Volley框架解析(六)—–Cache接口及其默認實現類解析
1. 前言(可直接無視跳過
不知不覺Volley的源碼分析到了Cache接口部分了，前面涉及到網絡的部分都介紹完了，在處理網絡請求返回的數據時，會根據request結果是否需要緩存來進行不同的處理。如需要緩存結果，就涉及到了Cache.java及其默認實現類DiskBasedCache.java。 其實在之前什麼都不知道的時候，認爲緩存是個非常神祕的東西，可能是人類對於未知的恐懼和敬畏嘛orz，後來在用到另外一個牛掰的網絡請求框架ion的時候，在自己app的目錄下面看到一個ion的文件夾= =，進去之後發現了好多balabala亂七八糟的打不開的文件，突然明白了，估計是看到本地緩存的文件了。扯遠了= =，開始源代碼的分析。
2. Cache.java
用於處理緩存的接口，裏面有很多抽象的方法等着被實現，裏面還有一個static類Entry,裏面有些關於緩存的單元信息。(例如，緩存的內容，緩存過期的時間，緩存需要刷新的時間等等）。
package com.android.volley;
/**
* An interface for a cache keyed by a String with * a byte array as data.
* 一個用於緩存的接口
*/
public interface Cache {
/**
* Retrieves an entry from the cache.
* 用來獲取緩存的入口，通過傳入的key
*
* @param key Cache key
* 這個key應該是request對應其緩存的唯一key
*
* @return An {@link Entry} or null in the event of a cache miss
*/
public Entry get(String key);
/**
* Adds or replaces an entry to the cache.
* 添加或者給request更新緩存
* @param key Cache key
* @param entry Data to store and metadata for cache coherency, TTL, etc.
*/
public void put(String key, Entry entry);
/**
* Performs any potentially long-running actions needed to initialize the cache;
* will be called from a worker thread.
*/
public void initialize();
/**
* Invalidates an entry in the cache.
* 這個函數是將key對應的緩存置於過期
* 分爲fully expire和soft expire， 目前還不知道是什麼意思 = =
*
* @param key Cache key
* @param fullExpire True to fully expire the entry, false to soft expire
*/
public void invalidate(String key, boolean fullExpire);
/**
* Removes an entry from the cache.
* 將key對應的緩存直接移除掉
*
* @param key Cache key
*/
public void remove(String key);
/**
* Empties the cache.
* 清除所有的緩存
*/
public void clear();
/**
* Data and metadata for an entry returned by the cache.
*/
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.
* 根據後面的isExpired()函數來看
* 該條數據的意思應該是緩存過期的時間
*/
public long ttl;
/** Soft TTL for this record.
* 根據refreshNeeded()函數來看
* 意思是需要更新緩存的時間點
*/
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.
* 用來查看緩存是否過期了
*/
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();
}
}
}
3. DiskBasedCache.java
實現了Cache.java接口，專門和本地存儲的文件打交道，負責緩存的寫入與讀取。涉及到了一部分InputStream和OutputStream還有File的知識。
/**
* Cache implementation that caches files directly onto the hard disk in the specified
* directory. The default disk usage size is 5MB, but is configurable.
* 實現了Cache接口
* 專門用於和本地文件交互的一個類
* 存入緩存和取出緩存等功能
*/
public class DiskBasedCache implements Cache {
/**
* Map of the Key, CacheHeader pairs
* CacheHeader.java爲本類中的一個static類
* 裏面存放着一些
*/
private final Map<String, CacheHeader> mEntries =
new LinkedHashMap<String, CacheHeader>(16, .75f, true);
/**
* Total amount of space currently used by the cache in bytes.
* 當前緩存的總大小
*/
private long mTotalSize = 0;
/**
* The root directory to use for the cache.
* 緩存的根目錄
*/
private final File mRootDirectory;
/**
* The maximum size of the cache in bytes.
* 緩存能接受的最大字節數
*/
private final int mMaxCacheSizeInBytes;
/**
* Default maximum disk usage in bytes.
