputStream(entry.getDirtyFile(index)));
820             }
821         }
822 
823         /**
824          * Sets the value at {@code index} to {@code value}.
825          */
826         public void set(int index, String value) throws IOException {
827             Writer writer = null;
828             try {
829                 writer = new OutputStreamWriter(newOutputStream(index), UTF_8);
830                 writer.write(value);
831             } finally {
832                 closeQuietly(writer);
833             }
834         }
835 
836         /**
837          * Commits this edit so it is visible to readers.  This releases the
838          * edit lock so another edit may be started on the same key.
839          */
840         public void commit() throws IOException {
841             if (hasErrors) {
842                 completeEdit(this, false);
843                 remove(entry.key); // the previous entry is stale
844             } else {
845                 completeEdit(this, true);
846             }
847         }
848 
849         /**
850          * Aborts this edit. This releases the edit lock so another edit may be
851          * started on the same key.
852          */
853         public void abort() throws IOException {
854             completeEdit(this, false);
855         }
856 
857         private class FaultHidingOutputStream extends FilterOutputStream {
858             private FaultHidingOutputStream(OutputStream out) {
859                 super(out);
860             }
861 
862             @Override public void write(int oneByte) {
863                 try {
864                     out.write(oneByte);
865                 } catch (IOException e) {
866                     hasErrors = true;
867                 }
868             }
869 
870             @Override public void write(byte[] buffer, int offset, int length) {
871                 try {
872                     out.write(buffer, offset, length);
873                 } catch (IOException e) {
874                     hasErrors = true;
875                 }
876             }
877 
878             @Override public void close() {
879                 try {
880                     out.close();
881                 } catch (IOException e) {
882                     hasErrors = true;
883                 }
884             }
885 
886             @Override public void flush() {
887                 try {
888                     out.flush();
889                 } catch (IOException e) {
890                     hasErrors = true;
891                 }
892             }
893         }
894     }
895 
896     private final class Entry {
897         private final String key;
898 
899         /** Lengths of this entry's files. */
900         private final long[] lengths;
901 
902         /** True if this entry has ever been published */
903         private boolean readable;
904 
905         /** The ongoing edit or null if this entry is not being edited. */
906         private Editor currentEditor;
907 
908         /** The sequence number of the most recently committed edit to this entry. */
909         private long sequenceNumber;
910 
911         private Entry(String key) {
912             this.key = key;
913             this.lengths = new long[valueCount];
914         }
915 
916         public String getLengths() throws IOException {
917             StringBuilder result = new StringBuilder();
918             for (long size : lengths) {
919                 result.append(' ').append(size);
920             }
921             return result.toString();
922         }
923 
924         /**
925          * Set lengths using decimal numbers like "10123".
926          */
927         private void setLengths(String[] strings) throws IOException {
928             if (strings.length != valueCount) {
929                 throw invalidLengths(strings);
930             }
931 
932             try {
933                 for (int i = 0; i < strings.length; i++) {
934                     lengths[i] = Long.parseLong(strings[i]);
935                 }
936             } catch (NumberFormatException e) {
937                 throw invalidLengths(strings);
938             }
939         }
940 
941         private IOException invalidLengths(String[] strings) throws IOException {
942             throw new IOException("unexpected journal line: " + Arrays.toString(strings));
943         }
944 
945         public File getCleanFile(int i) {
946             return new File(directory, key + "." + i);
947         }
948 
949         public File getDirtyFile(int i) {
950             return new File(directory, key + "." + i + ".tmp");
951         }
952     }
953 }

DiskLruCache磁盘缓存类

 

