概述
從OkHttp問世以來，度娘，google上關于OkHttp的講解說明數不勝數，各種解讀思想不盡相同，一千個讀者就有一千個哈默雷特。本篇文章從源碼出發向你介紹Okhttp的基本使用以及底層實現原理，讓你從會寫轉向會用，學習Android頂尖源碼的設計理念和開源擴展性，如果解讀有誤，還望提出探討糾正。
文章鏈接：
Android OkHttp源碼解析入門教程：同步和異步（一）
Android OkHttp源碼解析入門教程：攔截器和責任鏈（二）
前言
上一篇文章我們主要講解OkHttp的基本使用以及同步請求，異步請求的源碼分析，相信大家也對其內部的基本流程以及作用有了大致的了解，還記得上一篇我們提到過getResponseWithInterceptorChain責任鏈，那么這篇文章就帶你深入OkHttp內部5大攔截器(Interceptors)和責任鏈模式的源碼分析，滴滴滴。
正文
首先，我們要清楚OkHttp攔截器(Interceptors)是干什么用的，來看看官網的解釋
Interceptors are a powerful mechanism that can monitor, rewrite, and retry calls
攔截器是一種強大的機制,可以做網絡監視、重寫和重試調用
這句話怎么理解呢？簡單來說，就好比如你帶著盤纏上京趕考，遇到了五批賊寇，但其奇怪的是他們的目標有的是為財，有的是為色，各不相同；例子中的你就相當于一個正在執行任務的網絡請求，賊寇就是攔截器，它們的作用就是取走請求所攜帶的參數拿過去修改，判斷，校驗然后放行，其實際是實現了AOP（面向切面編程），關于AOP的了解，相信接觸過Spring框架的是最熟悉不過的。
我們再來看官方給出的攔截器圖
攔截器
攔截器有兩種：APP層面的攔截器（Application Interception）、網絡攔截器(Network Interception)，而我們這篇文章注重講解OkHttp core(系統內部五大攔截器）
那么OkHttp系統內部的攔截器有哪些呢？作用分別是干什么的？
緊接上篇文章，我們到getResponseWithInterceptorChain()源碼中分析，這些攔截器是怎么發揮各自作用的？
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
// 責任鏈
List<Interceptor> interceptors = new ArrayList<>();
// 添加自定義攔截器（Application Interception）
interceptors.addAll(client.interceptors());
// 添加 負責處理錯誤，失敗重試，重定向攔截器 RetryAndFollowUpInterceptor
interceptors.add(retryAndFollowUpInterceptor);
// 添加 負責補充用戶創建請求中缺少一些必須的請求頭以及壓縮處理的 BridgeInterceptor
interceptors.add(new BridgeInterceptor(client.cookieJar()));
// 添加 負責進行緩存處理的 CacheInterceptor
interceptors.add(new CacheInterceptor(client.internalCache()));
// 添加 負責與服務器建立鏈接的 ConnectInterceptor
interceptors.add(new ConnectInterceptor(client));
if (!forWebSocket) {
// 添加網絡攔截器(Network Interception)
interceptors.addAll(client.networkInterceptors());
}
//添加 負責向服務器發送請求數據、從服務器讀取響應數據的 CallServerInterceptor
interceptors.add(new CallServerInterceptor(forWebSocket));
// 將interceptors集合以及相應參數傳到RealInterceptorChain構造方法中完成責任鏈的創建
Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
originalRequest, this, eventListener, client.connectTimeoutMillis(),
client.readTimeoutMillis(), client.writeTimeoutMillis());
// 調用責任鏈的執行
return chain.proceed(originalRequest);
}
從這里可以看出，getResponseWithInterceptorChain方法首先創建了一系列攔截器，并整合到一個Interceptor集合當中，同時每個攔截器各自負責不同的部分，處理不同的功能，然后把集合加入到RealInterceptorChain構造方法中完成攔截器鏈的創建，而責任鏈模式就是管理多個攔截器鏈。
關于責任鏈模式的理解，簡單來說，其實日常的開發代碼中隨處可見
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
if (resultCode == RESULT_OK) {
switch (requestCode) {
case 1:
System.out.println("我是第一個攔截器: " + requestCode);
break;
case 2:
System.out.println("我是第二個攔截器: " + requestCode);
break;
case 3:
System.out.println("我是第三個攔截器: " + requestCode);
break;
default:
break;
}
}
}
相信當你看到這段代碼就明白責任鏈模式是干什么的吧，根據requestCode的狀態處理不同的業務，不屬于自己的任務傳遞給下一個，如果還不明白，你肯定是個假的程序員，當然這是個非常簡化的責任鏈模式，很多定義都有錯誤，這里只是給大家做個簡單介紹下責任鏈模式的流程是怎樣的，要想深入并應用到實際開發中，還需要看相關文檔，你懂我意思吧。
我們直奔主題，上文也說到，getResponseWithInterceptorChain最終調用的RealInterceptorChain的proceed方法，我們接著從源碼出發
public interface Interceptor {
// 每個攔截器會根據Chain攔截器鏈觸發對下一個攔截器的調用，直到最后一個攔截器不觸發
// 當攔截器鏈中所有的攔截器被依次執行完成后，就會將每次生成后的結果進行組裝
Response intercept(Chain chain) throws IOException;
interface Chain {
// 返回請求
Request request();
// 處理請求
Response proceed(Request request) throws IOException;
}
}
public final class RealInterceptorChain implements Interceptor.Chain {
private final List<Interceptor> interceptors;
private final StreamAllocation streamAllocation;
private final HttpCodec httpCodec;
private final RealConnection connection;
private final int index;
private final Request request;
private final Call call;
private final EventListener eventListener;
private final int connectTimeout;
private final int readTimeout;
private final int writeTimeout;
private int calls;
public RealInterceptorChain(List<Interceptor> interceptors, StreamAllocation streamAllocation,
