在開發(fā)安卓應(yīng)用中避免不了要使用到網(wǎng)絡(luò)圖片，獲取網(wǎng)絡(luò)圖片很簡單，但是需要付出一定的代價——流量。對于少數(shù)的圖片而言問題不大，但如果手機(jī)應(yīng)用中包含大量的圖片，這勢必會耗費用戶的一定流量，如果我們不加以處理，每次打開應(yīng)用都去網(wǎng)絡(luò)獲取圖片，那么用戶可就不樂意了，這里的處理就是指今天要講的緩存策略（緩存層分為三層：內(nèi)存層，磁盤層，網(wǎng)絡(luò)層）。

　　關(guān)于緩存層的工作，當(dāng)我們第一次打開應(yīng)用獲取圖片時，先到網(wǎng)絡(luò)去下載圖片，然后依次存入內(nèi)存緩存，磁盤緩存，當(dāng)我們再一次需要用到剛才下載的這張圖片時，就不需要再重復(fù)的到網(wǎng)絡(luò)上去下載，直接可以從內(nèi)存緩存和磁盤緩存中找，由于內(nèi)存緩存速度較快，我們優(yōu)先到內(nèi)存緩存中尋找該圖片，如果找到則運用，如果沒有找到（內(nèi)存緩存大小有限），那么我們再到磁盤緩存中去找。只要我們合理的去協(xié)調(diào)這三層緩存運用，便可以提升應(yīng)用性能和用戶體驗。

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1、內(nèi)存層：（手機(jī)內(nèi)存）

內(nèi)存緩存相對于磁盤緩存而言，速度要來的快很多，但缺點容量較小且會被系統(tǒng)回收，這里的實現(xiàn)我用到了LruCache。

LruCache這個類是Android3.1版本中提供的，如果你是在更早的Android版本中開發(fā)，則需要導(dǎo)入android-support-v4的jar包。

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磁盤層：（SD卡）

相比內(nèi)存緩存而言速度要來得慢很多，但容量很大，這里的實現(xiàn)我用到了DiskLruCache類。

DiskLruCache是非Google官方編寫，但獲得官方認(rèn)證的硬盤緩存類，該類沒有限定在Android內(nèi)，所以理論上java應(yīng)用也可以使用DiskLreCache來緩存。

這是DiskLruCache類的下載地址：http://pan.baidu.com/s/1hq0D53m?

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網(wǎng)絡(luò)層：（移動網(wǎng)絡(luò)，無線網(wǎng)絡(luò)）

這個就沒什么解釋的了，就是我們上網(wǎng)用的流量。這里的網(wǎng)絡(luò)訪問實現(xiàn)我用到了開源框架Volley。

開源框架Volley是2013年Google I/O大會發(fā)布的，Volley是Android平臺上的網(wǎng)絡(luò)通信庫，能使網(wǎng)絡(luò)通信更快，更簡單，更健壯。它的設(shè)計目標(biāo)就是非常適合去進(jìn)行數(shù)據(jù)量不大，但通信頻繁的網(wǎng)絡(luò)操作，而對于大數(shù)據(jù)量的網(wǎng)絡(luò)操作，比如說下載文件等，Volley的表現(xiàn)就會非常糟糕。

這是Volley的下載地址：http://pan.baidu.com/s/1hq1t2yo

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先來看下效果圖：

正常網(wǎng)絡(luò)下： ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?　　　　斷開網(wǎng)絡(luò)，飛行模式下：

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Log日志打印：

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來看下代碼實現(xiàn)：

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1、由于應(yīng)用中很多地方需要用到上下文對象，這里我自定義了一個全局的Application，用來提供上下文對象

1 package com.lcw.rabbit.image.utils; 2 3 import android.app.Application; 4 /** 5 * Application類，提供全局上下文對象 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 }

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2、Volley請求隊列處理類，用來管理Rquest請求對象操作

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 * 請求隊列處理類 10 * 獲取RequestQueue對象 11 */ 12 public class VolleyRequestQueueManager { 13 // 獲取請求隊列類 14 public static RequestQueue mRequestQueue = Volley.newRequestQueue(MyApplication.newInstance()); 15 16 //添加任務(wù)進(jìn)任務(wù)隊列 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 //取消任務(wù) 25 public static void cancelRequest(Object tag){ 26 mRequestQueue.cancelAll(tag); 27 } 28 29 30 31 }

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3、這里附上2個工具類（生成MD5序列幫助類，DiskLruCache磁盤緩存類）

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的算法進(jìn)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("沒有md5這個算法！"); 18 } 19 String md5code = new BigInteger(1, secretBytes).toString(16);// 16進(jìn)制數(shù)字 20 // 如果生成數(shù)字未滿32位，需要前面補(bǔ)0 21 for (int i = 0; i < 32 - md5code.length(); i++) { 22 md5code = "0" + md5code; 23 } 24 return md5code; 25 } 26 27 } MD5轉(zhuǎn)換類

