写在开头
okHttp目前可以称的上是Android主流网络框架,甚至连谷歌官方也将网络请求的实现替换成okHttp.
网上也有很多人对okHttp的源码进行了分析,不过基于每个人的分析思路都不尽相同,读者看起来的收获也各不相同,所以我还是整理了下思路,写了点自己的分析感悟。
本文基于okhttp3.11.0版本分析
基本用法
String url = "http://www.baidu.com";//'1. 生成OkHttpClient实例对象'OkHttpClient okHttpClient = new OkHttpClient();//'2. 生成Request对象'Request request = new Request.Builder() .url(url) .post(RequestBody.create(MediaType.parse("application/json; charset=utf-8"),"test content")) .build();Call call = okHttpClient.newCall(request);call.enqueue(new Callback() { @Override public void onFailure(@NonNull Call call, @NonNull IOException e) { } @Override public void onResponse(@NonNull Call call, @NonNull Response response) { }});复制代码 整体流程
借用别人的一张流程图来概括一下okHttp的请求走向
okHttp的整体流程大致分为以下几个阶段
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创建请求对象 (url, method,body)-->request-->Call
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请求事件队列,线程池分发 enqueue-->Runnable-->ThreadPoolExecutor
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递归
Interceptor拦截器,发送请求。 InterceptorChain -
请求回调,数据解析。 Respose-->(code,message,requestBody)
创建请求对象
其中 Request维护请求对象的属性
public final class Request { final HttpUrl url; final String method; final Headers headers; final @Nullable RequestBody body; //请求的标记,在okHttp2.x的时候，okHttpClint提供Cancel(tag)的方法来批量取消请求 //不过在3.x上批量请求的api被删除了,要取消请求只能在Callback中调用 call.cancel() //因此这个tags参数只能由开发者自己编写函数来实现批量取消请求的操作 final Map , Object> tags; }复制代码 请求响应的包装接口Call
public interface Call extends Cloneable { Request request(); Response execute() throws IOException; void enqueue(Callback responseCallback); void cancel();}复制代码 请求事件队列,线程池分发
Call的实现类RealCall和AsyncCall
@Override public void enqueue(Callback responseCallback) { synchronized (this) { if (executed) throw new IllegalStateException("Already Executed"); executed = true; } captureCallStackTrace(); eventListener.callStart(this); client.dispatcher().enqueue(new AsyncCall(responseCallback)); }//其中AsyncCall是RealCall的一个内部类,继承自Runnable,这样就能通过线程池来回调AsyncCall的execute函数final class AsyncCall extends NamedRunnable { @Override protected void execute() { boolean signalledCallback = false; try { //getResponseWithInterceptorChain 拦截链的逻辑,也是发起请求的真正入口 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) { ... } ... }}复制代码 递归Interceptor拦截器,发送请求
Response getResponseWithInterceptorChain() throws IOException { // Build a full stack of interceptors. List interceptors = new ArrayList<>(); //用户自定义的拦截器(注意addAll 所以可以添加多个自定义的拦截器) interceptors.addAll(client.interceptors()); //重试与重定向拦截器 interceptors.add(retryAndFollowUpInterceptor); //内容拦截器 interceptors.add(new BridgeInterceptor(client.cookieJar())); //缓存拦截器 interceptors.add(new CacheInterceptor(client.internalCache())); //网络连接拦截器 interceptors.add(new ConnectInterceptor(client)); if (!forWebSocket) { //用户自定义的网络拦截器 interceptors.addAll(client.networkInterceptors()); } //服务请求的拦截器 interceptors.add(new CallServerInterceptor(forWebSocket)); Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0, originalRequest, this, eventListener, client.connectTimeoutMillis(), client.readTimeoutMillis(), client.writeTimeoutMillis()); return chain.proceed(originalRequest); }复制代码 okHttp的核心部分就是这个Interceptor拦截链,每个Interceptor各自负责一部分功能,内部通过递归的方式遍历每一个Interceptor拦截器。递归逻辑在RealInterceptorChain类下