HttpCodec httpCodec, RealConnection connection, int index, Request request, Call call,
EventListener eventListener, int connectTimeout, int readTimeout, int writeTimeout) {
this.interceptors = interceptors;
this.connection = connection;
this.streamAllocation = streamAllocation;
this.httpCodec = httpCodec;
this.index = index;
this.request = request;
this.call = call;
this.eventListener = eventListener;
this.connectTimeout = connectTimeout;
this.readTimeout = readTimeout;
this.writeTimeout = writeTimeout;
}
@Override public Response proceed(Request request) throws IOException {
return proceed(request, streamAllocation, httpCodec, connection);
}
}
上述代碼中可以看出Interceptor是個接口類，RealInterceptorChain實現了Interceptor.chain方法，在這里我們也看到了之前我們初始化時傳進來的參數，可見我們實際調用的是RealInterceptorChain的proceed方法，接著往下看
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection connection) throws IOException {
if (index >= interceptors.size()) throw new AssertionError();
calls++;
// 去掉一些邏輯判斷處理，留下核心代碼
// Call the next interceptor in the chain.（調用鏈中的下一個攔截器）
// 創建下一個攔截器鏈，index+1表示如果要繼續訪問攔截器鏈中的攔截器，只能從下一個攔截器訪問，而不能從當前攔截器開始
// 即根據index光標不斷創建新的攔截器鏈，新的攔截器鏈會比之前少一個攔截器，這樣就可以防止重復執行
RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
writeTimeout);
// 獲取當前攔截器
Interceptor interceptor = interceptors.get(index);
// 執行當前攔截器，并將下一個攔截器鏈傳入
Response response = interceptor.intercept(next);
return response;
}
看到這里你會覺得一頭霧水，簡單了解后，你的疑問可能如下
1.index+1的作用是干什么的？
2.當前攔截器是怎么執行的？
3.攔截器鏈是怎么依次調用執行的？
4.上面3點都理解，但每個攔截器返回的結果都不一樣，它是怎么返回給我一個最終結果？
首先，我們要清楚責任鏈模式的流程，如同上文所說的，書生攜帶書籍，軟銀，家眷上京趕考，途遇強盜，第一批強盜搶走了書籍并且放行，第二批強盜搶走了軟銀放行，一直到最后一無所有才會停止。
書生就是最開始的RealInterceptorChain，強盜就是Interceptor，，index+1的作用就是創建一個新的攔截器鏈，簡單來說，就是
書生（書籍，軟銀，家眷） →劫書籍強盜→書生（軟銀，家眷）→劫財強盜→書生（家眷）→...
即通過不斷創建新的RealInterceptorChain鏈輪循執行interceptors中的攔截器形成一個責任鏈（搶劫鏈）模式直到全部攔截器鏈中的攔截器處理完成后返回最終結果
// 獲取當前攔截器
Interceptor interceptor = interceptors.get(index);
index+1不斷執行，list.get(0),list.get(1),... 這樣就能取出interceptors中所有的攔截器，我們之前也說過Interceptor是一個接口，okhttp內部的攔截器都實現了這個接口處理各自的業務
如此，每當 Interceptor interceptor = interceptors.get(index)執行時，就可以根據interceptor.intercept(next)執行相應攔截器實現的intercept方法處理相應的業務，同時把創建好新的攔截器鏈傳進來，這樣就可以避免重復執行一個攔截器。
RetryAndFollowUpInterceptor（重定向攔截器）
負責處理錯誤，失敗重試，重定向
/**
* This interceptor recovers from failures and follows redirects as necessary. It may throw an
* {@link IOException} if the call was canceled.
*/
public final class RetryAndFollowUpInterceptor implements Interceptor {
/**
* How many redirects and auth challenges should we attempt? Chrome follows 21 redirects; Firefox,
* curl, and wget follow 20; Safari follows 16; and HTTP/1.0 recommends 5.
*/
//最大失敗重連次數：
private static final int MAX_FOLLOW_UPS = 20;
public RetryAndFollowUpInterceptor(OkHttpClient client, boolean forWebSocket) {
this.client = client;
this.forWebSocket = forWebSocket;
}
@Override public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
// 建立執行Http請求所需要的對象，，通過責任鏈模式不斷傳遞，直到在ConnectInterceptor中具體使用，
// 主要用于 ①獲取連接服務端的Connection ②連接用于服務端進行數據傳輸的輸入輸出流
// 1.全局的連接池，2.連接線路Address，3.堆棧對象
streamAllocation = new StreamAllocation(
client.connectionPool(), createAddress(request.url()), callStackTrace);
int followUpCount = 0;
Response priorResponse = null; // 最終response
while (true) {
if (canceled) {
streamAllocation.release();
throw new IOException("Canceled");
}
Response response = null;
boolean releaseConnection = true;
try {
// 執行下一個攔截器，即BridgeInterceptor
// 將初始化好的連接對象傳遞給下一個攔截器，通過proceed方法執行下一個攔截器鏈
// 這里返回的response是下一個攔截器處理返回的response，通過priorResponse不斷結合，最終成為返回給我們的結果
response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);
releaseConnection = false;
} catch (RouteException e) {
// The attempt to connect via a route failed. The request will not have been sent.
