ImageLoader的圖片緩存分成磁盤和內存兩種，這裏分析一下磁盤緩存以及圖片文件名算法的實現

默認是不存儲在磁盤上的，需要手動打開開關

如下

DisplayImageOptions options = new DisplayImageOptions.Builder()
                .cacheInMemory(true) // default false
                .cacheOnDisk(true) // default false

imageLoader.displayImage("", imageView, options, null, null);
 

二. 磁盤文件命名

/**
 * Generates names for files at disk cache
 *
 * @author Sergey Tarasevich (nostra13[at]gmail[dot]com)
 * @since 1.3.1
 */
public interface FileNameGenerator {

   /** Generates unique file name for image defined by URI */
   String generate(String imageUri);
}

接口是FileNameGenerator，此接口非常簡單明了，只有一個根據圖片uri產生一個圖片文件名稱的方法。

它包含兩個實現類

1. HashCodeFileNameGenerator
2. Md5FileNameGenerator

接下來，分別看這兩個類的實現

2.1 HashCodeFileNameGenerator

/**
 * Names image file as image URI {@linkplain String#hashCode() hashcode}
 *
 * @author Sergey Tarasevich (nostra13[at]gmail[dot]com)
 * @since 1.3.1
 */
public class HashCodeFileNameGenerator implements FileNameGenerator {
   @Override
   public String generate(String imageUri) {
      return String.valueOf(imageUri.hashCode());
   }
}
 

實現比較簡單，根據uri的hashcode轉化成String即可，默認就是Hashcode命名。

2.2 Md5FileNameGenerator

/**
 * Names image file as MD5 hash of image URI
 *
 * @author Sergey Tarasevich (nostra13[at]gmail[dot]com)
 * @since 1.4.0
 */
public class Md5FileNameGenerator implements FileNameGenerator {

   private static final String HASH_ALGORITHM = "MD5";
   private static final int RADIX = 10 + 26; // 10 digits + 26 letters

   @Override
   public String generate(String imageUri) {
      byte[] md5 = getMD5(imageUri.getBytes());
      BigInteger bi = new BigInteger(md5).abs();
      return bi.toString(RADIX);
   }

   private byte[] getMD5(byte[] data) {
      byte[] hash = null;
      try {
         MessageDigest digest = MessageDigest.getInstance(HASH_ALGORITHM);
         digest.update(data);
         hash = digest.digest();
      } catch (NoSuchAlgorithmException e) {
         L.e(e);
      }
      return hash;
   }
}

通過imageUri得到byte數組，然後通過MD5算法得到文件名

三. 磁盤目錄選擇

一般默認優先選擇sdk/android/data/packageName/cache/uil-images卡，如果sdk目錄創建失敗，那麽會選擇/data/data/packageName目錄

四. 圖片緩存示例

其中-1557665659.0和1238391484.0兩個就是圖片存儲文件

journal是操作記錄描述性文件，內容如下

1. DIRTY: 操作記錄創建，如果DIRTY後面沒有CLEAN或者REMOVE，那麽這個圖片會被刪除。
2. CLEAN: 記錄成功創建和訪問
3. READ: 記錄成功訪問
4. REMOVE: 記錄刪除

五. 磁盤緩存接口

磁盤緩存算法的接口是DiskCache，接口很簡單明了。

public interface DiskCache {

   File getDirectory();

   File get(String imageUri);

   boolean save(String imageUri, InputStream imageStream, IoUtils.CopyListener listener) throws IOException;

   boolean save(String imageUri, Bitmap bitmap) throws IOException;

   boolean remove(String imageUri);

   void close();

   void clear();
}

5.1 實現類

下面詳細看每個類的實現

六. LruDiskCache

public class LruDiskCache implements DiskCache {
    protected DiskLruCache cache;
    ...
    protected final FileNameGenerator fileNameGenerator;
    ...

    public LruDiskCache(File cacheDir, File reserveCacheDir, FileNameGenerator fileNameGenerator, long cacheMaxSize,
            int cacheMaxFileCount) throws IOException {
        ...
        initCache(cacheDir, reserveCacheDir, cacheMaxSize, cacheMaxFileCount);
    }

    private void initCache(File cacheDir, File reserveCacheDir, long cacheMaxSize, int cacheMaxFileCount)
            throws IOException {
        try {
            cache = DiskLruCache.open(cacheDir, 1, 1, cacheMaxSize, cacheMaxFileCount);
        } catch (IOException e) {
            ...
        }
    }

