CacheInterceptor
CacheInterceptor 即缓存拦截器，和请求的缓存控制相关；
查看 CacheInterceptor 的 intercept 方法；
@Override public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null; // 获取 cache
long now = System.currentTimeMillis();
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get(); // 创建 CacheStrategy
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
if (cache != null) { // cache 不为空
cache.trackResponse(strategy);
}
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.
if (networkRequest == null && cacheResponse == null) { // 无网络且没有缓存，返回 504 响应无效
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.
if (networkRequest == null) { // 不需要调用请求，且有缓存，返回缓存
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
Response networkResponse = null;
try {
networkResponse = chain.proceed(networkRequest);// 调用下一个拦截器
} 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) { // 缓存不为null
if (networkResponse.code() == HTTP_NOT_MODIFIED) { //网络请求返回标志 缓存无需改变 存储原来缓存即可，同时更新相关时间
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());
}
}
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build(); // 创建 Response
if (cache != null) { // 设置 request 和 response 到 cache 中
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.
}
}
}
CacheInterceptor 的 intercept 方法首先会去获取 cache 里面的 Request 请求；
public CacheInterceptor(InternalCache cache) {
this.cache = cache;
}
InternalCache internalCache() {
return cache != null ? cache.internalCache : internalCache;
}
在 OkHttp3 中 InternalCache 默认使用的 DiskLruCache 方式进行实现；
public final class Cache implements Closeable, Flushable {
private static final int VERSION = 201105;
private static final int ENTRY_METADATA = 0;
private static final int ENTRY_BODY = 1;
private static final int ENTRY_COUNT = 2;
final InternalCache internalCache = new InternalCache() {
@Override public Response get(Request request) throws IOException {
return Cache.this.get(request);
}
@Override public CacheRequest put(Response response) throws IOException {
return Cache.this.put(response);
}
@Override public void remove(Request request) throws IOException {
Cache.this.remove(request);
}
@Override public void update(Response cached, Response network) {
Cache.this.update(cached, network);
}
@Override public void trackConditionalCacheHit() {
Cache.this.trackConditionalCacheHit();
}
@Override public void trackResponse(CacheStrategy cacheStrategy) {
Cache.this.trackResponse(cacheStrategy);
}
};
final DiskLruCache cache;
}
所以 这里的 Cache 默认使用 DiskLruCache 存储相关 Request 和 Response 进行实现；
当通过 Cache 获取到 cacheCandidate 不为空的时候，默认会进行创建 CacheStrategy 实例；
继续查看 CacheStrategy 是什么策略来控制缓存？
CacheStrategy
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);
}
}
}
}
如果此时获取到 Request 对应的缓存不为空，获取到 Last-Modified 、 ETag等相关数据进行赋值，这些数据后续用来判断缓存是否已经过期；
get 方法；
public CacheStrategy get() {
CacheStrategy candidate = getCandidate(); // 获取 CacheStrategy
if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) { // 没有网络的情况 而且 请求禁止缓存
// 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.
if (cacheResponse == null) { // 对应 Request 无缓存
return new CacheStrategy(request, null);
}
// Drop the cached response if it's missing a required handshake.
if (request.isHttps() && cacheResponse.handshake() == null) { // 缺少握手
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.
if (!isCacheable(cacheResponse, request)) { // 客户端设置不允许缓存 Response
return new CacheStrategy(request, null);
}
CacheControl requestCaching = request.cacheControl();
if (requestCaching.noCache() || hasConditions(request)) {// 请求没有缓存
return new CacheStrategy(request, null);
}
CacheControl responseCaching = cacheResponse.cacheControl();
if (responseCaching.immutable()) { // 可以一直使用缓存
return new CacheStrategy(null, cacheResponse);
}
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;
if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
}
//响应支持缓存
//持续时间+最短刷新时间<上次刷新时间+最大验证时间 则可以缓存
//现在时间(now)-已经过去的时间（sent）+可以存活的时间<最大存活时间(max-age)
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());
}
// 拿到上一次缓存的相应过期标签，添加到 Request 中
// 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);
}
CacheStrategy 主要去做了哪些事情呢？
1 、定义了一些不会缓存的情况，例如：响应没有握手、客户端不允许缓存等，返回 response 为 null；
2、通过上一次的响应情况，根据 E-tag、lastModified等定义缓存策略，重新设置 Request 的 Builder 进行返回；
拿到了 CacheStrategy 策略实例，再次回到 intercept 方法，根据返回分析就更容易理解缓存策略了；
1、networkRequest 和 cacheResponse 都为空，说明没有网络也没有缓存，返回 504 的响应状态；
2、networkRequest 为 null ，但是 cacheResponse 不为 null，说明可以直接使用缓存返回；
3、除掉上述两种情况，进行网络请求，同时对比返回的 Response 是否需要更新缓存进行相应的处理，存入到 Cache；
Cache 流程大致示意图如下：
Cache.png
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