// 如果有異常，判斷是否要恢復
if (!recover(e.getLastConnectException(), false, request)) {
throw e.getLastConnectException();
}
releaseConnection = false;
continue;
} catch (IOException e) {
// An attempt to communicate with a server failed. The request may have been sent.
boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
if (!recover(e, requestSendStarted, request)) throw e;
releaseConnection = false;
continue;
} finally {
// We're throwing an unchecked exception. Release any resources.
if (releaseConnection) {
streamAllocation.streamFailed(null);
streamAllocation.release();
}
}
// Attach the prior response if it exists. Such responses never have a body.
if (priorResponse != null) {
response = response.newBuilder()
.priorResponse(priorResponse.newBuilder()
.body(null)
.build())
.build();
}
// 檢查是否符合要求
Request followUp = followUpRequest(response);
if (followUp == null) {
if (!forWebSocket) {
streamAllocation.release();
}
// 返回結果
return response;
}
//不符合，關閉響應流
closeQuietly(response.body());
// 是否超過最大限制
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());
}
// 是否有相同的連接
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;
}
}
在這里我們看到response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null)，很明顯proceed執行的就是我們傳進來的新攔截器鏈，從而形成責任鏈，這樣也就能明白攔截器鏈是如何依次執行的。
其實RetryAndFollowUpInterceptor 主要負責的是失敗重連，但是要注意的，不是所有的網絡請求失敗后都可以重連，所以RetryAndFollowUpInterceptor內部會幫我們進行檢測網絡請求異常和響應碼情況判斷，符合條件即可進行失敗重連。
StreamAllocation： 建立執行Http請求所需的對象，主要用于
1. 獲取連接服務端的Connection2.連接用于服務端進行數據傳輸的輸入輸出流
通過責任鏈模式不斷傳遞，直到在ConnectInterceptor中具體使用，
(1)全局的連接池，(2)連接線路Address，(3)堆棧對象
streamAllocation = new StreamAllocation( client.connectionPool(), createAddress(request.url()), callStackTrace);
這里的createAddress(request.url())是根據Url創建一個基于Okio的Socket連接的Address對象。狀態處理情況流程如下
1、首先執行whie(true)循環，如果取消網絡請求canceled，則釋放streamAllocation 資源同時拋出異常結束
2、執行下一個攔截器鏈，如果發生異常，走到catch里面，判斷是否恢復請求繼續執行，否則退出釋放
3、 如果priorResponse不為空，則結合當前返回Response和之前響應返回后的Response
（這就是為什么最后返回的是一個完整的Response）
4、調用followUpRequest查看響應是否需要重定向，如果不需要重定向則返回當前請求
5、followUpCount 重定向次數+1，同時判斷是否達到最大重定向次數。符合則釋放streamAllocation并拋出異常
6、sameConnection檢查是否有相同的鏈接，相同則StreamAllocation釋放并重建
7、重新設置request，并把當前的Response保存到priorResponse，繼續while循環
由此可以看出RetryAndFollowUpInterceptor主要執行流程:
1）創建StreamAllocation對象
2）調用RealInterceptorChain.proceed(...)進行網絡請求
3）根據異常結果或則響應結果判斷是否要進行重新請求
4）調用下一個攔截器，處理response并返回給上一個攔截器
BridgeInterceptor（橋接攔截器）
負責設置編碼方式，添加頭部，Keep－Alive 連接以及應用層和網絡層請求和響應類型之間的相互轉換
/**
* 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
//1、cookie的處理, 2、Gzip壓縮
public Response intercept(Interceptor.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");
}
// 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");
}
// 所以返回的cookies不能為空，否則這里會報空指針
List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
if (!cookies.isEmpty()) {
// 創建Okhpptclitent時候配置的cookieJar，
requestBuilder.header("Cookie", cookieHeader(cookies));
}
if (userRequest.header("User-Agent") == null) {
requestBuilder.header("User-Agent", Version.userAgent());
}
// 以上為請求前的頭處理
Response networkResponse = chain.proceed(requestBuilder.build());
// 以下是請求完成，拿到返回后的頭處理
// 響應header， 如果沒有自定義配置cookie不會解析
HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
Response.Builder responseBuilder = networkResponse.newBuilder()
.request(userRequest);
}
從這里可以看出，BridgeInterceptor在發送網絡請求之前所做的操作都是幫我們傳進來的普通Request添加必要的頭部信息Content-Type、Content-Length、Transfer-Encoding、Host、Connection（默認Keep-Alive）、Accept-Encoding、User-Agent，使之變成可以發送網絡請求的Request。
我們具體來看HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers())，調用Http頭部的receiveHeaders靜態方法將服務器響應回來的Response轉化為用戶響應可以使用的Response
public static void receiveHeaders(CookieJar cookieJar, HttpUrl url, Headers headers) {
// 無配置則不解析
if (cookieJar == CookieJar.NO_COOKIES) return;
// 遍歷Cookie解析
List<Cookie> cookies = Cookie.parseAll(url, headers);
