Okhttp的缓存机制
Http协议下的缓存机制
强制缓存
通过http协议所传送的数据，会被保存到缓存数据库中，强制缓存的意思是指，若缓存数据库中的数据仍未失效，则直接通过缓存数据库获得数据，不再通过http向服务器发送请求。其中有两个比较重要的字段用于控制是否失效：
Expires
指过期的时间，其值由服务器所决定。当在缓存数据库取得相应的数据时，通过比较当前时间与Expires来决定是否直接使用缓存数据库中的数据。然而值得注意的是，服务端和客户端之间存在着延时，没有统一的时间标准，因此随着Http协议的发展，使用的机会也就越来越少
Cache-Control
字面理解为缓存的控制，其实际意思是缓存的属性。类似于java中的作用域，http中的缓存分为如下几种类型：
- public：表示其中的数据完全可以被存储，包括密码等隐私信息，且所有人就可以访问，其安全性也较低
- private： 存储到用户的私有cache中去，只有用户本身可以访问（默认）
- no-cache 仅在客户端与服务端建立认证后，才可以缓存（用于对比缓存）
- no-store 代表其中的请求和响应等信息都不会被缓存
- max-age 表示所返回的数据已经过期或失效
对比缓存
使用前要与服务器的缓存进行对比。通过服务器返回的状态码决定是否使用
304： 使用对比缓存的数据
200：使用服务器的最新数据
判定字段
Etag
资源的唯一标识，类似于人们的身份证号码，资源的内容一旦发生改动，Etag就会发生改变。客户端发送请求的时候格式为 If-None-Match +Etag，服务器收到后则会与缓存的Etag进行比对
Last-Modified
字面意思，最近修改的时间，由服务器所决定，客户端在发送请求的时候使用If-Modified-Since + 指定时间，若客户端所存的数据≤该时间则说明资源没有改动
To put into a nutshell :
Okhttp的缓存机制
可以看到，http本身协议的缓存机制较为简单，不能很好的满足实际的需求。okhttp相对来说便更加复杂。先说结论：
- 缓存基于文件存储
- 内部维护基于LRU算法的缓存清理线程
Okhttp读取缓存流程
Okhttp 存储缓存流程
源码解析
CacheControl
用于指定缓存的规则
public final class CacheControl {
//表示这是一个优先使用网络验证，验证通过之后才可以使用缓存的缓存控制，设置了noCache
public static final CacheControl FORCE_NETWORK = new Builder().noCache().build();
//表示这是一个优先先使用缓存的缓存控制，设置了onlyIfCached和maxStale的最大值
public static final CacheControl FORCE_CACHE = new Builder()
.onlyIfCached()
.maxStale(Integer.MAX_VALUE, TimeUnit.SECONDS)
.build();
//以下的字段都是HTTP中Cache-Control字段相关的值
private final boolean noCache;
private final boolean noStore;
private final int maxAgeSeconds;
private final int sMaxAgeSeconds;
private final boolean isPrivate;
private final boolean isPublic;
private final boolean mustRevalidate;
private final int maxStaleSeconds;
private final int minFreshSeconds;
private final boolean onlyIfCached;
private final boolean noTransform;
//解析头文件中的相关字段，得到该缓存控制类
public static CacheControl parse(Headers headers) {
...
