在RetryAndFollowUpInterceptor之后是BridgeInterceptor。BridgeInterceptor主要是为我们的请求添加了一些头部的信息,处理GZIP压缩解压缩等,暂且略过。 然后在BridgeInterceptor的后面就是本篇的主题了—— CacheInterceptor。
BridgeInterceptor的intercept
方法中调用了RealInterceptorChain的proceed
方法,内部会调用CacheInterceptor的intercept
方法。
先介绍一些关于响应缓存的知识点。当我们构建一个默认的OkHttpClient,默认是没有使用缓存的。OkHttpClient中的这两个成员都是为null。
final @Nullable Cache cache;
final @Nullable InternalCache internalCache;
为了缓存响应,你需要一个用于读取和写入的缓存目录,并且限制缓存的大小。缓存目录应该是私有的,不受信任的应用程序应该不可以读取该目录下的内容。
多个缓存实例同时访问同一个缓存目录是错误的。大多数应用程序应该只调用new OkHttpClient()
一次,并配置好他们的缓存,然后在应用中使用唯一的OkHttpClient实例。否则的话,两个缓存实例会互相踩踏,破坏响应缓存,并可能致程序崩溃。
响应缓存使用HTTP头进行所有的配置。你可添加请求头,像Cache-Control: max-stale=3600
,OkHttp的缓存会遵循它们。web服务器会在响应的头部配置响应应该被缓存多长时间,例如Cache-Control: max-age=9600
。有缓存头可以强制缓存响应,强制网络响应,或者使用有条件的GET来验证网络响应。
配置OkHttpClient的缓存目录
private final OkHttpClient client;
public CacheResponse(File cacheDirectory) throws Exception {
int cacheSize = 10 * 1024 * 1024; // 10 MiB
Cache cache = new Cache(cacheDirectory, cacheSize);
client = new OkHttpClient.Builder()
.cache(cache)//构建OkHttpClient的时候,可以传递一个Cache实例,用来缓存响应。
.build();
}
public void run() throws Exception {
Request request = new Request.Builder()
.url("http://publicobject.com/helloworld.txt")
.build();
String response1Body;
try (Response response1 = client.newCall(request).execute()) {
if (!response1.isSuccessful()) throw new IOException("Unexpected code " + response1);
response1Body = response1.body().string();
System.out.println("Response 1 response: " + response1);
System.out.println("Response 1 cache response: " + response1.cacheResponse());
System.out.println("Response 1 network response: " + response1.networkResponse());
}
String response2Body;
//注释1处,第二次请求的时候,就会从缓存里面取数据了。
try (Response response2 = client.newCall(request).execute()) {
if (!response2.isSuccessful()) throw new IOException("Unexpected code " + response2);
response2Body = response2.body().string();
System.out.println("Response 2 response: " + response2);
System.out.println("Response 2 cache response: " + response2.cacheResponse());
System.out.println("Response 2 network response: " + response2.networkResponse());
}
System.out.println("Response 2 equals Response 1? " +
response1Body.equals(response2Body));
}
注释1处,第二次请求的时候,就会从缓存里面取数据了。
RealCall的getResponseWithInterceptorChain方法4处,添加了CacheInterceptor。
Response getResponseWithInterceptorChain() throws IOException {
// 构建一整套拦截器
List interceptors = new ArrayList<>();
interceptors.addAll(client.interceptors());//1
interceptors.add(retryAndFollowUpInterceptor);//2
interceptors.add(new BridgeInterceptor(client.cookieJar()));//3
interceptors.add(new CacheInterceptor(client.internalCache()));//4
interceptors.add(new ConnectInterceptor(client));//5
//构建一个RealCall的时候我们传入的forWebSocket是false
if (!forWebSocket) {
interceptors.addAll(client.networkInterceptors());//6
}
interceptors.add(new CallServerInterceptor(forWebSocket));//7
//构建拦截器链
Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
originalRequest, this, eventListener, client.connectTimeoutMillis(),
client.readTimeoutMillis(), client.writeTimeoutMillis());
//拦截器链处理请求
return chain.proceed(originalRequest);
}
OkHttpClient的internalCache方法
InternalCache internalCache() {
return cache != null ? cache.internalCache : internalCache;
}
因为我们设置了cache,这里返回的是cache的internalCache。
okhttp3.Cache类部分代码。
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;
//内部还是调用了Cache的方法。
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);
}
};
//...