if (cookies.isEmpty()) return;
// 然后保存，即自定義
cookieJar.saveFromResponse(url, cookies);
}
當我們自定義Cookie配置后，receiveHeaders方法就會幫我們解析Cookie并添加到header頭部中保存
// 前面解析完header后，判斷服務器是否支持Gzip壓縮格式，如果支持將交給Okio處理
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);
// 處理完成后，重新生成一個response
responseBuilder.body(new RealResponseBody(strippedHeaders, Okio.buffer(responseBody)));
}
return responseBuilder.build();
}
- transparentGzip判斷服務器是否支持Gzip壓縮
- 滿足判斷當前頭部Content-Encoding是否支持gzip
- 判斷Http頭部是否有body體
滿足上述條件，就將Response.body輸入流轉換成GzipSource類型，獲得解壓過后的數據流后，移除響應中的header Content-Encoding和Content-Length，構造新的響應返回。
由此可以看出BridgeInterceptor主要執行流程:
1）負責將用戶構建的Request請求轉化成能夠進行網絡訪問的請求
2）將符合條件的Request執行網絡請求
3）將網絡請求響應后的Response轉化（Gzip壓縮，Gzip解壓縮）為用戶可用的Response
CacheInterceptor（緩存攔截器）
負責進行緩存處理
/** Serves requests from the cache and writes responses to the cache. */
public final class CacheInterceptor implements Interceptor {
final InternalCache cache;
public CacheInterceptor(InternalCache cache) {
this.cache = cache;
}
@Override public Response intercept(Chain chain) throws IOException {
// 通過Request從緩存中獲取Response
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis(); // 獲取系統時間
// 緩存策略類，該類決定了是使用緩存還是進行網絡請求
// 根據請求頭獲取用戶指定的緩存策略，并根據緩存策略來獲取networkRequest，cacheResponse;
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
// 網絡請求，如果為null就代表不用進行網絡請求
Request networkRequest = strategy.networkRequest;
// 獲取CacheStrategy緩存中的Response，如果為null，則代表不使用緩存
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {//根據緩存策略，更新統計指標：請求次數、使用網絡請求次數、使用緩存次數
cache.trackResponse(strategy);
}
if (cacheCandidate != null && cacheResponse == null) {
//cacheResponse不讀緩存，那么cacheCandidate不可用，關閉它
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.
// 如果我們禁止使用網絡和緩存不足，則返回504。
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.
// 不使用網絡請求 且存在緩存 直接返回響應
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
如果開始之前，你對Http緩存協議還不太懂，建議先去了解在回來看講解會更好理解
上面的代碼可以看出，CacheInterceptor的主要作用就是負責緩存的管理，期間也涉及到對網絡狀態的判斷，更新緩存等，流程如下
1.首先根據Request中獲取緩存的Response來獲取緩存（Chain就是Interceptor接口類中的接口方法，主要處理請求和返回請求）
2.獲取當前時間戳，同時通過 CacheStrategy.Factory工廠類來獲取緩存策略
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;
CacheStrategy(Request networkRequest, Response cacheResponse) {
this.networkRequest = networkRequest;
this.cacheResponse = cacheResponse;
}
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() {
//如果緩存沒有命中(即null),網絡請求也不需要加緩存Header了
if (cacheResponse == null) {
//沒有緩存的網絡請求,查上文的表可知是直接訪問
return new CacheStrategy(request, null);
}
// Drop the cached response if it's missing a required handshake.
// 如果緩存的TLS握手信息丟失,返回進行直接連接
if (request.isHttps() && cacheResponse.handshake() == null) {
return new CacheStrategy(request, null);
}
//檢測response的狀態碼,Expired時間,是否有no-cache標簽
if (!isCacheable(cacheResponse, request)) {
return new CacheStrategy(request, null);
}
CacheControl requestCaching = request.cacheControl();
// 如果請求指定不使用緩存響應并且當前request是可選擇的get請求
if (requestCaching.noCache() || hasConditions(request)) {
// 重新請求
return new CacheStrategy(request, null);
}
CacheControl responseCaching = cacheResponse.cacheControl();
// 如果緩存的response中的immutable標志位為true，則不請求網絡
if (responseCaching.immutable()) {
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());
}
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());
}
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緩存策略維護兩個變量networkRequest和cacheResponse，其內部工廠類Factory中的getCandidate方法會通過相應的邏輯判斷對比選擇最好的策略，如果返回networkRequest為null，則表示不進行網絡請求；而如果返回cacheResponse為null，則表示沒有有效的緩存。
3.緊接上文，判斷緩存是否為空，則調用trackResponse（如果有緩存，根據緩存策略，更新統計指標：請求次數、使用網絡請求次數、使用緩存次數）
4.緩存無效，則關閉緩存
5.如果networkRequest和cacheResponse都為null，則表示不請求網絡而緩存又為null，那就返回504，請求失敗
6.如果當前返回網絡請求為空，且存在緩存，直接返回響應
// 緊接上文
Response networkResponse = null;
try {
//執行下一個攔截器
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.
