什么是OKHttp?
OKHttp是一個非常優秀的網絡操作框架，它已經被谷歌收錄到Android源碼。目前比較流行的Retrofit也是默認使用OkHttp的。
怎樣使用它？
OK HTTP使用流程
OkHttpClient(流程的總控制者)
在Client對象里你會發現它是通過構建者模式來創建的
它里面主要有哪些參數呢？
參數.png
final Dispatcher dispatcher;//調度對象（也可以稱為分發器）
final @Nullable Proxy proxy;// 代理
final List<Protocol> protocols;//協議
final List<ConnectionSpec> connectionSpecs;//連接規格（傳輸層版本，連接協議）
final List<Interceptor> interceptors;//攔截器
final List<Interceptor> networkInterceptors;//網絡攔截器
final EventListener.Factory eventListenerFactory;//事件列表工廠
final ProxySelector proxySelector;//主動選擇器（代理選擇）
final CookieJar cookieJar;//cookie
final @Nullable Cache cache;//緩存
final @Nullable InternalCache internalCache;//內存緩存
final SocketFactory socketFactory;//socket工廠
final @Nullable SSLSocketFactory sslSocketFactory;//SSLSocket工廠
final @Nullable CertificateChainCleaner certificateChainCleaner;// 驗證確認響應證書 適用 HTTPS 請求連接的主機名。
final HostnameVerifier hostnameVerifier; // 主機名字確認
final CertificatePinner certificatePinner;// 證書鏈
final Authenticator proxyAuthenticator; //代理身份驗證
final Authenticator authenticator; // 本地身份驗證
final ConnectionPool connectionPool; //連接池,復用連接
final Dns dns;//域名
final boolean followSslRedirects;//安全套接層重定向
final boolean followRedirects; //本地重定向
final boolean retryOnConnectionFailure;//重試連接失敗
final int connectTimeout;//連接超時
final int readTimeout;//read 超時
final int writeTimeout;//write 超時
final int pingInterval;//平區間
Builder方法
OKHttp真正的流程是從newCall方法開始的
@Override
public Call newCall(Request request) {
return RealCall.newRealCall(this, request, false /* for web socket */);
}
它的返回值是一個RealCall，下面我們看看RealCall的構造方法
private RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
this.client = client;
this.originalRequest = originalRequest;
this.forWebSocket = forWebSocket;
this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket);
}
static RealCall newRealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
// Safely publish the Call instance to the EventListener.
RealCall call = new RealCall(client, originalRequest, forWebSocket);
call.eventListener = client.eventListenerFactory().create(call);
return call;
}
RealCall中構造方法中OriginalRequest對象就是request對象。接下來就是execute()方法
@Override
public Response execute() throws IOException {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
captureCallStackTrace();
eventListener.callStart(this);
try {
client.dispatcher().executed(this);
Response result = getResponseWithInterceptorChain();
if (result == null) throw new IOException("Canceled");
return result;
} catch (IOException e) {
eventListener.callFailed(this, e);
throw e;
} finally {
client.dispatcher().finished(this);
}
}
1.檢查這個 call是否已經被執行了，每個 call 只能被執行一次，如果想要一個完全一樣的 call，可以利用 clone 方法進行克隆。
@Override
public RealCall clone() {
return RealCall.newRealCall(client, originalRequest, forWebSocket);
}
2.利用 client.dispatcher().executed(this) 來進行實際執行，dispatcher 是剛才看到的 OkHttpClient.Builder 的成員之一，它的文檔說自己是異步 HTTP請求的執行策略，現在看來，同步請求它也有摻和。
3.調用 getResponseWithInterceptorChain() 函數獲取 HTTP 返回結果，從函數名可以看出，這一步還會進行一系列“攔截”操作。
4.最后還要通知 dispatcher 自己已經執行完畢。dispatcher在同步執行的流程中，涉及到 dispatcher 的內容只不過是告知它我們的執行狀態，比如開始執行了（調用 executed），比如執行完畢了（調用 finished），在異步執行流程中它會有更多的參與。真正發出網絡請求，解析返回結果的，還是 getResponseWithInterceptorChain：
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
List<Interceptor> 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）
if (!forWebSocket) {
interceptors.addAll(client.networkInterceptors());//（6）
}
interceptors.add(new CallServerInterceptor(forWebSocket));//（7）
Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
originalRequest, this, eventListener, client.connectTimeoutMillis(),
client.readTimeoutMillis(), client.writeTimeoutMillis());
return chain.proceed(originalRequest);
}
（1）在配置 OkHttpClient 時設置的 interceptors；
（2）負責失敗重試以及重定向的 RetryAndFollowUpInterceptor（后繼攔截器）；
（3）負責把用戶構造的請求轉換為發送到服務器的請求、把服務器返回的響應轉換為用戶友好的響應的 BridgeInterceptor（橋氏攔截器）；
（4）負責讀取緩存直接返回、更新緩存的 CacheInterceptor（緩存攔截器）；
（5）負責和服務器建立連接的 ConnectInterceptor（連接攔截器）；
（6）配置 OkHttpClient 時設置的 networkInterceptors（網絡攔截器）；
（7）負責向服務器發送請求數據、從服務器讀取響應數據的 CallServerInterceptor（調用攔截器）。
（8）在return chain.proceed(originalRequest);中開啟鏈式調用：
RealInterceptorChain
public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
Connection connection) throws IOException {
if (index >= interceptors.size()) throw new AssertionError();
calls++;
// If we already have a stream, confirm that the incoming request will use it.