* 默認緩存能使用的最大空間
*/
private static final int DEFAULT_DISK_USAGE_BYTES = 5 * 1024 * 1024;
/**
* High water mark percentage for the cache
* 類似於水位警戒線一樣的標識
*/
private static final float HYSTERESIS_FACTOR = 0.9f;
/**
* Magic number for current version of cache file format.
*
*/
private static final int CACHE_MAGIC = 0x20150306;
/**
* Constructs an instance of the DiskBasedCache at the specified directory.
* 在指定的目錄下面創建一個DiskBasedCache
*
* @param rootDirectory The root directory of the cache.
* @param maxCacheSizeInBytes The maximum size of the cache in bytes.
*/
public DiskBasedCache(File rootDirectory, int maxCacheSizeInBytes) {
mRootDirectory = rootDirectory;
mMaxCacheSizeInBytes = maxCacheSizeInBytes;
}
/**
* Constructs an instance of the DiskBasedCache at the specified directory using
* the default maximum cache size of 5MB.
* @param rootDirectory The root directory of the cache.
*/
public DiskBasedCache(File rootDirectory) {
this(rootDirectory, DEFAULT_DISK_USAGE_BYTES);
}
/**
* Clears the cache. Deletes all cached files from disk.
* 清除當前目錄下的緩存，刪除所有緩存文件
*/
@Override
public synchronized void clear() {
File[] files = mRootDirectory.listFiles();
if (files != null) {
for (File file : files) {
file.delete();
}
}
mEntries.clear();
mTotalSize = 0;
VolleyLog.d("Cache cleared.");
}
/**
* Returns the cache entry with the specified key if it exists, null otherwise.
* 通過特殊的key，來獲取與緩存交流的接口(entry)
* 如果沒有的話則返回null
*/
@Override
public synchronized Entry get(String key) {
CacheHeader entry = mEntries.get(key);
// if the entry does not exist, return.
if (entry == null) {
return null;
}
//依據key獲取緩存的文件，如果不存在則創建一個
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;
}
}
}
}
/**
* Initializes the DiskBasedCache by scanning for all files currently in the
* specified root directory. Creates the root directory if necessary.
* 對緩存目錄的初始化工作，檢查目錄是否存在
* 如果不存在就給重新創建一個
*/
@Override
public synchronized void initialize() {
if (!mRootDirectory.exists()) {
if (!mRootDirectory.mkdirs()) {
VolleyLog.e("Unable to create cache dir %s", mRootDirectory.getAbsolutePath());
}
return;
}
/**
* 如果緩存目錄已經存在了
* 則將緩存目錄下面的文件都掃描一遍
* 將關於緩存文件的部分信息加載到內存中來
* 方便後面對緩存的查詢等工作
*/
File[] files = mRootDirectory.listFiles();
if (files == null) {
return;
}
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);
} catch (IOException e) {
if (file != null) {
file.delete();
}
} finally {
try {
if (fis != null) {
fis.close();
}
} catch (IOException ignored) { }
}
}
}
/**
* Invalidates an entry in the cache.
* 將key對應的緩存作廢
* 如果fullExpire爲true，則將整個entry作廢
* 如果爲false,則只是軟作廢，也就是將緩存置於需要刷新的狀態
*
* @param key Cache key
* @param fullExpire True to fully expire the entry, false to soft expire
*/
@Override
public synchronized void invalidate(String key, boolean fullExpire) {
Entry entry = get(key);
if (entry != null) {
entry.softTtl = 0;
if (fullExpire) {
entry.ttl = 0;
}
put(key, entry);
}
}
/**
* Puts the entry with the specified key into the cache.
* 將entry中包含的信息存放到key對應的緩存文件中去
*/
@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);
boolean success = e.writeHeader(fos);
if (!success) {
fos.close();
VolleyLog.d("Failed to write header for %s", file.getAbsolutePath());
throw new IOException();
}
fos.write(entry.data);
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());
}
}
/**
* Removes the specified key from the cache if it exists.
