1. 前言
Okhttp3 俨然已成为Android的主流网络请求开源框架,它的设计非常巧妙,而且非常灵活,功能强大.它有如下默认特性:
- 支持HTTP/2，允许所有同一个主机地址的请求共享同一个Socket连接
- 连接池减少请求延时
- 透明的GZIP压缩减少响应数据的大小
- 缓存响应内容，避免一些完全重复的请求
现在的Android项目基本上都是以OkHttp来进行高效的网络请求.当然,在使用的同时我们需要去研究它的底层实现,从而让我们写出更好的代码.
2. 基本使用
这里简单介绍2种,GET和POST.推荐让 OkHttpClient 保持单例，用同一个 OkHttpClient 实例来执行你的所有请求，因为每一个 OkHttpClient 实例都拥有自己的连接池和线程池，重用这些资源可以减少延时和节省资源，如果为每个请求创建一个 OkHttpClient实例，显然就是一种资源的浪费。
1. 使用GET方式请求
public static final String URL = "http://www.baidu.com";
private OkHttpClient mOkHttpClient = new OkHttpClient();
private final Request mRequest = new Request.Builder().url(URL).build();
@Override
public void request() {
mOkHttpClient.newCall(mRequest)
//异步请求
.enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
e.printStackTrace();
}
@Override
public void onResponse(Call call, Response response) throws IOException {
Log.w(TAG, "onResponse: " + response.body().string());
}
});
}
2. 使用POST请求
public static final String URL = "https://api.github.com/markdown/raw";
private OkHttpClient mOkHttpClient = new OkHttpClient.Builder()
.build();
MediaType mMediaType = MediaType.parse("text/x-markdown; charset=utf-8");
String requestBody = "I am xfhy.";
private final Request mRequest = new Request.Builder()
.url(URL)
.post(RequestBody.create(mMediaType, requestBody))
.build();
@Override
public void request() {
//每一个Call（其实现是RealCall）只能执行一次，否则会报异常
mOkHttpClient.newCall(mRequest).enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
e.printStackTrace();
}
@Override
public void onResponse(Call call, Response response) throws IOException {
Log.w(TAG, "onResponse: " + response.body().string());
}
});
}
3. interceptor 拦截器-精髓
使用OkHttp3请求网络还是比较简单,而且异步请求也比较轻松.
3.1 构建OkHttpClient
正如名字所描述的,OkHttpClient像是一个请求网络的客户端.它内部有很多很多的配置信息(支持协议、任务调度器、连接池、超时时间等),通过构造器模式初始化的这些配置信息.(这里穿插一下,正如你所看到的这种一个类里面很多很多属性需要初始化的,一般就用构造器模式)
public OkHttpClient() {
this(new Builder());
}
public Builder() {
//任务调度器
dispatcher = new Dispatcher();
//支持的协议
protocols = DEFAULT_PROTOCOLS;
connectionSpecs = DEFAULT_CONNECTION_SPECS;
eventListenerFactory = EventListener.factory(EventListener.NONE);
proxySelector = ProxySelector.getDefault();
if (proxySelector == null) {
proxySelector = new NullProxySelector();
}
cookieJar = CookieJar.NO_COOKIES;
socketFactory = SocketFactory.getDefault();
hostnameVerifier = OkHostnameVerifier.INSTANCE;
certificatePinner = CertificatePinner.DEFAULT;
proxyAuthenticator = Authenticator.NONE;
authenticator = Authenticator.NONE;
//连接池
connectionPool = new ConnectionPool();
dns = Dns.SYSTEM;
followSslRedirects = true;
followRedirects = true;
retryOnConnectionFailure = true;
callTimeout = 0;
//超时时间
connectTimeout = 10_000;
readTimeout = 10_000;
writeTimeout = 10_000;
pingInterval = 0;
}
其中Dispatcher有一个线程池,用于执行异步的请求.并且内部还维护了3个双向任务队列,分别是:准备异步执行的任务队列、正在异步执行的任务队列、正在同步执行的任务队列.