//如果我們已經有一個stream。確定即將到來的request會使用它
if (this.httpCodec != null && !sameConnection(request.url())) {
throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
+ " must retain the same host and port");
}
// If we already have a stream, confirm that this is the only call to chain.proceed().
//如果我們已經有一個stream， 確定chain.proceed()唯一的call
if (this.httpCodec != null && calls > 1) {
throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
+ " must call proceed() exactly once");
}
// Call the next interceptor in the chain.
//調用鏈的下一個攔截器
RealInterceptorChain next = new RealInterceptorChain(
interceptors, streamAllocation, httpCodec, connection, index + 1, request);
Interceptor interceptor = interceptors.get(index);
Response response = interceptor.intercept(next);
// Confirm that the next interceptor made its required call to chain.proceed().
if (httpCodec != null && index + 1 < interceptors.size() && next.calls != 1) {
throw new IllegalStateException("network interceptor " + interceptor
+ " must call proceed() exactly once");
}
// Confirm that the intercepted response isn't null.
if (response == null) {
throw new NullPointerException("interceptor " + interceptor + " returned null");
}
return response;
}
最主要的代碼是這點
// Call the next interceptor in the chain.
//調用鏈的下一個攔截器
RealInterceptorChain next = new RealInterceptorChain(
interceptors, streamAllocation, httpCodec, connection, index + 1, request); //(1)
Interceptor interceptor = interceptors.get(index); //(2)
Response response = interceptor.intercept(next); //(3)
- 實例化下一個攔截器對應的RealIterceptorChain對象，這個對象會在傳遞給當前的攔截器
- 得到當前的攔截器interceptors是存放攔截器的ArryList
- 調用當前攔截器的Intercept()方法，并將下一個攔截器的RealIterceptorChain對象傳遞下去
除了在client中自己設置的interceptor,第一個調用的就是retryAndFollowUpInterceptor
RetryAndFollowUpInterceptor:負責失敗重試以及重定向
直接上代碼
@Override
public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
streamAllocation = new StreamAllocation(
client.connectionPool(), createAddress(request.url()));
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); //(1)
releaseConnection = false;
} catch (RouteException e) {
// The attempt to connect via a route failed. The request will not have been sent.
//通過路線連接失敗，請求將不會再發送
if (!recover(e.getLastConnectException(), true, request)) throw e.getLastConnectException();
releaseConnection = false;
continue;
} catch (IOException e) {
// An attempt to communicate with a server failed. The request may have been sent.
// 與服務器嘗試通信失敗，請求不會再發送。
if (!recover(e, false, request)) throw e;
releaseConnection = false;
continue;
} finally {
// We're throwing an unchecked exception. Release any resources.
//拋出未檢查的異常，釋放資源
if (releaseConnection) {
streamAllocation.streamFailed(null);
streamAllocation.release();
}
}
// Attach the prior response if it exists. Such responses never have a body.