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

  1 package com.lcw.rabbit.image.utils;
  2 
  3 import java.io.File;
  4 import java.io.IOException;
  5 import java.io.OutputStream;
  6 
  7 import android.content.Context;
  8 import android.content.pm.PackageInfo;
  9 import android.content.pm.PackageManager.NameNotFoundException;
 10 import android.graphics.Bitmap;
 11 import android.graphics.Bitmap.CompressFormat;
 12 import android.graphics.BitmapFactory;
 13 import android.os.Environment;
 14 import android.support.v4.util.LruCache;
 15 import android.util.Log;
 16 
 17 import com.android.volley.toolbox.ImageLoader.ImageCache;
 18 import com.lcw.rabbit.image.utils.DiskLruCache.Snapshot;
 19 
 20 /**
 21  * 图片缓存帮助类
 22  * 
 23  * 包含内存缓存LruCache和磁盘缓存DiskLruCache
 24  * 
 25  * @author Rabbit_Lee
 26  * 
 27  */
 28 public class ImageCacheUtil implements ImageCache {
 29     
 30     private String TAG=ImageCacheUtil.this.getClass().getSimpleName();
 31 
 32     //缓存类
 33     private static LruCache mLruCache;
 34     private static DiskLruCache mDiskLruCache;
 35 
 36     //磁盘缓存大小
 37     private static final int DISKMAXSIZE = 10 * 1024 * 1024;
 38 
 39     public ImageCacheUtil() {
 40         // 获取应用可占内存的1/8作为缓存
 41         int maxSize = (int) (Runtime.getRuntime().maxMemory() / 8);
 42         // 实例化LruCaceh对象
 43         mLruCache = new LruCache(maxSize) {
 44             @Override
 45             protected int sizeOf(String key, Bitmap bitmap) {
 46                 return bitmap.getRowBytes() * bitmap.getHeight();
 47             }
 48         };
 49         try {
 50             // 获取DiskLruCahce对象
 51             mDiskLruCache = DiskLruCache.open(getDiskCacheDir(MyApplication.newInstance(), "Rabbit"), getAppVersion(MyApplication.newInstance()), 1, DISKMAXSIZE);
 52         } catch (IOException e) {
 53             e.printStackTrace();
 54         }
 55     }
 56 
 57     /**
 58      * 从缓存（内存缓存，磁盘缓存）中获取Bitmap
 59      */
 60     @Override
 61     public Bitmap getBitmap(String url) {
 62         if (mLruCache.get(url) != null) {
 63             // 从LruCache缓存中取
 64             Log.i(TAG,"从LruCahce获取");
 65             return mLruCache.get(url);
 66         } else {
 67             String key = MD5Utils.md5(url);
 68             try {
 69                 if (mDiskLruCache.get(key) != null) {
 70                     // 从DiskLruCahce取
 71                     Snapshot snapshot = mDiskLruCache.get(key);
 72                     Bitmap bitmap = null;
 73                     if (snapshot != null) {
 74                         bitmap = BitmapFactory.decodeStream(snapshot.getInputStream(0));
 75                         // 存入LruCache缓存
 76                         mLruCache.put(url, bitmap);
 77                         Log.i(TAG,"从DiskLruCahce获取");
 78                     }
 79                     return bitmap;
 80                 }
 81             } catch (IOException e) {
 82                 e.printStackTrace();
 83             }
 84         }
 85         return null;
 86     }
 87 
 88     /**
 89      * 存入缓存（内存缓存，磁盘缓存）
 90      */
 91     @Override
 92     public void putBitmap(String url, Bitmap bitmap) {
 93         // 存入LruCache缓存
 94         mLruCache.put(url, bitmap);
 95         // 判断是否存在DiskLruCache缓存，若没有存入
 96         String key = MD5Utils.md5(url);
 97         try {
 98             if (mDiskLruCache.get(key) == null) {
 99                 DiskLruCache.Editor editor = mDiskLruCache.edit(key);
100                 if (editor != null) {
101                     OutputStream outputStream = editor.newOutputStream(0);
102                     if (bitmap.compress(CompressFormat.JPEG, 100, outputStream)) {
103                         editor.commit();
104                     } else {
105                         editor.abort();
106                     }
107                 }
108                 mDiskLruCache.flush();
109             }
110         } catch (IOException e) {
111             e.printStackTrace();
112         }
113 
114     }
115 
116     /**
117      * 该方法会判断当前sd卡是否存在，然后选择缓存地址
118      * 
119      * @param context
120      * @param uniqueName
121      * @return
122      */
123     public static File getDiskCacheDir(Context context, String uniqueName) {
124         String cachePath;
125         if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()) || !Environment.isExternalStorageRemovable()) {
126             cachePath = context.getExternalCacheDir().getPath();
127         } else {
128             cachePath = context.getCacheDir().getPath();
129         }
130         return new File(cachePath + File.separator + uniqueName);
131     }
132 
133     /**
134      * 获取应用版本号
135      * 
136      * @param context
137      * @return
138      */
139     public int getAppVersion(Context context) {
140         try {
141             PackageInfo info = context.getPackageManager().getPackageInfo(context.getPackageName(), 0);
142             return info.versionCode;
143         } catch (NameNotFoundException e) {
144             e.printStackTrace();
145         }
146         return 1;
147     }
148 
149 }

