时间一晃而过，今天想给大家带来OkHttp的zuihou最后一篇文章，主要讲一下OkHttp的缓存机制。OkHttp的责任链中有一个拦截器就是专门应对OkHttp的缓存的，那就是CacheInterceptor拦截器。
CacheInterceptor
其对应的方法如下，我们就从这个方法讲起：
public Response intercept(Chain chain) throws IOException {
//假如有缓存，会得到拿到缓存，否则为null
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
//获取缓存策略
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
//缓存策略请求
Request networkRequest = strategy.networkRequest;
//缓存策略响应
Response cacheResponse = strategy.cacheResponse;
//缓存非空判断
if (cache != null) {
cache.trackResponse(strategy);
}
//本地缓存不为空并且缓存策略响应为空
if (cacheCandidate != null && cacheResponse == null) {
closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
}
//缓存策略请求和缓存策略响应为空,禁止使用网络直接返回
// If we're forbidden from using the network and the cache is insufficient, fail.
if (networkRequest == null && cacheResponse == null) {
return new Response.Builder()
.request(chain.request())
.protocol(Protocol.HTTP_1_1)
.code(504)
.message("Unsatisfiable Request (only-if-cached)")
.body(Util.EMPTY_RESPONSE)
.sentRequestAtMillis(-1L)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
}
//缓存策略请求为空，即缓存有效则直接使用缓存不使用网络
// If we don't need the network, we're done.
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
//缓存无效，则执行下一个拦截器以获取请求
Response networkResponse = null;
try {
networkResponse = chain.proceed(networkRequest);
} finally {
// If we're crashing on I/O or otherwise, don't leak the cache body.
if (networkResponse == null && cacheCandidate != null) {
closeQuietly(cacheCandidate.body());
}
}
//假如本地也有缓存，则根据条件选择使用哪个响应
// If we have a cache response too, then we're doing a conditional get.
if (cacheResponse != null) {
if (networkResponse.code() == HTTP_NOT_MODIFIED) {
Response response = cacheResponse.newBuilder()
.headers(combine(cacheResponse.headers(), networkResponse.headers()))
.sentRequestAtMillis(networkResponse.sentRequestAtMillis())
.receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
networkResponse.body().close();
// Update the cache after combining headers but before stripping the
// Content-Encoding header (as performed by initContentStream()).
cache.trackConditionalCacheHit();
//更新缓存
cache.update(cacheResponse, response);
return response;
} else {
closeQuietly(cacheResponse.body());
}
}
//没有缓存，则直接使用网络响应
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
if (cache != null) {
//缓存到本地
if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
// Offer this request to the cache.
CacheRequest cacheRequest = cache.put(response);
return cacheWritingResponse(cacheRequest, response);
}
if (HttpMethod.invalidatesCache(networkRequest.method())) {
try {
cache.remove(networkRequest);
} catch (IOException ignored) {
// The cache cannot be written.
}
}
}
return response;
}
这就是整个缓存拦截器的主要方法，首先会从cache去拿缓存，没有则返回null,然后通过CacheStrategy来获取缓存策略，CacheStrategy根据之前缓存的结果与当前将要发送Request的header进行策略，并得出是否进行请求的结果。由于篇幅关系，这一块不细讲因为涉及网络协议，最终他的得出的规则如下如:
image.png
因为我把注释流程都写在代码了，大家可以看上面方法代码理解，其整体缓存流程如下：
- 如果有缓存，则取出缓存否则为null
- 根据CacheStrategy拿到它的缓存策略请求和响应
- 缓存策略请求和缓存策略响应为空,禁止使用网络直接返回
- 缓存策略请求为空，即缓存有效则直接使用缓存不使用网络
- 缓存无效，则执行下一个拦截器以获取请求
- 假如本地也有缓存，则根据条件选择使用哪个响应，更新缓存
- 没有缓存，则直接使用网络响应
- 添加缓存
到这里我们可以看到，缓存的“增删改查”都是cache(Cache)类来进行操作的。下面让我们来看一下这个类吧。
Cache
Cache的“增删改查”其实都是基于DiskLruCache，下面我们会继续讲，先来看一下“增删改查”的各个方法吧
- 添加缓存
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;
}
}
- 得到缓存
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
void remove(Request request) throws IOException {
//通过url转化成的key去删除缓存
cache.remove(key(request.url()));
}
Cache的"增删改查"大体通过注释代码的方式给出，Cache还有一个更重要的缓存处理类就是DiskLruCache。
DiskLruCache
不仔细看还以为这个类和JakeWharton写的DiskLruCache:[https://link.jianshu.com/t=https://github.com/JakeWharton/DiskLruCache(https://link.jianshu.com/t=https://github.com/JakeWharton/DiskLruCache)是一样的，其实主体架构差不多，只不过OkHttp的DiskLruCache结合了Ok.io，用Ok.io处理数据文件的储存.
