什么是OKHttp?
OKHttp是一个非常优秀的网络操作框架，它已经被谷歌收录到Android源码。目前比较流行的Retrofit也是默认使用OkHttp的。
怎样使用它？
OK HTTP使用流程
OkHttpClient(流程的总控制者)
在Client对象里你会发现它是通过构建者模式来创建的
它里面主要有哪些参数呢？
参数.png
final Dispatcher dispatcher;//调度对象（也可以称为分发器）
final @Nullable Proxy proxy;// 代理
final List protocols;//协议
final List connectionSpecs;//连接规格（传输层版本，连接协议）
final List interceptors;//拦截器
final List networkInterceptors;//网络拦截器
final EventListener.Factory eventListenerFactory;//事件列表工厂
final ProxySelector proxySelector;//主动选择器（代理选择）
final CookieJar cookieJar;//cookie
final @Nullable Cache cache;//缓存
final @Nullable InternalCache internalCache;//内存缓存
final SocketFactory socketFactory;//socket工厂
final @Nullable SSLSocketFactory sslSocketFactory;//SSLSocket工厂
final @Nullable CertificateChainCleaner certificateChainCleaner;// 验证确认响应证书 适用 HTTPS 请求连接的主机名。
final HostnameVerifier hostnameVerifier; // 主机名字确认
final CertificatePinner certificatePinner;// 证书链
final Authenticator proxyAuthenticator; //代理身份验证
final Authenticator authenticator; // 本地身份验证
final ConnectionPool connectionPool; //连接池,复用连接
final Dns dns;//域名
final boolean followSslRedirects;//安全套接层重定向
final boolean followRedirects; //本地重定向
final boolean retryOnConnectionFailure;//重试连接失败
final int connectTimeout;//连接超时
final int readTimeout;//read 超时
final int writeTimeout;//write 超时
final int pingInterval;//平区间
Builder方法
OKHttp真正的流程是从newCall方法开始的
@Override
public Call newCall(Request request) {
return RealCall.newRealCall(this, request, false /* for web socket */);
}
它的返回值是一个RealCall，下面我们看看RealCall的构造方法
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中构造方法中OriginalRequest对象就是request对象。接下来就是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.检查这个 call是否已经被执行了，每个 call 只能被执行一次，如果想要一个完全一样的 call，可以利用 clone 方法进行克隆。
@Override
public RealCall clone() {
return RealCall.newRealCall(client, originalRequest, forWebSocket);
}
2.利用 client.dispatcher().executed(this) 来进行实际执行，dispatcher 是刚才看到的 OkHttpClient.Builder 的成员之一，它的文档说自己是异步 HTTP请求的执行策略，现在看来，同步请求它也有掺和。
3.调用 getResponseWithInterceptorChain() 函数获取 HTTP 返回结果，从函数名可以看出，这一步还会进行一系列“拦截”操作。
4.最后还要通知 dispatcher 自己已经执行完毕。dispatcher在同步执行的流程中，涉及到 dispatcher 的内容只不过是告知它我们的执行状态，比如开始执行了（调用 executed），比如执行完毕了（调用 finished），在异步执行流程中它会有更多的参与。真正发出网络请求，解析返回结果的，还是 getResponseWithInterceptorChain：
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
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）
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 时设置的 interceptors；
（2）负责失败重试以及重定向的 RetryAndFollowUpInterceptor（后继拦截器）；
（3）负责把用户构造的请求转换为发送到服务器的请求、把服务器返回的响应转换为用户友好的响应的 BridgeInterceptor（桥氏拦截器）；
（4）负责读取缓存直接返回、更新缓存的 CacheInterceptor（缓存拦截器）；
（5）负责和服务器建立连接的 ConnectInterceptor（连接拦截器）；
（6）配置 OkHttpClient 时设置的 networkInterceptors（网络拦截器）；
（7）负责向服务器发送请求数据、从服务器读取响应数据的 CallServerInterceptor（调用拦截器）。
（8）在return chain.proceed(originalRequest);中开启链式调用：
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.
//如果我们已经有一个stream。确定即将到来的request会使用它
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().
//如果我们已经有一个stream， 确定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.
//调用链的下一个拦截器
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;
}
最主要的代码是这点
// Call the next interceptor in the chain.
//调用链的下一个拦截器
RealInterceptorChain next = new RealInterceptorChain(
interceptors, streamAllocation, httpCodec, connection, index + 1, request); //(1)
Interceptor interceptor = interceptors.get(index); //(2)
Response response = interceptor.intercept(next); //(3)
- 实例化下一个拦截器对应的RealIterceptorChain对象，这个对象会在传递给当前的拦截器
- 得到当前的拦截器interceptors是存放拦截器的ArryList
- 调用当前拦截器的Intercept()方法，并将下一个拦截器的RealIterceptorChain对象传递下去
除了在client中自己设置的interceptor,第一个调用的就是retryAndFollowUpInterceptor
RetryAndFollowUpInterceptor:负责失败重试以及重定向
直接上代码
@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.
//通过路线连接失败，请求将不会再发送
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.
// 与服务器尝试通信失败，请求不会再发送。
if (!recover(e, false, request)) throw e;
releaseConnection = false;
continue;
} finally {
// We're throwing an unchecked exception. Release any resources.
