android volley缓存设计分析

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本文学习一下volley的缓存设计写法。

首先看下哪些类使用了cache。

Cache.java

public interface Cache {

public Entry get(String key);

public void put(String key, Entry entry);

public void initialize();

public void invalidate(String key, boolean fullExpire);

public void remove(String key);

public void clear();

public static class Entry {

/** The data returned from cache. */

public byte[] data;

/** ETag for cache coherency. */

public String etag;

/** Date of this response as reported by the server. */

public long serverDate;

/** The last modified date for the requested object. */

public long lastModified;

/** TTL for this record. */

public long ttl;

/** Soft TTL for this record. */

public long softTtl;

/** Immutable response headers as received from server; must be non-null. */

public Map<String, String> responseHeaders = Collections.emptyMap();

/** True if the entry is expired. */

public boolean isExpired() {

return this.ttl < System.currentTimeMillis();

}

/** True if a refresh is needed from the original data source. */

public boolean refreshNeeded() {

return this.softTtl < System.currentTimeMillis();

}

DiskBasedCache是唯一的实现类。

DiskBasedCache.java

public synchronized void initialize() {

if (!mRootDirectory.exists()) {

if (!mRootDirectory.mkdirs()) {

VolleyLog.e("Unable to create cache dir %s", mRootDirectory.getAbsolutePath());

}

return; //路径不存在则只需创建路径即可返回

}

File[] files = mRootDirectory.listFiles();

if (files == null) {

return; //路径存在但不存在子目录即可返回

}

for (File file : files) {

BufferedInputStream fis = null;

try {

fis = new BufferedInputStream(new FileInputStream(file));

CacheHeader entry = CacheHeader.readHeader(fis);

entry.size = file.length();

putEntry(entry.key, entry); //若路径下的子路径不为空则将子路径处理成cache

} catch (IOException e) {

if (file != null) {

file.delete();

}

} finally {

try {

if (fis != null) {

fis.close();

}

} catch (IOException ignored) { }

}

private void putEntry(String key, CacheHeader entry) {

if (!mEntries.containsKey(key)) {

mTotalSize += entry.size;

} else {

CacheHeader oldEntry = mEntries.get(key);

mTotalSize += (entry.size - oldEntry.size);

}

mEntries.put(key, entry); //mEntries—>new LinkedHashMap<String,CacheHeader>(16,.75f,true);

//volley是这样做的在内存中保留一个map 在硬盘中存储实际的内容

}

@Override

public synchronized void put(String key, Entry entry) {

pruneIfNeeded(entry.data.length);

File file = getFileForKey(key);

try {

BufferedOutputStream fos = new BufferedOutputStream(new FileOutputStream(file));

CacheHeader e = new CacheHeader(key, entry);//将除data外的信息保存到cacheHeader进而保存到map 内存

boolean success = e.writeHeader(fos); //将header信息写入硬盘

if (!success) {

fos.close();

VolleyLog.d("Failed to write header for %s", file.getAbsolutePath());

throw new IOException();

}

fos.write(entry.data); //将data写入硬盘

fos.close();

putEntry(key, e);

return;

} catch (IOException e) {

}

boolean deleted = file.delete();

if (!deleted) {

VolleyLog.d("Could not clean up file %s", file.getAbsolutePath());

}

看一个cache put行为的实例。

NetworkDispatcher.java

public void run()

Response<?> response = request.parseNetworkResponse(networkResponse);

request.addMarker("network-parse-complete");

// Write to cache if applicable.

// TODO: Only update cache metadata instead of entire record for 304s.

if (request.shouldCache() && response.cacheEntry != null) {

mCache.put(request.getCacheKey(), response.cacheEntry);

request.addMarker("network-cache-written");

}

看一个Response实现parseNetworkResponse的例子。

ImageRequest.java

protected Response<Bitmap> parseNetworkResponse(NetworkResponse response)

return doParse(response);

private Response<Bitmap> doParse(NetworkResponse response)

return Response.success(bitmap, HttpHeaderParser.parseCacheHeaders(response));

//由下面的接口知第二个参数就是cache.Entry—>回忆一下自定义GsonRequest最后也用到了这个

//具体实现类HttpHeaderParser 在com.android.volley.toolbox包下此处不做分析就是构造出一个entry而已

/////////////******

Response.java

public static <T> Response<T> success(T result, Cache.Entry cacheEntry) {

return new Response<T>(result, cacheEntry);

}

*****///////////

回到DiskBasedCache.java

static class CacheHeader {

public long size; //观察到这些字段和Cache Entry是一样的唯独少了data字段

/** The key that identifies the cache entry. */

public String key;

/** ETag for cache coherence. */

public String etag;

/** Date of this response as reported by the server. */

public long serverDate;

/** The last modified date for the requested object. */

public long lastModified;

/** TTL for this record. */

public long ttl;

/** Soft TTL for this record. */

public long softTtl;

/** Headers from the response resulting in this cache entry. */

public Map<String, String> responseHeaders; //观察到这些字段和Cache Entry是一样的唯独少了data字段

public CacheHeader(String key, Entry entry); //构造函数

public static CacheHeader readHeader(InputStream is); //读取header 其实是剔除文件中header部分的内容

public Entry toCacheEntry(byte[] data); //根据data还原出一个Entry对象

public boolean writeHeader(OutputStream os); //写header

}

最后看一下get的实现

@Override

public synchronized Entry get(String key) {

CacheHeader entry = mEntries.get(key);

// if the entry does not exist, return.

if (entry == null) {

return null;

}

File file = getFileForKey(key);

CountingInputStream cis = null;

try {

cis = new CountingInputStream(new BufferedInputStream(new FileInputStream(file)));

CacheHeader.readHeader(cis); // eat header

byte[] data = streamToBytes(cis, (int) (file.length() - cis.bytesRead));

return entry.toCacheEntry(data);

} catch (IOException e) {

VolleyLog.d("%s: %s", file.getAbsolutePath(), e.toString());

remove(key);

return null;

} catch (NegativeArraySizeException e) {

VolleyLog.d("%s: %s", file.getAbsolutePath(), e.toString());

remove(key);

return null;

} finally {

if (cis != null) {

try {

cis.close();

} catch (IOException ioe) {

return null;

}

} //没啥特别

ok~这篇文章就先到这里啦~

这篇文章分析了volley关于cache的实现。

1 volley封装了一个Cache接口用于操作cache比如插入cache 清除cache等

2 volley封装了的Cache.Entry实体用于保存cache实际的内容比如cache中内容从服务器获取的时间 cache

的内容等

3 为了构建cache.Entry实体 volley封装了一个HttpHeaderParser类在http请求成功时将http请求比如请求头 data封装成一个Cache.Entry。

4 volley为了加快cache的查找速度保存了一个类型为LinkedHashMap<String,CacheHeader>的变量保存在内容中。

5 volley再次封装Cache.Entry 封装出一个CacheHeader保存cache.Entry除data外的内容(这样可以作为map的value) 而将实际data存储在硬盘。

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