*/
@Override
public synchronized void remove(String key) {
boolean deleted = getFileForKey(key).delete();
removeEntry(key);
if (!deleted) {
VolleyLog.d("Could not delete cache entry for key=%s, filename=%s",
key, getFilenameForKey(key));
}
}
/**
* Creates a pseudo-unique filename for the specified cache key.
* 通過給定的key，前半段的hashCode和後半段的hashCode連接起來
* 作爲一個獨一無二的文件名
* @param key The key to generate a file name for.
* @return A pseudo-unique filename.
*/
private String getFilenameForKey(String key) {
int firstHalfLength = key.length() / 2;
String localFilename = String.valueOf(key.substring(0, firstHalfLength).hashCode());
localFilename += String.valueOf(key.substring(firstHalfLength).hashCode());
return localFilename;
}
/**
* Returns a file object for the given cache key.
* 通過調用getFilenameForKey()方法來獲取相對路徑
*/
public File getFileForKey(String key) {
return new File(mRootDirectory, getFilenameForKey(key));
}
/**
* Prunes the cache to fit the amount of bytes specified.
* 從已有的緩存中清除數據
* 直到掃出了一片neededSapce大小的空地爲止
* @param neededSpace The amount of bytes we are trying to fit into the cache.
*/
private void pruneIfNeeded(int neededSpace) {
if ((mTotalSize + neededSpace) < mMaxCacheSizeInBytes) {
return;
}
if (VolleyLog.DEBUG) {
VolleyLog.v("Pruning old cache entries.");
}
long before = mTotalSize;
int prunedFiles = 0;
long startTime = SystemClock.elapsedRealtime();
Iterator<Map.Entry<String, CacheHeader>> iterator = mEntries.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry<String, CacheHeader> entry = iterator.next();
CacheHeader e = entry.getValue();
boolean deleted = getFileForKey(e.key).delete();
if (deleted) {
mTotalSize -= e.size;
} else {
VolleyLog.d("Could not delete cache entry for key=%s, filename=%s",
e.key, getFilenameForKey(e.key));
}
iterator.remove();
prunedFiles++;
/**
* 一直清除緩存
* 直到存入這個neededSapce之後還有一小部分空餘的地方
*/
if ((mTotalSize + neededSpace) < mMaxCacheSizeInBytes * HYSTERESIS_FACTOR) {
break;
}
}
if (VolleyLog.DEBUG) {
VolleyLog.v("pruned %d files, %d bytes, %d ms",
prunedFiles, (mTotalSize - before), SystemClock.elapsedRealtime() - startTime);
}
}
/**
* Puts the entry with the specified key into the cache.
* 將目錄下指定的緩存加載到mEntries中去
* 爲了方便之後對緩存的讀寫操作
* 全部讀寫一遍放在內存裏面，對查詢什麼的都會方便很多
*
* @param key The key to identify the entry by.
* @param entry The entry to cache.
*/
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);
}
/**
* Removes the entry identified by 'key' from the cache.
*/
private void removeEntry(String key) {
CacheHeader entry = mEntries.get(key);
if (entry != null) {
mTotalSize -= entry.size;
mEntries.remove(key);
}
}
/**
* Reads the contents of an InputStream into a byte[].
* 從InputStream中讀取指定長度的數據
*
*/
private static byte[] streamToBytes(InputStream in, int length) throws IOException {
byte[] bytes = new byte[length];
int count;
int pos = 0;
while (pos < length && ((count = in.read(bytes, pos, length - pos)) != -1)) {
pos += count;
}
if (pos != length) {
throw new IOException("Expected " + length + " bytes, read " + pos + " bytes");
}
return bytes;
}
/**
* Handles holding onto the cache headers for an entry.
*/
// Visible for testing.
static class CacheHeader {
/**
* The size of the data identified by this CacheHeader. (This is not
* serialized to disk.
