安卓开发笔记——关于照片墙的实现（完美缓存策略LruCache+DiskLruCache）
2024-08-24 09:08:24
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：
package com.example.photoswall; /**
* 图片资源类
* @author Balla_兔子
*
*/
public class Images { public final static String[] imageThumbUrls = new String[] {
"http://img.my.csdn.net/uploads/201407/26/1406383299_1976.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383291_6518.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383291_8239.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383290_9329.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383290_1042.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383275_3977.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383265_8550.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383264_3954.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383264_4787.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383264_8243.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383248_3693.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383243_5120.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383242_3127.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383242_9576.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383242_1721.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383219_5806.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383214_7794.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383213_4418.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383213_3557.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383210_8779.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383172_4577.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383166_3407.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383166_2224.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383166_7301.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383165_7197.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383150_8410.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383131_3736.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383130_5094.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383130_7393.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383129_8813.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383100_3554.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383093_7894.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383092_2432.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383092_3071.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383091_3119.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383059_6589.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383059_8814.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383059_2237.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383058_4330.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406383038_3602.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382942_3079.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382942_8125.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382942_4881.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382941_4559.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382941_3845.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382924_8955.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382923_2141.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382923_8437.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382922_6166.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382922_4843.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382905_5804.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382904_3362.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382904_2312.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382904_4960.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382900_2418.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382881_4490.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382881_5935.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382880_3865.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382880_4662.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382879_2553.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382862_5375.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382862_1748.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382861_7618.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382861_8606.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382861_8949.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382841_9821.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382840_6603.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382840_2405.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382840_6354.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382839_5779.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382810_7578.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382810_2436.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382809_3883.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382809_6269.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382808_4179.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382790_8326.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382789_7174.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382789_5170.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382789_4118.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382788_9532.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382767_3184.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382767_4772.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382766_4924.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382766_5762.jpg",
"http://img.my.csdn.net/uploads/201407/26/1406382765_7341.jpg"
};
}
DiskLruCache（磁盘缓存类）：
/*
* Copyright (C) 2011 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/ package com.example.photoswall; import java.io.BufferedInputStream;
import java.io.BufferedWriter;
import java.io.Closeable;
import java.io.EOFException;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileWriter;
import java.io.FilterOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.Reader;
import java.io.StringWriter;
import java.io.Writer;
import java.lang.reflect.Array;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit; /**
******************************************************************************
* Taken from the JB source code, can be found in:
* libcore/luni/src/main/java/libcore/io/DiskLruCache.java
* or direct link:
* https://android.googlesource.com/platform/libcore/+/android-4.1.1_r1/luni/src/main/java/libcore/io/DiskLruCache.java
******************************************************************************
*
* A cache that uses a bounded amount of space on a filesystem. Each cache
* entry has a string key and a fixed number of values. Values are byte
* sequences, accessible as streams or files. Each value must be between {@code
* 0} and {@code Integer.MAX_VALUE} bytes in length.
*
* <p>The cache stores its data in a directory on the filesystem. This
* directory must be exclusive to the cache; the cache may delete or overwrite
* files from its directory. It is an error for multiple processes to use the
* same cache directory at the same time.
*
* <p>This cache limits the number of bytes that it will store on the
* filesystem. When the number of stored bytes exceeds the limit, the cache will
* remove entries in the background until the limit is satisfied. The limit is
* not strict: the cache may temporarily exceed it while waiting for files to be
* deleted. The limit does not include filesystem overhead or the cache
* journal so space-sensitive applications should set a conservative limit.
*
* <p>Clients call {@link #edit} to create or update the values of an entry. An
* entry may have only one editor at one time; if a value is not available to be
* edited then {@link #edit} will return null.
* <ul>
* <li>When an entry is being <strong>created</strong> it is necessary to
* supply a full set of values; the empty value should be used as a
* placeholder if necessary.
* <li>When an entry is being <strong>edited</strong>, it is not necessary
* to supply data for every value; values default to their previous
* value.
* </ul>
* Every {@link #edit} call must be matched by a call to {@link Editor#commit}
* or {@link Editor#abort}. Committing is atomic: a read observes the full set
* of values as they were before or after the commit, but never a mix of values.
*
* <p>Clients call {@link #get} to read a snapshot of an entry. The read will
* observe the value at the time that {@link #get} was called. Updates and
* removals after the call do not impact ongoing reads.
*
* <p>This class is tolerant of some I/O errors. If files are missing from the
* filesystem, the corresponding entries will be dropped from the cache. If
* an error occurs while writing a cache value, the edit will fail silently.
