从上篇文章《OkHttp源码深入》中,我们知道okhttp最后所有的请求都会经过getResponseWithInterceptorChain方法,进行拦截处理。面试中常问的拦截策略必属缓存拦截器CacheInterceptor的实现。
OKHttp中缓存策略
- 强制缓存,即缓存在有效期内就直接返回缓存,不进行网络请求。
- 对比缓存,即缓存超过有效期,进行网络请求。若数据未修改,服务端返回不带body的304响应,表示客户端缓存仍有效可用;否则返回完整最新数据,客户端取网络请求的最新数据
这里先做一个总结:
OkHttp中缓存机制实现大致流程是先判断是否执行强制缓存策略,不执行则请求服务端获取数据,然后判断执行对比缓存策略,接着更新缓存或新增缓存,最后返回response到上层拦截器
CacheInterceptor源码解析
@Override public Response intercept(Chain chain) throws IOException {
// cache即构建OkHttpClient时传入的Cache对象的内部成员,用request的url作为key,查找缓存的response作为候选缓存
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
//todo === 1.生成缓存策略 ===
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
// 获取缓存策略生成的networkRequest和cacheResponse(合法缓存)
// 下面流程会根据生成的networkRequest和cacheResponse来决定执行什么缓存策略
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {
// cache内部的计数器加1
cache.trackResponse(strategy);
}
// 若候选缓存存在但是缓存策略生成的cache不存在,关闭cacheCandidate中的BufferedSource的输入输出流
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.
// 即无网络请求又无合法缓存,返回状态码504的response
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.
// 强制缓存策略
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
// networkResponse表示当前网络请求的最新response,cacheResponse表示由缓存策略获取的合法缓存response
Response networkResponse = null;
try {
// 调用下层拦截器进行网络请求
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) {
// 对比缓存策略
if (networkResponse.code() == HTTP_NOT_MODIFIED) {
// 服务端返回304状态码,表示本地缓存仍有效
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内计数器加1
cache.trackConditionalCacheHit();
// 更新本地缓存信息
cache.update(cacheResponse, response);
return response;
} else {
closeQuietly(cacheResponse.body());
}
}
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
//todo === 2.判断能否缓存networkResponse ===
if (cache != null) {
// OkHttpClient设置了Cache
if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
// Offer this request to the cache.
// 保存拥有body且符合缓存条件的响应
// put方法中只缓存GET请求的response
CacheRequest cacheRequest = cache.put(response);
// 通过该方法返回新建的repsonse,保存缓存response的body能够正确的写入和关闭流
return cacheWritingResponse(cacheRequest, response);
}
// 判断是否时无效缓存,若请求方式是POST、PATCH、PUT、DELETE、MOVE,则移除缓存
if (HttpMethod.invalidatesCache(networkRequest.method())) {
try {
cache.remove(networkRequest);
} catch (IOException ignored) {
// The cache cannot be written.
}
}
}
return response;
}
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