// 附加上先前存在的response。這樣的response從來沒有body
// TODO: 2016/8/23 這里沒賦值，豈不是一直為空？
if (priorResponse != null) { // (2)
response = response.newBuilder()
.priorResponse(priorResponse.newBuilder()
.body(null)
.build())
.build();
}
Request followUp = followUpRequest(response); //判斷狀態碼 (3)
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) {
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()));
} 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;
}
}
- 這里是最關鍵的代碼，可以看出在response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);中直接調用了下一個攔截器，然后捕獲可能的異常來進行操作
- 這里沒看太懂，有點坑，以后補
- 這里對于返回的response的狀態碼進行判斷，然后進行處理
BridgeInterceptor：
負責把用戶構造的請求轉換為發送到服務器的請求、把服務器返回的響應轉換為用戶友好的響應的 。
@Override
public Response intercept(Chain chain) throws IOException {
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();
//檢查request。將用戶的request轉換為發送到server的請求
RequestBody body = userRequest.body(); //(1)
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.
//GZIP壓縮
boolean transparentGzip = false;
if (userRequest.header("Accept-Encoding") == 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()); //(2)
HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers()); //(3)
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();
}
- 在（1）和（2）之間，BridgeInterceptor對于request的格式進行檢查，讓構建了一個新的request
- 調用下一個interceptor來得到response
- （3）下面就是對得到的response進行一些判斷操作，最后將結果返回。
@Override
public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null //=============(1)
? cache.get(chain.request()) //通過request得到緩存
: null;
long now = System.currentTimeMillis();
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get(); //根據request來得到緩存策略===========(2)
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {
cache.trackResponse(strategy);
}
if (cacheCandidate != null && cacheResponse == null) { //存在緩存的response，但是不允許緩存
closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it. 緩存不適合，關閉
}
// If we're forbidden from using the network and the cache is insufficient, fail.
//如果我們禁止使用網絡，且緩存為null，失敗
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(EMPTY_BODY)
.sentRequestAtMillis(-1L)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
}
// If we don't need the network, we're done.
if (networkRequest == null) { //沒有網絡請求，跳過網絡，返回緩存
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
Response networkResponse = null;
try {
networkResponse = chain.proceed(networkRequest);//網絡請求攔截器 //======(3)
} finally {
// If we're crashing on I/O or otherwise, don't leak the cache body.
//如果我們因為I/O或其他原因崩潰，不要泄漏緩存體
if (networkResponse == null && cacheCandidate != null) {
closeQuietly(cacheCandidate.body());
}
}
// If we have a cache response too, then we're doing a conditional get.========(4)
//如果我們有一個緩存的response，然后我們正在做一個條件GET
if (cacheResponse != null) {
if (validate(cacheResponse, networkResponse)) { //比較確定緩存response可用
Response response = cacheResponse.newBuilder()
.headers(combine(cacheResponse.headers(), networkResponse.headers()))
.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()).
//更新緩存，在剝離content-Encoding之前
cache.trackConditionalCacheHit();
cache.update(cacheResponse, response);
return response;
} else {
closeQuietly(cacheResponse.body());
}
}
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
if (HttpHeaders.hasBody(response)) { // =========(5)
CacheRequest cacheRequest = maybeCache(response, networkResponse.request(), cache);
response = cacheWritingResponse(cacheRequest, response);
}
return response;
}
- 首先，根據request來判斷cache中是否有緩存的response，如果有，得到這個response，然后進行判斷當前response是否有效，沒有將cacheCandate賦值為空。
- 根據request判斷緩存的策略，是否要使用了網絡，緩存 或兩者都使用
- 調用下一個攔截器，決定從網絡上來得到response
- 如果本地已經存在cacheResponse，那么讓它和網絡得到的networkResponse做比較，決定是否來更新緩存的cacheResponse
- 緩存未經緩存過的response
ConnectInterceptor:建立連接
@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);
}
實際上建立連接就是創建了一個HttpCodec對象，它將在后面的步驟中被使用，那它又是何方神圣呢？它是對 HTTP協議操作的抽象，有兩個實現：Http1Codec和Http2Codec，顧名思義，它們分別對應 HTTP/1.1 和 HTTP/2 版本的實現。
在Http1Codec中，它利用 Okio 對Socket的讀寫操作進行封裝，Okio以后有機會再進行分析，現在讓我們對它們保持一個簡單地認識：它對java.io和java.nio進行了封裝，讓我們更便捷高效的進行 IO 操作。
而創建HttpCodec對象的過程涉及到StreamAllocation、RealConnection，代碼較長，這里就不展開，這個過程概括來說，就是找到一個可用的RealConnection，再利用RealConnection的輸入輸出（BufferedSource和BufferedSink）創建HttpCodec對象，供后續步驟使用。
NetworkInterceptors
配置OkHttpClient時設置的 NetworkInterceptors。
CallServerInterceptor：發送和接收數據
@Override public Response intercept(Chain chain) throws IOException {