}
}
CacheStrategy
主要用于判断是否使用缓存数据
public final class CacheStrategy {
public Factory(long nowMillis, Request request, Response cacheResponse) {
this.nowMillis = nowMillis;
//网络请求和缓存响应
this.request = request;
this.cacheResponse = cacheResponse;
if (cacheResponse != null) {
//找到缓存响应的响应头信息
Headers headers = cacheResponse.headers();
for (int i = 0, size = headers.size(); i < size; i++) {
//查看响应头信息中是否有以下字段信息
String fieldName = headers.name(i);
String value = headers.value(i);
if ("Date".equalsIgnoreCase(fieldName)) {
servedDate = HttpDate.parse(value);
servedDateString = value;
} else if ("Expires".equalsIgnoreCase(fieldName)) {
expires = HttpDate.parse(value);
} else if ("Last-Modified".equalsIgnoreCase(fieldName)) {
lastModified = HttpDate.parse(value);
lastModifiedString = value;
} else if ("ETag".equalsIgnoreCase(fieldName)) {
etag = value;
} else if ("Age".equalsIgnoreCase(fieldName)) {
ageSeconds = HeaderParser.parseSeconds(value, -1);
} else if (OkHeaders.SENT_MILLIS.equalsIgnoreCase(fieldName)) {
sentRequestMillis = Long.parseLong(value);
} else if (OkHeaders.RECEIVED_MILLIS.equalsIgnoreCase(fieldName)) {
receivedResponseMillis = Long.parseLong(value);
}
}
}
}
public CacheStrategy get() {
//获取判定的缓存策略
CacheStrategy candidate = getCandidate();
if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) {
// 如果判定的缓存策略的网络请求不为空，但是只使用缓存，则返回两者都为空的缓存策略。
return new CacheStrategy(null, null);
}
return candidate;
}
/** Returns a strategy to use assuming the request can use the network. */
private CacheStrategy getCandidate() {
// No cached response.
//如果没有缓存响应，则返回没有缓存响应的策略
if (cacheResponse == null) {
return new CacheStrategy(request, null);
}
// Drop the cached response if it's missing a required handshake.
//如果请求是https，而缓存响应的握手信息为空，则返回没有缓存响应的策略
if (request.isHttps() && cacheResponse.handshake() == null) {
return new CacheStrategy(request, null);
}
// If this response shouldn't have been stored, it should never be used
// as a response source. This check should be redundant as long as the
// persistence store is well-behaved and the rules are constant.
//如果请求对应的响应不能被缓存，则返回没有缓存响应的策略
if (!isCacheable(cacheResponse, request)) {
return new CacheStrategy(request, null);
}
//获取请求头中的CacheControl信息
CacheControl requestCaching = request.cacheControl();
//如果请求头中的CacheControl信息是不缓存的，则返回没有缓存响应的策略
if (requestCaching.noCache() || hasConditions(request)) {
return new CacheStrategy(request, null);
}
//获取响应的年龄
long ageMillis = cacheResponseAge();
//计算上次响应刷新的时间
long freshMillis = computeFreshnessLifetime();
//如果请求里有最大持续时间要求，则取较小的值作为上次响应的刷新时间
if (requestCaching.maxAgeSeconds() != -1) {
freshMillis = Math.min(freshMillis, SECONDS.toMillis(requestCaching.maxAgeSeconds()));
}
//如果请求里有最短刷新时间要求，则用它来作为最短刷新时间
long minFreshMillis = 0;
if (requestCaching.minFreshSeconds() != -1) {
minFreshMillis = SECONDS.toMillis(requestCaching.minFreshSeconds());
}
//最大过期时间
long maxStaleMillis = 0;
//获取缓存响应头中的CacheControl信息
CacheControl responseCaching = cacheResponse.cacheControl();
//如果缓存响应不是必须要再验证，并且请求有最大过期时间，则用请求的最大过期时间作为最大过期时间
if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
}
//如果支持缓存，并且持续时间+最短刷新时间<上次刷新时间+最大验证时间 则可以缓存
if (!responseCaching.noCache() && ageMillis + minFreshMillis < freshMillis + maxStaleMillis) {
Response.Builder builder = cacheResponse.newBuilder();
if (ageMillis + minFreshMillis >= freshMillis) {
builder.addHeader("Warning", "110 HttpURLConnection \\"Response is stale\\"");
}
long oneDayMillis = 24 * 60 * 60 * 1000L;
if (ageMillis > oneDayMillis && isFreshnessLifetimeHeuristic()) {
builder.addHeader("Warning", "113 HttpURLConnection \\"Heuristic expiration\\"");
}
//返回响应缓存
return new CacheStrategy(null, builder.build());
}
//构造一个新的有条件的Request，添加If-None-Match，If-Modified-Since等信息
Request.Builder conditionalRequestBuilder = request.newBuilder();
if (etag != null) {
conditionalRequestBuilder.header("If-None-Match", etag);
} else if (lastModified != null) {
conditionalRequestBuilder.header("If-Modified-Since", lastModifiedString);
} else if (servedDate != null) {
conditionalRequestBuilder.header("If-Modified-Since", servedDateString);
}
Request conditionalRequest = conditionalRequestBuilder.build();
//根据是否有If-None-Match，If-Modified-Since信息，返回不同的缓存策略
return hasConditions(conditionalRequest)
? new CacheStrategy(conditionalRequest, cacheResponse)
: new CacheStrategy(conditionalRequest, null);
}
/**
* Returns true if the request contains conditions that save the server from sending a response
* that the client has locally. When a request is enqueued with its own conditions, the built-in
* response cache won't be used.