public final class RealInterceptorChain implements Interceptor.Chain { //拦截器递归的入口 public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec, RealConnection connection) throws IOException { ... //拦截器递归的核心代码,根据interceptors列表执行每一个拦截器的intercept函数 RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec, connection, index + 1, request, call, eventListener, connectTimeout, readTimeout, writeTimeout); Interceptor interceptor = interceptors.get(index); Response response = interceptor.intercept(next); .... return response; }}复制代码 递归结束后会获得请求响应,那么说明我们的request行为就在这个拦截链中,接下来我们先看看负责网络请求的那部分拦截器,从类名上就能比较容易的看出 ConnectInterceptor和 CallServerInterceptor这两个拦截器的主要工作。
网络连接拦截器ConnectInterceptor
public final class ConnectInterceptor implements Interceptor { @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, chain, doExtensiveHealthChecks); RealConnection connection = streamAllocation.connection(); return realChain.proceed(request, streamAllocation, httpCodec, connection); }}复制代码 其中有几个对象说明一下
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**StreamAllocation:**内存流的存储空间,这个对象可以直接从realChain中直接获取,说明在之前的拦截链中就已经赋值过
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HttpCodec(Encodes HTTP requests and decodes HTTP responses): 对请求的编码以及对响应数据的解码
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**realChain.proceed():**通知下一个拦截器执行
接下来看创建HttpCodec对象的newStream函数中做了些什么
//HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);public HttpCodec newStream( OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) { ... try { //findHealthyConnection内部通过一个死循环查找一个可用的连接,优先使用存在的可用连接,否则就通过 //线程池来生成,其中多处使用 synchronized关键字,防止因为多并发导致问题 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); } }复制代码 沿着代码往下走,你会发现实际上负责网络连接功能的类是一个叫RealConnection的类,该类中有一个connect的函数
RealConnection#connect
public void connect(int connectTimeout, int readTimeout, int writeTimeout, int pingIntervalMillis, boolean connectionRetryEnabled, Call call, EventListener eventListener) { ... while (true) { try { if (route.requiresTunnel()) { //这个函数最终还是会走到connectSocket()函数中 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); } } ... } //最终调用的还是Socket对象来创建网络连接,包括connectTimeout,readTimeout等参数也是这个时候真正设置的。复制代码 网络请求拦截器 CallServerInterceptor
This is the last interceptor in the chain. It makes a network call to the server.
直接看CallServerInterceptor的intercept函数
@Overridepublic Response intercept(Chain chain) throws IOException{ //下面的各参数都是之前几个拦截器所生成的 RealInterceptorChain realChain = (RealInterceptorChain) chain; HttpCodec httpCodec = realChain.httpStream(); StreamAllocation streamAllocation = realChain.streamAllocation(); RealConnection connection = (RealConnection) realChain.connection(); Request request = realChain.request(); //发送请求头,也是网络请求的开始 httpCodec.writeRequestHeaders(request); Response.Builder responseBuilder = null; //请求不是get,并且有添加了请求体,写入请求体信息 if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) { //如果请求头中有Expect:100-continue这么一个属性 //会先发送一个header部分给服务器,并询问服务器是否支持Expect:100-continue 这么一个扩展域 //okhttp3提供这么个判断是为了兼容http2的连接复用行为的 if ("100-continue".equalsIgnoreCase(request.header("Expect"))) { //刷新缓存区,可以理解为向服务端写入数据 httpCodec.flushRequest(); realChain.eventListener().responseHeadersStart(realChain.call()); responseBuilder = httpCodec.readResponseHeaders(true); } //写入请求body if (responseBuilder == null) { realChain.eventListener().requestBodyStart(realChain.call()); long contentLength = request.body().contentLength(); CountingSink requestBodyOut = new CountingSink(httpCodec.createRequestBody(request, contentLength)); BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut); request.body().writeTo(bufferedRequestBody); bufferedRequestBody.close(); realChain.eventListener() .requestBodyEnd(realChain.call(), requestBodyOut.successfulCount); } ... httpCodec.finishRequest(); //响应相关的代码 ...}复制代码 写入请求body的核心代码
