　　DiskLruCache是谷歌推荐的用来实现硬盘缓存的类，今天我们开始对于DiskLruCache的学习。DiskLruCache的测试代码：DiskLruCache的测试代码下载。关于FidkLruCache的使用，请参见我的博客：android基础---->LruCache的使用及原理

目录导航

DiskLruCache缓存的代码实例
DiskLruCache的原理分析
友情链接

DiskLruCache缓存的代码实例

我们通过一个案例来体会DiskLruCache的使用及执行的流程，在我们的项目中包含DiskLruCache文件：http://pan.baidu.com/s/1slR6pg5。项目结构如下：

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" alt="" />

一、 DiskLruCache的是一个final类，不能继承。如果我们要创建一个DiskLruCache的实例，则需要调用它的open()方法，如下：

public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)

在AndroidManifest.xml加入网络权限和sd卡写入权限：

<uses-permission android:name="android.permission.INTERNET"/>

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

项目中创建并打开缓存：

try {

    File cacheDir = getDiskCacheDir(this, "bitmap");

    if (!cacheDir.exists()) {

        cacheDir.mkdirs();

    }

    mDiskLruCache = DiskLruCache.open(cacheDir, getAppVersion(this), 1, 10 * 1024 * 1024);

} catch (Exception e) {

    e.printStackTrace();

}

二、从网络得到图片资源，并写入缓存：

private void saveCache() {

    new Thread(new Runnable() {

        @Override

        public void run() {

            try {

                String key = hashKeyForDisk(imageUrl);

                DiskLruCache.Editor editor = null;

                editor = mDiskLruCache.edit(key);

                if (editor != null) {

                    OutputStream outputStream = editor.newOutputStream(0);

                    if (downloadUrlToStream(imageUrl, outputStream)) {

                        editor.commit();

                    } else {

                        editor.abort();

                    }

                }

                //频繁的flush

                mDiskLruCache.flush();

                handler.post(new Runnable() {

                    @Override

                    public void run() {

                        textView.setText("saveCache done,the bitmap is ready");

                    }

                });

            } catch (IOException e) {

                e.printStackTrace();

            }

        }

    }).start();

}

downloadUrlToStream方法用于从网络上获取图片资源：

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;

}

三、由于涉及到key的因素，我们写一个MD5对key进行编码：

public String hashKeyForDisk(String key) {

    String cacheKey;

    try {

        final MessageDigest mDigest = MessageDigest.getInstance("MD5");

        mDigest.update(key.getBytes());

        cacheKey = bytesToHexString(mDigest.digest());

    } catch (NoSuchAlgorithmException e) {

        cacheKey = String.valueOf(key.hashCode());

    }

    return cacheKey;

}

private String bytesToHexString(byte[] bytes) {

    StringBuilder sb = new StringBuilder();

    for (int i = 0; i < bytes.length; i++) {

        String hex = Integer.toHexString(0xFF & bytes[i]);

        if (hex.length() == 1) {

            sb.append('0');

        }

        sb.append(hex);

    }

    return sb.toString();

}

四、从缓存中读取数据：

private void readCache() {

    //读取缓存

    try {

        DiskLruCache.Snapshot snapshot = mDiskLruCache.get(hashKeyForDisk(imageUrl));

        if (snapshot != null) {

            InputStream is = snapshot.getInputStream(0);

            Bitmap bitmap = BitmapFactory.decodeStream(is);

            imageView.setImageBitmap(bitmap);

        } else {

            imageView.setImageBitmap(null);

        }

    } catch (IOException e) {

        e.printStackTrace();

    }

}

五、移除缓存：

private void removeCache() {

    try {

        mDiskLruCache.remove(hashKeyForDisk(imageUrl));

    } catch (IOException e) {

        e.printStackTrace();

    }

}

六、清空缓存：

private void deleteCache() {

    try {

        //delete()方法内部会调用close()

        mDiskLruCache.delete();

    } catch (IOException e) {

        e.printStackTrace();