HttpCodec httpCodec = ((RealInterceptorChain) chain).httpStream();
StreamAllocation streamAllocation = ((RealInterceptorChain) chain).streamAllocation();
Request request = chain.request();
long sentRequestMillis = System.currentTimeMillis();
httpCodec.writeRequestHeaders(request);
if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) { //===(1)
Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());
BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
request.body().writeTo(bufferedRequestBody);
bufferedRequestBody.close();
}
httpCodec.finishRequest();
Response response = httpCodec.readResponseHeaders() //====(2)
.request(request)
.handshake(streamAllocation.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
if (!forWebSocket || response.code() != 101) {
response = response.newBuilder()
.body(httpCodec.openResponseBody(response))
.build();
}
if ("close".equalsIgnoreCase(response.request().header("Connection"))
|| "close".equalsIgnoreCase(response.header("Connection"))) {
streamAllocation.noNewStreams();
}
int code = response.code();
if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
throw new ProtocolException(
"HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
}
return response;
}
- 檢查請求方法，用Httpcodec處理request
- 進行網絡請求得到response
- 返回response
總結
前面說了攔截器用了責任鏈設計模式,它將請求一層一層向下傳，知道有一層能夠得到Resposne就停止向下傳遞，然后將response向上面的攔截器傳遞，然后各個攔截器會對respone進行一些處理，最后會傳到RealCall類中通過execute來得到esponse。
異步請求的流程：
異步get請求示例如下：
private final OkHttpClient client = new OkHttpClient();
public void run() throws Exception {
Request request = new Request.Builder()
.url("http://publicobject.com/helloworld.txt")
.build();
client.newCall(request).enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
e.printStackTrace();
}
@Override
public void onResponse(Call call, Response response) throws IOException {
if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);
Headers responseHeaders = response.headers();
for (int i = 0, size = responseHeaders.size(); i < size; i++) {
System.out.println(responseHeaders.name(i) + ": " + responseHeaders.value(i));
}
System.out.println(response.body().string());
}
});
}
由代碼中client.newCall(request).enqueue(Callback)，開始我們知道client.newCall(request)方法返回的是RealCall對象，接下來繼續向下看enqueue()方法:
//異步任務使用
@Override
public void enqueue(Callback responseCallback) {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
client.dispatcher().enqueue(new AsyncCall(responseCallback));
}
調用了上面我們沒有詳細說的Dispatcher類中的enqueue(Call )方法.接著繼續看：
synchronized void enqueue(AsyncCall call) {
if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
runningAsyncCalls.add(call);
executorService().execute(call);
} else {
readyAsyncCalls.add(call);
}
}
如果中的runningAsynCalls不滿，且call占用的host小于最大數量，則將call加入到runningAsyncCalls中執行，同時利用線程池執行call；否者將call加入到readyAsyncCalls中。runningAsyncCalls和readyAsyncCalls是什么呢？看下面：
/** Ready async calls in the order they'll be run. */
private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>(); //正在準備中的異步請求隊列
/** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>(); //運行中的異步請求
/** Running synchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>(); //同步請求
call加入到線程池中執行了。現在再看AsynCall的代碼，它是RealCall中的內部類:
//異步請求
final class AsyncCall extends NamedRunnable {
private final Callback responseCallback;
private AsyncCall(Callback responseCallback) {
super("OkHttp %s", redactedUrl());
this.responseCallback = responseCallback;
}
String host() {
return originalRequest.url().host();
}
Request request() {
return originalRequest;
}
RealCall get() {
return RealCall.this;
}
@Override protected void execute() {
boolean signalledCallback = false;
try {
Response response = getResponseWithInterceptorChain();
if (retryAndFollowUpInterceptor.isCanceled()) {
signalledCallback = true;
responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
} else {
signalledCallback = true;
responseCallback.onResponse(RealCall.this, response);
}
} catch (IOException e) {
if (signalledCallback) {
// Do not signal the callback twice!
Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
} else {
responseCallback.onFailure(RealCall.this, e);
}
} finally {
client.dispatcher().finished(this);
}
}
}
AysncCall中的execute()中的方法，同樣是通過Response response = getResponseWithInterceptorChain();來獲得response，這樣異步任務也同樣通過了interceptor，剩下的流程就和上面一樣了。