//抛出未检查的异常，释放资源
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。这样的response从来没有body
// TODO: 2016/8/23 这里没赋值，岂不是一直为空？
if (priorResponse != null) { // (2)
response = response.newBuilder()
.priorResponse(priorResponse.newBuilder()
.body(null)
.build())
.build();
}
Request followUp = followUpRequest(response); //判断状态码 (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;
}
}
- 这里是最关键的代码，可以看出在response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);中直接调用了下一个拦截器，然后捕获可能的异常来进行操作
- 这里没看太懂，有点坑，以后补
- 这里对于返回的response的状态码进行判断，然后进行处理
BridgeInterceptor：
负责把用户构造的请求转换为发送到服务器的请求、把服务器返回的响应转换为用户友好的响应的 。
@Override
public Response intercept(Chain chain) throws IOException {
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();
//检查request。将用户的request转换为发送到server的请求
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压缩
boolean transparentGzip = false;
if (userRequest.header("Accept-Encoding") == null) {
transparentGzip = true;
requestBuilder.header("Accept-Encoding", "gzip");
}
List 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）之间，BridgeInterceptor对于request的格式进行检查，让构建了一个新的request
- 调用下一个interceptor来得到response
- （3）下面就是对得到的response进行一些判断操作，最后将结果返回。
@Override
public Response intercept(Chain chain) throws IOException {
Response cacheCandidate = cache != null //=============(1)
? cache.get(chain.request()) //通过request得到缓存
: null;
long now = System.currentTimeMillis();
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get(); //根据request来得到缓存策略===========(2)
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {
cache.trackResponse(strategy);
}
if (cacheCandidate != null && cacheResponse == null) { //存在缓存的response，但是不允许缓存
closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it. 缓存不适合，关闭
}
// If we're forbidden from using the network and the cache is insufficient, fail.
//如果我们禁止使用网络，且缓存为null，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(EMPTY_BODY)
.sentRequestAtMillis(-1L)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
}
// If we don't need the network, we're done.
if (networkRequest == null) { //没有网络请求，跳过网络，返回缓存
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
Response networkResponse = null;
try {
networkResponse = chain.proceed(networkRequest);//网络请求拦截器 //======(3)
} finally {
// If we're crashing on I/O or otherwise, don't leak the cache body.
//如果我们因为I/O或其他原因崩溃，不要泄漏缓存体
if (networkResponse == null && cacheCandidate != null) {
closeQuietly(cacheCandidate.body());
}
}
// If we have a cache response too, then we're doing a conditional get.========(4)
//如果我们有一个缓存的response，然后我们正在做一个条件GET
if (cacheResponse != null) {
if (validate(cacheResponse, networkResponse)) { //比较确定缓存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()).
//更新缓存，在剥离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，根据request来判断cache中是否有缓存的response，如果有，得到这个response，然后进行判断当前response是否有效，没有将cacheCandate赋值为空。
- 根据request判断缓存的策略，是否要使用了网络，缓存 或两者都使用
- 调用下一个拦截器，决定从网络上来得到response
- 如果本地已经存在cacheResponse，那么让它和网络得到的networkResponse做比较，决定是否来更新缓存的cacheResponse
- 缓存未经缓存过的response
ConnectInterceptor:建立连接
@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);
}
实际上建立连接就是创建了一个HttpCodec对象，它将在后面的步骤中被使用，那它又是何方神圣呢？它是对 HTTP协议操作的抽象，有两个实现：Http1Codec和Http2Codec，顾名思义，它们分别对应 HTTP/1.1 和 HTTP/2 版本的实现。
在Http1Codec中，它利用 Okio 对Socket的读写操作进行封装，Okio以后有机会再进行分析，现在让我们对它们保持一个简单地认识：它对java.io和java.nio进行了封装，让我们更便捷高效的进行 IO 操作。
而创建HttpCodec对象的过程涉及到StreamAllocation、RealConnection，代码较长，这里就不展开，这个过程概括来说，就是找到一个可用的RealConnection，再利用RealConnection的输入输出（BufferedSource和BufferedSink）创建HttpCodec对象，供后续步骤使用。
NetworkInterceptors
配置OkHttpClient时设置的 NetworkInterceptors。
CallServerInterceptor：发送和接收数据
@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;
}
- 检查请求方法，用Httpcodec处理request
- 进行网络请求得到response
- 返回response
总结
前面说了拦截器用了责任链设计模式,它将请求一层一层向下传，知道有一层能够得到Resposne就停止向下传递，然后将response向上面的拦截器传递，然后各个拦截器会对respone进行一些处理，最后会传到RealCall类中通过execute来得到esponse。
异步请求的流程：
异步get请求示例如下：
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());
}
});
}
由代码中client.newCall(request).enqueue(Callback)，开始我们知道client.newCall(request)方法返回的是RealCall对象，接下来继续向下看enqueue()方法:
//异步任务使用
@Override
public void enqueue(Callback responseCallback) {
synchronized (this) {
if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
client.dispatcher().enqueue(new AsyncCall(responseCallback));
}
调用了上面我们没有详细说的Dispatcher类中的enqueue(Call )方法.接着继续看：
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不满，且call占用的host小于最大数量，则将call加入到runningAsyncCalls中执行，同时利用线程池执行call；否者将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 readyAsyncCalls = new ArrayDeque<>(); //正在准备中的异步请求队列
/** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque runningAsyncCalls = new ArrayDeque<>(); //运行中的异步请求
/** Running synchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque runningSyncCalls = new ArrayDeque<>(); //同步请求
call加入到线程池中执行了。现在再看AsynCall的代码，它是RealCall中的内部类:
//异步请求
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，同样是通过Response response = getResponseWithInterceptorChain();来获得response，这样异步任务也同样通过了interceptor，剩下的流程就和上面一样了。