Okhttp3源码解析

首先是Okhttp的使用：

//缓存文件夹
File cacheFile = new File(getExternalCacheDir().toString(), "cache");
//缓存大小为10M
int cacheSize = 10 * 1024 * 1024;
//创建缓存对象
Cache cache = new Cache(cacheFile, cacheSize);
OkHttpClient client = new OkHttpClient
        .Builder()
        //.addInterceptor(new LoggingInterceptor())//日志拦截器
.cache(cache)
        .build();
Request request = new Request.Builder()
        .url("http://www.baidu.com")
        .build();
client.newCall(request).enqueue(new Callback() {
    @Override
public void onFailure(Call call, IOException e) {
        Log.d("OkHttp", "Call Failed:" + e.getMessage());
}

    @Override
public void onResponse(Call call, Response response) throws IOException {
        Log.d("OkHttp", "Call succeeded:" + response.message());
}
});

先来看一下Request包含的有通讯的信息

url：发起通讯的地址

method：通讯方式

public final class Request {
  final HttpUrl url;
  final String method;
  final Headers headers;
  final RequestBody body;
  final Object tag;

  private volatile CacheControl cacheControl; // Lazily initialized.

Request(Builder builder) {
    this.url = builder.url;
    this.method = builder.method;
    this.headers = builder.headers.build();
    this.body = builder.body;
    this.tag = builder.tag != null ? builder.tag : this;
}

主体流程

下面我们先来摸清这个框架的主干部分，刚开始看的时候，先沿一条线下来，细枝末节先不要太关心：

我们来看看newCall：

client.newCall(request).enqueue(new Callback() {

@Override public Call newCall(Request request) {
  return new RealCall(this, request, false /* for web socket */);
}

final class RealCall implements Call {

RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
  final EventListener.Factory eventListenerFactory = client.eventListenerFactory();

  this.client = client;
  this.originalRequest = originalRequest;
  this.forWebSocket = forWebSocket;
  this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket);

// TODO(jwilson): this is unsafe publication and not threadsafe.
this.eventListener = eventListenerFactory.create(this);
}

可以看出：Call是一个接口，而RealCall是Call的一个具体实现。client.newCall(request)里面是生成一个RealCall的实例

Recall同时持有client和request，

所以：

client.newCall(request).enqueue(new Callback() {

实际上执行到的是RealCall的enqueue：

@Override public void enqueue(Callback responseCallback) {
  synchronized (this) {
    if (executed) throw new IllegalStateException("Already Executed");
executed = true;
}
  captureCallStackTrace();
client.dispatcher().enqueue(new AsyncCall(responseCallback));
}

这里的responseCallback就是我们最外层的回调接口，暂时先不管这个。

重点看这行代码：

client.dispatcher().enqueue(new AsyncCall(responseCallback));

dispatcher是client持有的一个调度器，我们找到Dispatcher这个类里面的enqueue方法：

synchronized void enqueue(AsyncCall call) {
  if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
    runningAsyncCalls.add(call);
executorService().execute(call);
} else {
    readyAsyncCalls.add(call);
}
}

可以看出，runningAsyncCalls，这里是线程池里面的概念，先看executorService().execute(call）

来看一下call：

final class AsyncCall extends NamedRunnable {

public abstract class NamedRunnable implements Runnable {

可以看出，AsyncCall是一个Runnable，所以executorService().execute(call）执行的时候，实际上执行的是AsnycCall里面的execute()方法：

@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);
}
}

找到关键的语句：

Response result = getResponseWithInterceptorChain();

这句是这个框架比较核心的思想：拦截器，通过一个个的拦截器，通过递归串联起来：

Response getResponseWithInterceptorChain() throws IOException {
  // Build a full stack of interceptors.
List<Interceptor> interceptors = new ArrayList<>();
interceptors.addAll(client.interceptors());
interceptors.add(retryAndFollowUpInterceptor);
interceptors.add(new BridgeInterceptor(client.cookieJar()));
interceptors.add(new CacheInterceptor(client.internalCache()));
interceptors.add(new ConnectInterceptor(client));
  if (!forWebSocket) {
    interceptors.addAll(client.networkInterceptors());
}
  interceptors.add(new CallServerInterceptor(forWebSocket));

Interceptor.Chain chain = new RealInterceptorChain(
      interceptors, null, null, null, 0, originalRequest);
  return chain.proceed(originalRequest);
}

首先是interceptors.addAll(client.interceptors())，这里面的intterceptors是哪里来的呢，从代码可以看出是由client来的，其实就是我们初始化client的时候，由用户

添加的new OkHttpClient.Builder().addInterceptor(new LoggingInterceptor()).build();这里面的LoggingInterceptor就是我们用户自己定义的interceptor，从上面的代码

我们可以看到interceptors是一个list，说明client的addInterceptor可以添加多个。

我们看client的interceptors，是分为两种类型的， addInterceptor()和addNetworkInterceptor() ，通过 addInterceptor() 方法添加的拦截器是放在最前面的。通过 addNetworkInterceptor() 方法添加的网络拦截器，则是在非 WebSocket 请求时，添加在 ConnectInterceptor 和 CallServerInterceptor 之间的。这两个的区别，可以看

这里：https://blog.csdn.net/qq_38219041/article/details/72935142?utm_source=blogxgwz1

addInterceptor() 添加应用拦截器
● 不需要担心中间过程的响应,如重定向和重试.
● 总是只调用一次,即使HTTP响应是从缓存中获取.
● 观察应用程序的初衷. 不关心OkHttp注入的头信息如: If-None-Match.
● 允许短路而不调用 Chain.proceed(),即中止调用.
● 允许重试,使 Chain.proceed()调用多次.
addNetworkInterceptor() 添加网络拦截器
● 能够操作中间过程的响应,如重定向和重试.
● 当网络短路而返回缓存响应时不被调用.
● 只观察在网络上传输的数据.
● 携带请求来访问连接.