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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 * <p>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 * <p>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 * <p>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 * <ul> 79 * <li>When an entry is being <strong>created</strong> it is necessary to 80 * supply a full set of values; the empty value should be used as a 81 * placeholder if necessary. 82 * <li>When an entry is being <strong>edited</strong>, it is not necessary 83 * to supply data for every value; values default to their previous 84 * value. 85 * </ul> 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 * <p>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 * <p>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<String, Entry> lruEntries 163 = new LinkedHashMap<String, Entry>(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> 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<Runnable>()); 272 private final Callable<Void> cleanupCallable = new Callable<Void>() { 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<Entry> 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<Entry>(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<String, Entry> toEvict = lruEntries.eldest(); 701 final Map.Entry<String, 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磁盤緩存類

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4、圖片緩存類，包含（LruCache內(nèi)存緩存，DiskLruCache磁盤緩存）

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 * 圖片緩存幫助類 22 * 23 * 包含內(nèi)存緩存LruCache和磁盤緩存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 //緩存類 33 private static LruCache<String, Bitmap> mLruCache; 34 private static DiskLruCache mDiskLruCache; 35 36 //磁盤緩存大小 37 private static final int DISKMAXSIZE = 10 * 1024 * 1024; 38 39 public ImageCacheUtil() { 40 // 獲取應(yīng)用可占內(nèi)存的1/8作為緩存 41 int maxSize = (int) (Runtime.getRuntime().maxMemory() / 8); 42 // 實例化LruCaceh對象 43 mLruCache = new LruCache<String, Bitmap>(maxSize) { 44 @Override 45 protected int sizeOf(String key, Bitmap bitmap) { 46 return bitmap.getRowBytes() * bitmap.getHeight(); 47 } 48 }; 49 try { 50 // 獲取DiskLruCahce對象 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 * 從緩存（內(nèi)存緩存，磁盤緩存）中獲取Bitmap 59 */ 60 @Override 61 public Bitmap getBitmap(String url) { 62 if (mLruCache.get(url) != null) { 63 // 從LruCache緩存中取 64 Log.i(TAG,"從LruCahce獲取"); 65 return mLruCache.get(url); 66 } else { 67 String key = MD5Utils.md5(url); 68 try { 69 if (mDiskLruCache.get(key) != null) { 70 // 從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緩存 76 mLruCache.put(url, bitmap); 77 Log.i(TAG,"從DiskLruCahce獲取"); 78 } 79 return bitmap; 80 } 81 } catch (IOException e) { 82 e.printStackTrace(); 83 } 84 } 85 return null; 86 } 87 88 /** 89 * 存入緩存（內(nèi)存緩存，磁盤緩存） 90 */ 91 @Override 92 public void putBitmap(String url, Bitmap bitmap) { 93 // 存入LruCache緩存 94 mLruCache.put(url, bitmap); 95 // 判斷是否存在DiskLruCache緩存，若沒有存入 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 * 該方法會判斷當(dāng)前sd卡是否存在，然后選擇緩存地址 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 * 獲取應(yīng)用版本號 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 }

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5、圖片緩存管理類

這里的圖片加載運用到了Volley自帶的ImageLoader，在它的構(gòu)造方法中需要一個ImageCache對象，在上面的圖片緩存類已實現(xiàn)了該接口。

這里向外部提供了一個loadImage的重載方法，一個傳入加載圖片的寬高，一個默認(rèn)加載原圖，使得外部不再需要關(guān)注任何關(guān)于緩存的操作。

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 * 圖片緩存管理類 獲取ImageLoader對象 15 * 16 * @author Rabbit_Lee 17 * 18 */ 19 public class ImageCacheManager { 20 21 private static String TAG = ImageCacheManager.class.getSimpleName(); 22 23 // 獲取圖片緩存類對象 24 private static ImageCache mImageCache = new ImageCacheUtil(); 25 // 獲取ImageLoader對象 26 public static ImageLoader mImageLoader = new ImageLoader(VolleyRequestQueueManager.mRequestQueue, mImageCache); 27 28 /** 29 * 獲取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 // 回調(diào)失敗 43 if (errorImage != null) { 44 view.setImageBitmap(errorImage); 45 } 46 } 47 48 @Override 49 public void onResponse(ImageContainer response, boolean isImmediate) { 50 // 回調(diào)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 * 提供給外部調(diào)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 * 提供給外部調(diào)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 }

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6、MainActivity類

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 }

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到這里代碼就結(jié)束了，由于主要是講關(guān)于緩存層的運用，關(guān)于Volley，LruCache，DiskCache的介紹使用，這里就不再闡述了，網(wǎng)上資料很多，大家查閱下便是。

有任何疑問或者建議，大家可以在文章評論給我留言，一起交流！

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作者：李晨瑋
出處：http://www.cnblogs.com/lichenwei/
本文版權(quán)歸作者和博客園共有,歡迎轉(zhuǎn)載,但未經(jīng)作者同意必須保留此段聲明,且在文章頁面明顯位置給出原文鏈接。
正在看本人博客的這位童鞋，我看你氣度不凡，談吐間隱隱有王者之氣，日后必有一番作為！旁邊有“推薦”二字，你就順手把它點了吧，相得準(zhǔn)，我分文不收；相不準(zhǔn)，你也好回來找我！

轉(zhuǎn)載于:https://www.cnblogs.com/lichenwei/p/4651576.html

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