* Callers should handle other problems by catching {@code IOException} and
* responding appropriately.
*/
public final class DiskLruCache implements Closeable {
static final String JOURNAL_FILE = "journal";
static final String JOURNAL_FILE_TMP = "journal.tmp";
static final String MAGIC = "libcore.io.DiskLruCache";
static final String VERSION_1 = "1";
static final long ANY_SEQUENCE_NUMBER = -1;
private static final String CLEAN = "CLEAN";
private static final String DIRTY = "DIRTY";
private static final String REMOVE = "REMOVE";
private static final String READ = "READ"; private static final Charset UTF_8 = Charset.forName("UTF-8");
private static final int IO_BUFFER_SIZE = 8 * 1024; /*
* This cache uses a journal file named "journal". A typical journal file
* looks like this:
* libcore.io.DiskLruCache
* 1
* 100
* 2
*
* CLEAN 3400330d1dfc7f3f7f4b8d4d803dfcf6 832 21054
* DIRTY 335c4c6028171cfddfbaae1a9c313c52
* CLEAN 335c4c6028171cfddfbaae1a9c313c52 3934 2342
* REMOVE 335c4c6028171cfddfbaae1a9c313c52
* DIRTY 1ab96a171faeeee38496d8b330771a7a
* CLEAN 1ab96a171faeeee38496d8b330771a7a 1600 234
* READ 335c4c6028171cfddfbaae1a9c313c52
* READ 3400330d1dfc7f3f7f4b8d4d803dfcf6
*
* The first five lines of the journal form its header. They are the
* constant string "libcore.io.DiskLruCache", the disk cache's version,
* the application's version, the value count, and a blank line.
*
* Each of the subsequent lines in the file is a record of the state of a
* cache entry. Each line contains space-separated values: a state, a key,
* and optional state-specific values.
* o DIRTY lines track that an entry is actively being created or updated.
* Every successful DIRTY action should be followed by a CLEAN or REMOVE
* action. DIRTY lines without a matching CLEAN or REMOVE indicate that
* temporary files may need to be deleted.
* o CLEAN lines track a cache entry that has been successfully published
* and may be read. A publish line is followed by the lengths of each of
* its values.
* o READ lines track accesses for LRU.
* o REMOVE lines track entries that have been deleted.
*
* The journal file is appended to as cache operations occur. The journal may
* occasionally be compacted by dropping redundant lines. A temporary file named
* "journal.tmp" will be used during compaction; that file should be deleted if
* it exists when the cache is opened.
*/ private final File directory;
private final File journalFile;
private final File journalFileTmp;
private final int appVersion;
private final long maxSize;
private final int valueCount;
private long size = 0;
private Writer journalWriter;
private final LinkedHashMap<String, Entry> lruEntries
= new LinkedHashMap<String, Entry>(0, 0.75f, true);
private int redundantOpCount; /**
* To differentiate between old and current snapshots, each entry is given
* a sequence number each time an edit is committed. A snapshot is stale if
* its sequence number is not equal to its entry's sequence number.
*/
private long nextSequenceNumber = 0; /* From java.util.Arrays */
@SuppressWarnings("unchecked")
private static <T> T[] copyOfRange(T[] original, int start, int end) {
final int originalLength = original.length; // For exception priority compatibility.
if (start > end) {
throw new IllegalArgumentException();
}
if (start < 0 || start > originalLength) {
throw new ArrayIndexOutOfBoundsException();
}
final int resultLength = end - start;
final int copyLength = Math.min(resultLength, originalLength - start);
final T[] result = (T[]) Array
.newInstance(original.getClass().getComponentType(), resultLength);
System.arraycopy(original, start, result, 0, copyLength);
return result;
} /**
* Returns the remainder of 'reader' as a string, closing it when done.
*/
public static String readFully(Reader reader) throws IOException {
try {
StringWriter writer = new StringWriter();
char[] buffer = new char[1024];
int count;
while ((count = reader.read(buffer)) != -1) {
writer.write(buffer, 0, count);
}
return writer.toString();
} finally {
reader.close();
}
} /**
* Returns the ASCII characters up to but not including the next "\r\n", or
* "\n".
*
* @throws java.io.EOFException if the stream is exhausted before the next newline
* character.
