在最近的工作中接触到了okhttp的缓存，借此机会记录下。网上已经有很多相关文章了，比如参考文章一还包括了http头缓存的相关介绍；但是一来自己跟代码记得比较牢，二来我比较关心缓存中lastModified和etag的设置所以需要挑重点，所以还是自己下手了。
本文基于okhttp 3.12版本
开启缓存
okhttp开启缓存非常方便，只需要设置cache参数即可
//3mb缓存
final int cacheSize = 1024 * 1024 * 3;
defaultBuilder = new OkHttpClient.Builder()
.cache(new Cache(CmGameSdkConstant.getAppContext().getCacheDir(), cacheSize))
.connectTimeout(DEFAULT_CONNECT_TIME_SEC, TimeUnit.SECONDS);
这样设置之后，okhttp就开启了缓存，包括本地缓存以及浏览器相关缓存（etag、last-modify），这一点我一开始是不信的（原先没有关注这方面），看完之后就。。。真香
缓存类Cache内部设置
在第一步中我们开启了缓存，也就是开启了cache选项，其中传入了Cache对象。我们看看这个对象做了些什么，首先就是get，这里根据请求拿到缓存对应的回复，这里是使用LruCache做本地缓存的。
@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;
}
// 这里是将请求映射为url，使用md5计算
public static String key(HttpUrl url) {
return ByteString.encodeUtf8(url.toString()).md5().hex();
}
缓存任务拦截器 CacheInterceptor
在请求流程中，是用链式分发的形式走的，大概如下：
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
List<Interceptor> interceptors = new ArrayList<>();
interceptors.addAll(client.interceptors());
interceptors.add(retryAndFollowUpInterceptor);
interceptors.add(new BridgeInterceptor(client.cookieJar()));
interceptors.add(new CacheInterceptor(client.internalCache()));
interceptors.add(new ConnectInterceptor(client));
if (!forWebSocket) {
interceptors.addAll(client.networkInterceptors());
}
interceptors.add(new CallServerInterceptor(forWebSocket));
Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
originalRequest, this, eventListener, client.connectTimeoutMillis(),
client.readTimeoutMillis(), client.writeTimeoutMillis());
return chain.proceed(originalRequest);
}
其中缓存的关键类就是CacheInterceptor
我们来看看他的拦截关键方法
@Override public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
一开始根据请求去获取缓存的回复
拿到历史缓存后，加上当前时间构建Request以及Response对象
long now = System.currentTimeMillis();
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
这个缓存策略CacheStrategy很重要，基本上后面的匹配都与他有关。这里根据请求和缓存回复构建，后面关于http的匹配都是在里面。
在构造方法里，我们看到他将解决请求时间、etag以及lastModify等信息都保存下来了，为之后的缓存氢气做准备
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
//看下Factory的构造方法
public Factory(long nowMillis, Request request, Response cacheResponse) {
this.nowMillis = nowMillis;
this.request = request;
this.cacheResponse = cacheResponse;
if (cacheResponse != null) {
this.sentRequestMillis = cacheResponse.sentRequestAtMillis();
this.receivedResponseMillis = cacheResponse.receivedResponseAtMillis();
Headers headers = cacheResponse.headers();
for (int i = 0, size = headers.size(); i < size; i++) {
String fieldName = headers.name(i);
String value = headers.value(i);
//这里用来保存服务器时间，为之后的缓存过期做准备
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;
//表示收到的时间，同样是为了后面过期做准备
} else if ("Age".equalsIgnoreCase(fieldName)) {
ageSeconds = HttpHeaders.parseSeconds(value, -1);
}
}
}
}
随后我们会开始构建CacheStrategy，也就是缓存策略，缓存会最终体现在CacheStrategy的networkRequest和cacheResponse中，也就是他的构造函数。最终实现方法在getCandidate()中：
private CacheStrategy getCandidate() {
// No cached response.
// 没有缓存，那直接返回
if (cacheResponse == null) {
return new CacheStrategy(request, null);
}
// Drop the cached response if it's missing a required handshake.
