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：/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：/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 11public static String TAG;12public static MyApplication myApplication;13 14public static MyApplication newInstance() {15 return myApplication;16}17 18@Override19public 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14public static RequestQueue mRequestQueue = Volley.newRequestQueue(MyApplication.newInstance());15 16//添加任务进任务队列17public static void addRequest(Request<?> request, Object tag) {18 if (tag != null) {19 request.setTag(tag);20 }21 mRequestQueue.add(request);22}2324//取消任务25public static void cancelRequest(Object tag){26 mRequestQueue.cancelAll(tag);27}282930 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*/11public 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021 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) 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 */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 * /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 {101static final String JOURNAL_FILE = "journal";102static final String JOURNAL_FILE_TMP = "journal.tmp";103static final String MAGIC = "libcore.io.DiskLruCache";104static final String VERSION_1 = "1";105static final long ANY_SEQUENCE_NUMBER = -1;106private static final String CLEAN = "CLEAN";107private static final String DIRTY = "DIRTY";108private static final String REMOVE = "REMOVE";109private static final String READ = "READ";110 111private static final Charset UTF_8 = Charset.forName("UTF-8");112private static final int IO_BUFFER_SIZE = 8 * 1024;113 114/*115* This cache uses a journal file named "journal". A typical journal file116* looks like this:117*libcore.io.DiskLruCache118*1119*100120*2121*122*CLEAN 3400330d1dfc7f3f7f4b8d4d803dfcf6 832 21054123*DIRTY 335c4c6028171cfddfbaae1a9c313c52124*CLEAN 335c4c6028171cfddfbaae1a9c313c52 3934 2342125*REMOVE 335c4c6028171cfddfbaae1a9c313c52126*DIRTY 1ab96a171faeeee38496d8b330771a7a127*CLEAN 1ab96a171faeeee38496d8b330771a7a 1600 234128*READ 335c4c6028171cfddfbaae1a9c313c52129*READ 3400330d1dfc7f3f7f4b8d4d803dfcf6130*131* The first five lines of the journal form its header. They are the132* 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 a136* 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 REMOVE140*action. DIRTY lines without a matching CLEAN or REMOVE indicate that141*temporary files may need to be deleted.142* o CLEAN lines track a cache entry that has been successfully published143*and may be read. A publish line is followed by the lengths of each of144*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 may149* occasionally be compacted by dropping redundant lines. A temporary file named150* "journal.tmp" will be used during compaction; that file should be deleted if151* it exists when the cache is opened.152*/153 154private final File directory;155private final File journalFile;156private final File journalFileTmp;157private final int appVersion;158private final long maxSize;159private final int valueCount;160private long size = 0;161private Writer journalWriter;162private final LinkedHashMap<String, Entry> lruEntries163 = new LinkedHashMap<String, Entry>(0, 0.75f, true);164private int redundantOpCount;165 166/**167* To differentiate between old and current snapshots, each entry is given168* a sequence number each time an edit is committed. A snapshot is stale if169* its sequence number is not equal to its entry's sequence number.170*/171private long nextSequenceNumber = 0;172 173/* From java.util.Arrays */174@SuppressWarnings("unchecked")175private 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[]) Array186 .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*/194public 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", or210* "\n".211*212* @throws java.io.EOFException if the stream is exhausted before the next newline213*character.214*/215public static String readAsciiLine(InputStream in) throws IOException {216 // TODO: support UTF-8 here instead217 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*/239public 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 Files254public 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. */270private final ExecutorService executorService = new ThreadPoolExecutor(0, 1,271 60L, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>());272private 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; // closed277 }278 trimToSize();279 if (journalRebuildRequired()) {280 rebuildJournal();281 redundantOpCount = 0;282 }283 }284 return null;285 }286};287 288private 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 exists299* there.300*301* @param directory a writable directory302* @param appVersion303* @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 store305* @throws java.io.IOException if reading or writing the cache directory fails306*/307public 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 off317 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),323IO_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 cache333 directory.mkdirs();334 cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);335 cache.rebuildJournal();336 return cache;337}338 339private 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 368private 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 the401* cache. Dirty entries are assumed to be inconsistent and will be deleted.402*/403private 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 the424* current journal if it exists.425*/426private 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 455private 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't470* exist is not currently readable. If a value is returned, it is moved to471* the head of the LRU queue.472*/473public 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 published487* snapshot. If we opened streams lazily then the streams could come488* 