/** Executes calls. Created lazily. */
//这个线程池是需要的时候才会被初始化
private @Nullable ExecutorService executorService;
/** Ready async calls in the order they'll be run. */
private final Deque readyAsyncCalls = new ArrayDeque<>();
/** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque runningAsyncCalls = new ArrayDeque<>();
/** Running synchronous calls. Includes canceled calls that haven't finished yet. */
private final Deque runningSyncCalls = new ArrayDeque<>();
public synchronized ExecutorService executorService() {
if (executorService == null) {
//注意,该线程池没有核心线程,线程数量可以是Integer.MAX_VALUE个(相当于没有限制),超过60秒没干事就要被回收
executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
new SynchronousQueue<>(), Util.threadFactory("OkHttp Dispatcher", false));
}
return executorService;
}
3.2 构建Request
Request感觉就是一个请求的封装.它里面封装了url、method、header、body,该有的都有了.而且它也是用构造器模式来构建的,它默认的请求方式是GET
public final class Request {
final HttpUrl url;
final String method;
final Headers headers;
final @Nullable RequestBody body;
final Map, Object> tags;
public Builder() {
this.method = "GET";
this.headers = new Headers.Builder();
}
public static class Builder {
@Nullable HttpUrl url;
String method;
Headers.Builder headers;
@Nullable RequestBody body;
/** A mutable map of tags, or an immutable empty map if we don't have any. */
Map, Object> tags = Collections.emptyMap();
public Builder() {
this.method = "GET";
this.headers = new Headers.Builder();
}
}
3.3 开始请求
我们进入mOkHttpClient的newCall方法,它构造的是一个Call对象,实际上是一个RealCall
/**
* Prepares the {@code request} to be executed at some point in the future.
*/
@Override public Call newCall(Request request) {
return RealCall.newRealCall(this, request, false /* for web socket */);
}
RealCall#enqueue(Callback)
所以示例中的enqueue实际上是RealCall中的方法
@Override public void enqueue(Callback responseCallback) {
......
//将AsyncCall传入任务调度器,
client.dispatcher().enqueue(new AsyncCall(responseCallback));
}
将AsyncCall(这个我们稍后再说)传入任务调度器,任务任务调度器会将其存入待执行的请求队列(上面提到的readyAsyncCalls)中,然后条件允许的话再加入到运行中的请求队列(runningAsyncCalls)中,然后将这个请求放到任务调度器中的线程池中进行消费.下面是详细代码
----Dispatcher#enqueue(AsyncCall)
void enqueue(AsyncCall call) {
synchronized (this) {
readyAsyncCalls.add(call);
// Mutate the AsyncCall so that it shares the AtomicInteger of an existing running call to
// the same host.
if (!call.get().forWebSocket) {
AsyncCall existingCall = findExistingCallWithHost(call.host());
if (existingCall != null) call.reuseCallsPerHostFrom(existingCall);
}
}
promoteAndExecute();
}
private boolean promoteAndExecute() {
List executableCalls = new ArrayList<>();
boolean isRunning;
synchronized (this) {
//从待执行队列中取出来
for (Iterator i = readyAsyncCalls.iterator(); i.hasNext(); ) {
AsyncCall asyncCall = i.next();
//如果正在执行的任务>=64 那么就算了,先缓一缓
if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.
if (asyncCall.callsPerHost().get() >= maxRequestsPerHost) continue; // Host max capacity.
i.remove();
asyncCall.callsPerHost().incrementAndGet();
executableCalls.add(asyncCall);
//加入到运行队列中
runningAsyncCalls.add(asyncCall);
}
isRunning = runningCallsCount() > 0;
}
for (int i = 0, size = executableCalls.size(); i < size; i++) {
AsyncCall asyncCall = executableCalls.get(i);
//一个个地开始执行 executorService方法是获取线程池
asyncCall.executeOn(executorService());
}
return isRunning;
}
//获取线程池代码
public synchronized ExecutorService executorService() {
if (executorService == null) {
executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
new SynchronousQueue<>(), Util.threadFactory("OkHttp Dispatcher", false));
}
return executorService;
}
上面我们提到了很多次AsyncCall,它其实是一个RealCall的非静态内部类,所以能直接访问到RealCall的属性啥的,方便.同时,AsyncCall继承自NamedRunnable,NamedRunnable实现了NamedRunnable.
public abstract class NamedRunnable implements Runnable {
protected final String name;
public NamedRunnable(String format, Object... args) {
this.name = Util.format(format, args);
}
@Override public final void run() {
String oldName = Thread.currentThread().getName();
Thread.currentThread().setName(name);
try {
execute();
} finally {
Thread.currentThread().setName(oldName);
}
}
protected abstract void execute();
}
NamedRunnable中使用了模板方法模式,子类必须实现execute方法,并且将逻辑放在execute中.并且NamedRunnable中还设置了自己线程的名字,实属方便管理.
