OKhttp的缓存是通过CacheInterceptor这个拦截器实现的,它的实现如下:
public final class CacheInterceptor implements Interceptor {
final InternalCache cache;
public CacheInterceptor(InternalCache cache) {
this.cache = cache;
}
@Override public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;//先从缓存中取出Request对应的Response,可能为null
long now = System.currentTimeMillis();
//通过Request和Response构造CacheStrategy
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
//缓存监控
if (cache != null) {
cache.trackResponse(strategy);
}
//cacheCandidate无效,关闭它
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.
//networkReqeust和cacheResponse都为null表示禁止使用网络且缓存不够用,这是返回504响应
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.
//缓存有效的情况,不需要进行网络请求
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))//配置Response的cacheResponse,但并不需要持有body所以这里剔除掉body
.build();
}
Response networkResponse = null;
try {
//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.
//如果缓存也存在
if (cacheResponse != null) {
//响应304码表示服务器认为资源未改变
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;//返回cacheResponse的内容
} else {
closeQuietly(cacheResponse.body());
}
}
//网络请求的Response
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
if (cache != null) {
//将最新的请求存储到缓存中
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.
}
}
}
return response;//返回结果
}
//将Response的body剔除
private static Response stripBody(Response response) {
return response != null && response.body() != null
? response.newBuilder().body(null).build()
: response;
}
……
}
CacheInterceptor内部持有一个InternalCache,它负责缓存的存取,而CacheStrategy用来控制缓存的存取,决定什么时候用缓存,什么时候使用网络进行请求。它是通过其内部的networkRequest和cacheResponse决定的,当networkRequest的值为null表示缓存有效不进行网络请求,而当cacheResponse为null是表示缓存失效或者未命中,需要进行网络请求。当它们都不为null,则根据响应结果来判断缓存是否失效。
CacehStrategy通过Request和候选的缓存进行构造的,我们看看它的内部是如何实现的
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
public final class CacheStrategy {
/** The request to send on the network, or null if this call doesn't use the network. */
public final @Nullable Request networkRequest;
/** The cached response to return or validate; or null if this call doesn't use a cache. */
public final @Nullable Response cacheResponse;
public static class Factory {
final long nowMillis;
final Request request;
final Response cacheResponse;
/** The server's time when the cached response was served, if known. */
private Date servedDate;
private String servedDateString;
/** The last modified date of the cached response, if known. */
private Date lastModified;
private String lastModifiedString;
/**
* The expiration date of the cached response, if known. If both this field and the max age are
* set, the max age is preferred.
*/
private Date expires;
/**
* Extension header set by OkHttp specifying the timestamp when the cached HTTP request was
* first initiated.
*/
private long sentRequestMillis;
/**
* Extension header set by OkHttp specifying the timestamp when the cached HTTP response was
* first received.
*/
private long receivedResponseMillis;
/** Etag of the cached response. */
private String etag;
/** Age of the cached response. */
private int ageSeconds = -1;
public Factory(long nowMillis, Request request, Response cacheResponse) {
this.nowMillis = nowMillis;
this.request = request;
this.cacheResponse = cacheResponse;//将候选的缓存保存下来
//从候选的缓存中解析出一些信息
if (cacheResponse != null) {
this.sentRequestMillis = cacheResponse.sentRequestAtMillis();
this.receivedResponseMillis = cacheResponse.receivedResponseAtMillis();
Headers headers = cacheResponse.headers();//获取到缓存响应头部
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);//得到服务器的日期
servedDateString = value;
} else if ("Expires".equalsIgnoreCase(fieldName)) {
expires = HttpDate.parse(value);//得到失效时间
} else if ("Last-Modified".equalsIgnoreCase(fieldName)) {
lastModified = HttpDate.parse(value);//得到资源上次修改的时间
lastModifiedString = value;
} else if ("ETag".equalsIgnoreCase(fieldName)) {
etag = value;//得到资源对应的ETag
} 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();
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.
//没有命中的缓存,使用网络进行请求
if (cacheResponse == null) {
return new CacheStrategy(request, null);
}
// Drop the cached response if it's missing a required handshake.
//如果是https,但是缓存中缺少了握手信息,同样进行网络请求
if (request.isHttps() && cacheResponse.handshake() == null) {
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.
//如果该缓存是不应该被缓存的,则同样进行网络请求
if (!isCacheable(cacheResponse, request)) {
return new CacheStrategy(request, null);
}
//得到请求头部的CacheControl
CacheControl requestCaching = request.cacheControl();
//如果不进行缓存则直接进行网络请求
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());
}
//可以缓存且缓存有效,不需要进行网络请求
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) {//etag不为null
conditionName = "If-None-Match";
conditionValue = etag;//将If-None-Match配合etag进行请求,服务端可以基于ETag判断资源是否匹配。
} else if (lastModified != null) {//在请求头部添加If-Modified-Since结合lastModifiedString判断资源是否被修改,如果未修改返回304
conditionName = "If-Modified-Since";
conditionValue = lastModifiedString;
} else if (servedDate != null) {//在请求头部添加If-Modified-Since
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);
}
}
}
在getCandidate方法中实现了构造CacheStrategy实例的逻辑,总的来说:
一 使用网络进行请求的包括,即networkRequest != null :
- 缓存未命中
- https请求缺少handshake信息
- request指定的CacheControl不进行缓存
- 缓存的Response头部没有缓存相关的信息如ETag,If-None-Match,If-Modified-Since等
二 使用缓存请求的情况,即cacheResponse != null
- 缓存有效,未超过其失效时间
三 networkRequest != null 且 cacheResponse != null
- 缓存是否有效,要根据响应结果来判断
缓存的写入
在CacheInterceptor内部它对Response进行缓存的实现如下:
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.
}
}
}
写入缓存是在Cache类中实现的,它内部持有一个internalCache,它正是CacheInterceptor拦截器内部的缓存引用。这里我们先看cache put方法的实现
@Nullable CacheRequest put(Response response) {
String requestMethod = response.request().method();//获取requestMethod
//判断是否为失效的缓存
if (HttpMethod.invalidatesCache(response.request().method())) {
try {
remove(response.request());
} catch (IOException ignored) {
// The cache cannot be written.
}
return null;
}
//只对GET响应进行缓存
if (!requestMethod.equals("GET")) {
// Don't cache non-GET responses. We're technically allowed to cache
// HEAD requests and some POST requests, but the complexity of doing
// so is high and the benefit is low.
return null;
}
if (HttpHeaders.hasVaryAll(response)) {
return null;
}
//构造Entry
Entry entry = new Entry(response);
DiskLruCache.Editor editor = null;
try {
//得到DiskLruCache.Editor 可以看到OkHttp缓存是通过DiskLruCache实现的
editor = cache.edit(key(response.request().url()));
if (editor == null) {
return null;
}
entry.writeTo(editor);//写入缓存
return new CacheRequestImpl(editor);
} catch (IOException e) {
abortQuietly(editor);
return null;
}
}
关于DiskLruCache将在另外的篇章中介绍。