    @Override
    public File getDirectory() {
        return cache.getDirectory();
    }

    @Override
    public File get(String imageUri) {
        DiskLruCache.Snapshot snapshot = null;
        try {
            snapshot = cache.get(getKey(imageUri));
            return snapshot == null ? null : snapshot.getFile(0);
        } catch (IOException e) {
            L.e(e);
            return null;
        } finally {
            if (snapshot != null) {
                snapshot.close();
            }
        }
    }

    @Override
    public boolean save(String imageUri, InputStream imageStream, IoUtils.CopyListener listener) throws IOException {
        DiskLruCache.Editor editor = cache.edit(getKey(imageUri));
        if (editor == null) {
            return false;
        }

        OutputStream os = new BufferedOutputStream(editor.newOutputStream(0), bufferSize);
        boolean copied = false;
        try {
            copied = IoUtils.copyStream(imageStream, os, listener, bufferSize);
        } finally {
            IoUtils.closeSilently(os);
            if (copied) {
                editor.commit();
            } else {
                editor.abort();
            }
        }
        return copied;
    }
    ...

    @Override
    public boolean remove(String imageUri) {
        try {
            return cache.remove(getKey(imageUri));
        } catch (IOException e) {
            L.e(e);
            return false;
        }
    }

    @Override
    public void close() {
        try {
            cache.close();
        } catch (IOException e) {
            L.e(e);
        }
        cache = null;
    }

    @Override
    public void clear() {
        try {
            cache.delete();
        } catch (IOException e) {
            L.e(e);
        }
        try {
            initCache(cache.getDirectory(), reserveCacheDir, cache.getMaxSize(), cache.getMaxFileCount());
        } catch (IOException e) {
            L.e(e);
        }
    }

    private String getKey(String imageUri) {
        return fileNameGenerator.generate(imageUri);
    }
}

LruDiskCache有幾個比較重要的屬性，

protected DiskLruCache cache;
protected final FileNameGenerator fileNameGenerator;

FileNameGenerator就是上面說的文件命名生成器，包括hashcode和md5算法。我們思考下，為什麽需要FileNameGenerator？

個人以為網絡上面的uri可能是千奇百怪的，甚至包括特殊字符，那作為文件名顯然不合適。所以，這個時候來一次hashcode，或者md5轉換，獲取文件名是最好的。

DiskLruCache，竊以為這個命名不是很好，因為跟LruDiskCache很類似(我第一眼就看成一個東西了！)

這個DiskLruCache很重要，它維護了磁盤圖片文件緩存的操作記錄，緩存和文件對應關系等。

而且如果你仔細看LruDiskCache的各個方法時會發現，基本都是調用cache的對應方法。

所以，我們主要接下來看DiskLruCache代碼

final class DiskLruCache implements Closeable {
   ...
   private final File directory;
   private final File journalFile;
   ...
   private Writer journalWriter;
   private final LinkedHashMap<String, Entry> lruEntries =
         new LinkedHashMap<String, Entry>(0, 0.75f, true);

   ...
}

DiskLruCache包含了journalFile，文件裏面具體的含義可以第四點的樣例。包含了

LinkedHashMap<String, Entry> lruEntries 

表示每個圖片的緩存記錄，String表示key， Entry表示圖片的描述信息

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;

   public File getCleanFile(int i) {
      return new File(directory, key + "." + i);
   }

   public File getDirtyFile(int i) {
      return new File(directory, key + "." + i + ".tmp");
   }
}

我們以保存圖片緩存為例，分析下LruDiskCache的工作流程，首先看LruDiskCache的save方法

public boolean save(String imageUri, InputStream imageStream, IoUtils.CopyListener listener) throws IOException {
   DiskLruCache.Editor editor = cache.edit(getKey(imageUri));
   if (editor == null) {
      return false;
   }

   OutputStream os = new BufferedOutputStream(editor.newOutputStream(0), bufferSize);
   boolean copied = false;
   try {
      copied = IoUtils.copyStream(imageStream, os, listener, bufferSize);
   } finally {
      IoUtils.closeSilently(os);
      if (copied) {
         editor.commit();
      } else {
         editor.abort();
      }
   }
   return copied;
}