}
接下来我们看CacheInterceptor是怎么工作的。
@Override
public Response intercept(Interceptor.Chain chain) throws IOException {
//注释1处,从缓存中获取响应候选者。
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
//注释2处,根据当前时间、请求、响应候选者构建缓存策略
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
//注释3处
Request networkRequest = strategy.networkRequest;
//如果返回的cacheResponse不为null,说明缓存是可用的。
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {
cache.trackResponse(strategy);
}
//注释4处,缓存不满足缓存策略,缓存不可用
if (cacheCandidate != null && cacheResponse == null) {
closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
}
// 注释5处,如果禁止使用网络,并且缓存不满足,失败。
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();
}
// 注释6处,缓存可用。返回缓存。
if (networkRequest == null) {
return cacheResponse.newBuilder()
//给返回的Response添加cacheResponse字段,cacheResponse字段是去掉了响应体的。
.cacheResponse(stripBody(cacheResponse))
.build();
}
Response networkResponse = null;
try {
//注释7处,缓存不满足,交给后面的拦截器发起网络请求
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());
}
}
// 注释8处,condition get的情况If we have a cache response too, then we're doing a conditional get.
if (cacheResponse != null) {
if (networkResponse.code() == HTTP_NOT_MODIFIED) {
//304,响应未修改,
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();
//注释9处,更新缓存
cache.update(cacheResponse, response);
//返回响应。
return response;
} else {
closeQuietly(cacheResponse.body());
}
}
//构建响应
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
if (cache != null) {
if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
// 注释10处,添加缓存
CacheRequest cacheRequest = cache.put(response);
return cacheWritingResponse(cacheRequest, response);
}
if (HttpMethod.invalidatesCache(networkRequest.method())) {
try {
//注释11处,缓存不合法了,移除缓存
cache.remove(networkRequest);
} catch (IOException ignored) {
// The cache cannot be written.
}
}
}
//最终返回响应。
return response;
}
注释1处,从缓存中获取响应候选者。
注释2处,根据当前时间、请求、响应候选者构建缓存策略。
-------------------------- 跳过开始 --------------------------
CacheStrategy.Factory的构造函数。可以跳过,涉及到http一些知识,后面再复习研究。
public Factory(long nowMillis, Request request, Response cacheResponse) {
this.nowMillis = nowMillis;
this.request = request;
this.cacheResponse = cacheResponse;
if (cacheResponse != null) {
//请求发出的时间
this.sentRequestMillis = cacheResponse.sentRequestAtMillis();
//响应获取的时间
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)) {
//响应的过期时间,如果有设置了max age ,max age 优先。
expires = HttpDate.parse(value);
} else if ("Last-Modified".equalsIgnoreCase(fieldName)) {
//响应的最后修改时间。
lastModified = HttpDate.parse(value);
lastModifiedString = value;
} else if ("ETag".equalsIgnoreCase(fieldName)) {
etag = value;
} else if ("Age".equalsIgnoreCase(fieldName)) {
ageSeconds = HttpHeaders.parseSeconds(value, -1);
}
}
}
}
CacheStrategy.Factory的get方法。可以跳过,涉及到http一些知识,后面再复习研究。
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;
}
可以跳过,涉及到http一些知识,后面再复习研究。
private CacheStrategy getCandidate() {
// 没有缓存的响应
if (cacheResponse == null) {
return new CacheStrategy(request, null);
}
// 如果缺少所需的握手,请删除缓存的响应。
if (request.isHttps() && cacheResponse.handshake() == null) {
return new CacheStrategy(request, null);
}
//兜底策略,防止错误的存储了不应该被缓存的响应。
//如果不应该存储此响应,则永远不要将其用作响应源。只要持久性存储行为良好且规则不变,该检查就应该是多余的。
if (!isCacheable(cacheResponse, request)) {
return new CacheStrategy(request, null);
}
CacheControl requestCaching = request.cacheControl();
if (requestCaching.noCache() || hasConditions(request)) {
return new CacheStrategy(request, null);
}
CacheControl responseCaching = cacheResponse.cacheControl();
if (responseCaching.immutable()) {
//存在缓存的响应且响应是不可变的
return new CacheStrategy(null, cacheResponse);
}
//缓存响应的年龄
long ageMillis = cacheResponseAge();
//缓存还是新鲜的时间
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());
}