    }

}

七、得到缓存的大小：

private void getCacheSize() {

    textView.setText("cache size : " + mDiskLruCache.size() + "B");

}

八、在onDesctory方法中关闭缓存：

@Override

protected void onDestroy() {

    super.onDestroy();

    try {

        //关闭DiskLruCache,与open对应

        mDiskLruCache.close();

    } catch (IOException e) {

        e.printStackTrace();

    }

}

DiskLruCache的原理分析

经过上述实例的说明，我们对DiskLruCache的使用已经有了一些认识。现在我们开始DiskLruCache的原理分析：

创建打开缓存

一、当创建打开缓存：mDiskLruCache = DiskLruCache.open(cacheDir, getAppVersion(this), 1, 10 * 1024 * 1024);

创建日志journal文件，并且初始化journalWriter：

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) {

            cache.delete();

        }

    }

    // create a new empty cache

    directory.mkdirs();

    cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);

    cache.rebuildJournal();

    return cache;

}

在DiskLruCache的构造方法中，创建journal和journal.tmp文件。

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;

}

在rebuildJournal方法中，在journalFileTmp临时文件中，写入一些数据，最后重命名为journalFile：

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);

}

二、创建完成之后，在sd中存在journal文件:内容如下：

libcore.io.DiskLruCache  // MAGIC

1                        // VERSION_1

1                        // appVersion

1                        // valueCount

写入缓存

二、然后是这个代码：DiskLruCache.Editor editor = mDiskLruCache.edit(key);其中的lruEntries是一个LinkedHashMap，用于记录资源的key与value。

当第一次edit时，lruEntries.get(key)返回的是空。这里会创建一个Entry对象，并在日志文件中写入DIRTY + ' ' + key + '\n'内容。最后返回包含这个entry的Editor。

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;

}

三、 OutputStream outputStream = editor.newOutputStream(0);downloadUrlToStream(imageUrl, outputStream);得到文件输出流，将从网络上请求到的资源写入到文件中。

这里关注下outputStream，它是由newOutputStream方法得到的：

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)));

    }

}

getDirtyFile文件是一个临时文件，也就是网络文件写入到这个临时文件当中：

public File getDirtyFile(int i) {

    return new File(directory, key + "." + i + ".tmp");

}

四、接着执行到了editor.commit()方法，如果没有出现错误的话：
创建一个key + "." + i的文件，将上述的dirty文件重命名为key + "." + i的文件。更新已经缓存的大小，并且删除上述的dirty文件。

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);

    }

}

五、当执行完写入之后，日志文件如下内容：abb7d9d7add6b9fba5314aec6e60c9e6就是上述MD5生成的key，15417是代表缓存的大小。并生成一个abb7d9d7add6b9fba5314aec6e60c9e6.0文件.

libcore.io.DiskLruCache

1

1

1

DIRTY abb7d9d7add6b9fba5314aec6e60c9e6

CLEAN abb7d9d7add6b9fba5314aec6e60c9e6 15417

从缓存中读取

一、 mDiskLruCache.get(key，以下是关键代码，中间省略了代码);

Entry entry = lruEntries.get(key); // 根据key从map中得到相应的value

　　....

InputStream[] ins = new InputStream[valueCount];

try {

    for (int i = 0; i < valueCount; i++) {

        ins[i] = new FileInputStream(entry.getCleanFile(i)); // 得到clearn文件输入流

    }

} catch (FileNotFoundException e) {

    // a file must have been deleted manually!

    return null;

}

journalWriter.append(READ + ' ' + key + '\n');  // 在日志文件中写入内容

　　....

return new Snapshot(key, entry.sequenceNumber, ins); // 返回一个Snapshot对象

二、 snapshot.getInputStream(0)得到clean文件的输入流，然后通过Bitmap bitmap = BitmapFactory.decodeStream(is);方法得到缓存在硬盘的图片。

友情链接

DiskLruCache的测试代码下载

巴特西

android基础---->DiskLruCache的使用及原理