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;
}
下部分代碼主要做了這幾件事：
1）執行下一個攔截器，即ConnectInterceptor
2）責任鏈執行完畢后，會返回最終響應數據，如果返回結果為空，即無網絡情況下，關閉緩存
3）如果cacheResponse緩存不為空并且最終響應數據的返回碼為304，那么就直接從緩存中讀取數據，否則關閉緩存
4）有網絡狀態下，直接返回最終響應數據
5）如果Http頭部是否有響應體且緩存策略是可以緩存的，true=將響應體寫入到Cache，下次直接調用
6）判斷最終響應數據的是否是無效緩存方法，true,則從Cache清除掉
7）返回Response
ConnectInterceptor
負責與服務器建立鏈接
/** 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();
// 建立執行Http請求所需要的對象
// 主要用于 ①獲取連接服務端的Connection ②連接用于服務端進行數據傳輸的輸入輸出流
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, chain, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();
return realChain.proceed(request, streamAllocation, httpCodec, connection);
}
}
上面我們講到重定向攔截器時，發現RetryAndFollowUpInterceptor創建初始化StreamAllocation，但沒有使用，只是跟著攔截器鏈傳遞給下一個攔截器，最終會傳到ConnectInterceptor中使用。從上面代碼可以看出ConnectInterceptor主要執行的流程：
1.ConnectInterceptor獲取Interceptor傳過來的StreamAllocation，streamAllocation.newStream。
2.將剛才創建的用于網絡IO的RealConnection對象以及對于與服務器交互最為關鍵的HttpCodec等對象傳遞給后面的攔截。
ConnectInterceptor的Interceptor代碼很簡單，但是關鍵代碼還是在streamAllocation.newStream(…)方法里，在這個方法完成所有鏈接的建立。
public HttpCodec newStream(
OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
int connectTimeout = chain.connectTimeoutMillis();
int readTimeout = chain.readTimeoutMillis();
int writeTimeout = chain.writeTimeoutMillis();
int pingIntervalMillis = client.pingIntervalMillis();
boolean connectionRetryEnabled = client.retryOnConnectionFailure();
try {
RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);
HttpCodec resultCodec = resultConnection.newCodec(client, chain, this);
synchronized (connectionPool) {
codec = resultCodec;
return resultCodec;
}
} catch (IOException e) {
throw new RouteException(e);
}
}
這里的流程主要就是
1.調用findHealthyConnection來創建RealConnection進行實際的網絡連接（能復用就復用，不能復用就新建）
2.通過獲取到的RealConnection來創建HttpCodec對象并通過同步代碼中返回
我們具體往findHealthyConnection看
/**
* Finds a connection and returns it if it is healthy. If it is unhealthy the process is repeated
* until a healthy connection is found.
* 找到一個健康的連接并返回它。 如果不健康，則重復該過程直到發現健康的連接。
*/
private RealConnection findHealthyConnection(int connectTimeout, int readTimeout,
int writeTimeout, boolean connectionRetryEnabled, boolean doExtensiveHealthChecks)
throws IOException {
while (true) {
RealConnection candidate = findConnection(connectTimeout, readTimeout, writeTimeout,
connectionRetryEnabled);
// If this is a brand new connection, we can skip the extensive health checks.
// 如果這是一個全新的聯系，我們可以跳過廣泛的健康檢查
synchronized (connectionPool) {
if (candidate.successCount == 0) {
return candidate;
}
}
// Do a (potentially slow) check to confirm that the pooled connection is still good. If it
// isn't, take it out of the pool and start again.
// 做一個（潛在的慢）檢查，以確認匯集的連接是否仍然良好。 如果不是，請將其從池中取出并重新開始。
if (!candidate.isHealthy(doExtensiveHealthChecks)) {
noNewStreams();
continue;
}
return candidate;
}
}
1.首先開啟while(true){}循環，循環不斷地從findConnection方法中獲取Connection對象
2.然后在同步代碼塊，如果滿足candidate.successCount ==0(就是整個網絡連接結束了)，就返回
3.接著判斷如果這個candidate是不健康的，則執行銷毀并且重新調用findConnection獲取Connection對象
不健康的RealConnection條件為如下幾種情況：
①RealConnection對象的socket沒有關閉 ②socket的輸入流沒有關閉 ③socket的輸出流沒有關閉 ④http2時連接沒有關閉
我們接著看findConnection中具體做了什么操作
/**
* Returns a connection to host a new stream. This prefers the existing connection if it exists,
* then the pool, finally building a new connection.
*/
private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
boolean connectionRetryEnabled) throws IOException {
boolean foundPooledConnection = false;
RealConnection result = null;
Route selectedRoute = null;
Connection releasedConnection;
Socket toClose;
synchronized (connectionPool) {
if (released) throw new IllegalStateException("released");
if (codec != null) throw new IllegalStateException("codec != null");
if (canceled) throw new IOException("Canceled");
// Attempt to use an already-allocated connection. We need to be careful here because our
// already-allocated connection may have been restricted from creating new streams.
//選擇嘗試復用Connection
releasedConnection = this.connection;
toClose = releaseIfNoNewStreams();
//判斷可復用的Connection是否為空
if (this.connection != null) {
// We had an already-allocated connection and it's good.
result = this.connection;
releasedConnection = null;
}
if (!reportedAcquired) {
// If the connection was never reported acquired, don't report it as released!
// 如果連接從未被報告獲得，不要報告它被釋放
releasedConnection = null;
}
//如果RealConnection不能復用，就從連接池中獲取
if (result == null) {
// Attempt to get a connection from the pool.
Internal.instance.get(connectionPool, address, this, null);
if (connection != null) {
foundPooledConnection = true;
result = connection;
} else {
selectedRoute = route;
}
}
}
closeQuietly(toClose);
if (releasedConnection != null) {
eventListener.connectionReleased(call, releasedConnection);
}
if (foundPooledConnection) {
eventListener.connectionAcquired(call, result);
}
if (result != null) {
// If we found an already-allocated or pooled connection, we're done.
return result;
}
// If we need a route selection, make one. This is a blocking operation.
boolean newRouteSelection = false;
if (selectedRoute == null && (routeSelection == null || !routeSelection.hasNext())) {
newRouteSelection = true;
routeSelection = routeSelector.next();
}
synchronized (connectionPool) {
if (canceled) throw new IOException("Canceled");
if (newRouteSelection) {
// Now that we have a set of IP addresses, make another attempt at getting a connection from
// the pool. This could match due to connection coalescing.