*/
public static String readAsciiLine(InputStream in) throws IOException {
// TODO: support UTF-8 here instead StringBuilder result = new StringBuilder(80);
while (true) {
int c = in.read();
if (c == -1) {
throw new EOFException();
} else if (c == '\n') {
break;
} result.append((char) c);
}
int length = result.length();
if (length > 0 && result.charAt(length - 1) == '\r') {
result.setLength(length - 1);
}
return result.toString();
} /**
* Closes 'closeable', ignoring any checked exceptions. Does nothing if 'closeable' is null.
*/
public static void closeQuietly(Closeable closeable) {
if (closeable != null) {
try {
closeable.close();
} catch (RuntimeException rethrown) {
throw rethrown;
} catch (Exception ignored) {
}
}
} /**
* Recursively delete everything in {@code dir}.
*/
// TODO: this should specify paths as Strings rather than as Files
public static void deleteContents(File dir) throws IOException {
File[] files = dir.listFiles();
if (files == null) {
throw new IllegalArgumentException("not a directory: " + dir);
}
for (File file : files) {
if (file.isDirectory()) {
deleteContents(file);
}
if (!file.delete()) {
throw new IOException("failed to delete file: " + file);
}
}
} /** This cache uses a single background thread to evict entries. */
private final ExecutorService executorService = new ThreadPoolExecutor(0, 1,
60L, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>());
private final Callable<Void> cleanupCallable = new Callable<Void>() {
@Override public Void call() throws Exception {
synchronized (DiskLruCache.this) {
if (journalWriter == null) {
return null; // closed
}
trimToSize();
if (journalRebuildRequired()) {
rebuildJournal();
redundantOpCount = 0;
}
}
return null;
}
}; private DiskLruCache(File directory, int appVersion, int valueCount, long maxSize) {
this.directory = directory;
this.appVersion = appVersion;
this.journalFile = new File(directory, JOURNAL_FILE);
this.journalFileTmp = new File(directory, JOURNAL_FILE_TMP);
this.valueCount = valueCount;
this.maxSize = maxSize;
} /**
* Opens the cache in {@code directory}, creating a cache if none exists
* there.
*
* @param directory a writable directory
* @param appVersion
* @param valueCount the number of values per cache entry. Must be positive.
* @param maxSize the maximum number of bytes this cache should use to store
* @throws java.io.IOException if reading or writing the cache directory fails
*/
public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)
throws IOException {
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
if (valueCount <= 0) {
throw new IllegalArgumentException("valueCount <= 0");
} // prefer to pick up where we left off
DiskLruCache cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
if (cache.journalFile.exists()) {
try {
cache.readJournal();
cache.processJournal();
cache.journalWriter = new BufferedWriter(new FileWriter(cache.journalFile, true),
IO_BUFFER_SIZE);
return cache;
} catch (IOException journalIsCorrupt) {
// System.logW("DiskLruCache " + directory + " is corrupt: "
// + journalIsCorrupt.getMessage() + ", removing");
cache.delete();
}
} // create a new empty cache
directory.mkdirs();
cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
cache.rebuildJournal();
return cache;
} private void readJournal() throws IOException {
InputStream in = new BufferedInputStream(new FileInputStream(journalFile), IO_BUFFER_SIZE);
try {
String magic = readAsciiLine(in);
String version = readAsciiLine(in);
String appVersionString = readAsciiLine(in);
String valueCountString = readAsciiLine(in);
String blank = readAsciiLine(in);
if (!MAGIC.equals(magic)
|| !VERSION_1.equals(version)
|| !Integer.toString(appVersion).equals(appVersionString)
|| !Integer.toString(valueCount).equals(valueCountString)
|| !"".equals(blank)) {
throw new IOException("unexpected journal header: ["
+ magic + ", " + version + ", " + valueCountString + ", " + blank + "]");
} while (true) {
try {
readJournalLine(readAsciiLine(in));
} catch (EOFException endOfJournal) {
break;
}
}
} finally {
closeQuietly(in);
}
} private void readJournalLine(String line) throws IOException {
String[] parts = line.split(" ");
if (parts.length < 2) {
throw new IOException("unexpected journal line: " + line);
} String key = parts[1];
if (parts[0].equals(REMOVE) && parts.length == 2) {
lruEntries.remove(key);
return;
} Entry entry = lruEntries.get(key);
if (entry == null) {
entry = new Entry(key);
lruEntries.put(key, entry);
} if (parts[0].equals(CLEAN) && parts.length == 2 + valueCount) {
entry.readable = true;
entry.currentEditor = null;
entry.setLengths(copyOfRange(parts, 2, parts.length));
} else if (parts[0].equals(DIRTY) && parts.length == 2) {
entry.currentEditor = new Editor(entry);
} else if (parts[0].equals(READ) && parts.length == 2) {
// this work was already done by calling lruEntries.get()
} else {
throw new IOException("unexpected journal line: " + line);
}
} /**
* Computes the initial size and collects garbage as a part of opening the
* cache. Dirty entries are assumed to be inconsistent and will be deleted.