上面的任务调度器中执行的AsyncCall,相当于就是执行的AsyncCall的execute的逻辑
@Override protected void execute() {
boolean signalledCallback = false;
transmitter.timeoutEnter();
try {
//-----------------------重点代码 华丽的分割线围起来---------------------------------
//1. 通过拦截器链条,获取最终的网络请求结果
Response response = getResponseWithInterceptorChain();
//2. 标记已执行 不能再执行第二次了
signalledCallback = true;
//3. 将结果回调给调用处
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);
}
}
开始了,开始了,重点来了,通过getResponseWithInterceptorChain方法这条拦截器链路可以获取到网络请求的结果.然后我们通过CallBack接口回调回调用处.
start
在开始之前,大家先看两张图,这张图是整个拦截器的流程,也是OkHttp的精华,设计之巧妙.
image
image
从上面的代码也可以看到,getResponseWithInterceptorChain方法是获取到了网络请求的最终数据的.紧接着根据我画了两张图,这两张图主要是描绘了从getResponseWithInterceptorChain进去之后发生的事,它内部会串行的执行一些特定的拦截器(interceptors),每个拦截器负责一个特殊的职责.最后那个拦截器负责请求服务器,然后服务器返回了数据再根据这个拦截器的顺序逆序返回回去,最终就得到了网络数据.
下面先简单介绍一下这些拦截器,方便后面的源码梳理
- RetryAndFollowUpInterceptor 负责请求的重定向操作，用于处理网络请求中，请求失败后的重试机制。
- BridgeInterceptor 主要是添加一些header
- CacheInterceptor 负责缓存
- ConnectInterceptor 打开与目标服务器的连接
- CallServerInterceptor 最后一个拦截器,负责请求网络
3.4 进入拦截器调用链
有了上面的简单介绍,我们直接进入getResponseWithInterceptorChain方法一探究竟.
Response getResponseWithInterceptorChain() throws IOException {
// Build a full stack of interceptors.
//用来盛放所有的拦截器的
List interceptors = new ArrayList<>();
//1. 添加用户定义的拦截器
interceptors.addAll(client.interceptors());
//2. 添加一些OkHttp自带的拦截器
interceptors.add(new RetryAndFollowUpInterceptor(client));
interceptors.add(new BridgeInterceptor(client.cookieJar()));
interceptors.add(new CacheInterceptor(client.internalCache()));
interceptors.add(new ConnectInterceptor(client));
if (!forWebSocket) {
//这里还有一个网络拦截器,也是可以用户自定义的
interceptors.addAll(client.networkInterceptors());
}
//最终访问服务器的拦截器
interceptors.add(new CallServerInterceptor(forWebSocket));
//3. 将拦截器,当前拦截器索引等传入Interceptor.Chain
Interceptor.Chain chain = new RealInterceptorChain(interceptors, transmitter, null, 0,
originalRequest, this, client.connectTimeoutMillis(),
client.readTimeoutMillis(), client.writeTimeoutMillis());
boolean calledNoMoreExchanges = false;
try {
//4. 请求访问下一个拦截器
Response response = chain.proceed(originalRequest);
if (transmitter.isCanceled()) {
closeQuietly(response);
throw new IOException("Canceled");
}
return response;
} catch (IOException e) {
calledNoMoreExchanges = true;
throw transmitter.noMoreExchanges(e);
} finally {
if (!calledNoMoreExchanges) {
transmitter.noMoreExchanges(null);
}
}
}
可以看到,OkHttp这个拦截器链的大体流程,最开始是用户自定义的拦截器,然后才是OkHttp自己默认的拦截器(需要注意的是,最后一个拦截器是CallServerInterceptor).然后将拦截器集合和当前拦截器的索引等数据传入RealInterceptorChain,调用RealInterceptorChain对象的proceed,并最终得到执行结果.看来逻辑在RealInterceptorChain的proceed方法内部
public final class RealInterceptorChain implements Interceptor.Chain {
private final List interceptors;
private final Transmitter transmitter;
private final @Nullable Exchange exchange;
private final int index;
private final Request request;
private final Call call;
private final int connectTimeout;
private final int readTimeout;
private final int writeTimeout;
private int calls;
public RealInterceptorChain(List interceptors, Transmitter transmitter,
@Nullable Exchange exchange, int index, Request request, Call call,
int connectTimeout, int readTimeout, int writeTimeout) {
this.interceptors = interceptors;
this.transmitter = transmitter;
this.exchange = exchange;
this.index = index;
this.request = request;
this.call = call;
this.connectTimeout = connectTimeout;
this.readTimeout = readTimeout;
this.writeTimeout = writeTimeout;
}
@Override
public Response proceed(Request request) throws IOException {
return proceed(request, transmitter, exchange);
}
public Response proceed(Request request, Transmitter transmitter, @Nullable Exchange exchange)
throws IOException {
calls++;
// Call the next interceptor in the chain.
