CacheInterceptor也是okhttp自带的核心功能的拦截器，它负责处理http请求的缓存。今天我们来仔细分析下相关源码。
Http协议规定的缓存
在分析源码之前，我们需要事先对http协议的缓存有所了解，这里有篇文章写的很好 《http协议缓存》，大家最好看明白这篇文章再继续往下分析。
为了避免大家不想看其他文章，我简单的总结下：http缓存分为两种，一种强制缓存，一种对比缓存，强制缓存生效时直接使用以前的请求结果，无需发起网络请求。对比缓存生效时，无论怎样都会发起网络请求，如果请求结果未改变，服务端会返回304，但不会返回数据，数据从缓存中取，如果改变了会返回数据。具体详细规则还是得看上面提到的文章。
缓存结果获取
@Override public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
//networkRequest 不为null，说明强制缓存规则不生效
Request networkRequest = strategy.networkRequest;
//该请求的上次的缓存的结果
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {
cache.trackResponse(strategy);
}
if (cacheCandidate != null && cacheResponse == null) {
closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
}
// If we're forbidden from using the network and the cache is insufficient, fail.
if (networkRequest == null && cacheResponse == null) {
//省略代码
}
// If we don't need the network, we're done.
if (networkRequest == null) {
//省略代码
}
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());
}
}
// If we have a cache response too, then we're doing a conditional get.
if (cacheResponse != null) {
if (networkResponse.code()==HTTP_NOT_MODIFIED) {
//省略代码
} else {
closeQuietly(cacheResponse.body());
}
}
//省略代码
return response;
}
在分析整体结构的时候，我们需要理解，这里的networkRequest如果不为null，则说明强制缓存生效，也就意味着这次请求直接使用上次的结果，无需发起真正的请求。而cacheResponse则是上次的缓存结果。整个缓存处理其实就是这两种情况的组合：
1.networkRequest == null && cacheResponse == null
// 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();
}
这种情况说明从缓存策略上来说强制缓存生效，应该直接取上次缓存结果，但由于未知原因缓存的结果没有了或者上次返回就没有，这里直接返回了失败的Response
2.networkRequest == null
// If we don't need the network, we're done.
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
这种情况下，强制缓存生效，按道理来说我们直接返回上次的cacheResponse就可以了，当然这里对Response的cacheResponse的body做了置空处理，不是什么大事。
3.networkRequest != null
强制缓存不生效，说明此时应该使用对比缓存，需要从服务端取数据：
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());
}
}
按照对比缓存规则，取完之后应该判断是否返回304，如果是应该使用缓存结果：
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();
cache.update(cacheResponse, response);
return response;
} else {
closeQuietly(cacheResponse.body());
}
可以看到确实是通过cacheResponse来构造Response的，而不是通过刚才请求的networkResponse来构造Response。当然，更新了一些必要数据而已。
4. 缓存失效
剩下的情况就是缓存完全失效了，这个时候应该利用刚才网络请求的结果：
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
//省略将这次请求结果加入缓存的代码
return response;
缓存策略判断
上面分析了缓存的使用方式，其实就是根据是否使用强制缓存（networkRequest为null），是否使用过对比缓存（networkRequest不为null，且cacheResponse不为null），那么具体使用哪种策略是怎么决定的呢，我们继续看源码：
@Override public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {
cache.trackResponse(strategy);
}
//省略代码
}
这段代码我们之前没有分析，这里有个CacheStrategy,就是由它来决定采用何种缓存规则的。okhttp为每一个请求都记录了一个Strategy。我们来看下这个get()方法：
public CacheStrategy get() {
CacheStrategy candidate = getCandidate();
if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) {
// We're forbidden from using the network and the cache is insufficient.
return new CacheStrategy(null, null);
}
return candidate;
}
继续跟到getCandidate()中去：
private CacheStrategy getCandidate() {
// No cached response.
if (cacheResponse == null) {
return new CacheStrategy(request, null);
}
// Drop the cached response if it's missing a required handshake.
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.
