安卓开发笔记——关于照片墙的实现（完美缓存策略LruCache+DiskLruCache）
这几天一直研究在安卓开发中图片应该如何处理，在网上翻了好多资料，这里做点小总结，如果朋友们有更好的解决方案，可以留言一起交流下。
内存缓存技术
在我们开发程序中要在界面上加载一张图片是件非常容易的事情，但如果是加载一堆图片呢？比如ListView，GridView这类的控件，随着屏幕滑动，图片加载也会越来越多，应用程序所可以使用的内存毕竟是有限的，如果一味的去加载图片，很容易导致OOM（Out Of Memory）内存溢出，导致程序崩溃。
这里我们一般的做法是将显示在屏幕之外的图片进行内存回收，此时的垃圾回收器会认为应用对这些图片不再持有引用，从而进行GC操作。但现实还需要考虑到问题是，如果用户又滑动屏幕回到之前我们已经回收掉的图片位置，这时候该怎么办？重新去加载一张图片肯定是不可取的，这样既浪费了时间，又浪费了用户的流量。
这里我们就会想到利用内存缓存来解决这个问题，利用内存缓存可以让应用快速的加载和处理图片，从而提高流畅性。
内存缓存技术对那些大量占用应用程序宝贵内存的图片提供了快速访问的方法，其中最核心的类是LruCache (此类在android-support-v4的包中提供) 。这个类非常适合用来缓存图片，它的主要算法原理是把最近使用的对象用强引用存储在 LinkedHashMap 中，并且把最近最少使用的对象在缓存值达到预设定值之前从内存中移除。
在过去，我们经常会使用一种非常流行的内存缓存技术的实现，即软引用或弱引用 (SoftReference or WeakReference)。但是现在已经不再推荐使用这种方式了，因为从 Android 2.3 (API Level 9)开始，垃圾回收器会更倾向于回收持有软引用或弱引用的对象，这让软引用和弱引用变得不再可靠。另外，Android 3.0 (API Level 11)中，图片的数据会存储在本地的内存当中，因而无法用一种可预见的方式将其释放，这就有潜在的风险造成应用程序的内存溢出并崩溃。
对于LruCache类不熟悉的朋友可以看看这篇文章《Android高效加载大图、多图解决方案，有效避免程序OOM》。
磁盘缓存技术
对于内存缓存LruCache只是管理了内存中图片的存储与释放，如果图片从内存中被移除的话，那么又需要从网络上重新加载一次图片，这显然非常耗时。对此，Google又提供了一套硬盘缓存的解决方案：DiskLruCache(非Google官方编写，但获得官方认证)。
关于磁盘缓存DisLruCache类不熟悉的朋友可以看看这篇文章的介绍《Android DiskLruCache完全解析，硬盘缓存的最佳方案》
完美结合LruCache+DiskLruCache
首先先来看下实现效果：
这是一个很简单的布局，大布局是GridView，小布局ImageView嵌套在大布局里，贴上代码再做分析吧，其实注释也挺全的。
既然要完成磁盘存储，那么必不可少的就是DisLruCache类的，先引进项目里再说，再来就是图片资源集合了。
图片资源类：
1 package com.example.photoswall; 2 3 4 /** 5 * 图片资源类 6 * @author Balla_兔子 7 * 8 */ 9 public class Images { 10 11 public final static String[] imageThumbUrls = new String[] { 12 "//img-my.csdn.net/uploads/201407/26/1406383299_1976.jpg", 13 "//img-my.csdn.net/uploads/201407/26/1406383291_6518.jpg", 14 "//img-my.csdn.net/uploads/201407/26/1406383291_8239.jpg", 15 "//img-my.csdn.net/uploads/201407/26/1406383290_9329.jpg", 16 "//img-my.csdn.net/uploads/201407/26/1406383290_1042.jpg", 17 "//img-my.csdn.net/uploads/201407/26/1406383275_3977.jpg", 18 "//img-my.csdn.net/uploads/201407/26/1406383265_8550.jpg", 19 "//img-my.csdn.net/uploads/201407/26/1406383264_3954.jpg", 20 "//img-my.csdn.net/uploads/201407/26/1406383264_4787.jpg", 21 "//img-my.csdn.net/uploads/201407/26/1406383264_8243.jpg", 22 "//img-my.csdn.net/uploads/201407/26/1406383248_3693.jpg", 23 "//img-my.csdn.net/uploads/201407/26/1406383243_5120.jpg", 24 "//img-my.csdn.net/uploads/201407/26/1406383242_3127.jpg", 25 "//img-my.csdn.net/uploads/201407/26/1406383242_9576.jpg", 26 "//img-my.csdn.net/uploads/201407/26/1406383242_1721.jpg", 27 "//img-my.csdn.net/uploads/201407/26/1406383219_5806.jpg", 28 "//img-my.csdn.net/uploads/201407/26/1406383214_7794.jpg", 29 "//img-my.csdn.net/uploads/201407/26/1406383213_4418.jpg", 30 "//img-my.csdn.net/uploads/201407/26/1406383213_3557.jpg", 