//调用下一个interceptor.注意到,这里的index索引+1了的,所以是下一个interceptor
RealInterceptorChain next = new RealInterceptorChain(interceptors, transmitter, exchange,
index + 1, request, call, connectTimeout, readTimeout, writeTimeout);
//当前interceptor
Interceptor interceptor = interceptors.get(index);
//调用interceptor的intercept方法
Response response = interceptor.intercept(next);
return response;
}
}
在proceed方法里面主要是将下一个拦截器的RealInterceptorChain构建出来,然后传入当前拦截器的intercept方法里面,方便在intercept方法里面执行下一个RealInterceptorChain的proceed方法.intercept方法返回的是获取数据之后的Response.
下面进入intercept方法内部,Interceptor其实是一个接口,然后所有的拦截器都实现了这个接口Interceptor.如果没有用户自定义的拦截器,那么第一个拦截器就是RetryAndFollowUpInterceptor
RetryAndFollowUpInterceptor#intercept
@Override public Response intercept(Chain chain) throws IOException {
Request request = chain.request();
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Transmitter transmitter = realChain.transmitter();
int followUpCount = 0;
Response priorResponse = null;
//死循环 直到达到重定向的最大次数
while (true) {
//准备一个流来承载request,如果存在则复用
transmitter.prepareToConnect(request);
if (transmitter.isCanceled()) {
throw new IOException("Canceled");
}
Response response;
boolean success = false;
try {
//调用下一个拦截器
response = realChain.proceed(request, transmitter, null);
success = true;
} catch (RouteException e) {
//下面是一些失败,然后又重新请求的代码
// The attempt to connect via a route failed. The request will not have been sent.
if (!recover(e.getLastConnectException(), transmitter, false, request)) {
throw e.getFirstConnectException();
}
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, transmitter, requestSendStarted, request)) throw e;
continue;
} finally {
// The network call threw an exception. Release any resources.
if (!success) {
transmitter.exchangeDoneDueToException();
}
}
// 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();
}
Exchange exchange = Internal.instance.exchange(response);
Route route = exchange != null ? exchange.connection().route() : null;
Request followUp = followUpRequest(response, route);
if (followUp == null) {
if (exchange != null && exchange.isDuplex()) {
transmitter.timeoutEarlyExit();
}
return response;
}
RequestBody followUpBody = followUp.body();
if (followUpBody != null && followUpBody.isOneShot()) {
return response;
}
closeQuietly(response.body());
if (transmitter.hasExchange()) {
exchange.detachWithViolence();
}
if (++followUpCount > MAX_FOLLOW_UPS) {
throw new ProtocolException("Too many follow-up requests: " + followUpCount);
}
request = followUp;
priorResponse = response;
}
}
RetryAndFollowUpInterceptor主要是负责错误处理,以及重定向.当然重定向是有最大次数的,OkHttp规定是20次.
RetryAndFollowUpInterceptor执行proceed方法是来到了BridgeInterceptor,它是一个连接桥.添加了很多header
@Override public Response intercept(Chain chain) throws IOException {
Request userRequest = chain.request();
Request.Builder requestBuilder = userRequest.newBuilder();
//进行header的包装
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");
}
//添加Accept-Encoding：gzip
// 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");
}
//创建OkhttpClient配置的cookieJar
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());
}
//执行下一个Interceptor
Response networkResponse = chain.proceed(requestBuilder.build());
HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
Response.Builder responseBuilder = networkResponse.newBuilder()
.request(userRequest);
//先判断服务器是否支持gzip压缩,支持则交给Okio处理
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);
String contentType = networkResponse.header("Content-Type");
responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
}
//最后将结果返回
return responseBuilder.build();
}
BridgeInterceptor就跟它的名字那样，它是一个连接桥.它负责把用户构造的请求转换成发送给服务器的请求,就是添加了不少的header,其中还有gzip等.
BridgeInterceptor的下一个拦截器是CacheInterceptor
@Override public Response intercept(Chain chain) throws IOException {
////如果配置了缓存：优先从缓存中读取Response
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
//缓存策略，该策略通过某种规则来判断缓存是否有效
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = strategy.networkRequest;
Response cacheResponse = strategy.cacheResponse;
if (cache != null) {
cache.trackResponse(strategy);
}
if (cacheCandidate != null && cacheResponse == null) {
closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
}
// If we're forbidden from using the network and the cache is insufficient, fail.