if (!isCacheable(cacheResponse, request)) {
return new CacheStrategy(request, null);
}
CacheControl requestCaching = request.cacheControl();
//省略代码
}
所谓的决定策略，其实就是决定是否要重新发起网络请求，上面的几种情况都是要发起请求的，包括：没有对应的缓存结果；https请求却没有握手信息；不允许缓存的请求（包括一些特殊状态码以及Header中明确禁止缓存）。
上述的几种情况其实都没有涉及到具体的缓存策略，没有通过Header中一些字段来判断，下面的几种情况就都是靠Header中的字段来决定的了，主要借助了一个类CacheControl，我们看下它是干什么的：
public final class CacheControl {
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;
private final boolean immutable;
}
我们看下成员变量，很明显，这里都是记录的http协议中的一些和缓存有关的字段，我们可以通过这些字段判断一个requeset是否允许缓存，一个Response采用何种缓存策略。
这个时候我们继续回到getCandidate()方法：
CacheControl requestCaching = request.cacheControl();
if (requestCaching.noCache() || hasConditions(request)) {
return new CacheStrategy(request, null);
}
CacheControl responseCaching = cacheResponse.cacheControl();
if (responseCaching.immutable()) {
return new CacheStrategy(null, cacheResponse);
}
可以看到这里分别获取了request的和Response的CacheControl,下面的代码就是按照标准的http缓存协议，从CacheControl中判断一些Header中的关键字决定缓存策略，这里就不再继续深入。
缓存的具体实现
上面讨论了是否使用缓存，什么时候采用何种策略，这节我们讨论下使用缓存时，缓存具体如何存的，如何获取的：
@Override public Response intercept(Chain chain) throws IOException {
//缓存的获取
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
//省略代码
if (cache != null) {
if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
// 存入到缓存中去
CacheRequest cacheRequest = cache.put(response);
}
if (HttpMethod.invalidatesCache(networkRequest.method())) {
try {
//从缓存中删除
cache.remove(networkRequest);
} catch (IOException ignored) {
// The cache cannot be written.
}
}
}
从上面可以看到，所有的缓存处理都是来自cache类：
final InternalCache cache;
最终传进来的是在OkHttpClient中设置的Cache类
先看下成员变量：
public final class Cache implements Closeable, Flushable {
private static final int VERSION = 201105;
private static final int ENTRY_METADATA = 0;
private static final int ENTRY_BODY = 1;
private static final int ENTRY_COUNT = 2;
final InternalCache internalCache = new InternalCache() {
@Override public Response get(Request request) throws IOException {
return Cache.this.get(request);
}
@Override public CacheRequest put(Response response) throws IOException {
return Cache.this.put(response);
}
@Override public void remove(Request request) throws IOException {
Cache.this.remove(request);
}
@Override public void update(Response cached, Response network) {
Cache.this.update(cached, network);
}
@Override public void trackConditionalCacheHit() {
Cache.this.trackConditionalCacheHit();
}
@Override public void trackResponse(CacheStrategy cacheStrategy) {
Cache.this.trackResponse(cacheStrategy);
}
};
final DiskLruCache cache;
/* read and write statistics, all guarded by 'this' */
int writeSuccessCount;
int writeAbortCount;
private int networkCount;
private int hitCount;
private int requestCount;
}
从成员变量中可以猜测缓存应该是借助DiskLruCache实现的
缓存的put
@Nullable CacheRequest put(Response response) {
String requestMethod = response.request().method();
if (HttpMethod.invalidatesCache(response.request().method())) {
try {
remove(response.request());
} catch (IOException ignored) {
// The cache cannot be written.
}
return null;
}
if (!requestMethod.equals("GET")) {
// Don't cache non-GET responses. We're technically allowed to cache
// HEAD requests and some POST requests, but the complexity of doing
// so is high and the benefit is low.
return null;
}
if (HttpHeaders.hasVaryAll(response)) {
return null;
}
Entry entry = new Entry(response);
DiskLruCache.Editor editor = null;
try {
editor = cache.edit(key(response.request().url()));
if (editor == null) {
return null;
}
entry.writeTo(editor);
return new CacheRequestImpl(editor);
} catch (IOException e) {
abortQuietly(editor);
return null;
}
}
总体逻辑很简单，首先做了个保险的判断，对方法是POST，PATCH，PUT，DELETE，MOVE的请求，将缓存清除掉，这些是不应该被缓存的。然后明确了一点，只有GET方法才会被缓存。
而真正的缓存写入到文件是通过一个叫Entry的辅助类来的，我们简单的看下writeTo()方法：
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');
//省略类似代码
sink.close();
}
其实就是将请求信息按顺序写入到DiskLruCache中，最终由DiskLruCache写入到磁盘中。
缓存的get
@Nullable Response get(Request request) {
String key = key(request.url());
DiskLruCache.Snapshot snapshot;
Entry entry;
try {
snapshot = cache.get(key);
if (snapshot == null) {
return null;
}
} catch (IOException e) {
// Give up because the cache cannot be read.
return null;
}
try {
entry = new Entry(snapshot.getSource(ENTRY_METADATA));
} catch (IOException e) {
Util.closeQuietly(snapshot);
return null;
}
Response response = entry.response(snapshot);
if (!entry.matches(request, response)) {
Util.closeQuietly(response.body());
return null;
}
return response;
}
获取Response的结果没有任何复杂逻辑，先获取一个DiskLruCache.Snashot,然后构造一个Entry，最后借助这个Entry构造一个Response。至于Snashot是什么东西，我们下篇文章分析。