*
* CacheHeader所表示的數據段的大小
*/
public long size;
/**
* The key that identifies the cache entry.
* 這個key應該是request對應其緩存的唯一key
*/
public String key;
/**
* ETag for cache coherence.
*
*/
public String etag;
/**
* Date of this response as reported by the server.
* 緩存起來的數據返回的日期
*/
public long serverDate;
/**
* The last modified date for the requested object.
* 最後一次更改的時間
*/
public long lastModified;
/**
* TTL for this record.
* ping時候返回的TTL=128的概念如下
* TTL：生存時間
* 指定數據報被路由器丟棄之前允許通過的網段數量。
* TTL 是由發送主機設置的，以防止數據包不斷在 IP 互聯網絡上永不終止地循環。轉發 IP 數據包時，要求路由器至少將 TTL 減小 1。
*
* 但是= =，注意這裏的和上面的那種不是一個概念，這裏只是模擬了上面的概念，但也是用來標誌緩存存活時間的。
*/
public long ttl;
/**
* Soft TTL for this record.
*
* 根據refreshNeeded()函數來看
* 意思是需要更新緩存的時間點
*/
public long softTtl;
/**
* Headers from the response resulting in this cache entry.
* 用來指向上一次response的header
*/
public Map<String, String> responseHeaders;
private CacheHeader() { }
/**
* Instantiates a new CacheHeader object
* @param key The key that identifies the cache entry
* @param entry The cache entry.
*/
public CacheHeader(String key, Entry entry) {
this.key = key;
this.size = entry.data.length;
this.etag = entry.etag;
this.serverDate = entry.serverDate;
this.lastModified = entry.lastModified;
this.ttl = entry.ttl;
this.softTtl = entry.softTtl;
this.responseHeaders = entry.responseHeaders;
}
/**
* Reads the header off of an InputStream and returns a CacheHeader object.
* 從InputStream中讀取數據並組建一個CacheHeader對象實例
* @param is The InputStream to read from.
* @throws IOException
*/
public static CacheHeader readHeader(InputStream is) throws IOException {
CacheHeader entry = new CacheHeader();
int magic = readInt(is);
if (magic != CACHE_MAGIC) {
// don't bother deleting, it'll get pruned eventually
throw new IOException();
}
entry.key = readString(is);
entry.etag = readString(is);
if (entry.etag.equals("")) {
entry.etag = null;
}
entry.serverDate = readLong(is);
entry.lastModified = readLong(is);
entry.ttl = readLong(is);
entry.softTtl = readLong(is);
entry.responseHeaders = readStringStringMap(is);
return entry;
}
/**
* Creates a cache entry for the specified data.
* 從CacheHeader轉換成Entry類的實例
*/
public Entry toCacheEntry(byte[] data) {
Entry e = new Entry();
e.data = data;
e.etag = etag;
e.serverDate = serverDate;
e.lastModified = lastModified;
e.ttl = ttl;
e.softTtl = softTtl;
e.responseHeaders = responseHeaders;
return e;
}
/**
* Writes the contents of this CacheHeader to the specified OutputStream.
* 將CacheHeader裏面的數據寫入指定的OutputStream中
*/
public boolean writeHeader(OutputStream os) {
try {
writeInt(os, CACHE_MAGIC);
writeString(os, key);
writeString(os, etag == null ? "" : etag);
writeLong(os, serverDate);
writeLong(os, lastModified);
writeLong(os, ttl);
writeLong(os, softTtl);
writeStringStringMap(responseHeaders, os);
os.flush();
return true;
} catch (IOException e) {
VolleyLog.d("%s", e.toString());
return false;
}
}
}
/**
* 繼承了FilterInputStream
* 沒啥特別的= =
*/
private static class CountingInputStream extends FilterInputStream {
private int bytesRead = 0;
private CountingInputStream(InputStream in) {
super(in);
}
@Override
public int read() throws IOException {
int result = super.read();
if (result != -1) {
bytesRead++;
}
return result;
}
@Override
public int read(byte[] buffer, int offset, int count) throws IOException {
int result = super.read(buffer, offset, count);
if (result != -1) {
bytesRead += result;
}
return result;
}
}
/*
* Homebrewed simple serialization system used for reading and writing cache
* headers on disk. Once upon a time, this used the standard Java
* Object{Input,Output}Stream, but the default implementation relies heavily
* on reflection (even for standard types) and generates a ton of garbage.