// 遍歷所有路由地址，再次嘗試從ConnectionPool中獲取connection
List<Route> routes = routeSelection.getAll();
for (int i = 0, size = routes.size(); i < size; i++) {
Route route = routes.get(i);
Internal.instance.get(connectionPool, address, this, route);
if (connection != null) {
foundPooledConnection = true;
result = connection;
this.route = route;
break;
}
}
}
if (!foundPooledConnection) {
if (selectedRoute == null) {
selectedRoute = routeSelection.next();
}
// Create a connection and assign it to this allocation immediately. This makes it possible
// for an asynchronous cancel() to interrupt the handshake we're about to do.
route = selectedRoute;
refusedStreamCount = 0;
result = new RealConnection(connectionPool, selectedRoute);
acquire(result, false);
}
}
// If we found a pooled connection on the 2nd time around, we're done.
if (foundPooledConnection) {
eventListener.connectionAcquired(call, result);
return result;
}
// Do TCP + TLS handshakes. This is a blocking operation.
//進行實際網絡連接
result.connect(
connectTimeout, readTimeout, writeTimeout, connectionRetryEnabled, call, eventListener);
routeDatabase().connected(result.route());
Socket socket = null;
synchronized (connectionPool) {
reportedAcquired = true;
// Pool the connection.
//獲取成功后放入到連接池當中
Internal.instance.put(connectionPool, result);
// If another multiplexed connection to the same address was created concurrently, then
// release this connection and acquire that one.
if (result.isMultiplexed()) {
socket = Internal.instance.deduplicate(connectionPool, address, this);
result = connection;
}
}
closeQuietly(socket);
eventListener.connectionAcquired(call, result);
return result;
}
代碼量很多，重點我都標識出來方便理解，大致流程如下：
1.首先判斷當前StreamAllocation對象是否存在Connection對象，有則返回（能復用就復用）
2.如果1沒有獲取到，就去ConnectionPool中獲取
3.如果2也沒有拿到，則會遍歷所有路由地址，并再次嘗試從ConnectionPool中獲取
4.如果3還是拿不到，就嘗試重新創建一個新的Connection進行實際的網絡連接
5.將新的Connection添加到ConnectionPool連接池中去，返回結果
從流程中可以，findConnection印證了其命名，主要都是在如何尋找可復用的Connection連接，能復用就復用，不能復用就重建，這里我們注重關注result.connect()，它是怎么創建一個可進行實際網絡連接的Connection
public void connect(int connectTimeout, int readTimeout, int writeTimeout,
int pingIntervalMillis, boolean connectionRetryEnabled, Call call,
EventListener eventListener) {
//檢查鏈接是否已經建立，protocol 標識位代表請求協議，在介紹OkHttpClient的Builder模式下參數講過
if (protocol != null) throw new IllegalStateException("already connected");
RouteException routeException = null;
//Socket鏈接的配置
List<ConnectionSpec> connectionSpecs = route.address().connectionSpecs();
//用于選擇鏈接（隧道鏈接還是Socket鏈接）
ConnectionSpecSelector connectionSpecSelector = new ConnectionSpecSelector(connectionSpecs);
while (true) {
try {
//是否建立Tunnel隧道鏈接
if (route.requiresTunnel()) {
connectTunnel(connectTimeout, readTimeout, writeTimeout, call, eventListener);
if (rawSocket == null) {
// We were unable to connect the tunnel but properly closed down our resources.
break;
}
} else {
connectSocket(connectTimeout, readTimeout, call, eventListener);
}
establishProtocol(connectionSpecSelector, pingIntervalMillis, call, eventListener);
eventListener.connectEnd(call, route.socketAddress(), route.proxy(), protocol);
break;
} catch (IOException e) {
closeQuietly(socket);
closeQuietly(rawSocket);
socket = null;
rawSocket = null;
source = null;
sink = null;
handshake = null;
protocol = null;
http2Connection = null;
eventListener.connectFailed(call, route.socketAddress(), route.proxy(), null, e);
}
}
}
我們這里留下重點代碼來講解
1.首先判斷當前protocol協議是否為空，如果不為空，則拋出異常（鏈接已存在)
2.創建Socket鏈接配置的list集合
3.通過2創建好的list構造了一個ConnectionSpecSelector(用于選擇隧道鏈接還是Socket鏈接)
4.接著執行while循環，route.requiresTunnel()判斷是否需要建立隧道鏈接,否則建立Socket鏈接
5.執行establishProtocol方法設置protocol協議并執行網絡請求
ConnectionPool
不管protocol協議是http1.1的Keep-Live機制還是http2.0的Multiplexing機制都需要引入連接池來維護整個鏈接，而OkHttp會將客戶端和服務端的鏈接作為一個Connection類，RealConnection就是Connection的實現類，ConnectionPool連接池就是負責維護管理所有Connection，并在有限時間內選擇Connection是否復用還是保持打開狀態，超時則及時清理掉
Get方法
//每當要選擇復用Connection時，就會調用ConnectionPool的get方法獲取
@Nullable RealConnection get(Address address, StreamAllocation streamAllocation, Route route) {
assert (Thread.holdsLock(this));
for (RealConnection connection : connections) {
if (connection.isEligible(address, route)) {
streamAllocation.acquire(connection, true);
return connection;
}
}
return null;
}
首先執行for循環遍歷Connections隊列，根據地址address和路由route判斷鏈接是否為可復用，true則執行streamAllocation.acquire返回connection，這里也就明白之前講解findConnection時，ConnectionPool時怎么查找并返回可復用的Connection的
/**
* Use this allocation to hold {@code connection}. Each call to this must be paired with a call to
* {@link #release} on the same connection.
