系列文章
OKHttp原始碼解析(2)----攔截器RetryAndFollowUpInterceptor
OKHttp原始碼解析(3)----攔截器BridgeInterceptor
OKHttp原始碼解析(4)----攔截器CacheInterceptor
OKHttp原始碼解析(5)----攔截器ConnectInterceptor
OKHttp原始碼解析(6)----攔截器CallServerInterceptor
1.簡介
Serves requests from the cache and writes responses to the cache.
快取攔截器，負責讀取快取直接返回、更新快取。當網路請求有符合要求的Cache時，直接返回Cache。如果當前Cache失效，則刪除。CacheStrategy：快取策略，CacheStrategy類是一個非常重要的類，用於控制請求是網路獲取還是快取獲取
2.原始碼解析
@Override public Response intercept(Chain chain) throws IOException {
// 得到 request 對應快取中的 response
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
// 獲取當前時間，會和之前快取的時間進行比較
long now = System.currentTimeMillis();
// 得到快取策略
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
// 追蹤快取，其實就是計數
if (cache != null) {
cache.trackResponse(strategy);
}
// 快取不適用，關閉
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.
// 禁止網路並且沒有快取的話，返回失敗
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.
//不用網路請求，返回快取
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.
// 如果我們同時有快取和 networkResponse ，根據情況使用
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()).
// 更新原來的快取至最新
cache.trackConditionalCacheHit();
cache.update(cacheResponse, response);
return response;
} else {
closeQuietly(cacheResponse.body());
}
}
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
// 儲存之前未快取的快取
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;
}
複製程式碼主要做三件事：
- 根據Request和之前快取的Response得到CacheStrategy
- 根據CacheStrategy決定是請求網路還是直接返回快取
- 如果2中決定請求網路，則在這一步將返回的網路響應和本地快取對比，對本地快取進行增刪改操作
| networkRequest | cacheResponse | 結 果 |
|---|---|---|
| null | null | 禁止進行網路請求，但快取不存在或者過期，只能返回503錯誤 |
| null | non-null | 快取可以使用，直接返回快取，不用請求網路 |
| non-null | null | 快取不存在或者過期，直接訪問網路 |
| non-null | non-null | 條件get，請求網路 |
對於快取攔截器來說，最重要的地方在於快取策略的生成邏輯：
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
複製程式碼根據now(此刻時間),chain.request()(請求),cacheCandidate(快取)，來生成相應的請求和響應。下面我們來看一下快取策略的生成邏輯：
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;
//響應物件在代理快取中存在的時間，以秒為單位
} else if ("ETag".equalsIgnoreCase(fieldName)) {
etag = value;
//響應物件在代理快取中存在的時間，以秒為單位
} else if ("Age".equalsIgnoreCase(fieldName)) {
ageSeconds = HttpHeaders.parseSeconds(value, -1);
}
}
}
}
複製程式碼Factory()： Factory()方法記錄一些資料：現在的時間、請求體、快取響應。如果快取響應有資料，則記錄快取的請求傳送時間、收到響應的時間、伺服器時間、有效期、上次修改、存活時間等。
/**
* 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;
}
複製程式碼get()： get()方法獲取快取策略，其中的核心方法為getCandidate()。
/** Returns a strategy to use assuming the request can use the network. */
private CacheStrategy getCandidate() {
// No cached response.
//無快取
if (cacheResponse == null) {
return new CacheStrategy(request, null);
}
// Drop the cached response if it is missing a required handshake.
//丟棄請求是https且沒有進行握手的快取
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.
//檢測這個響應是否可以被快取
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);
}
複製程式碼小結
本章對原始碼進行了基本的講解，但是OkHttp的快取策略還是比較複雜的，我們將在新的一章中對其進行詳細分析。