31 "//img-my.csdn.net/uploads/201407/26/1406383210_8779.jpg", 32 "//img-my.csdn.net/uploads/201407/26/1406383172_4577.jpg", 33 "//img-my.csdn.net/uploads/201407/26/1406383166_3407.jpg", 34 "//img-my.csdn.net/uploads/201407/26/1406383166_2224.jpg", 35 "//img-my.csdn.net/uploads/201407/26/1406383166_7301.jpg", 36 "//img-my.csdn.net/uploads/201407/26/1406383165_7197.jpg", 37 "//img-my.csdn.net/uploads/201407/26/1406383150_8410.jpg", 38 "//img-my.csdn.net/uploads/201407/26/1406383131_3736.jpg", 39 "//img-my.csdn.net/uploads/201407/26/1406383130_5094.jpg", 40 "//img-my.csdn.net/uploads/201407/26/1406383130_7393.jpg", 41 "//img-my.csdn.net/uploads/201407/26/1406383129_8813.jpg", 42 "//img-my.csdn.net/uploads/201407/26/1406383100_3554.jpg", 43 "//img-my.csdn.net/uploads/201407/26/1406383093_7894.jpg", 44 "//img-my.csdn.net/uploads/201407/26/1406383092_2432.jpg", 45 "//img-my.csdn.net/uploads/201407/26/1406383092_3071.jpg", 46 "//img-my.csdn.net/uploads/201407/26/1406383091_3119.jpg", 47 "//img-my.csdn.net/uploads/201407/26/1406383059_6589.jpg", 48 "//img-my.csdn.net/uploads/201407/26/1406383059_8814.jpg", 49 "//img-my.csdn.net/uploads/201407/26/1406383059_2237.jpg", 50 "//img-my.csdn.net/uploads/201407/26/1406383058_4330.jpg", 51 "//img-my.csdn.net/uploads/201407/26/1406383038_3602.jpg", 52 "//img-my.csdn.net/uploads/201407/26/1406382942_3079.jpg", 53 "//img-my.csdn.net/uploads/201407/26/1406382942_8125.jpg", 54 "//img-my.csdn.net/uploads/201407/26/1406382942_4881.jpg", 55 "//img-my.csdn.net/uploads/201407/26/1406382941_4559.jpg", 56 "//img-my.csdn.net/uploads/201407/26/1406382941_3845.jpg", 57 "//img-my.csdn.net/uploads/201407/26/1406382924_8955.jpg", 58 "//img-my.csdn.net/uploads/201407/26/1406382923_2141.jpg", 59 "//img-my.csdn.net/uploads/201407/26/1406382923_8437.jpg", 60 "//img-my.csdn.net/uploads/201407/26/1406382922_6166.jpg", 61 "//img-my.csdn.net/uploads/201407/26/1406382922_4843.jpg", 62 "//img-my.csdn.net/uploads/201407/26/1406382905_5804.jpg", 63 "//img-my.csdn.net/uploads/201407/26/1406382904_3362.jpg", 64 "//img-my.csdn.net/uploads/201407/26/1406382904_2312.jpg", 65 "//img-my.csdn.net/uploads/201407/26/1406382904_4960.jpg", 66 "//img-my.csdn.net/uploads/201407/26/1406382900_2418.jpg", 67 "//img-my.csdn.net/uploads/201407/26/1406382881_4490.jpg", 68 "//img-my.csdn.net/uploads/201407/26/1406382881_5935.jpg", 69 "//img-my.csdn.net/uploads/201407/26/1406382880_3865.jpg", 70 "//img-my.csdn.net/uploads/201407/26/1406382880_4662.jpg", 71 "//img-my.csdn.net/uploads/201407/26/1406382879_2553.jpg", 72 "//img-my.csdn.net/uploads/201407/26/1406382862_5375.jpg", 73 "//img-my.csdn.net/uploads/201407/26/1406382862_1748.jpg", 74 "//img-my.csdn.net/uploads/201407/26/1406382861_7618.jpg", 75 "//img-my.csdn.net/uploads/201407/26/1406382861_8606.jpg", 76 "//img-my.csdn.net/uploads/201407/26/1406382861_8949.jpg", 77 "//img-my.csdn.net/uploads/201407/26/1406382841_9821.jpg", 78 "//img-my.csdn.net/uploads/201407/26/1406382840_6603.jpg", 79 "//img-my.csdn.net/uploads/201407/26/1406382840_2405.jpg", 80 "//img-my.csdn.net/uploads/201407/26/1406382840_6354.jpg", 81 "//img-my.csdn.net/uploads/201407/26/1406382839_5779.jpg", 82 "//img-my.csdn.net/uploads/201407/26/1406382810_7578.jpg", 83 "//img-my.csdn.net/uploads/201407/26/1406382810_2436.jpg", 84 "//img-my.csdn.net/uploads/201407/26/1406382809_3883.jpg", 85 "//img-my.csdn.net/uploads/201407/26/1406382809_6269.jpg", 86 "//img-my.csdn.net/uploads/201407/26/1406382808_4179.jpg", 87 "//img-my.csdn.net/uploads/201407/26/1406382790_8326.jpg", 88 "//img-my.csdn.net/uploads/201407/26/1406382789_7174.jpg", 89 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DiskLruCache（磁盘缓存类）：
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.example.photoswall; 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 }
MD5Utils（MD5转换工具类）：
1 package com.example.photoswall; 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 }
上面三个类直接引入项目就行了，接下来说说核心实现代码了。
PhotoWallAdapter（GridView适配器类）：
说下思路：由于我们的图片源是单纯的字符串（网址），这里给GridView适配的Adpter采用ArrayAdapter，当然如果你想用BaseAdatper也是可以的，思路不变。
1、继承ArrayAdatper，在构造函数传入必要参数后，需要进行2个操作：1、对内存缓存类的初始化 2、对磁盘缓存的初始化
2、然后在getView方法中来设置图片源，首先先设置成一张默认的图片，然后根据图片的URL去缓存中找是否有相关联的资源，如果没找到在磁盘缓存中找，如果还是没找到再去网络上下载，然后保存在磁盘缓存里，在以上的任一环节（磁盘，网络）里，只要我们找到了相对的图片资源，我们就把它添加到内存缓存中，以便下一次的引用。为了避免异步下载图片造成的图片错位现象，我们在每一个ImageView里设置了一个标识符Tag，Tag为图片的唯一标志：URL地址。
3、由于磁盘缓存属于I/O操作，网络属于下载操作都是属于耗时性的工作，这里我们开启了一个内部类Async异步类去完成，把所有的耗时操作都安排在doInBackground里执行（这里选择Async而不选择直接new Thread的原因是，Async运用了线程池的概念，会比单开子线程会更省资源，而且所有的任务会按照队列的顺序去执行）
由于这里的图片都是比较小的，在实际开发中，大家可以对利用BitmapFactory的Options类对图片进行压缩再展示。
1 package com.example.photoswall; 2 3 import java.io.BufferedInputStream; 4 import java.io.BufferedOutputStream; 5 import java.io.File; 6 import java.io.IOException; 7 import java.io.OutputStream; 8 import java.net.HttpURLConnection; 9 import java.net.URL; 10 import java.util.HashSet; 11 import java.util.Set; 12 13 import android.content.Context; 14 import android.content.pm.PackageInfo; 15 import android.content.pm.PackageManager.NameNotFoundException; 16 import android.graphics.Bitmap; 17 import android.graphics.BitmapFactory; 18 import android.os.AsyncTask; 19 import android.os.Environment; 20 import android.support.v4.util.LruCache; 21 import android.util.Log; 22 import android.view.LayoutInflater; 23 import android.view.View; 24 import android.view.ViewGroup; 