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 another511* edit is in progress.512*/513public Editor edit(String key) throws IOException {514 return edit(key, ANY_SEQUENCE_NUMBER);515}516 517private 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_NUMBER522 && (entry == null || entry.sequenceNumber != expectedSequenceNumber)) {523 return null; // snapshot is stale524 }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 progress530 }531 532 Editor editor = new Editor(entry);533 entry.currentEditor = editor;534 535 // flush the journal before creating files to prevent file leaks536 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*/544public File getDirectory() {545 return directory;546}547 548/**549* Returns the maximum number of bytes that this cache should use to store550* its data.551*/552public long maxSize() {553 return maxSize;554}555 556/**557* Returns the number of bytes currently being used to store the values in558* this cache. This may be greater than the max size if a background559* deletion is pending.560*/561public synchronized long size() {562 return size;563}564 565private 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 value572 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 journal617* and eliminate at least 2000 ops.618*/619private boolean journalRebuildRequired() {620 final int REDUNDANT_OP_COMPACT_THRESHOLD = 2000;621 return redundantOpCount >= REDUNDANT_OP_COMPACT_THRESHOLD622 && redundantOpCount >= lruEntries.size();623}624 625/**626* Drops the entry for {@code key} if it exists and can be removed. Entries627* actively being edited cannot be removed.628*629* @return true if an entry was removed.630*/631public 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*/662public boolean isClosed() {663 return journalWriter == null;664}665 666private 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*/675public 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*/684public synchronized void close() throws IOException {685 if (journalWriter == null) {686 return; // already closed687 }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 698private 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 delete708* all files in the cache directory including files that weren't created by709* the cache.710*/711public void delete() throws IOException {712 close();713 deleteContents(directory);714}715 716private 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 723private 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*/730public 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 the743* entry has changed since this snapshot was created or if another edit744* 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*/774public 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 value800* 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 at809* {@code index}. If the underlying output stream encounters errors810* when writing to the filesystem, this edit will be aborted when811* {@link #commit} is called. The returned output stream does not throw812* 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 the838* 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 stale844 } else {845 completeEdit(this, true);846 }847 }848 849 /**850* Aborts this edit. This releases the edit lock so another edit may be851* 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 896private 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.pressFormat; 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 { 2930private String TAG=ImageCacheUtil.this.getClass().getSimpleName(); 31 32//缓存类 33private static LruCache<String, Bitmap> mLruCache; 34private static DiskLruCache mDiskLruCache; 35 36//磁盘缓存大小 37private static final int DISKMAXSIZE = 10 * 1024 * 1024; 38 39public 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<String, Bitmap>(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 61public 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) { 74bitmap = BitmapFactory.decodeStream(snapshot.getInputStream(0)); 75// 存入LruCache缓存 76mLruCache.put(url, bitmap); 77Log.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 92public 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 (press(CompressFormat.JPEG, 100, outputStream)) {103mit();104 } else {105editor.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 context120* @param uniqueName121* @return122*/123public 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 context137* @return138*/139public 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_Lee17 * 18 */19 public class ImageCacheManager {20 21private static String TAG = ImageCacheManager.class.getSimpleName();22 23// 获取图片缓存类对象24private static ImageCache mImageCache = new ImageCacheUtil();25// 获取ImageLoader对象26public static ImageLoader mImageLoader = new ImageLoader(VolleyRequestQueueManager.mRequestQueue, mImageCache);27 28/**29* 获取ImageListener30* 31* @param view32* @param defaultImage33* @param errorImage34* @return35*/36public static ImageListener getImageListener(final ImageView view, final Bitmap defaultImage, final Bitmap errorImage) {37 38 return new ImageListener() {39 40 @Override41 public void onErrorResponse(VolleyError error) {42 // 回调失败43 if (errorImage != null) {44 view.setImageBitmap(errorImage);45 }46 }47 48 @Override49 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 url65* @param view66* @param defaultImage67* @param errorImage68*/69public 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 url77* @param view78* @param defaultImage79* @param errorImage80*/81public 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 15private Button mButton;16private ImageView mImageView;17 18@Override19protected 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 @Override29 public void onClick(View v) {30 String url = "https://img-/0702124537953?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 37public 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：/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！