*/
private void processJournal() throws IOException {
deleteIfExists(journalFileTmp);
for (Iterator<Entry> i = lruEntries.values().iterator(); i.hasNext(); ) {
Entry entry = i.next();
if (entry.currentEditor == null) {
for (int t = 0; t < valueCount; t++) {
size += entry.lengths[t];
}
} else {
entry.currentEditor = null;
for (int t = 0; t < valueCount; t++) {
deleteIfExists(entry.getCleanFile(t));
deleteIfExists(entry.getDirtyFile(t));
}
i.remove();
}
}
} /**
* Creates a new journal that omits redundant information. This replaces the
* current journal if it exists.
*/
private synchronized void rebuildJournal() throws IOException {
if (journalWriter != null) {
journalWriter.close();
} Writer writer = new BufferedWriter(new FileWriter(journalFileTmp), IO_BUFFER_SIZE);
writer.write(MAGIC);
writer.write("\n");
writer.write(VERSION_1);
writer.write("\n");
writer.write(Integer.toString(appVersion));
writer.write("\n");
writer.write(Integer.toString(valueCount));
writer.write("\n");
writer.write("\n"); for (Entry entry : lruEntries.values()) {
if (entry.currentEditor != null) {
writer.write(DIRTY + ' ' + entry.key + '\n');
} else {
writer.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\n');
}
} writer.close();
journalFileTmp.renameTo(journalFile);
journalWriter = new BufferedWriter(new FileWriter(journalFile, true), IO_BUFFER_SIZE);
} private static void deleteIfExists(File file) throws IOException {
// try {
// Libcore.os.remove(file.getPath());
// } catch (ErrnoException errnoException) {
// if (errnoException.errno != OsConstants.ENOENT) {
// throw errnoException.rethrowAsIOException();
// }
// }
if (file.exists() && !file.delete()) {
throw new IOException();
}
} /**
* Returns a snapshot of the entry named {@code key}, or null if it doesn't
* exist is not currently readable. If a value is returned, it is moved to
* the head of the LRU queue.
*/
public synchronized Snapshot get(String key) throws IOException {
checkNotClosed();
validateKey(key);
Entry entry = lruEntries.get(key);
if (entry == null) {
return null;
} if (!entry.readable) {
return null;
} /*
* Open all streams eagerly to guarantee that we see a single published
* snapshot. If we opened streams lazily then the streams could come
* from different edits.
*/
InputStream[] ins = new InputStream[valueCount];
try {
for (int i = 0; i < valueCount; i++) {
ins[i] = new FileInputStream(entry.getCleanFile(i));
}
} catch (FileNotFoundException e) {
// a file must have been deleted manually!
return null;
} redundantOpCount++;
journalWriter.append(READ + ' ' + key + '\n');
if (journalRebuildRequired()) {
executorService.submit(cleanupCallable);
} return new Snapshot(key, entry.sequenceNumber, ins);
} /**
* Returns an editor for the entry named {@code key}, or null if another
* edit is in progress.
*/
public Editor edit(String key) throws IOException {
return edit(key, ANY_SEQUENCE_NUMBER);
} private synchronized Editor edit(String key, long expectedSequenceNumber) throws IOException {
checkNotClosed();
validateKey(key);
Entry entry = lruEntries.get(key);
if (expectedSequenceNumber != ANY_SEQUENCE_NUMBER
&& (entry == null || entry.sequenceNumber != expectedSequenceNumber)) {
return null; // snapshot is stale
}
if (entry == null) {
entry = new Entry(key);
lruEntries.put(key, entry);
} else if (entry.currentEditor != null) {
return null; // another edit is in progress
} Editor editor = new Editor(entry);
entry.currentEditor = editor; // flush the journal before creating files to prevent file leaks
journalWriter.write(DIRTY + ' ' + key + '\n');
journalWriter.flush();
return editor;
} /**
* Returns the directory where this cache stores its data.
*/
public File getDirectory() {
return directory;
} /**
* Returns the maximum number of bytes that this cache should use to store
* its data.