//将请求体写入到BufferedSink中,而BufferedSink是另外一个类库Okio中的类CountingSink requestBodyOut = new CountingSink(httpCodec.createRequestBody(request, contentLength));BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);request.body().writeTo(bufferedRequestBody);//httpCodec.finishRequest 最终会调用 sink.flush(),sink是BufferedSink的对象,BufferedSink在底层//会将其内的数据推给服务端,相当于是一个刷新缓冲区的功能httpCodec.finishRequest();复制代码
响应相关的代码
if (responseBuilder == null) { realChain.eventListener().responseHeadersStart(realChain.call()); //读取响应头,实际的返回流存放位置在okio库下的buffer对象中,读取过程中做了判断,只有当code==100时才会 //有返回,不然抛出异常并拦截,所以下面这段代码肯定有响应头返回,不然直到超时也不会回调 responseBuilder = httpCodec.readResponseHeaders(false);}Response response = responseBuilder .request(request) .handshake(streamAllocation.connection().handshake()) .sentRequestAtMillis(sentRequestMillis) .receivedResponseAtMillis(System.currentTimeMillis()) .build();int code = response.code();if (code == 100) { //如果服务端响应码为100,需要我们再次请求,注意这里的100是响应码和之前的100不同 //之前的100是headerLine的标识码 responseBuilder = httpCodec.readResponseHeaders(false); response = responseBuilder .request(request) .handshake(streamAllocation.connection().handshake()) .sentRequestAtMillis(sentRequestMillis) .receivedResponseAtMillis(System.currentTimeMillis()) .build(); 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 { //读取响应body response = response.newBuilder() .body(httpCodec.openResponseBody(response)) .build();}return response;复制代码 读取响应body HttpCodec#openResponseBody
public ResponseBody openResponseBody(Response response) throws IOException { ... Source source = newFixedLengthSource(contentLength); return new RealResponseBody(contentType, contentLength, Okio.buffer(source)); ...}//openResponseBody将Socket的输入流InputStream对象交给OkIo的Source对象，然后封装成RealResponseBody（该类是ResponseBody的子类）作为Response的body.//具体读取是在RealResponseBody父类ResponseBody中,其中有个string()函数//响应主体存放在内存中,然后调用source.readString来读取服务器的数据。需要注意的是该方法最后调用closeQuietly来关闭了当前请求的InputStream输入流，所以string()方法只能调用一次,再次调用的话会报错public final String string() throws IOException { BufferedSource source = source(); try { Charset charset = Util.bomAwareCharset(source, charset()); return source.readString(charset); } finally { Util.closeQuietly(source); }}复制代码 请求回调,数据解析
拿到请求回调的Response之后,再回到我们最开始调用的代码,
String url = "http://www.baidu.com";//'1. 生成OkHttpClient实例对象'OkHttpClient okHttpClient = new OkHttpClient();//'2. 生成Request对象'Request request = new Request.Builder() .url(url) .post(RequestBody.create(MediaType.parse("application/json; charset=utf-8"),"test content")) .build();Call call = okHttpClient.newCall(request);call.enqueue(new Callback() { @Override public void onFailure(@NonNull Call call, @NonNull IOException e) { } @Override public void onResponse(@NonNull Call call, @NonNull Response 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()); }});复制代码 我们可以从Response对象中获取所有我们所需要的数据,包括header,body.至此,okHttp的网络请求的大致流程已经分析完成,至于还有部分没有讲到的拦截器就不再本文缀述了.有兴趣的可以看下文末的参考连接或者自行谷歌。
参考文章
扩展阅读
关于Http的请求头 Expect:100-Continue
Expect请求头部域，用于指出客户端要求的特殊服务器行为。若服务器不能理解或者满足Expect域中的任何期望值，则必须返回417(Expectation Failed)状态，或者如果请求有其他问题，返回4xx状态。Expect:100-Continue握手的目的，是为了允许客户端在发送请求内容之前，判断源服务器是否愿意接受请求（基于请求头部）。Expect:100-Continue握手需谨慎使用，因为遇到不支持HTTP/1.1协议的服务器或者代理时会引起问题。复制代码
http2比起http1.x的有点主要体现在以下几点
- 新的数据格式, http基于文件协议解析,http2基于二进制协议解析,
- 连接共享,多路复用（MultiPlexing）
- header压缩,减小header的体积,使得请求更快
- 压缩算法从gzip改成HPACK的算法,防破解
- 重置连接表现更好,http1.x取消请求的是直接断开连接，http2则是断开某个连接的stream流
- 更安全的SSL