*/
public void acquire(RealConnection connection, boolean reportedAcquired) {
assert (Thread.holdsLock(connectionPool));
if (this.connection != null) throw new IllegalStateException();
this.connection = connection;
this.reportedAcquired = reportedAcquired;
connection.allocations.add(new StreamAllocationReference(this, callStackTrace));
}
public final List<Reference<StreamAllocation>> allocations = new ArrayList<>();
首先就是把從連接池中獲取到的RealConnection對象賦值給StreamAllocation的connection屬性,然后把StreamAllocation對象的弱引用添加到RealConnection對象的allocations集合中去，判斷出當前鏈接對象所持有的StreamAllocation數目，該數組的大小用來判定一個鏈接的負載量是否超過OkHttp指定的最大次數。
Put方法
我們在講到findConnection時，ConnectionPool如果找不到可復用的Connection，就會重新創建一個Connection并通過Put方法添加到ConnectionPool中
void put(RealConnection connection) {
assert (Thread.holdsLock(this));
if (!cleanupRunning) {
cleanupRunning = true;
// 異步回收線程
executor.execute(cleanupRunnable);
}
connections.add(connection);
}
private final Deque<RealConnection> connections = new ArrayDeque<>();
在這里我們可以看到，首先在進行添加隊列之前，代碼會先判斷當前的cleanupRunning（當前清理是否正在執行），符合條件則執行cleanupRunnable異步清理任務回收連接池中的無效連接，其內部時如何執行的呢？
private final Runnable cleanupRunnable = new Runnable() {
@Override public void run() {
while (true) {
// 下次清理的間隔時間
long waitNanos = cleanup(System.nanoTime());
if (waitNanos == -1) return;
if (waitNanos > 0) {
long waitMillis = waitNanos / 1000000L;
waitNanos -= (waitMillis * 1000000L);
synchronized (ConnectionPool.this) {
try {
// 等待釋放鎖和時間間隔
ConnectionPool.this.wait(waitMillis, (int) waitNanos);
} catch (InterruptedException ignored) {
}
}
}
}
}
};
這里我們重點看cleanUp方法是如何清理無效連接的，請看代碼
long cleanup(long now) {
int inUseConnectionCount = 0;
int idleConnectionCount = 0;
RealConnection longestIdleConnection = null;
long longestIdleDurationNs = Long.MIN_VALUE;
// Find either a connection to evict, or the time that the next eviction is due.
synchronized (this) {
// 循環遍歷RealConnection
for (Iterator<RealConnection> i = connections.iterator(); i.hasNext(); ) {
RealConnection connection = i.next();
// If the connection is in use, keep searching.
// 判斷當前connection是否正在使用，true則inUseConnectionCount+1,false則idleConnectionCount+1
if (pruneAndGetAllocationCount(connection, now) > 0) {
// 正在使用的連接數
inUseConnectionCount++;
continue;
}
// 空閑連接數
idleConnectionCount++;
// If the connection is ready to be evicted, we're done.
long idleDurationNs = now - connection.idleAtNanos;
if (idleDurationNs > longestIdleDurationNs) {
longestIdleDurationNs = idleDurationNs;
longestIdleConnection = connection;
}
}
//
if (longestIdleDurationNs >= this.keepAliveDurationNs
|| idleConnectionCount > this.maxIdleConnections) {
// 如果被標記的連接數超過5個，就移除這個連接
// We've found a connection to evict. Remove it from the list, then close it below (outside
// of the synchronized block).
connections.remove(longestIdleConnection);
} else if (idleConnectionCount > 0) {
//如果上面處理返回的空閑連接數目大于0。則返回保活時間與空閑時間差
// A connection will be ready to evict soon.
return keepAliveDurationNs - longestIdleDurationNs;
} else if (inUseConnectionCount > 0) {
// 如果上面處理返回的都是活躍連接。則返回保活時間
// All connections are in use. It'll be at least the keep alive duration 'til we run again.
return keepAliveDurationNs;
} else {
// 跳出死循環
// No connections, idle or in use.
cleanupRunning = false;
return -1;
}
}
closeQuietly(longestIdleConnection.socket());
// Cleanup again immediately.
return 0;
}
- for循環遍歷connections隊列，如果當前的connection對象正在被使用，則
inUseConnectionCount+1 （活躍連接數），跳出當前判斷邏輯，執行下一個判斷邏輯，否則idleConnectionCount+1（空閑連接數) - 判斷
當前的connection對象的空閑時間是否比已知的長，true就記錄 - 如果
空閑連接的時間超過5min，空閑連接的數量超過5，就直接從連接池中移除，并關閉底層socket，返回等待時間0，直接再次循環遍歷連接池（這是個死循環） - 如果3不滿足，
判斷idleConnectionCount是否大于0 （返回的空閑連接數目大于0）。則返回保活時間與空閑時間差 - 如果4不滿足，判斷有沒有正在使用的連接，有的話返回保活時間
- 如果5也不滿足，當前說明連接池中沒有連接，
跳出死循環返回-1
這里可以算是核心代碼，引用Java GC算法中的標記-擦除算法，標記處最不活躍的連接進行清除。每當要創建一個新的Connection時，ConnectionPool都會循環遍歷整個connections隊列，標記查找不活躍的連接，當不活躍連接達到一定數量時及時清除，這也是OkHttp 連接復用的核心。
還有一點要注意，pruneAndGetAllocationCount方法就是判斷當前connection對象是空閑還是活躍的
private int pruneAndGetAllocationCount(RealConnection connection, long now) {
List<Reference<StreamAllocation>> references = connection.allocations;
for (int i = 0; i < references.size(); ) {
Reference<StreamAllocation> reference = references.get(i);
if (reference.get() != null) {
i++;
continue;
}
// We've discovered a leaked allocation. This is an application bug.