我们可以看到上面的DiskLruCache有shang三个内部类，分别是Entry，Snapshot，Editor。
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);
}
}
//省略
......
实际上只是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，代码如下:
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;
}
}
即一个Entry对应着一个Snapshot对象，在看一下Snapshot的内部代码：
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);
}
}
}
初始化的Snapshot仅仅只是存储了一些变量而已。
Editor
在Editor的初始化中要传入Editor，其实Editor就是编辑entry的类。源码如下:
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];
}
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;
}
}
//返回指定index的cleanFile的读入流
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;
}
}
}
//向指定index的dirtyFiles文件写入数据
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();
}
}
};
}
}
//这里执行的工作是提交数据，并释放锁，最后通知DiskLruCache刷新相关数据
public void commit() throws IOException {
synchronized (DiskLruCache.this) {
if (done) {
throw new IllegalStateException();
}
if (entry.currentEditor == this) {
completeEdit(this, true);
}
done = true;
}
}
//终止编辑，并释放锁
public void abort() throws IOException {
synchronized (DiskLruCache.this) {
if (done) {
throw new IllegalStateException();
}
if (entry.currentEditor == this) {
completeEdit(this, false);
}
done = true;
}
}
//除非正在编辑，否则终止
public void abortUnlessCommitted() {
synchronized (DiskLruCache.this) {
if (!done && entry.currentEditor == this) {
try {
completeEdit(this, false);
} catch (IOException ignored) {
}
}
}
}
}
各个方法对应作用如下：
- Source newSource(int index)：返回指定index的cleanFile的读入流
- Sink newSink(int index)：向指定index的dirtyFiles文件写入数据
- commit()：这里执行的工作是提交数据，并释放锁，最后通知DiskLruCache刷新相关数据
- abort()：终止编辑，并释放锁
- abortUnlessCommitted()：除非正在编辑，否则终止
剩下关键来了，还记得上面我们讲Cache添加有一行代码entry.writeTo(editor);，里面操作如下：
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();
}
上面的都是Ok.io的操作了，不懂OK.io的可以去看一下相关知识。BufferedSink sink = Okio.buffer(editor.newSink(ENTRY_METADATA));editor.newSink拿到ok.io版的OutputStream(Sink)生成Ok.io的输入类，剩下的就是把数据用ok.io写入文件，然后关闭输出类。
同理我看们可以一下上面Cache获取缓存的代码 Response response = entry.response(snapshot);，在response方法里又有一个方法：CacheResponseBody()就是获取缓存的方法，代码如下：
CacheResponseBody(final DiskLruCache.Snapshot snapshot,String contentType, String contentLength) {
this.snapshot = snapshot;
this.contentType = contentType;
this.contentLength = contentLength;
Source source = snapshot.getSource(ENTRY_BODY);
bodySource = Okio.buffer(new ForwardingSource(source) {
@Override public void close() throws IOException {
snapshot.close();
super.close();
}
});
}
new ForwardingSource(source)相当于传入ok.io版的InputStream(Source)生成Ok.io的读取类，剩下的都是读取缓存数据然后生成Response.
而上面Cache的Update()方法，其写入过程也和上面的添加是一样的，不同的只不过先构造成一个就得Entry然后再把新的缓存写上去更新而已，因为涉及我重要的Ok.io是一样的，所以不细讲。
剩下就是删除了，在Cache的delete方法里，在removeEntry就是执行删除操作，代码如下：
boolean removeEntry(Entry entry) throws IOException {
//省略
journalWriter.writeUtf8(REMOVE).writeByte(' ').writeUtf8(entry.key).writeByte('\n');
lruEntries.remove(entry.key);
//省略
return true;
}
上面这两句代码就是删除的关键， journalWriter.writeUtf8表示在DiskLruCache的本地缓存清单列表里删除，lruEntries.remove表示在缓存内存里删除。
到此增删给查的流程基本结束，其实DiskLruCache还有很多可以讲，但是我的重心是OKhttp的缓存底层是用Ok.io,为此在这里点到为止。
内容有点多，如有错误请多多指出
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