if (!responseCaching.noCache() && ageMillis + minFreshMillis < freshMillis + maxStaleMillis) {
//这里返回的响应应该是可用的。
Response.Builder builder = cacheResponse.newBuilder();
if (ageMillis + minFreshMillis >= freshMillis) {
//添加响应不新鲜的head
builder.addHeader("Warning", "110 HttpURLConnection \"Response is stale\"");
}
long oneDayMillis = 24 * 60 * 60 * 1000L;
if (ageMillis > oneDayMillis && isFreshnessLifetimeHeuristic()) {
//添加响应快要过期的head
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);
}
-------------------------- 跳过结束 --------------------------
继续CacheInterceptor的intercept方法
注释3处,CacheStrategy返回的networkRequest和cacheResponse。总之,返回的策略一共有四种。
- 如果CacheStrategy的networkRequest, cacheResponse都为null,则不使用网络策略,也不使用缓存策略。
- 如果CacheStrategy的networkRequest不为null, cacheResponse为null,使用网络策略。
- 如果CacheStrategy的networkRequest为null, cacheResponse不为null,使用缓存策略
- 如果CacheStrategy的networkRequest, cacheResponse都不为null,使用有条件的网络策略,如果请求条件验证通过的话,服务器只会发送响应头,用来更新缓存的响应head信息,缓存的响应的body还可以继续使用。
释4处,cacheResponse为null,说明缓存cacheCandidate不满足缓存策略,不可用。
注释5处,如果networkRequest为null,禁止发起网络请求,并且缓存不满足,失败。
注释6处,缓存可用。返回缓存。
注释7处,缓存不满足,交给后面的拦截器发起网络请求。
注释8处,condition get的情况。如果返回304,说明服务端的内容未修改,我们只需要更新本地缓存的头部信息和时间信息即可。
注释9处,更新缓存。
注释10处,添加缓存。
注释11处,缓存不合法了,移除缓存。
接下来,我们看一下okhttp3.Cache是怎么工作的。
Cache的构造函数创建了DiskLruCache
对象。
final DiskLruCache cache;
public Cache(File directory, long maxSize) {
this(directory, maxSize, FileSystem.SYSTEM);
}
Cache(File directory, long maxSize, FileSystem fileSystem) {
this.cache = DiskLruCache.create(fileSystem, directory, VERSION, ENTRY_COUNT, maxSize);
}
Cache的put方法。
@Nullable
CacheRequest put(Response response) {
String requestMethod = response.request().method();
//不合法的http方法 POST,PATCH,PUT,DELETE,MOVE,直接移除缓存。
if (HttpMethod.invalidatesCache(response.request().method())) {
try {
remove(response.request());
} catch (IOException ignored) {
// The cache cannot be written.
}
return null;
}
//只缓存GET方法的响应。
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;
}
//不能缓存的响应
if (HttpHeaders.hasVaryAll(response)) {
return null;
}
//构建一个 Cache.Entry 对象
Cache.Entry entry = new Cache.Entry(response);
DiskLruCache.Editor editor = null;
try {
editor = cache.edit(key(response.request().url()));
if (editor == null) {
return null;
}
//写入缓存
entry.writeTo(editor);
return new Cache.CacheRequestImpl(editor);
} catch (IOException e) {
abortQuietly(editor);
return null;
}
}
Cache的update方法。
void update(Response cached, Response network) {
Entry entry = new Entry(network);
DiskLruCache.Snapshot snapshot = ((CacheResponseBody) cached.body()).snapshot;
DiskLruCache.Editor editor = null;
try {
editor = snapshot.edit(); // Returns null if snapshot is not current.
if (editor != null) {
//更新缓存。
entry.writeTo(editor);
//commit的时候,会整理DiskLruCache的大小
editor.commit();
}
} catch (IOException e) {
abortQuietly(editor);
}
}
Cache的remove方法。
void remove(Request request) throws IOException {
cache.remove(key(request.url()));
}
DiskLruCache 会在一些合适的时机,比如插入提交,更新提交,删除的时候会整理其使用的文件大小。
private final Runnable cleanupRunnable = new Runnable() {
public void run() {
//注意,这里加锁了。
synchronized (DiskLruCache.this) {
if (!initialized | closed) {
return; // Nothing to do
}
try {
//整理大小
trimToSize();
} catch (IOException ignored) {
mostRecentTrimFailed = true;
}
try {
if (journalRebuildRequired()) {
rebuildJournal();
redundantOpCount = 0;
}
} catch (IOException e) {
mostRecentRebuildFailed = true;
journalWriter = Okio.buffer(Okio.blackhole());
}
}
}
};
void trimToSize() throws IOException {
while (size > maxSize) {
Entry toEvict = lruEntries.values().iterator().next();
removeEntry(toEvict);
}
mostRecentTrimFailed = false;
}
就是循环移除Entry,直到满足大小限制。
总结:自己并没有去专门学习过HTTP方面的知识,所以有很多东西没法去更细的了解。等自己学习完HTTP相关的知识,在回来查漏补缺。DiskLruCache后面会单独写一篇文章。
- okhttp源码分析(三)-CacheInterceptor过滤器
- github-okhttp