*/
public long maxSize() {
return maxSize;
} /**
* Returns the number of bytes currently being used to store the values in
* this cache. This may be greater than the max size if a background
* deletion is pending.
*/
public synchronized long size() {
return size;
} private synchronized void completeEdit(Editor editor, boolean success) throws IOException {
Entry entry = editor.entry;
if (entry.currentEditor != editor) {
throw new IllegalStateException();
} // if this edit is creating the entry for the first time, every index must have a value
if (success && !entry.readable) {
for (int i = 0; i < valueCount; i++) {
if (!entry.getDirtyFile(i).exists()) {
editor.abort();
throw new IllegalStateException("edit didn't create file " + i);
}
}
} for (int i = 0; i < valueCount; i++) {
File dirty = entry.getDirtyFile(i);
if (success) {
if (dirty.exists()) {
File clean = entry.getCleanFile(i);
dirty.renameTo(clean);
long oldLength = entry.lengths[i];
long newLength = clean.length();
entry.lengths[i] = newLength;
size = size - oldLength + newLength;
}
} else {
deleteIfExists(dirty);
}
} redundantOpCount++;
entry.currentEditor = null;
if (entry.readable | success) {
entry.readable = true;
journalWriter.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\n');
if (success) {
entry.sequenceNumber = nextSequenceNumber++;
}
} else {
lruEntries.remove(entry.key);
journalWriter.write(REMOVE + ' ' + entry.key + '\n');
} if (size > maxSize || journalRebuildRequired()) {
executorService.submit(cleanupCallable);
}
} /**
* We only rebuild the journal when it will halve the size of the journal
* and eliminate at least 2000 ops.
*/
private boolean journalRebuildRequired() {
final int REDUNDANT_OP_COMPACT_THRESHOLD = 2000;
return redundantOpCount >= REDUNDANT_OP_COMPACT_THRESHOLD
&& redundantOpCount >= lruEntries.size();
} /**
* Drops the entry for {@code key} if it exists and can be removed. Entries
* actively being edited cannot be removed.
*
* @return true if an entry was removed.
*/
public synchronized boolean remove(String key) throws IOException {
checkNotClosed();
validateKey(key);
Entry entry = lruEntries.get(key);
if (entry == null || entry.currentEditor != null) {
return false;
} for (int i = 0; i < valueCount; i++) {
File file = entry.getCleanFile(i);
if (!file.delete()) {
throw new IOException("failed to delete " + file);
}
size -= entry.lengths[i];
entry.lengths[i] = 0;
} redundantOpCount++;
journalWriter.append(REMOVE + ' ' + key + '\n');
lruEntries.remove(key); if (journalRebuildRequired()) {
executorService.submit(cleanupCallable);
} return true;
} /**
* Returns true if this cache has been closed.
*/
public boolean isClosed() {
return journalWriter == null;
} private void checkNotClosed() {
if (journalWriter == null) {
throw new IllegalStateException("cache is closed");
}
} /**
* Force buffered operations to the filesystem.
*/
public synchronized void flush() throws IOException {
checkNotClosed();
trimToSize();
journalWriter.flush();
} /**
* Closes this cache. Stored values will remain on the filesystem.
*/
public synchronized void close() throws IOException {
if (journalWriter == null) {
return; // already closed
}
for (Entry entry : new ArrayList<Entry>(lruEntries.values())) {
if (entry.currentEditor != null) {
entry.currentEditor.abort();
}
}
trimToSize();
journalWriter.close();
journalWriter = null;
} private void trimToSize() throws IOException {
while (size > maxSize) {
// Map.Entry<String, Entry> toEvict = lruEntries.eldest();
final Map.Entry<String, Entry> toEvict = lruEntries.entrySet().iterator().next();
remove(toEvict.getKey());
}
} /**
* Closes the cache and deletes all of its stored values. This will delete
* all files in the cache directory including files that weren't created by
* the cache.
*/
public void delete() throws IOException {
close();
deleteContents(directory);
} private void validateKey(String key) {
if (key.contains(" ") || key.contains("\n") || key.contains("\r")) {
throw new IllegalArgumentException(
"keys must not contain spaces or newlines: \"" + key + "\"");
}
} private static String inputStreamToString(InputStream in) throws IOException {
return readFully(new InputStreamReader(in, UTF_8));
} /**
* A snapshot of the values for an entry.
