OkHttp3源码（九） ------ 拦截器

拦截器Interceptor的设计思想——创建拦截器链（后面简称链），执行指定拦截器的方法，根据旧链创建新链，再次执行上述过程，直至全部拦截器执行完毕。
拦截器方法(intercept(…))的主要实现：
(1)、对发起的request进行处理。
(2)、调用下一个拦截器，获取response。
(3)、对上一个拦截器返回的response进行处理，最后返回给上个拦截器。
.
OkHttp3执行网络请求及接受响应的过程中要经过
普通拦截器>RetryAndFollowUpInterceptor>BridgeInterceptor
.>CacheInterceptor>ConnectInterceptor>网络拦截器>CallServerInterceptor

接下来我们会按照如下顺序讲解拦截器的具体实现。
（1）、RetryAndFollowUpInterceptor
（2）、BridgeInterceptor
（3）、CacheInterceptor
（4）、ConnectInterceptor
（5）、CallServerInterceptor

(1)、RetryAndFollowUpInterceptor的intercept(…)

@Override public Response intercept(Chain chain) throws IOException {
Request request = chain.request();

streamAllocation = new StreamAllocation(
client.connectionPool(), createAddress(request.url()), callStackTrace);

int followUpCount = 0;
Response priorResponse = null;
while (true) {
if (canceled) {
streamAllocation.release();
throw new IOException("Canceled");
}

Response response = null;
boolean releaseConnection = true;
try {
response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);
releaseConnection = false;
} catch (RouteException e) {
releaseConnection = false;
continue;
} catch (IOException e) {
releaseConnection = false;
continue;
} finally {
if (releaseConnection) {
streamAllocation.streamFailed(null);
streamAllocation.release();
}
}
if (priorResponse != null) {
response = response.newBuilder()
.priorResponse(priorResponse.newBuilder()
.body(null)
.build())
.build();
}

Request followUp = followUpRequest(response);

if (followUp == null) {
if (!forWebSocket) {
streamAllocation.release();
}
return response;
}

closeQuietly(response.body());

if (++followUpCount > MAX_FOLLOW_UPS) {
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}

if (followUp.body() instanceof UnrepeatableRequestBody) {
streamAllocation.release();
throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
}

if (!sameConnection(response, followUp.url())) {
streamAllocation.release();
streamAllocation = new StreamAllocation(
client.connectionPool(), createAddress(followUp.url()), callStackTrace);
} else if (streamAllocation.codec() != null) {
throw new IllegalStateException("Closing the body of " + response
+ " didn't close its backing stream. Bad interceptor?");
}

request = followUp;
priorResponse = response;
}
}

主要做了4件事：
1、为连接流配置连接池，以便后面为连接流“找”连接时提供“地方”。
2、将连接流交由下一个拦截器处理。
3、判断是否重连接。
4、返回响应报文。

（2）、BridgeInterceptor的intercept(……)

（2）、BridgeInterceptor的intercept(......)
@Override public Response intercept(Chain chain) throws IOException {
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();

RequestBody body = userRequest.body();
if (body != null) {
MediaType contentType = body.contentType();
if (contentType != null) {
requestBuilder.header("Content-Type", contentType.toString());
}

long contentLength = body.contentLength();
if (contentLength != -1) {
requestBuilder.header("Content-Length", Long.toString(contentLength));
requestBuilder.removeHeader("Transfer-Encoding");
} else {
requestBuilder.header("Transfer-Encoding", "chunked");
requestBuilder.removeHeader("Content-Length");
}
}

if (userRequest.header("Host") == null) {
requestBuilder.header("Host", hostHeader(userRequest.url(), false));
}

if (userRequest.header("Connection") == null) {
requestBuilder.header("Connection", "Keep-Alive");
}

boolean transparentGzip = false;
if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
transparentGzip = true;
requestBuilder.header("Accept-Encoding", "gzip");
}

List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
if (!cookies.isEmpty()) {
requestBuilder.header("Cookie", cookieHeader(cookies));
}

if (userRequest.header("User-Agent") == null) {
requestBuilder.header("User-Agent", Version.userAgent());
}

Response networkResponse = chain.proceed(requestBuilder.build());

HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

Response.Builder responseBuilder = networkResponse.newBuilder()
.request(userRequest);

if (transparentGzip
&& "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))
&& HttpHeaders.hasBody(networkResponse)) {
GzipSource responseBody = new GzipSource(networkResponse.body().source());
Headers strippedHeaders = networkResponse.headers().newBuilder()
.removeAll("Content-Encoding")
.removeAll("Content-Length")
.build();
responseBuilder.headers(strippedHeaders);
responseBuilder.body(new RealResponseBody(strippedHeaders, Okio.buffer(responseBody)));
}

return responseBuilder.build();
}

主要做了2件事：
1、配置请求头部信息
2、如果响应报文中有Cookie则保存，
然后判断是否是“GIZP”编码类型，如果是则配置响应报文的首部，否则不配置，最后返回响应报文

（3）、CacheInterceptor的intercept(……)

@Override public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;

long now = System.currentTimeMillis();

CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;

if (cache != null) {
cache.trackResponse(strategy);
}
//缓存策略（1）
if (cacheCandidate != null && cacheResponse == null) {
closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
}

//缓存策略（2）
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();
}

// 缓存策略（3）
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());
}
}

//缓存策略（4）
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()).
cache.trackConditionalCacheHit();
cache.update(cacheResponse, response);
return response;
} else {
closeQuietly(cacheResponse.body());
}
}

Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();