25 import android.widget.ArrayAdapter; 26 import android.widget.GridView; 27 import android.widget.ImageView; 28 29 import com.example.photoswall.DiskLruCache.Snapshot; 30 31 public class PhotoWallAdapter extends ArrayAdapter<String> { 32 33 // 声明LruCache缓存对象 34 private LruCache<String, Bitmap> lruCache; 35 // 声明DiskLruCache硬盘缓存对象 36 private DiskLruCache diskLruCache; 37 // 任务队列 38 private Set<LoadImageAsync> tasks; 39 // 声明GridView对象 40 private GridView gridView; 41 42 public PhotoWallAdapter(Context context, int textViewResourceId, String[] objects, GridView gridView) { 43 super(context, textViewResourceId, objects); 44 this.gridView = gridView; 45 tasks = new HashSet<LoadImageAsync>(); 46 /** 47 * 初始化内存缓存LruCache 48 */ 49 // 获取应用程序最大可占内存值 50 int maxMemory = (int) Runtime.getRuntime().maxMemory(); 51 // 设置最大内存的八分之一作为缓存大小 52 int lruMemory = maxMemory / 8; 53 lruCache = new LruCache<String, Bitmap>(lruMemory) { 54 @Override 55 protected int sizeOf(String key, Bitmap bitmap) { 56 // 返回Bitmap对象所占大小，单位：kb 57 return bitmap.getByteCount(); 58 } 59 60 }; 61 /** 62 * 初始化硬盘缓存DiskLruCahce 63 */ 64 // 获取硬盘缓存路径,参数二为所在缓存路径的文件夹名称 65 File directory = getDiskCacheDir(getContext(), "bitmap"); 66 if (!directory.exists()) { 67 // 若文件夹不存在，建立文件夹 68 directory.mkdirs(); 69 } 70 int appVersion = getAppVersion(getContext()); 71 try { 72 // 参数1：缓存文件路径，参数2：系统版本号，参数3：一个缓存路径对于几个文件，参数4：缓存空间大小：字节 73 diskLruCache = DiskLruCache.open(directory, appVersion, 1, 1024 * 1024 * 10); 74 } catch (IOException e) { 75 e.printStackTrace(); 76 } 77 78 } 79 80 /** 81 * @param context 82 * @param uniqueName 83 * @return 84 * 当SD卡存在或者SD卡不可被移除的时候，就调用getExternalCacheDir()方法来获取缓存路径，否则就调用getCacheDir 85 * ()方法来获取缓存路径。 前者获取到的就是 /sdcard/Android/data/<application 86 * package>/cache 这个路径 而后者获取到的是 datadata/application package>/cache 87 * 这个路径。 88 */ 89 public File getDiskCacheDir(Context context, String uniqueName) { 90 String cachePath; 91 if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()) || !Environment.isExternalStorageRemovable()) { 92 cachePath = context.getExternalCacheDir().getPath(); 93 } else { 94 cachePath = context.getCacheDir().getPath(); 95 } 96 return new File(cachePath + File.separator + uniqueName); 97 } 98 99 /** 100 * @param context 101 * @return 获取系统版本号 102 */ 103 public int getAppVersion(Context context) { 104 try { 105 PackageInfo info = context.getPackageManager().getPackageInfo(context.getPackageName(), 0); 106 return info.versionCode; 107 } catch (NameNotFoundException e) { 108 e.printStackTrace(); 109 } 110 return 1; 111 } 112 113 @Override 114 public View getView(int position, View convertView, ViewGroup parent) { 115 // 获取图片资源URL地址 