*/
public final class Snapshot implements Closeable {
private final String key;
private final long sequenceNumber;
private final InputStream[] ins; private Snapshot(String key, long sequenceNumber, InputStream[] ins) {
this.key = key;
this.sequenceNumber = sequenceNumber;
this.ins = ins;
} /**
* Returns an editor for this snapshot's entry, or null if either the
* entry has changed since this snapshot was created or if another edit
* is in progress.
*/
public Editor edit() throws IOException {
return DiskLruCache.this.edit(key, sequenceNumber);
} /**
* Returns the unbuffered stream with the value for {@code index}.
*/
public InputStream getInputStream(int index) {
return ins[index];
} /**
* Returns the string value for {@code index}.
*/
public String getString(int index) throws IOException {
return inputStreamToString(getInputStream(index));
} @Override public void close() {
for (InputStream in : ins) {
closeQuietly(in);
}
}
} /**
* Edits the values for an entry.
*/
public final class Editor {
private final Entry entry;
private boolean hasErrors; private Editor(Entry entry) {
this.entry = entry;
} /**
* Returns an unbuffered input stream to read the last committed value,
* or null if no value has been committed.
*/
public InputStream newInputStream(int index) throws IOException {
synchronized (DiskLruCache.this) {
if (entry.currentEditor != this) {
throw new IllegalStateException();
}
if (!entry.readable) {
return null;
}
return new FileInputStream(entry.getCleanFile(index));
}
} /**
* Returns the last committed value as a string, or null if no value
* has been committed.
*/
public String getString(int index) throws IOException {
InputStream in = newInputStream(index);
return in != null ? inputStreamToString(in) : null;
} /**
* Returns a new unbuffered output stream to write the value at
* {@code index}. If the underlying output stream encounters errors
* when writing to the filesystem, this edit will be aborted when
* {@link #commit} is called. The returned output stream does not throw
* IOExceptions.
*/
public OutputStream newOutputStream(int index) throws IOException {
synchronized (DiskLruCache.this) {
if (entry.currentEditor != this) {
throw new IllegalStateException();
}
return new FaultHidingOutputStream(new FileOutputStream(entry.getDirtyFile(index)));
}
} /**
* Sets the value at {@code index} to {@code value}.
*/
public void set(int index, String value) throws IOException {
Writer writer = null;
try {
writer = new OutputStreamWriter(newOutputStream(index), UTF_8);
writer.write(value);
} finally {
closeQuietly(writer);
}
} /**
* Commits this edit so it is visible to readers. This releases the
* edit lock so another edit may be started on the same key.
*/
public void commit() throws IOException {
if (hasErrors) {
completeEdit(this, false);
remove(entry.key); // the previous entry is stale
} else {
completeEdit(this, true);
}
} /**
* Aborts this edit. This releases the edit lock so another edit may be
* started on the same key.
*/
public void abort() throws IOException {
completeEdit(this, false);
} private class FaultHidingOutputStream extends FilterOutputStream {
private FaultHidingOutputStream(OutputStream out) {
super(out);
} @Override public void write(int oneByte) {
try {
out.write(oneByte);
} catch (IOException e) {
hasErrors = true;
}
} @Override public void write(byte[] buffer, int offset, int length) {
try {
out.write(buffer, offset, length);
} catch (IOException e) {
hasErrors = true;
}
} @Override public void close() {
try {
out.close();
} catch (IOException e) {
hasErrors = true;
}
} @Override public void flush() {
try {
out.flush();
} catch (IOException e) {
hasErrors = true;
}
}
}
} private final class Entry {
private final String key; /** Lengths of this entry's files. */
private final long[] lengths; /** True if this entry has ever been published */
private boolean readable; /** The ongoing edit or null if this entry is not being edited. */
private Editor currentEditor; /** The sequence number of the most recently committed edit to this entry. */
private long sequenceNumber; private Entry(String key) {
this.key = key;
this.lengths = new long[valueCount];
} public String getLengths() throws IOException {
StringBuilder result = new StringBuilder();
for (long size : lengths) {
result.append(' ').append(size);
}
return result.toString();
} /**
* Set lengths using decimal numbers like "10123".