*/
private static boolean hasConditions(Request request) {
return request.header("If-Modified-Since") != null || request.header("If-None-Match") != null;
}
}
Cache
对外开放的缓存类，类似数据库能够增删改查
- 增添缓存
CacheRequest put(Response response) {
String requestMethod = response.request().method();
//如果请求是"POST","PUT","PATCH","PROPPATCH","REPORT"则移除这些缓存
if (HttpMethod.invalidatesCache(response.request().method())) {
try {
remove(response.request());
} catch (IOException ignored) {
}
return null;
}
//仅支持GET的请求缓存，其他请求不缓存
if (!requestMethod.equals("GET")) {
return null;
}
//判断请求中的http数据包中headers是否有符号"*"的通配符，有则不缓存
if (HttpHeaders.hasVaryAll(response)) {
return null;
}
//把response构建成一个Entry对象
Entry entry = new Entry(response);
DiskLruCache.Editor editor = null;
try {
//生成DiskLruCache.Editor对象
editor = cache.edit(key(response.request().url()));
if (editor == null) {
return null;
}
//对缓存进行写入
entry.writeTo(editor);
//构建一个CacheRequestImpl类，包含Ok.io的Sink对象
return new CacheRequestImpl(editor);
} catch (IOException e) {
abortQuietly(editor);
return null;
}
}
- 查找缓存
Response get(Request request) {
//获取url转换过来的key
String key = key(request.url());
DiskLruCache.Snapshot snapshot;
Entry entry;
try {
//根据key获取对应的snapshot
snapshot = cache.get(key);
if (snapshot == null) {
return null;
}
} catch (IOException e) {
return null;
}
try {
//创建一个Entry对象,并由snapshot.getSource()获取Sink
entry = new Entry(snapshot.getSource(ENTRY_METADATA));
} catch (IOException e) {
Util.closeQuietly(snapshot);
return null;
}
//通过entry和response生成respson，通过Okio.buffer获取请求体，然后封装各种请求信息
Response response = entry.response(snapshot);
if (!entry.matches(request, response)) {
//对request和Response进行比配检查，成功则返回该Response。
Util.closeQuietly(response.body());
return null;
}
return response;
}
- 更新缓存
void update(Response cached, Response network) {
//用Respon构建一个Entry
Entry entry = new Entry(network);
//从缓存中获取DiskLruCache.Snapshot
DiskLruCache.Snapshot snapshot = ((CacheResponseBody) cached.body()).snapshot;
DiskLruCache.Editor editor = null;
try {
//获取DiskLruCache.Snapshot.edit对象
editor = snapshot.edit(); // Returns null if snapshot is not current.