116 String path = getItem(position); 117 View view = null; 118 if (convertView == null) { 119 view = LayoutInflater.from(getContext()).inflate(R.layout.gridview_item, null); 120 } else { 121 view = convertView; 122 } 123 // 获取控件实例 124 ImageView imageView = (ImageView) view.findViewById(R.id.iv_photo); 125 // 设置一个唯一标识符，避免异步加载图片时错位 126 imageView.setTag(path); 127 // 设置默认图片 128 imageView.setImageResource(R.drawable.ic_launcher); 129 // 根据图片URL到缓存中去找图片资源并设置 130 setImageFromLruCache(path, imageView); 131 return view; 132 } 133 134 /** 135 * 根据图片URL地址获取缓存中图片，若不存在去磁盘缓存中查找=》网络下载 136 * 137 * @param path 138 * @param imageView 139 */ 140 private void setImageFromLruCache(String path, ImageView imageView) { 141 Bitmap bitmap = lruCache.get(path); 142 if (bitmap != null) { 143 // 缓存存在,取出设置图片 144 Log.i("PhotoWallAdapter", "在内存缓存中找到"); 145 imageView.setImageBitmap(bitmap); 146 } else { 147 // 缓存不存在，先找硬盘缓存，还不存在，就去网络下载（开启异步任务） 148 LoadImageAsync loadImageAsync = new LoadImageAsync(); 149 loadImageAsync.execute(path); 150 // 添加任务到任务队列 151 tasks.add(loadImageAsync); 152 } 153 } 154 155 /** 156 * 取消队列中准备下载和正在下载的任务 157 */ 158 public void cancelTask() { 159 for (LoadImageAsync task : tasks) { 160 task.cancel(false); 161 } 162 } 163 164 /** 165 * 同步内存操作到journal文件 166 */ 167 public void flushCache() { 168 if (diskLruCache != null) { 169 try { 170 diskLruCache.flush(); 171 } catch (IOException e) { 172 e.printStackTrace(); 173 } 174 } 175 176 } 177 178 class LoadImageAsync extends AsyncTask<String, Void, Bitmap> { 179 // 图片资源URL 180 String path = null; 181 182 @Override 183 protected Bitmap doInBackground(String... params) { 184 // 图片下载地址 185 this.path = params[0]; 186 Snapshot snapshot = null; 187 OutputStream outputStream = null; 188 Bitmap bitmap = null; 189 String pathMd5 = MD5Utils.md5(path); 190 // 根据图片url（md5）查找图片资源是否存在于硬盘缓存 191 try { 192 snapshot = diskLruCache.get(pathMd5); 193 if (snapshot == null) { 194 // 在磁盘缓存中没有找到图片资源 195 // 获取一个DiskLruCache写入对象 196 DiskLruCache.Editor editor = diskLruCache.edit(pathMd5); 197 if (editor != null) { 198 outputStream = editor.newOutputStream(0); 199 // 开启异步网络任务获取图片，并存入磁盘缓存 200 if (downloadUrlToStream(path, outputStream)) { 201 // 下载成功 202 Log.i("PhotoWallAdapter", "下载文件成功"); 203 editor.commit(); 204 } else { 205 editor.abort(); 206 } 207 } 208 } 209 // 图片写入磁盘缓存后，再一次的查找磁盘缓存 210 snapshot = diskLruCache.get(pathMd5); 211 if (snapshot != null) { 212 // 若查找到，获取图片，并把图片资源写入内存缓存 213 bitmap = BitmapFactory.decodeStream(snapshot.getInputStream(0)); 214 Log.i("PhotoWallAdapter", "在磁盘缓存中找到"); 215 } 216 if (bitmap != null) { 217 // 将Bitmap对象添加到内存缓存当中 218 lruCache.put(path, bitmap); 219 } 220 return bitmap; 221 } catch (IOException e) { 222 e.printStackTrace(); 223 } finally { 224 if (outputStream != null) { 225 try { 226 outputStream.close(); 227 } catch (IOException e) { 228 e.printStackTrace(); 229 } 230 } 231 } 232 return null; 233 } 234 235 @Override 236 protected void onPostExecute(Bitmap bitmap) { 237 super.onPostExecute(bitmap); 238 // 根据Tag获取控件对象并设置图片 239 ImageView imageView = (ImageView) gridView.findViewWithTag(path); 240 if (imageView != null && bitmap != null) { 241 // 加载图片 242 imageView.setImageBitmap(bitmap); 243 } 244 tasks.remove(this); 245 246 } 247 248 /** 249 * 根据图片URL地址下载图片,成功返回true，失败false 250 * 251 * @param urlString 252 * @param outputStream 253 * @return 254 */ 255 private boolean downloadUrlToStream(String urlString, OutputStream outputStream) { 256 HttpURLConnection urlConnection = null; 257 BufferedOutputStream out = null; 258 BufferedInputStream in = null; 259 try { 260 final URL url = new URL(urlString); 261 urlConnection = (HttpURLConnection) url.openConnection(); 262 in = new BufferedInputStream(urlConnection.getInputStream(), 8 * 1024); 263 out = new BufferedOutputStream(outputStream, 8 * 1024); 264 int b; 265 while ((b = in.read()) != -1) { 266 out.write(b); 267 } 268 return true; 269 } catch (final IOException e) { 270 e.printStackTrace(); 271 } finally { 272 if (urlConnection != null) { 273 urlConnection.disconnect(); 274 } 275 try { 276 if (out != null) { 277 out.close(); 278 } 279 if (in != null) { 280 in.close(); 281 } 282 } catch (final IOException e) { 283 e.printStackTrace(); 284 } 285 } 286 return false; 287 } 288 289 } 290 291 }
MainActivity类：
相比之下这个类就简单许多了，这里需要注意的两点:
1、在onPause方法里进行flush操作（更新磁盘缓存操作日志，磁盘缓存之所以能够被读取取决于日志文件）
2、在Activity销毁之前取消所有正在下载和准备下载的任务。
1 package com.example.photoswall; 2 3 import android.app.Activity; 4 import android.os.Bundle; 5 import android.widget.GridView; 6 7 public class MainActivity extends Activity { 8 9 private GridView gv_photo; 10 private PhotoWallAdapter adapter; 11 12 @Override 13 protected void onCreate(Bundle savedInstanceState) { 14 super.onCreate(savedInstanceState); 15 setContentView(R.layout.activity_main); 16 gv_photo = (GridView) findViewById(R.id.gv_photo); 17 adapter = new PhotoWallAdapter(MainActivity.this, 0, Images.imageThumbUrls, gv_photo); 18 gv_photo.setAdapter(adapter); 19 20 } 21 22 @Override 23 protected void onPause() { 24 super.onPause(); 25 adapter.flushCache(); 26 } 27 28 @Override 29 protected void onDestroy() { 30 super.onDestroy(); 31 adapter.cancelTask(); 32 } 33 34 }
剩下的一些细节，代码注释的很详细，还有不清楚的可以看看上文提到附带的2篇文章或者评论给我留言。
作者：Balla_兔子
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。
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