StreamAllocation.StreamAllocationReference streamAllocRef =
(StreamAllocation.StreamAllocationReference) reference;
String message = "A connection to " + connection.route().address().url()
+ " was leaked. Did you forget to close a response body?";
Platform.get().logCloseableLeak(message, streamAllocRef.callStackTrace);
references.remove(i);
connection.noNewStreams = true;
// If this was the last allocation, the connection is eligible for immediate eviction.
if (references.isEmpty()) {
connection.idleAtNanos = now - keepAliveDurationNs;
return 0;
}
}
return references.size();
}
1.首先for循環遍歷RealConnection的StreamAllocation List列表，判斷每個StreamAllocation是否為空，false則表示無對象引用這個StreamAllocation，及時清除，執行下一個邏輯判斷（這里有個坑，當作彩蛋吧）
2.接著判斷如果當前StreamAllocation集合因為1的remove而導致列表為空，即沒有任何對象引用，返回0結束
3.返回結果
CallServerInterceptor
負責發起網絡請求和接受服務器返回響應
/** This is the last interceptor in the chain. It makes a network call to the server. */
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 {
// 攔截器鏈
RealInterceptorChain realChain = (RealInterceptorChain) chain;
// HttpCodec接口，封裝了底層IO可以直接用來收發數據的組件流對象
HttpCodec httpCodec = realChain.httpStream();
// 用來Http網絡請求所需要的組件
StreamAllocation streamAllocation = realChain.streamAllocation();
// Connection類的具體實現
RealConnection connection = (RealConnection) realChain.connection();
Request request = realChain.request();
long sentRequestMillis = System.currentTimeMillis();
realChain.eventListener().requestHeadersStart(realChain.call());
// 先向Socket中寫請求頭部信息
httpCodec.writeRequestHeaders(request);
realChain.eventListener().requestHeadersEnd(realChain.call(), request);
Response.Builder responseBuilder = null;
//詢問服務器是否可以發送帶有請求體的信息
if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
// If there's a "Expect: 100-continue" header on the request, wait for a "HTTP/1.1 100
// Continue" response before transmitting the request body. If we don't get that, return
// what we did get (such as a 4xx response) without ever transmitting the request body.
//特殊處理，如果服務器允許請求頭部可以攜帶Expect或則100-continue字段，直接獲取響應信息
if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
httpCodec.flushRequest();
realChain.eventListener().responseHeadersStart(realChain.call());
responseBuilder = httpCodec.readResponseHeaders(true);
}
if (responseBuilder == null) {
// Write the request body if the "Expect: 100-continue" expectation was met.
realChain.eventListener().requestBodyStart(realChain.call());
long contentLength = request.body().contentLength();
CountingSink requestBodyOut =
new CountingSink(httpCodec.createRequestBody(request, contentLength));
BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
// 向Socket中寫入請求信息
request.body().writeTo(bufferedRequestBody);
bufferedRequestBody.close();
realChain.eventListener()
.requestBodyEnd(realChain.call(), requestBodyOut.successfulCount);
} else if (!connection.isMultiplexed()) {
// If the "Expect: 100-continue" expectation wasn't met, prevent the HTTP/1 connection
// from being reused. Otherwise we're still obligated to transmit the request body to
// leave the connection in a consistent state.
streamAllocation.noNewStreams();
}
}
// 完成網絡請求的寫入
httpCodec.finishRequest();
if (responseBuilder == null) {
realChain.eventListener().responseHeadersStart(realChain.call());
//讀取響應信息的頭部
responseBuilder = httpCodec.readResponseHeaders(false);
}
Response response = responseBuilder
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
realChain.eventListener()
.responseHeadersEnd(realChain.call(), response);
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;
}
其實代碼很簡單，我們這里只做OkHttp上的講解，關于HttpCodec（HttpCodec接口，封裝了底層IO可以直接用來收發數據的組件流對象）可以去相關文獻中了解。流程如下
1.首先初始化對象，同時調用httpCodec.writeRequestHeaders(request)向Socket中寫入Header信息
2.判斷服務器是否可以發送帶有請求體的信息，返回為True時會做一個特殊處理，判斷服務器是否允許請求頭部可以攜帶Expect或則100-continue字段（握手操作），可以就直接獲取響應信息
3.當前面的"100-continue"，需要握手，但又握手失敗，如果body信息為空，并寫入請求body信息，否則判斷是否是多路復用，滿足則關閉寫入流和Connection
4.完成網絡請求的寫入
5.判斷body信息是否為空（即表示沒有處理特殊情況），就直接讀取響應的頭部信息，并通過構造者模式一個寫入原請求，握手情況，請求時間，得到的結果時間的Response
6.通過狀態碼判斷以及是否webSocket判斷，是否返回一個空的body，true則返回空的body，否則讀取body信息
7.如果設置了連接 close ,斷開連接，關閉寫入流和Connection
8.返回Response
到此，OkHttp內部5大攔截器的作用已經講解完成，流程大致如下
流程圖