 

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。

 1 package com.lcw.rabbit.image;
 2 
 3 import android.graphics.Bitmap;
 4 import android.widget.ImageView;
 5 
 6 import com.android.volley.VolleyError;
 7 import com.android.volley.toolbox.ImageLoader;
 8 import com.android.volley.toolbox.ImageLoader.ImageCache;
 9 import com.android.volley.toolbox.ImageLoader.ImageContainer;
10 import com.android.volley.toolbox.ImageLoader.ImageListener;
11 import com.lcw.rabbit.image.utils.ImageCacheUtil;
12 
13 /**
14  * 图片缓存管理类 获取ImageLoader对象
15  * 
16  * @author Rabbit_Lee
17  * 
18  */
19 public class ImageCacheManager {
20 
21     private static String TAG = ImageCacheManager.class.getSimpleName();
22 
23     // 获取图片缓存类对象
24     private static ImageCache mImageCache = new ImageCacheUtil();
25     // 获取ImageLoader对象
26     public static ImageLoader mImageLoader = new ImageLoader(VolleyRequestQueueManager.mRequestQueue, mImageCache);
27 
28     /**
29      * 获取ImageListener
30      * 
31      * @param view
32      * @param defaultImage
33      * @param errorImage
34      * @return
35      */
36     public static ImageListener getImageListener(final ImageView view, final Bitmap defaultImage, final Bitmap errorImage) {
37 
38         return new ImageListener() {
39 
40             @Override
41             public void onErrorResponse(VolleyError error) {
42                 // 回调失败
43                 if (errorImage != null) {
44                     view.setImageBitmap(errorImage);
45                 }
46             }
47 
48             @Override
49             public void onResponse(ImageContainer response, boolean isImmediate) {
50                 // 回调成功
51                 if (response.getBitmap() != null) {
52                     view.setImageBitmap(response.getBitmap());
53                 } else if (defaultImage != null) {
54                     view.setImageBitmap(defaultImage);
55                 }
56             }
57         };
58 
59     }
60 
61     /**
62      * 提供给外部调用方法
63      * 
64      * @param url
65      * @param view
66      * @param defaultImage
67      * @param errorImage
68      */
69     public static void loadImage(String url, ImageView view, Bitmap defaultImage, Bitmap errorImage) {
70         mImageLoader.get(url, ImageCacheManager.getImageListener(view, defaultImage, errorImage), 0, 0);
71     }
72 
73     /**
74      * 提供给外部调用方法
75      * 
76      * @param url
77      * @param view
78      * @param defaultImage
79      * @param errorImage
80      */
81     public static void loadImage(String url, ImageView view, Bitmap defaultImage, Bitmap errorImage, int maxWidth, int maxHeight) {
82         mImageLoader.get(url, ImageCacheManager.getImageListener(view, defaultImage, errorImage), maxWidth, maxHeight);
83     }
84 }

 

6、MainActivity类

 1 package com.lcw.rabbit.image;
 2 
 3 import android.app.Activity;
 4 import android.content.res.Resources;
 5 import android.graphics.Bitmap;
 6 import android.graphics.BitmapFactory;
 7 import android.os.Bundle;
 8 import android.view.View;
 9 import android.view.View.OnClickListener;
10 import android.widget.Button;
11 import android.widget.ImageView;
12 
13 public class MainActivity extends Activity {
14 
15     private Button mButton;
16     private ImageView mImageView;
17 
18     @Override
19     protected void onCreate(Bundle savedInstanceState) {
20         super.onCreate(savedInstanceState);
21         setContentView(R.layout.activity_main);
22         mButton = (Button) findViewById(R.id.button);
23         mImageView= (ImageView) findViewById(R.id.image);
24         
25 
26         mButton.setOnClickListener(new OnClickListener() {
27 
28             @Override
29             public void onClick(View v) {
30                 String url = "https://img-blog.csdn.net/20130702124537953?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvdDEyeDM0NTY=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast";
31                 ImageCacheManager.loadImage(url, mImageView, getBitmapFromRes(R.drawable.ic_launcher), getBitmapFromRes(R.drawable.ic_launcher));
32 
33             }
34         });
35     }
36 
37     public Bitmap getBitmapFromRes(int resId) {
38         Resources res = this.getResources();
39         return BitmapFactory.decodeResource(res, resId);
40 
41     }
42 
43 }

 

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！