*/
private void setLengths(String[] strings) throws IOException {
if (strings.length != valueCount) {
throw invalidLengths(strings);
} try {
for (int i = 0; i < strings.length; i++) {
lengths[i] = Long.parseLong(strings[i]);
}
} catch (NumberFormatException e) {
throw invalidLengths(strings);
}
} private IOException invalidLengths(String[] strings) throws IOException {
throw new IOException("unexpected journal line: " + Arrays.toString(strings));
} public File getCleanFile(int i) {
return new File(directory, key + "." + i);
} public File getDirtyFile(int i) {
return new File(directory, key + "." + i + ".tmp");
}
}
}
MD5Utils（MD5转换工具类）：
package com.example.photoswall; import java.math.BigInteger;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException; public class MD5Utils {
/**
* 使用md5的算法进行加密
*/
public static String md5(String plainText) {
byte[] secretBytes = null;
try {
secretBytes = MessageDigest.getInstance("md5").digest(
plainText.getBytes());
} catch (NoSuchAlgorithmException e) {
throw new RuntimeException("没有md5这个算法！");
}
String md5code = new BigInteger(1, secretBytes).toString(16);// 16进制数字
// 如果生成数字未满32位，需要前面补0
for (int i = 0; i < 32 - md5code.length(); i++) {
md5code = "0" + md5code;
}
return md5code;
} }
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。
package com.example.photoswall; import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.IOException;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.HashSet;
import java.util.Set; import android.content.Context;
import android.content.pm.PackageInfo;
import android.content.pm.PackageManager.NameNotFoundException;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Environment;
import android.support.v4.util.LruCache;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.GridView;
import android.widget.ImageView; import com.example.photoswall.DiskLruCache.Snapshot; public class PhotoWallAdapter extends ArrayAdapter<String> { // 声明LruCache缓存对象
private LruCache<String, Bitmap> lruCache;
// 声明DiskLruCache硬盘缓存对象
private DiskLruCache diskLruCache;
// 任务队列
private Set<LoadImageAsync> tasks;
// 声明GridView对象
private GridView gridView; public PhotoWallAdapter(Context context, int textViewResourceId, String[] objects, GridView gridView) {
super(context, textViewResourceId, objects);
this.gridView = gridView;
tasks = new HashSet<LoadImageAsync>();
/**
* 初始化内存缓存LruCache
*/
// 获取应用程序最大可占内存值
int maxMemory = (int) Runtime.getRuntime().maxMemory();
// 设置最大内存的八分之一作为缓存大小
int lruMemory = maxMemory / 8;
lruCache = new LruCache<String, Bitmap>(lruMemory) {
@Override
protected int sizeOf(String key, Bitmap bitmap) {
// 返回Bitmap对象所占大小，单位：kb
return bitmap.getByteCount();
} };
/**
* 初始化硬盘缓存DiskLruCahce
*/
// 获取硬盘缓存路径,参数二为所在缓存路径的文件夹名称
File directory = getDiskCacheDir(getContext(), "bitmap");
if (!directory.exists()) {
// 若文件夹不存在，建立文件夹
directory.mkdirs();
}
int appVersion = getAppVersion(getContext());
try {
// 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
diskLruCache = DiskLruCache.open(directory, appVersion, 1, 1024 * 1024 * 10);
} catch (IOException e) {
e.printStackTrace();
} } /**
* @param context
* @param uniqueName
* @return
* 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
* ()方法来获取缓存路径。 前者获取到的就是 /sdcard/Android/data/<application
* package>/cache 这个路径 而后者获取到的是 datadata/application package>/cache
* 这个路径。
*/
public File getDiskCacheDir(Context context, String uniqueName) {
String cachePath;
if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()) || !Environment.isExternalStorageRemovable()) {
cachePath = context.getExternalCacheDir().getPath();
} else {
cachePath = context.getCacheDir().getPath();
}
return new File(cachePath + File.separator + uniqueName);
} /**
* @param context
* @return 获取系统版本号
*/
public int getAppVersion(Context context) {
try {
PackageInfo info = context.getPackageManager().getPackageInfo(context.getPackageName(), 0);
return info.versionCode;
} catch (NameNotFoundException e) {
e.printStackTrace();
}
return 1;
} @Override
public View getView(int position, View convertView, ViewGroup parent) {
// 获取图片资源URL地址
String path = getItem(position);
View view = null;
if (convertView == null) {
view = LayoutInflater.from(getContext()).inflate(R.layout.gridview_item, null);
} else {
