CacheInterceptor(缓存拦截器)
public final class CacheInterceptor implements Interceptor {
final InternalCache cache;
public CacheInterceptor(InternalCache cache) {
this.cache = cache;
}
/**
*入口
*/
@Override
public Response intercept(Chain chain) throws IOException {
//todo 通过url的md5数据 从文件缓存查找 （GET请求才有缓存）
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
//todo 缓存策略:根据各种条件(请求头)组成 请求与缓存
CacheStrategy strategy =
new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
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.
}
//todo 没有网络请求也没有缓存
//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();
}
//todo 没有请求，肯定就要使用缓存
//If we don't need the network, we're done.
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
//todo 去发起请求
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) {
//todo 服务器返回304无修改，那就使用缓存的响应修改了时间等数据后作为本次请求的响应
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());
}
}
//todo 走到这里说明缓存不可用 那就使用网络的响应
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
//todo 进行缓存
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;
}
CacheStrategy 缓存策略工厂
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);
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;
} else if ("Age".equalsIgnoreCase(fieldName)) {
ageSeconds = HttpHeaders.parseSeconds(value, -1);
}
}
}
}
/**
* Returns a strategy to satisfy {@code request} using the a cached response {@code
* response}.
*/
public CacheStrategy get() {
CacheStrategy candidate = getCandidate();
//todo 如果可以使用缓存，那networkRequest必定为null；指定了只使用缓存但是networkRequest又不为null，冲突。那就gg(拦截器返回504)
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;
}
/**
* Returns a strategy to use assuming the request can use the network.
*/
private CacheStrategy getCandidate() {
// No cached response.
//todo 1、没有缓存,进行网络请求
if (cacheResponse == null) {
return new CacheStrategy(request, null);
}
//todo 2、https请求，但是没有握手信息,进行网络请求
//Drop the cached response if it's missing a required handshake.
if (request.isHttps() && cacheResponse.handshake() == null) {
return new CacheStrategy(request, null);
}
//todo 3、主要是通过响应码以及头部缓存控制字段判断响应能不能缓存，不能缓存那就进行网络请求
//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();
//todo 4、如果 请求包含：CacheControl:no-cache 需要与服务器验证缓存有效性
// 或者请求头包含 If-Modified-Since：时间 值为lastModified或者data 如果服务器没有在该头部指定的时间之后修改了请求的数据，服务器返回304(无修改)
// 或者请求头包含 If-None-Match：值就是Etag（资源标记）服务器将其与存在服务端的Etag值进行比较；如果匹配，返回304
// 请求头中只要存在三者中任意一个，进行网络请求
if (requestCaching.noCache() || hasConditions(request)) {
return new CacheStrategy(request, null);
}
//todo 5、如果缓存响应中存在 Cache-Control:immutable 响应内容将一直不会改变,可以使用缓存
CacheControl responseCaching = cacheResponse.cacheControl();
if (responseCaching.immutable()) {
return new CacheStrategy(null, cacheResponse);
}
//todo 6、根据 缓存响应的 控制缓存的响应头 判断是否允许使用缓存
// 6.1、获得缓存的响应从创建到现在的时间
long ageMillis = cacheResponseAge();
//todo
// 6.2、获取这个响应有效缓存的时长
long freshMillis = computeFreshnessLifetime();
if (requestCaching.maxAgeSeconds() != -1) {
//todo 如果请求中指定了 max-age 表示指定了能拿的缓存有效时长，就需要综合响应有效缓存时长与请求能拿缓存的时长，获得最小的能够使用响应缓存的时长
freshMillis = Math.min(freshMillis,
SECONDS.toMillis(requestCaching.maxAgeSeconds()));
}
//todo
// 6.3 请求包含 Cache-Control:min-fresh=[秒] 能够使用还未过指定时间的缓存 （请求认为的缓存有效时间）
long minFreshMillis = 0;
if (requestCaching.minFreshSeconds() != -1) {
minFreshMillis = SECONDS.toMillis(requestCaching.minFreshSeconds());
}
//todo
// 6.4
// 6.4.1、Cache-Control:must-revalidate 可缓存但必须再向源服务器进行确认
// 6.4.2、Cache-Control:max-stale=[秒] 缓存过期后还能使用指定的时长 如果未指定多少秒，则表示无论过期多长时间都可以；如果指定了，则只要是指定时间内就能使用缓存
// 前者会忽略后者，所以判断了不必须向服务器确认，再获得请求头中的max-stale
long maxStaleMillis = 0;
if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
}
//todo
// 6.5 不需要与服务器验证有效性 && 响应存在的时间+请求认为的缓存有效时间 小于 缓存有效时长+过期后还可以使用的时间
// 允许使用缓存
if (!responseCaching.noCache() && ageMillis + minFreshMillis < freshMillis + maxStaleMillis) {
Response.Builder builder = cacheResponse.newBuilder();
//todo 如果已过期，但未超过 过期后继续使用时长，那还可以继续使用，只用添加相应的头部字段
if (ageMillis + minFreshMillis >= freshMillis) {
builder.addHeader("Warning", "110 HttpURLConnection \"Response is stale\"");
}
//todo 如果缓存已超过一天并且响应中没有设置过期时间也需要添加警告
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.