6.1 getkey(imageUri)

首先根據imageUri生成文件名，也就是key，目前我們用的是hashCode

private String getKey(String imageUri) {
   return fileNameGenerator.generate(imageUri);
}

6.2 cache.edit

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;
}

從lruEntries裏面根據key獲取到對應的圖片Entry對象，如果沒有就新建一個。

然後利用journalWriter寫入一條DIRTY記錄。

6.3 DiskLruCache 打開Dirty圖片文件流

public OutputStream newOutputStream(int index) throws IOException {
   synchronized (DiskLruCache.this) {
      if (entry.currentEditor != this) {
         throw new IllegalStateException();
      }
      if (!entry.readable) {
         written[index] = true;
      }
      File dirtyFile = entry.getDirtyFile(index);
      FileOutputStream outputStream;
      try {
         outputStream = new FileOutputStream(dirtyFile);
      } catch (FileNotFoundException e) {
         // Attempt to recreate the cache directory.
         directory.mkdirs();
         try {
            outputStream = new FileOutputStream(dirtyFile);
         } catch (FileNotFoundException e2) {
            // We are unable to recover. Silently eat the writes.
            return NULL_OUTPUT_STREAM;
         }
      }
      return new FaultHidingOutputStream(outputStream);
   }
}

public File getDirtyFile(int i) {
   return new File(directory, key + "." + i + ".tmp");
}

註意這裏打開的是drity文件，就是正常的文件後面加上一個.tmp後綴。

6.4 copyStream把網絡圖片流寫入Dirty文件

public static boolean copyStream(InputStream is, OutputStream os, CopyListener listener, int bufferSize)
      throws IOException {
   int current = 0;
   int total = is.available();
   if (total <= 0) {
      total = DEFAULT_IMAGE_TOTAL_SIZE;
   }

   final byte[] bytes = new byte[bufferSize];
   int count;
   if (shouldStopLoading(listener, current, total)) return false;
   while ((count = is.read(bytes, 0, bufferSize)) != -1) {
      os.write(bytes, 0, count);
      current += count;
      if (shouldStopLoading(listener, current, total)) return false;
   }
   os.flush();
   return true;
}

private static boolean shouldStopLoading(CopyListener listener, int current, int total) {
   if (listener != null) {
      boolean shouldContinue = listener.onBytesCopied(current, total);
      if (!shouldContinue) {
         if (100 * current / total < CONTINUE_LOADING_PERCENTAGE) {
            return true; // if loaded more than 75% then continue loading anyway
         }
      }
   }
   return false;
}

很普通的文件流讀寫，有意思的是shouldStopLoading，它給了我們一個使用listener終止copy的時機。

public static interface CopyListener {
   /**
    * @param current Loaded bytes
    * @param total   Total bytes for loading
    * @return <b>true</b> - if copying should be continued; <b>false</b> - if copying should be interrupted
    */
   boolean onBytesCopied(int current, int total);
}

6.5 關閉Dirty文件流

IoUtils.closeSilently(os);

6.6 寫入圖片文件

假設沒有出錯，completeEdit裏面，會把dirty文件正式名稱成圖片緩存文件

dirty.renameTo(clean);

然後寫入一條CLEAN或者REMOVE操作日誌到journal文件中。

具體可以看代碼

editor.commit();

public void commit() throws IOException {
   if (hasErrors) {
      completeEdit(this, false);
      remove(entry.key); // The previous entry is stale.
   } else {
      completeEdit(this, true);
   }
   committed = true;
}

private synchronized void completeEdit(Editor editor, boolean success) throws IOException {
   ...

   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); //保存dirty到正式圖片文件
            long oldLength = entry.lengths[i];
            long newLength = clean.length();
            entry.lengths[i] = newLength;
            size = size - oldLength + newLength;
            fileCount++;
         }
      } else {
         deleteIfExists(dirty);
      }
   }

   redundantOpCount++;
   entry.currentEditor = null;
   if (entry.readable | success) {// 寫入CLEAN操作日誌
      entry.readable = true;
      journalWriter.write(CLEAN + ‘ ‘ + entry.key + entry.getLengths() + ‘\n‘);
      if (success) {
         entry.sequenceNumber = nextSequenceNumber++;
      }
   } else {
      lruEntries.remove(entry.key); //操作失敗，寫入REMOVE操作日誌
      journalWriter.write(REMOVE + ‘ ‘ + entry.key + ‘\n‘);
   }
   journalWriter.flush();

   if (size > maxSize || fileCount > maxFileCount || journalRebuildRequired()) {
      executorService.submit(cleanupCallable);
   }
}

這樣一次文件保存操作就完成了。

七. BaseDiskCache

BaseDiskCache是抽象類，實現了基本的圖片文件存儲，獲取，刪除等操作，並沒有做什麽限制。

如save和get, remove等操作

public abstract class BaseDiskCache implements DiskCache {
   ...

   protected final FileNameGenerator fileNameGenerator;
   ...