if (editor != null) {
//将entry写入editor中
entry.writeTo(editor);
editor.commit();
}
} catch (IOException e) {
abortQuietly(editor);
}
}
- 删除缓存
主体位于DiskLruCache之中
void remove(Request request) throws IOException {
//通过url转化成的key去删除缓存
cache.remove(key(request.url()));
}
- writeTo ok.io
public void writeTo(DiskLruCache.Editor editor) throws IOException {
BufferedSink sink = Okio.buffer(editor.newSink(ENTRY_METADATA));
sink.writeUtf8(url)
.writeByte('\\n');
sink.writeUtf8(requestMethod)
.writeByte('\\n');
sink.writeDecimalLong(varyHeaders.size())
.writeByte('\\n');
for (int i = 0, size = varyHeaders.size(); i < size; i++) {
sink.writeUtf8(varyHeaders.name(i))
.writeUtf8(": ")
.writeUtf8(varyHeaders.value(i))
.writeByte('\\n');
}
sink.writeUtf8(new StatusLine(protocol, code, message).toString())
.writeByte('\\n');
sink.writeDecimalLong(responseHeaders.size() + 2)
.writeByte('\\n');
for (int i = 0, size = responseHeaders.size(); i < size; i++) {
sink.writeUtf8(responseHeaders.name(i))
.writeUtf8(": ")
.writeUtf8(responseHeaders.value(i))
.writeByte('\\n');
}
sink.writeUtf8(SENT_MILLIS)
.writeUtf8(": ")
.writeDecimalLong(sentRequestMillis)
.writeByte('\\n');
sink.writeUtf8(RECEIVED_MILLIS)
.writeUtf8(": ")
.writeDecimalLong(receivedResponseMillis)
.writeByte('\\n');
if (isHttps()) {
sink.writeByte('\\n');
sink.writeUtf8(handshake.cipherSuite().javaName())
.writeByte('\\n');
writeCertList(sink, handshake.peerCertificates());
writeCertList(sink, handshake.localCertificates());
sink.writeUtf8(handshake.tlsVersion().javaName()).writeByte('\\n');
}
sink.close();
}
DiskLruCache
真实存储（文件格式）的缓存功能类，使用了基于LinkedHashedMap。可以看到除了一些关键的方法之外其主要包括了三个重要的内部类。
- Entry
用于存储缓存数据的实体类，一个url对应一个实体,在Entry还有Snapshot对象
private final class Entry {
final String key;
/** Lengths of this entry's files. */
final long[] lengths;
final File[] cleanFiles;
final File[] dirtyFiles;
/** True if this entry has ever been published. */
boolean readable;
/** The ongoing edit or null if this entry is not being edited. */
Editor currentEditor;
/** The sequence number of the most recently committed edit to this entry. */
long sequenceNumber;
Entry(String key) {
this.key = key;
lengths = new long[valueCount];
cleanFiles = new File[valueCount];
dirtyFiles = new File[valueCount];
// The names are repetitive so re-use the same builder to avoid allocations.
StringBuilder fileBuilder = new StringBuilder(key).append('.');
int truncateTo = fileBuilder.length();
for (int i = 0; i < valueCount; i++) {
fileBuilder.append(i);
cleanFiles[i] = new File(directory, fileBuilder.toString());
fileBuilder.append(".tmp");
dirtyFiles[i] = new File(directory, fileBuilder.toString());
fileBuilder.setLength(truncateTo);
}
}
/** Set lengths using decimal numbers like "10123". */
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);
}
}
/** Append space-prefixed lengths to {@code writer}. */
void writeLengths(BufferedSink writer) throws IOException {
for (long length : lengths) {
writer.writeByte(' ').writeDecimalLong(length);
}
}
private IOException invalidLengths(String[] strings) throws IOException {
throw new IOException("unexpected journal line: " + Arrays.toString(strings));
}
/**
* Returns a snapshot of this entry. This opens 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.
*/
Snapshot snapshot() {
if (!Thread.holdsLock(DiskLruCache.this)) throw new AssertionError();
Source[] sources = new Source[valueCount];
long[] lengths = this.lengths.clone(); // Defensive copy since these can be zeroed out.
try {
for (int i = 0; i < valueCount; i++) {
sources[i] = fileSystem.source(cleanFiles[i]);
}
return new Snapshot(key, sequenceNumber, sources, lengths);
} catch (FileNotFoundException e) {
// A file must have been deleted manually!
for (int i = 0; i < valueCount; i++) {
if (sources[i] != null) {
Util.closeQuietly(sources[i]);
} else {
break;
}
}
// Since the entry is no longer valid, remove it so the metadata is accurate (i.e. the cache
// size.)