//todo 7、缓存过期了
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.
}
//todo 如果设置了 If-None-Match/If-Modified-Since 服务器是可能返回304(无修改)的,使用缓存的响应体
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);
}
/**
* Returns the number of milliseconds that the response was fresh for, starting from the
* served
* date.
*/
private long computeFreshnessLifetime() {
CacheControl responseCaching = cacheResponse.cacheControl();
/**
* todo
* max-age：资源最大有效时间/秒
* expires:缓存过期的时间(服务器时间,可能客户端时间和服务器不一致)
* 服务器如果发出了Data(服务器发出这个消息的时间) 则expires-data为有效时间
* 没发出Data 则用 expires-收到响应的时间 为有效时间
* Last-Modified: 请求的数据最后修改的时间 进行试探性过期时间的计算
* 服务器如果发出了Data(服务器发出这个消息的时间) 则data-lastModified 作为有效时间
* 没发出Data 则用 对应缓存响应发起请求的时间-lastModified 作为有效时间
*/
if (responseCaching.maxAgeSeconds() != -1) {
return SECONDS.toMillis(responseCaching.maxAgeSeconds());
} else if (expires != null) {
long servedMillis = servedDate != null
? servedDate.getTime()
: receivedResponseMillis;
long delta = expires.getTime() - servedMillis;
return delta > 0 ? delta : 0;
} else if (lastModified != null
&& cacheResponse.request().url().query() == null) {
// As recommended by the HTTP RFC and implemented in Firefox, the
// max age of a document should be defaulted to 10% of the
// document's age at the time it was served. Default expiration
// dates aren't used for URIs containing a query.
long servedMillis = servedDate != null
? servedDate.getTime()
: sentRequestMillis;
long delta = servedMillis - lastModified.getTime();
return delta > 0 ? (delta / 10) : 0;
}
return 0;
}
/**
* Returns the current age of the response, in milliseconds. The calculation is specified
* by RFC
* 7234, 4.2.3 Calculating Age.
*/
private long cacheResponseAge() {
//todo
// 响应Data字段：服务器发出这个消息的时间
// 响应Age字段: 当代理服务器用自己缓存的实体去响应请求时，会用该头部表明该实体从产生到现在经过多长时间了
// 计算收到响应的时间与服务器创建发出这个消息的时间差值:apparentReceivedAge
// 取出apparentReceivedAge 与ageSeconds最大值并赋予receivedAge （代理服务器缓存的时间，意义如上的结果，但是要拿最大值）
// 计算从发起请求到收到响应的时间差responseDuration
// 计算现在与收到响应的时间差residentDuration
// 三者加起来就是响应已存在的总时长
long apparentReceivedAge = servedDate != null
? Math.max(0, receivedResponseMillis - servedDate.getTime())
: 0;
long receivedAge = ageSeconds != -1
? Math.max(apparentReceivedAge, SECONDS.toMillis(ageSeconds))
: apparentReceivedAge;
long responseDuration = receivedResponseMillis - sentRequestMillis;
long residentDuration = nowMillis - receivedResponseMillis;
return receivedAge + responseDuration + residentDuration;
}
/**
* Returns true if computeFreshnessLifetime used a heuristic. If we used a heuristic to
* serve a
* cached response older than 24 hours, we are required to attach a warning.
*/
private boolean isFreshnessLifetimeHeuristic() {
return cacheResponse.cacheControl().maxAgeSeconds() == -1 && expires == null;
}
/**
* Returns true if the request contains conditions that save the server from sending a
* response
* that the client has locally. When a request is enqueued with its own conditions, the
* built-in
* response cache won't be used.
*/
private static boolean hasConditions(Request request) {
return request.header("If-Modified-Since") != null || request.header("If-None-Match") != null;
}
核心代码就这2个了