//如果根据缓存策略strategy禁止使用网络，并且缓存无效，直接返回空的Response
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.
//如果根据缓存策略strategy禁止使用网络，且有缓存则直接使用缓存
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
//需要网络
Response networkResponse = null;
try {
//执行下一个拦截器,发起网路请求
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());
}
}
//本地有缓存，
// If we have a cache response too, then we're doing a conditional get.
if (cacheResponse != null) {
//并且服务器返回304状态码（说明缓存还没过期或服务器资源没修改）
if (networkResponse.code() == HTTP_NOT_MODIFIED) {
//使用缓存数据
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());
}
}
//如果网络资源已经修改：使用网络响应返回的最新数据
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
//将最新的数据缓存起来
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.
}
}
}
//返回最新的数据
return response;
}
CacheInterceptor是进行一些缓存上面的处理,接下来是ConnectInterceptor
@Override
public Response intercept(Chain chain) throws IOException {
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Request request = realChain.request();
Transmitter transmitter = realChain.transmitter();
// We need the network to satisfy this request. Possibly for validating a conditional GET.
//判断请求是不是GET方法, 不是的情况下,需要进行有效监测
boolean doExtensiveHealthChecks = !request.method().equals("GET");
Exchange exchange = transmitter.newExchange(chain, doExtensiveHealthChecks);
//执行下一个拦截器
return realChain.proceed(request, transmitter, exchange);
}
ConnectInterceptor的下一个拦截器就是最好一个拦截器CallServerInterceptor了.
@Override public Response intercept(Chain chain) throws IOException {
RealInterceptorChain realChain = (RealInterceptorChain) chain;
Exchange exchange = realChain.exchange();
Request request = realChain.request();
long sentRequestMillis = System.currentTimeMillis();
//整理请求头并写入
exchange.writeRequestHeaders(request);
boolean responseHeadersStarted = false;
Response.Builder responseBuilder = null;
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.
if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
exchange.flushRequest();
responseHeadersStarted = true;
exchange.responseHeadersStart();
responseBuilder = exchange.readResponseHeaders(true);
}
if (responseBuilder == null) {
if (request.body().isDuplex()) {
// Prepare a duplex body so that the application can send a request body later.
exchange.flushRequest();
BufferedSink bufferedRequestBody = Okio.buffer(
exchange.createRequestBody(request, true));
request.body().writeTo(bufferedRequestBody);
} else {
// Write the request body if the "Expect: 100-continue" expectation was met.
BufferedSink bufferedRequestBody = Okio.buffer(
exchange.createRequestBody(request, false));
request.body().writeTo(bufferedRequestBody);
bufferedRequestBody.close();
}
} else {
exchange.noRequestBody();
if (!exchange.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.
exchange.noNewExchangesOnConnection();
}
}
} else {
exchange.noRequestBody();
}
if (request.body() == null || !request.body().isDuplex()) {
//发送最终的请求
exchange.finishRequest();
}
if (!responseHeadersStarted) {
exchange.responseHeadersStart();
}
if (responseBuilder == null) {
//响应头
responseBuilder = exchange.readResponseHeaders(false);
}
Response response = responseBuilder
.request(request)
.handshake(exchange.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
int code = response.code();
if (code == 100) {
// server sent a 100-continue even though we did not request one.
// try again to read the actual response
response = exchange.readResponseHeaders(false)
.request(request)
.handshake(exchange.connection().handshake())
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(System.currentTimeMillis())
.build();
code = response.code();
}
exchange.responseHeadersEnd(response);
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(exchange.openResponseBody(response))
.build();
}
//断开连接
if ("close".equalsIgnoreCase(response.request().header("Connection"))
|| "close".equalsIgnoreCase(response.header("Connection"))) {
exchange.noNewExchangesOnConnection();
}
//抛出协议异常
if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
throw new ProtocolException(
"HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
}
return response;
}
这是链中最后一个拦截器，它向 服务器 发起了一次网络访问.负责向服务器发送请求数据、从服务器读取响应数据.拿到数据之后再沿着链返回.
4. 总结
OkHttp的拦截器链设计得非常巧妙,是典型的责任链模式.并最终由最后一个链处理了网络请求,并拿到结果.本文主要是对OkHttp主流程进行了梳理,通过本文能对OkHttp有一个整体的了解.