view = convertView;
}
// 获取控件实例
ImageView imageView = (ImageView) view.findViewById(R.id.iv_photo);
// 设置一个唯一标识符，避免异步加载图片时错位
imageView.setTag(path);
// 设置默认图片
imageView.setImageResource(R.drawable.ic_launcher);
// 根据图片URL到缓存中去找图片资源并设置
setImageFromLruCache(path, imageView);
return view;
} /**
* 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
*
* @param path
* @param imageView
*/
private void setImageFromLruCache(String path, ImageView imageView) {
Bitmap bitmap = lruCache.get(path);
if (bitmap != null) {
// 缓存存在,取出设置图片
Log.i("PhotoWallAdapter", "在内存缓存中找到");
imageView.setImageBitmap(bitmap);
} else {
// 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）
LoadImageAsync loadImageAsync = new LoadImageAsync();
loadImageAsync.execute(path);
// 添加任务到任务队列
tasks.add(loadImageAsync);
}
} /**
* 取消队列中准备下载和正在下载的任务
*/
public void cancelTask() {
for (LoadImageAsync task : tasks) {
task.cancel(false);
}
} /**
* 同步内存操作到journal文件
*/
public void flushCache() {
if (diskLruCache != null) {
try {
diskLruCache.flush();
} catch (IOException e) {
e.printStackTrace();
}
} } class LoadImageAsync extends AsyncTask<String, Void, Bitmap> {
// 图片资源URL
String path = null; @Override
protected Bitmap doInBackground(String... params) {
// 图片下载地址
this.path = params[0];
Snapshot snapshot = null;
OutputStream outputStream = null;
Bitmap bitmap = null;
String pathMd5 = MD5Utils.md5(path);
// 根据图片url（md5）查找图片资源是否存在于硬盘缓存
try {
snapshot = diskLruCache.get(pathMd5);
if (snapshot == null) {
// 在磁盘缓存中没有找到图片资源
// 获取一个DiskLruCache写入对象
DiskLruCache.Editor editor = diskLruCache.edit(pathMd5);
if (editor != null) {
outputStream = editor.newOutputStream(0);
// 开启异步网络任务获取图片，并存入磁盘缓存
if (downloadUrlToStream(path, outputStream)) {
// 下载成功
Log.i("PhotoWallAdapter", "下载文件成功");
editor.commit();
} else {
editor.abort();
}
}
}
// 图片写入磁盘缓存后，再一次的查找磁盘缓存
snapshot = diskLruCache.get(pathMd5);
if (snapshot != null) {
// 若查找到，获取图片，并把图片资源写入内存缓存
bitmap = BitmapFactory.decodeStream(snapshot.getInputStream(0));
Log.i("PhotoWallAdapter", "在磁盘缓存中找到");
}
if (bitmap != null) {
// 将Bitmap对象添加到内存缓存当中
lruCache.put(path, bitmap);
}
return bitmap;
} catch (IOException e) {
e.printStackTrace();
} finally {
if (outputStream != null) {
try {
outputStream.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
return null;
} @Override
protected void onPostExecute(Bitmap bitmap) {
super.onPostExecute(bitmap);
// 根据Tag获取控件对象并设置图片
ImageView imageView = (ImageView) gridView.findViewWithTag(path);
if (imageView != null && bitmap != null) {
// 加载图片
imageView.setImageBitmap(bitmap);
}
tasks.remove(this); } /**
* 根据图片URL地址下载图片,成功返回true，失败false
*
* @param urlString
* @param outputStream
* @return
*/
private boolean downloadUrlToStream(String urlString, OutputStream outputStream) {
HttpURLConnection urlConnection = null;
BufferedOutputStream out = null;
BufferedInputStream in = null;
try {
final URL url = new URL(urlString);
urlConnection = (HttpURLConnection) url.openConnection();
in = new BufferedInputStream(urlConnection.getInputStream(), 8 * 1024);
out = new BufferedOutputStream(outputStream, 8 * 1024);
int b;
while ((b = in.read()) != -1) {
out.write(b);
}
return true;
} catch (final IOException e) {
e.printStackTrace();
} finally {
if (urlConnection != null) {
urlConnection.disconnect();
}
try {
if (out != null) {
out.close();
}
if (in != null) {
in.close();
}
} catch (final IOException e) {
e.printStackTrace();
}
}
return false;
} } }
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。
package com.example.photoswall; import android.app.Activity;
import android.os.Bundle;
import android.widget.GridView; public class MainActivity extends Activity { private GridView gv_photo;
private PhotoWallAdapter adapter; @Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
gv_photo = (GridView) findViewById(R.id.gv_photo);
adapter = new PhotoWallAdapter(MainActivity.this, 0, Images.imageThumbUrls, gv_photo);
gv_photo.setAdapter(adapter); } @Override
protected void onPause() {
super.onPause();
adapter.flushCache();
} @Override
protected void onDestroy() {
super.onDestroy();
adapter.cancelTask();
} }
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。
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！
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