try {
removeEntry(this);
} catch (IOException ignored) {
}
return null;
}
}
}
- Snapshot
public final class Snapshot implements Closeable {
private final String key;
private final long sequenceNumber;
private final Source[] sources;
private final long[] lengths;
Snapshot(String key, long sequenceNumber, Source[] sources, long[] lengths) {
this.key = key;
this.sequenceNumber = sequenceNumber;
this.sources = sources;
this.lengths = lengths;
}
public String key() {
return key;
}
/**
* 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 @Nullable Editor edit() throws IOException {
return DiskLruCache.this.edit(key, sequenceNumber);
}
/** Returns the unbuffered stream with the value for {@code index}. */
public Source getSource(int index) {
return sources[index];
}
/** Returns the byte length of the value for {@code index}. */
public long getLength(int index) {
return lengths[index];
}
public void close() {
for (Source in : sources) {
Util.closeQuietly(in);
}
}
}
- Editor
在Editor的初始化中要传入Editor，其实Editor就是编辑entry的类
public final class Editor {
final Entry entry;
final boolean[] written;
private boolean done;
Editor(Entry entry) {
this.entry = entry;
this.written = (entry.readable) ? null : new boolean[valueCount];
}
/**
* Prevents this editor from completing normally. This is necessary either when the edit causes
* an I/O error, or if the target entry is evicted while this editor is active. In either case
* we delete the editor's created files and prevent new files from being created. Note that once
* an editor has been detached it is possible for another editor to edit the entry.
*/
void detach() {
if (entry.currentEditor == this) {
for (int i = 0; i < valueCount; i++) {
try {
fileSystem.delete(entry.dirtyFiles[i]);
} catch (IOException e) {
// This file is potentially leaked. Not much we can do about that.
}
}
entry.currentEditor = null;
}
}
/**
* Returns an unbuffered input stream to read the last committed value, or null if no value has
* been committed.
*/
public Source newSource(int index) {
synchronized (DiskLruCache.this) {
if (done) {
throw new IllegalStateException();
}
if (!entry.readable || entry.currentEditor != this) {
return null;
}
try {
return fileSystem.source(entry.cleanFiles[index]);
} catch (FileNotFoundException e) {
return 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 Sink newSink(int index) {
synchronized (DiskLruCache.this) {
if (done) {
throw new IllegalStateException();
}
if (entry.currentEditor != this) {
return Okio.blackhole();
}
if (!entry.readable) {
written[index] = true;
}
File dirtyFile = entry.dirtyFiles[index];
Sink sink;
try {
sink = fileSystem.sink(dirtyFile);
} catch (FileNotFoundException e) {
return Okio.blackhole();
}
return new FaultHidingSink(sink) {
@Override protected void onException(IOException e) {
synchronized (DiskLruCache.this) {
detach();
}
}
};
}
}
/**
* 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 {
synchronized (DiskLruCache.this) {
if (done) {
throw new IllegalStateException();
}
if (entry.currentEditor == this) {
completeEdit(this, true);
}
done = true;
}
}
/**
* Aborts this edit. This releases the edit lock so another edit may be started on the same
* key.
*/
public void abort() throws IOException {
synchronized (DiskLruCache.this) {
if (done) {
throw new IllegalStateException();
}
if (entry.currentEditor == this) {
completeEdit(this, false);
}
done = true;
}
}
public void abortUnlessCommitted() {
synchronized (DiskLruCache.this) {
if (!done && entry.currentEditor == this) {
try {
completeEdit(this, false);
} catch (IOException ignored) {
}
}
}
}
}
- 删除
boolean removeEntry(Entry entry) throws IOException {
if (entry.currentEditor != null) {
entry.currentEditor.detach(); // Prevent the edit from completing normally.
}
for (int i = 0; i < valueCount; i++) {
fileSystem.delete(entry.cleanFiles[i]);
size -= entry.lengths[i];
entry.lengths[i] = 0;
}
redundantOpCount++;
journalWriter.writeUtf8(REMOVE).writeByte(' ').writeUtf8(entry.key).writeByte('\\n');
lruEntries.remove(entry.key);
if (journalRebuildRequired()) {
executor.execute(cleanupRunnable);
}
return true;
}