*
*/
/**
* Simple wrapper around {@link InputStream#read()} that throws EOFException
* instead of returning -1.
* 如果文件讀到了末尾直接拋出異常
*/
private static int read(InputStream is) throws IOException {
int b = is.read();
if (b == -1) {
throw new EOFException();
}
return b;
}
/**
* 剛開始看到這裏的時候沒有明白是什麼意思= =
* 就不明白了，好好的一個int類型的數據
* 爲什麼非要分段寫入呢，一個字節一個字節的寫入
* 後來查了資料才發現，OutputStream及其子類的write()方法
* 一次都只能寫入一個byte，int類型有4個byte，分四次寫入沒什麼問題咯
*/
static void writeInt(OutputStream os, int n) throws IOException {
os.write((n >> 0) & 0xff);
os.write((n >> 8) & 0xff);
os.write((n >> 16) & 0xff);
os.write((n >> 24) & 0xff);
}
static int readInt(InputStream is) throws IOException {
int n = 0;
n |= (read(is) << 0);
n |= (read(is) << 8);
n |= (read(is) << 16);
n |= (read(is) << 24);
return n;
}
static void writeLong(OutputStream os, long n) throws IOException {
os.write((byte)(n >>> 0));
os.write((byte)(n >>> 8));
os.write((byte)(n >>> 16));
os.write((byte)(n >>> 24));
os.write((byte)(n >>> 32));
os.write((byte)(n >>> 40));
os.write((byte)(n >>> 48));
os.write((byte)(n >>> 56));
}
static long readLong(InputStream is) throws IOException {
long n = 0;
n |= ((read(is) & 0xFFL) << 0);
n |= ((read(is) & 0xFFL) << 8);
n |= ((read(is) & 0xFFL) << 16);
n |= ((read(is) & 0xFFL) << 24);
n |= ((read(is) & 0xFFL) << 32);
n |= ((read(is) & 0xFFL) << 40);
n |= ((read(is) & 0xFFL) << 48);
n |= ((read(is) & 0xFFL) << 56);
return n;
}
static void writeString(OutputStream os, String s) throws IOException {
byte[] b = s.getBytes("UTF-8");
writeLong(os, b.length);
os.write(b, 0, b.length);
}
static String readString(InputStream is) throws IOException {
int n = (int) readLong(is);
byte[] b = streamToBytes(is, n);
return new String(b, "UTF-8");
}
static void writeStringStringMap(Map<String, String> map, OutputStream os) throws IOException {
if (map != null) {
writeInt(os, map.size());
for (Map.Entry<String, String> entry : map.entrySet()) {
writeString(os, entry.getKey());
writeString(os, entry.getValue());
}
} else {
writeInt(os, 0);
}
}
/**
* 從InputStream中讀取key類型爲String，值類型也爲String的Map
*/
static Map<String, String> readStringStringMap(InputStream is) throws IOException {
int size = readInt(is);
Map<String, String> result = (size == 0)
? Collections.<String, String>emptyMap()
: new HashMap<String, String>(size);
for (int i = 0; i < size; i++) {
//將讀出來的byte[]轉換成String
String key = readString(is).intern();
String value = readString(is).intern();
result.put(key, value);
}
return result;
}
}
涉及到緩存讀寫的這個實現類當時閱讀的時候還是花了不少時間的= =，人太笨了沒辦法orz，有什麼不妥的地方還望各位juju多多指教，小達感激不盡0.0。下面還有Request.java等類等着去解析呢，Volley中的主角要登場了╭(╯^╰)╮。