// handshake是握手的意思，如果请求为https而且没有经过握手，那也不缓存
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.
// 这里是判断是否不允许缓存，比如返回头中的no-store，注意这里同步比较了请求和返回的cache策略
if (!isCacheable(cacheResponse, request)) {
return new CacheStrategy(request, null);
}
// 第一个判断是请求是否允许缓存，第二个是请求中如果已经带了If-Modified-Since或者If-None-Match，说明上层自己做了缓存
CacheControl requestCaching = request.cacheControl();
if (requestCaching.noCache() || hasConditions(request)) {
return new CacheStrategy(request, null);
}
CacheControl responseCaching = cacheResponse.cacheControl();
//cacheResponseAge 缓存时间的计算大概如下
//return receivedAge + responseDuration + residentDuration;
//也就是请求到收到时间+现在使用时间
long ageMillis = cacheResponseAge();
//通过之前缓存的expires、last-modify等计算缓存有效期
long freshMillis = computeFreshnessLifetime();
// 如果请求中已经包含了max-age,那么我们取相对最小值
if (requestCaching.maxAgeSeconds() != -1) {
freshMillis = Math.min(freshMillis, SECONDS.toMillis(requestCaching.maxAgeSeconds()));
}
// Cache-Control相关
// min-fresh是指在期望的时间内响应有效
long minFreshMillis = 0;
if (requestCaching.minFreshSeconds() != -1) {
minFreshMillis = SECONDS.toMillis(requestCaching.minFreshSeconds());
}
// 这个同上，mustRevalidate是指在本地副本未过期前可以使用，否则必须刷新
long maxStaleMillis = 0;
if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
}
// 还没过期，加个warning信息返回上层，随即复用缓存
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\"");
}
return new CacheStrategy(null, builder.build());
}
// Find a condition to add to the request. If the condition is satisfied, the response body
// will not be transmitted.
// 这里也有点意思，etag优先，然后是If-Modified-Since，然后是服务器时间，进行拼接，向服务器请求
String conditionName;
String conditionValue;
if (etag != null) {
conditionName = "If-None-Match";
conditionValue = etag;
} else if (lastModified != null) {
conditionName = "If-Modified-Since";
conditionValue = lastModifiedString;
} else if (servedDate != null) {
conditionName = "If-Modified-Since";
conditionValue = servedDateString;
} else {
// 如果这些都没有了，那只能走普通的网络请求了
return new CacheStrategy(request, null); // No condition! Make a regular request.
}
// 最后就是我们的集大成者了，header内部是用键值对维护的
// 我们重新修改后，生成“附带属性”的conditionalRequest
Headers.Builder conditionalRequestHeaders = request.headers().newBuilder();
Internal.instance.addLenient(conditionalRequestHeaders, conditionName, conditionValue);
Request conditionalRequest = request.newBuilder()
.headers(conditionalRequestHeaders.build())
.build();
return new CacheStrategy(conditionalRequest, cacheResponse);
}
上面是缓存的大头，构建出了我们想要的内容，同时先过滤一次网络请求，如果本地缓存可用则先用本地缓存，这个时候request是为空的。
下面我们回到拦截方法CacheInterceptor.intercept
// If we're forbidden from using the network and the cache is insufficient, fail
// 这里是禁止使用网络的情况下缓存又无效，所以直接504错误
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();
}
// If we don't need the network, we're done.
// 如果缓存未过期同时又没有网络，直接返回缓存结果
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
// 走到这里说明能用的缓存都用了，还是需要走网络的，交给链条的下一个
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) {
// 如果是传说中的304，那么缓存可用，我们主动构建一个回复
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());
}
}
// 否则更新response信息
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
if (cache != null) {
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.
}
}
}
参考文章
- https://www.jianshu.com/p/00d281c226f6 okhttp缓存处理
- https://blog.csdn.net/aiynmimi/article/details/79807036 OkHttp3源码分析之缓存Cache
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