   @Override
   public File getDirectory() {
      return cacheDir;
   }

   @Override
   public File get(String imageUri) {
      return getFile(imageUri);
   }

   @Override
   public boolean save(String imageUri, InputStream imageStream, IoUtils.CopyListener listener) throws IOException {
      File imageFile = getFile(imageUri);
      File tmpFile = new File(imageFile.getAbsolutePath() + TEMP_IMAGE_POSTFIX);
      boolean loaded = false;
      try {
         OutputStream os = new BufferedOutputStream(new FileOutputStream(tmpFile), bufferSize);
         try {
            loaded = IoUtils.copyStream(imageStream, os, listener, bufferSize);
         } finally {
            IoUtils.closeSilently(os);
         }
      } finally {
         if (loaded && !tmpFile.renameTo(imageFile)) {
            loaded = false;
         }
         if (!loaded) {
            tmpFile.delete();
         }
      }
      return loaded;
   }

   @Override
   public boolean remove(String imageUri) {
      return getFile(imageUri).delete();
   }

   @Override
   public void close() {
      // Nothing to do
   }

   @Override
   public void clear() {
      File[] files = cacheDir.listFiles();
      if (files != null) {
         for (File f : files) {
            f.delete();
         }
      }
   }

   protected File getFile(String imageUri) {
      String fileName = fileNameGenerator.generate(imageUri);
      File dir = cacheDir;
      if (!cacheDir.exists() && !cacheDir.mkdirs()) {
         if (reserveCacheDir != null && (reserveCacheDir.exists() || reserveCacheDir.mkdirs())) {
            dir = reserveCacheDir;
         }
      }
      return new File(dir, fileName);
   }
 }

以save為例，首先會生成一個tmp文件，然後把網絡圖片文件流寫入tmp文件。

OutputStream os = new BufferedOutputStream(new FileOutputStream(tmpFile),
loaded = IoUtils.copyStream(imageStream, os, listener, bufferSize);

然後把tmp文件重新名稱成正式的文件

tmpFile.renameTo(imageFile)

八. UnlimitedDiskCache

和BaseDiskCache完全一樣，並沒有新的邏輯

九. LimitedAgeDiskCache

限制存儲時間的文件存儲管理，當我們嘗試獲取緩存文件的時候會去刪除時間過長的文件，存儲的空間沒有限制。

我們以save和get為例

private final Map<File, Long> loadingDates = Collections.synchronizedMap(new HashMap<File, Long>());

@Override
public boolean save(String imageUri, Bitmap bitmap) throws IOException {
   boolean saved = super.save(imageUri, bitmap);
   rememberUsage(imageUri);
   return saved;
}

private void rememberUsage(String imageUri) {
   File file = getFile(imageUri);
   long currentTime = System.currentTimeMillis();
   file.setLastModified(currentTime);
   loadingDates.put(file, currentTime);
}

save的時候，會調用rememberUsage方法，使用一個HashMap來存儲緩存時間。

get

@Override
public File get(String imageUri) {
   File file = super.get(imageUri);
   if (file != null && file.exists()) {
      boolean cached;
      Long loadingDate = loadingDates.get(file);
      if (loadingDate == null) {
         cached = false;
         loadingDate = file.lastModified();
      } else {
         cached = true;
      }

      if (System.currentTimeMillis() - loadingDate > maxFileAge) {
         file.delete();
         loadingDates.remove(file);
      } else if (!cached) {
         loadingDates.put(file, loadingDate);
      }
   }
   return file;
}

get的時候會根據當前時間和緩存時間比較，如果大於maxFileAge，那麽就刪除它，從而實現了限制時間文件存儲。

64.ImageLoader源代碼分析-磁盤命名和圖片緩存算法