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;
}

主要做了2件事：设置缓存怎么处理和没有设置缓存怎么处理
1、设置缓存怎么处理
根据请求拿到缓存响应。
依据缓存响应和请求“找到”缓存策略。
缓存策略有：
（1）有该请求的缓存响应，但是该缓存响应没有完成TTL或响应缓存禁止使用缓存或请求禁止使用缓存或缓存响应已过期
（2）请求只允许从缓存中取响应报文。
（3）有该请求的缓存响应，且该请求允许从缓存从获取，那么直接使用缓存响应报文。
（4）有该请求的缓存响应，但是使用缓存报文前需要先去服务器验证其有效性。
最后根据请求和响应的头部信息判断是否将响应存储到缓存中
2、没有设置缓存怎么处理
直接从服务器获取响应并返回。

OkHttp3用的是硬盘缓存DiskLruCache
硬盘缓存的日志文件：
libcore.io.DiskLruCache
1
100
2
.
CLEAN 3400330d1dfc7f3f7f4b8d4d803dfcf6 832 21054
DIRTY 335c4c6028171cfddfbaae1a9c313c52
CLEAN 335c4c6028171cfddfbaae1a9c313c52 3934 2342
REMOVE 335c4c6028171cfddfbaae1a9c313c52
DIRTY 1ab96a171faeeee38496d8b330771a7a
CLEAN 1ab96a171faeeee38496d8b330771a7a 1600 234
READ 335c4c6028171cfddfbaae1a9c313c52
READ 3400330d1dfc7f3f7f4b8d4d803dfcf6
.
前5行固定不变，分别为：常量：libcore.io.DiskLruCache；diskCache版本；应用程序版本；valueCount(一次缓存操作对应几个文件)，空行
接下来每一行对应一个cache entry的一次状态记录，其格式为：[状态（DIRTY,CLEAN,READ,REMOVE），key，状态相关value(可选)]:
- DIRTY:表明一个cache entry正在被创建或更新，每一个成功的DIRTY记录都应该对应一个CLEAN或REMOVE操作。如果一个DIRTY缺少预期匹配的CLEAN/REMOVE，则对应entry操作失败，需要将其从lruEntries中删除
- CLEAN:说明cache已经被成功操作，当前可以被正常读取。每一个CLEAN行还需要记录其每一个value的长度
- READ: 记录一次cache读取操作
- REMOVE:记录一次cache清除
.
OkHttp3的DiskLruCache主要有以下几个特点：
（1）通过LinkedHashMap实现LRU算法。
（2）通过本地维护Cache操作日志保证Cache原子性与可用性，同时为防止日志过分膨胀定时执行日志精简。
（3）每个Cache通过两种状态（DIRTY、ClEAN）形成四个文件，DIRTY标识的文件表示该文件不可读能写；CLEAN标识的文件表示该文件可读不可写。
每个状态对应两个文件，一个文件存储Cache meta数据，一个文件存储Cache内容数据。

（4）、ConnectInterceptor的intercept(……)

@Override public Response intercept(Chain chain) throws IOException {
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Request request = realChain.request();
StreamAllocation streamAllocation = realChain.streamAllocation();

// We need the network to satisfy this request. Possibly for validating a conditional GET.
boolean doExtensiveHealthChecks = !request.method().equals("GET");
HttpCodec httpCodec = streamAllocation.newStream(client, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();

return realChain.proceed(request, streamAllocation, httpCodec, connection);
}

主要做了2件事：
1、为这个连接流在连接池找到连接，
如果没有则为连接流创建新的连接并放入连接池
（注意：连接池中的连接都是已经和服务器连接好的，只是没有数据传递）
2、通过连接获得和服务器的IO流，并封装为HttpCodec。

（5）、CallServerInterceptor的intercept(……)

@Override public Response intercept(Chain chain) throws IOException {
RealInterceptorChain realChain = (RealInterceptorChain) chain;
HttpCodec httpCodec = realChain.httpStream();
StreamAllocation streamAllocation = realChain.streamAllocation();
RealConnection connection = (RealConnection) realChain.connection();
Request request = realChain.request();

long sentRequestMillis = System.currentTimeMillis();
//将请求头写入缓冲区
httpCodec.writeRequestHeaders(request);

Response.Builder responseBuilder = null;
//只要方法不是GET并响应体不为空就写入请求体到缓冲区
if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
httpCodec.flushRequest();
responseBuilder = httpCodec.readResponseHeaders(true);
}
//将请求正文写入缓存区。
if (responseBuilder == null) {
Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());
BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
request.body().writeTo(bufferedRequestBody);
bufferedRequestBody.close();
} else if (!connection.isMultiplexed()) {
streamAllocation.noNewStreams();
}
}
//释放写入流缓存区中的数据
httpCodec.finishRequest();

//读取响应头
if (responseBuilder == null) {
responseBuilder = httpCodec.readResponseHeaders(false);
}
//配置响应报文
Response response = responseBuilder
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
//配置响应报文的响应体
int code = response.code();
if (forWebSocket && code == 101) {
// Connection is upgrading, but we need to ensure interceptors see a non-null response body.
response = response.newBuilder()
.body(Util.EMPTY_RESPONSE)
.build();
} else {
response = response.newBuilder()
.body(httpCodec.openResponseBody(response))
.build();
}

if ("close".equalsIgnoreCase(response.request().header("Connection"))
|| "close".equalsIgnoreCase(response.header("Connection"))) {
streamAllocation.noNewStreams();
}

if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
throw new ProtocolException(
"HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
}
return response;
}

主要做了2件事：
1、将请求报文写入服务器
2、将读取响应头并获得响应正文的读取流

到这里，已经将OkHttp3的源码部分分析完了。
总共九篇博客，这都是我之前写在笔记本中的，今天有时间把它们都整理出来。
对于OkHttp3的连接池技术,请参见OkHttp源码分析——多路复用

学习心得：观看资料 + 思考问题 + 实践验证 + 总结概括 + 整理记录 = 有思想的技术大牛。