接下来的重点是这句话：

Interceptor.Chain chain = new RealInterceptorChain(
    interceptors, null, null, null, 0, originalRequest);
return chain.proceed(originalRequest);

interceptors的循环调用就在这里面了，那我们看一下这里面是如何实现的：

public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
RealConnection 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.
if (this.httpCodec != null && !this.connection.supportsUrl(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().
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.
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;
}

可以看到，RealInterceptorChain的参数index第一次传进来的时候为0，来看porceed这个方法里面，重点是下面这句话：

// Call the next interceptor in the chain.
RealInterceptorChain next = new RealInterceptorChain(
    interceptors, streamAllocation, httpCodec, connection, index + 1, request);
Interceptor interceptor = interceptors.get(index);
Response response = interceptor.intercept(next);

这里先new一个RealInterceptorChain，index为在原来的基础上+1，然后从前面的interceptors这个list里面，根据index取出相对应的一个interceptor，然后这个RealInterceptorChain作为一个参数传进interceptor.intercept

这时候我们就要回过头去看一下，说了这么久的interceptor到底是什么：

interceptors.add(new BridgeInterceptor(client.cookieJar()));

public final class BridgeInterceptor implements Interceptor { 

public interface Interceptor {
  Response intercept(Chain chain) throws IOException;

  interface Chain {
    Request request();

Response proceed(Request request) throws IOException;

Connection connection();
}
}

interceptort定义一个接口，各个具体的拦截器实现这个接口，我们用户自己定义的拦截器也都是实现这个接口，

前面提到的：

Response response = interceptor.intercept(next);

其实就是每个具体拦截器的方法intercept：

@Override public Response intercept(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");
}

  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());

HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

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();
}

以上代码我们先追踪是如何实现循环调用interceptor，主要是以下这一句：

Response networkResponse = chain.proceed(requestBuilder.build());

看到这里，又调用了chain的proceed，而这个chain就是RealInterceptorChain，又回到了chain的proceed里面：

// Call the next interceptor in the chain.
RealInterceptorChain next = new RealInterceptorChain(
    interceptors, streamAllocation, httpCodec, connection, index + 1, request);
Interceptor interceptor = interceptors.get(index);
Response response = interceptor.intercept(next);

通过index再次加1，调用下一个interceptor的intercept；所以拦截器list interceptors并不是通过for循环来获得每一个的拦截器，而是通过前面的index每次+1，然后

通过chain来串连起来。

由上可知，每个拦截器的重点位置就是看里面的intercept方法如何执行的，这里面都包括该拦截器的本身的业务处理以及调用下一个拦截器。

 拦截器

BridgeInterceptor：转换应用层request数据为真正的网络请求数据：

先看一下BridgeInterceptor,拦截器我们都是从intercept方法入手：

@Override public Response intercept(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");
    }

    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());

    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

    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();
  }

以上代码中，前半部分，chain.proceed(requestBuilder.build())前面，其实就是将最外层的代码，即用户层面定义的request：

Request request = new Request.Builder() .........

里面的各个参数转为真正的网络请求参数，包括Host、Connect、Accept-Encoding、User-Agent、Cookie等http的头部分头信息，通过：

Response networkResponse = chain.proceed(requestBuilder.build());

将这些信息传入到下一个拦截器去执行，请求阶段这个拦截器的作用已经结束，接下来就是调用：

Response.Builder responseBuilder = networkResponse.newBuilder()
    .request(userRequest);

来等待下级拦截器的返回结果。所以拦截器的作用就是，我尽管完成我的那一部分工作，把我得到的请求交给下一个拦截器，中间的过程不用管，我就等待下一个拦截器返回结果，然后再将结果同样返回上一个拦截器。

return responseBuilder.build();

CacheInterceptor：缓存拦截器，从网络还是缓存获取数据：

接下来看一下CacheInterceptor：

@Override public Response intercept(Chain chain) throws IOException {
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

    long now = System.currentTimeMillis();

    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.
    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.
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    Response networkResponse = null;
    try {
      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;
  }

首先是这一句：

Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

判断本地是否存在缓存，如果存在，直接用，如果不存在，则值为空继续往下，那这个cache又从哪里来呢，其实也是应用层定义的：

//缓存文件夹
File cacheFile = new File(getExternalCacheDir().toString(), "cache");
//缓存大小为10M
int cacheSize = 10 * 1024 * 1024;
Cache cache = new Cache(cacheFile, cacheSize);
OkHttpClient client = new OkHttpClient
        .Builder()
        //.addInterceptor(new LoggingInterceptor())//日志拦截器
.cache(cache)

这里定义了一个file存储cache，定义了名字、存储路径以及最大缓存容量，那是在哪个地方设置值进去呢：

// 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());
  }
}

我们先来看HTTP_NOT_MODIFIED看是什么意思：

/**
     * HTTP Status-Code 304: Not Modified.
     */
    public static final int HTTP_NOT_MODIFIED = 304;

返回这个意思就是，当客户端再次请求数据的时候，这个时间到上次请求的时间段内，服务器的资源数据没有更新，所以对于客户端来说，没必要再次去服务器获取具体数据，直接从本地获取上一次的数据，然后return response。

如果说是网络资源已修改，或者是第一次请求，则使用下一层的网络请求返回response，然后将该response缓存在cache里面供下次的请求使用。

RetryAndFollowUpInterceptor 重试和重定向拦截器
作者：chaychan
链接：http://www.imooc.com/article/details/id/271841
来源：慕课网

未完待续。。。