okhttp——2.拦截器原理
强烈建议阅读：
责任链模式
okhttp 基本使用及概要（todo:）
官网关于拦截器说明
(说明了普通拦截器和网络拦截器的使用及区别)
拦截器介绍
从下面的图看出，其实okhttp的核心就是围绕拦截器展开的。
拦截器的工作原理
打开Okhttp的源码，发出具体请求的地方为RealCall的execute方法
@Override public Response execute() throws IOException {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
captureCallStackTrace();
try {
client.dispatcher().executed(this);
// 这行代码是核心，用于获取返回体
Response result = getResponseWithInterceptorChain();
if (result == null) throw new IOException("Canceled");
return result;
} finally {
client.dispatcher().finished(this);
}
}
getResponseWithInterceptorChain方法就是一种责任链的处理模式，具体代码如下：
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.(构造拦截器列表)
List<Interceptor> interceptors = new ArrayList<>();
interceptors.addAll(client.interceptors());
interceptors.add(retryAndFollowUpInterceptor);
interceptors.add(new BridgeInterceptor(client.cookieJar()));
interceptors.add(new CacheInterceptor(client.internalCache()));
interceptors.add(new ConnectInterceptor(client));
if (!forWebSocket) {
interceptors.addAll(client.networkInterceptors());
}
interceptors.add(new CallServerInterceptor(forWebSocket));
// 构造责任链
Interceptor.Chain chain = new RealInterceptorChain(
interceptors, null, null, null, 0, originalRequest);
// 进行链式处理
return chain.proceed(originalRequest);
}
0. 拦截器接口
public interface Interceptor {
//当前拦截器对一次请求的处理，也是拦截器的核心接口
Response intercept(Chain chain) throws IOException;
// 拦截器链 接口
interface Chain {
Request request();
// 处理一次请求
Response proceed(Request request) throws IOException;
/**
* Returns the connection the request will be executed on. This is only available in the chains
* of network interceptors; for application interceptors this is always null.
*/
@Nullable Connection connection();
}
}
1. 拦截器——RetryAndFollowUpInterceptor
private static final int MAX_FOLLOW_UPS = 20;
@Override public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
streamAllocation = new StreamAllocation(
client.connectionPool(), createAddress(request.url()), callStackTrace);
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;
} finally {
// We're throwing an unchecked exception. Release any resources.
if (releaseConnection) {
streamAllocation.streamFailed(null);
streamAllocation.release();
}
}
if (++followUpCount > MAX_FOLLOW_UPS) {
streamAllocation.release();
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
}
}
1.创建streamAllocation对象（里面包含我们所有对http请求对组件）
2.调用(RealInterceptorChain)proceed进行网络请求
3.根据异常结果或者响应结果判断是否需要重新请求
4.调用下一个拦截器，对response进行处理，返回给上一个拦截器
2 拦截器——BridgeInterceptor
1.将用户构建Request请求转化为能够进行网络访问的请求
2.将这个符合网络请求的Request进行网络请求
3.将网络请求回来的响应Respond转化为用户可用的response（包括gzip解压）
/**
* Bridges from application code to network code. First it builds a network request from a user
* request. Then it proceeds to call the network. Finally it builds a user response from the network
* response.
*/
public final class BridgeInterceptor implements Interceptor {
private final CookieJar cookieJar;
public BridgeInterceptor(CookieJar cookieJar) {
this.cookieJar = cookieJar;
}
@Override public Response intercept(Chain chain) throws IOException {
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();
// 请求体的构造及header相关参数的构造
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");
}
// If we add an "Accept-Encoding: gzip" header field we're responsible for also decompressing
// the transfer stream.
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());
}
// 调用责任链的下一个拦截器
Response networkResponse = chain.proceed(requestBuilder.build());
HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
Response.Builder responseBuilder = networkResponse.newBuilder()
.request(userRequest);
// 对返回体的处理： 透明的gzip传输
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();
}
/** Returns a 'Cookie' HTTP request header with all cookies, like {@code a=b; c=d}. */
private String cookieHeader(List<Cookie> cookies) {
StringBuilder cookieHeader = new StringBuilder();
for (int i = 0, size = cookies.size(); i < size; i++) {
if (i > 0) {
cookieHeader.append("; ");
}
Cookie cookie = cookies.get(i);
cookieHeader.append(cookie.name()).append('=').append(cookie.value());
}
return cookieHeader.toString();
}
}
3 拦截器——CacheInterceptor
请求结构缓存的使用：
//okhttp的缓存使用非常简单，只需要OkHttpClient创建的时候设置cache就可以了
new OkHttpClient().newBuilder().cache(new Cache(new File("cache"),
1024*1024*10)).connectTimeout(5,TimeUnit.SECONDS).build();
核心代码如下：
@Override public Response intercept(Chain chain) throws IOException {
1 Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
2 CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {
3 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.
4 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.
5 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());
}
}
// If we have a cache response too, then we're doing a conditional get.
6 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();
7 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;
}
1.cache不为空苁中获取cacheCandidate
2.通过FacheStrategy.Factory获取到缓存策略，获取网络请求与响应缓存
3.调用cache的同步方法trackResponse，保证请求一致性
4. 没网和没缓存的请求下直接返回一个504错误响应
具体的是使用缓存还是进行网络请求可参考CacheStrategy中相关代码，主要是一些条件判断。
4 拦截器——ConnectInterceptor
获取一个RealConnection对象，然后创建Socket链接，并且调用下一个也是最后一个拦截器来完成Okhttp的整个操作。
/** Opens a connection to the target server and proceeds to the next interceptor. */
public final class ConnectInterceptor implements Interceptor {
public final OkHttpClient client;
public ConnectInterceptor(OkHttpClient client) {
this.client = client;
}
@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");
// newStream中会从连接池中获取RealConnection/创建一个RealConnection并进行连接。
HttpCodec httpCodec = streamAllocation.newStream(client, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();
return realChain.proceed(request, streamAllocation, httpCodec, connection);
}
}
5 拦截器——CallServerInterceptor
1.发起真正的网络请求
2.介绍服务器返回的响应
public final class CallServerInterceptor implements Interceptor {
private final boolean forWebSocket;
public CallServerInterceptor(boolean forWebSocket) {
this.forWebSocket = forWebSocket;
}
@Override public Response intercept(Chain chain) throws IOException {
1 HttpCodec httpCodec = ((RealInterceptorChain) chain).httpStream();
StreamAllocation streamAllocation = ((RealInterceptorChain) chain).streamAllocation();
Request request = chain.request();
long sentRequestMillis = System.currentTimeMillis();
2 httpCodec.writeRequestHeaders(request);
3 if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());
BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
request.body().writeTo(bufferedRequestBody);
bufferedRequestBody.close();
}
4 httpCodec.finishRequest();
Response response = httpCodec.readResponseHeaders()
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
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 {
5 response = response.newBuilder()
.body(httpCodec.openResponseBody(response))
.build();
}
6 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;
}
}
1.HttpCodec(http中所有流对象都封装到了HttpCodec实现类中),可以简单的理解为它能编码request和解码response
2.可以看到证实了上面说的HttpCodec写入了请求的头部信息
3.想socket写入请求体
4.表明完成了http请求request的写入工作
5.读取响应的body信息
6.关闭连接
参考：
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