简介
Serves requests from the cache and writes responses to the cache.
缓存拦截器,负责读取缓存直接返回、更新缓存。当网络请求有符合要求的Cache时,直接返回Cache。如果当前Cache失效,则删除。CacheStrategy:缓存策略,CacheStrategy类是一个非常重要的类,用于控制请求是网络获取还是缓存获取
源码解析
@Override public Response intercept(Chain chain) throws IOException {
// 得到 request 对应缓存中的 response
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
// 获取当前时间,会和之前缓存的时间进行比较
long now = System.currentTimeMillis();
// 得到缓存策略
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
// 追踪缓存,其实就是计数
if (cache != null) {
cache.trackResponse(strategy);
}
// 缓存不适用,关闭
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.
// 禁止网络并且没有缓存的话,返回失败
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.
//不用网络请求,返回缓存
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
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 are doing a conditional get.
// 如果我们同时有缓存和 networkResponse ,根据情况使用
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()).
// 更新原来的缓存至最新
cache.trackConditionalCacheHit();
cache.update(cacheResponse, response);
return response;
} else {
closeQuietly(cacheResponse.body());
}
}
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
// 保存之前未缓存的缓存
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.
}
}
}
return response;
}
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主要做三件事:
- 根据Request和之前缓存的Response得到CacheStrategy
- 根据CacheStrategy决定是请求网络还是直接返回缓存
- 如果2中决定请求网络,则在这一步将返回的网络响应和本地缓存对比,对本地缓存进行增删改操作
networkRequest | cacheResponse | 结 果 |
---|---|---|
null | null | 禁止进行网络请求,但缓存不存在或者过期,只能返回503错误 |
null | non-null | 缓存可以使用,直接返回缓存,不用请求网络 |
non-null | null | 缓存不存在或者过期,直接访问网络 |
non-null | non-null | 条件get,请求网络 |
对于缓存拦截器来说,最重要的地方在于缓存策略的生成逻辑:
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
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根据now(此刻时间),chain.request()(请求),cacheCandidate(缓存),来生成相应的请求和响应。下面我们来看一下缓存策略的生成逻辑:
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);
//日期
if ("Date".equalsIgnoreCase(fieldName)) {
servedDate = HttpDate.parse(value);
servedDateString = value;
//有效期
} else if ("Expires".equalsIgnoreCase(fieldName)) {
expires = HttpDate.parse(value);
//上次修改
} else if ("Last-Modified".equalsIgnoreCase(fieldName)) {
lastModified = HttpDate.parse(value);
lastModifiedString = value;
//响应对象在代理缓存中存在的时间,以秒为单位
} else if ("ETag".equalsIgnoreCase(fieldName)) {
etag = value;
//响应对象在代理缓存中存在的时间,以秒为单位
} else if ("Age".equalsIgnoreCase(fieldName)) {
ageSeconds = HttpHeaders.parseSeconds(value, -1);
}
}
}
}
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Factory(): Factory()方法记录一些数据:现在的时间、请求体、缓存响应。如果缓存响应有数据,则记录缓存的请求发送时间、收到响应的时间、服务器时间、有效期、上次修改、存活时间等。
/**
* Returns a strategy to satisfy {@code request} using the a cached response {@code response}.
*/
public CacheStrategy get() {
CacheStrategy candidate = getCandidate();
if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) {
// We are forbidden from using the network and the cache is insufficient.
return new CacheStrategy(null, null);
}
return candidate;
}
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get(): get()方法获取缓存策略,其中的核心方法为getCandidate()。
/** Returns a strategy to use assuming the request can use the network. */
private CacheStrategy getCandidate() {
// No cached response.
//无缓存
if (cacheResponse == null) {
return new CacheStrategy(request, null);
}
// Drop the cached response if it is missing a required handshake.
//丢弃请求是https且没有进行握手的缓存
if (request.isHttps() && cacheResponse.handshake() == null) {
return new CacheStrategy(request, null);
}
// If this response should not 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.
//检测这个响应是否可以被缓存
if (!isCacheable(cacheResponse, request)) {
return new CacheStrategy(request, null);
}
CacheControl requestCaching = request.cacheControl();
if (requestCaching.noCache() || hasConditions(request)) {
return new CacheStrategy(request, null);
}
long ageMillis = cacheResponseAge();
long freshMillis = computeFreshnessLifetime();
if (requestCaching.maxAgeSeconds() != -1) {
freshMillis = Math.min(freshMillis, SECONDS.toMillis(requestCaching.maxAgeSeconds()));
}
long minFreshMillis = 0;
if (requestCaching.minFreshSeconds() != -1) {
minFreshMillis = SECONDS.toMillis(requestCaching.minFreshSeconds());
}
long maxStaleMillis = 0;
CacheControl responseCaching = cacheResponse.cacheControl();
if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
}
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.
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 {
return new CacheStrategy(request, null); // No condition! Make a regular request.
}
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);
}
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小结
本章对源码进行了基本的讲解,但是OkHttp的缓存策略还是比较复杂的,我们将在新的一章中对其进行详细分析。