前言

Retrofit是Square公司开发的一款Android网络请求库，Retrofit仅负责网络请求接口的封装，它使用运行时注解的方式提供请求参数的配置功能，底层基于OkHttp实现真正的网络请求。

如果还不了解OkHttp框架，强烈建议先阅读Android 网络请求库OkHttp基本原理与实现再来阅读本篇文章，会更加酣畅淋漓哦~

本文所提及的网络请求如无特殊说明，都是异步网络请求

为什么选择Retrofit（与其他主流框架对比）

现在市场上比较主流的网络请求框架有：

• android-async-http（作者停止维护、不推荐使用）
• Volley（谷歌）
• OkHttp（Square）
• Retrofit（Square）

一图让你了解全部的网络请求库和他们之间的区别！

Retrofit基本用法

1. 导入库

    implementation("com.squareup.okhttp3:okhttp:4.9.3")
    // define a BOM and its version
    implementation(platform("com.squareup.okhttp3:okhttp-bom:4.9.3"))
    implementation("com.squareup.okhttp3:logging-interceptor")

    implementation 'com.squareup.retrofit2:retrofit:2.9.0'
    implementation("com.squareup.retrofit2:converter-gson:2.9.0") //支持返回值为Gson类型的数据

默认情况下，Retrofit只能将数据反序列化为OkHttp的ResponseBody类型，并且它只能接受@Body注解的RequestBody类型。

除了默认情况，我们也可以添加转换器以支持其他类型。

• Gson: com.squareup.retrofit2:converter-gson
• Jackson: com.squareup.retrofit2:converter-jackson
• Moshi: com.squareup.retrofit2:converter-moshi
• Protobuf: com.squareup.retrofit2:converter-protobuf
• Wire: com.squareup.retrofit2:converter-wire
• Simple XML: com.squareup.retrofit2:converter-simplexml
• JAXB: com.squareup.retrofit2:converter-jaxb
• Scalars (primitives, boxed, and String): com.squareup.retrofit2:converter-scalars

2. 注解分类

Retrofit注解分为三大类，分别是请求方法注解、标记类注解和参数类注解。

请求方法注解有8种：GET/POST/PUT/DELETE/HEAD/PATCH/OPTIONS/HTTP，前7种与HTTP请求方法是对应的，最后的HTTP可以替换前面七种，也可以扩展请求方法。

标记类注解有3种：FormUrlEncoded、Multipart、Streaming。

参数类注解有Header、Headers、Body、Path、Field、FieldMap、Query、QueryMap、Part、PartMap等。

3. GET网络请求

首先编写请求网络接口

open interface ApiService {
    @GET("users/{user}/repos")
    fun listRepos(@Path("user") user: String?): Call<List<Repo?>?>?
}

@GET 注解指定当前网络请求方式，注解参数是访问地址，方法返回Call<List<Repo?>?>类型的数据。@Path注解用来动态配置Url地址，@Path("user")对应@GET注解中的{user}，即使用传入的 user: String?值动态替换{user}

接下来创建Retrofit对象，创建接口类（动态代理）调用网络请求方法，得到Call并发起异步请求，回调的Callback运行在UI线程。

val retrofit = Retrofit.Builder()
        .baseUrl("https://api.github.com/")
        .client(okHttpClient)
        .addConverterFactory(GsonConverterFactory.create())  //数据转换器Factory
        .build()
val listRepos = retrofit.create(ApiService::class.java).listRepos("octocat")
listRepos?.enqueue(object : Callback<List<Repo?>?> {
    override fun onResponse(call: Call<List<Repo?>?>, response: Response<List<Repo?>?>) {
        Log.d("retrofit", response.toString())
    }

    override fun onFailure(call: Call<List<Repo?>?>, t: Throwable) {

    }
})

请求的Url是由baseUrl和@GET注解参数拼接出来的：https://api.github.com/users/octocat/repos

一般情况下为了更准确地去查找网络数据，我们需要在Url中添加查询参数，这种情况可以使用Query指定查询条件。例如根据id查找数据，我们修改代码如下：

@GET("users/{user}/repos")
fun listRepos(@Path("user") user: String?, 
              @Query("id") id: Int?): Call<List<Repo?>?>?

val listRepos = retrofit.create(ApiService::class.java).listRepos("octocat", 132935648)

这样请求Url变为：https://api.github.com/users/octocat/repos?id=132935648

如果有多个查询参数使用@QueryMap

@GET("users/{user}/repos")
fun listRepos(@Path("user") user: String?,
              @QueryMap map:HashMap<String, Any>): Call<List<Repo?>?>?

val hashMap = HashMap<String, Any>()
hashMap.put("id", 132935648)
val listRepos = retrofit.create(ApiService::class.java).listRepos("octocat", hashMap)

4. POST网络请求

POST请求最常见的情况是传输键值对的数据类型，使用@Field注解就可以为POST请求添加键值对

@FormUrlEncoded
@POST("user/edit")
fun listRepos(@Field("first_name") first: String,
              @Field("last_name") last: String, ): Call<List<Repo?>?>?

val listRepos = retrofit.create(ApiService::class.java).listRepos("张", "三")

@FormUrlEncoded表明这是一个表单请求，使用@Field注解标注对应的键值对。

POST请求除了传输键值对，还可以传输JSON字符串，对应注解@Body。

package android.com.retrofit
data class User(val id: Int,
                val age: Int,
                val school: String) {
    constructor() : this(999, 16, "abc")
}

@POST("user/edit")
fun listRepos(@Body user: User): Call<List<Repo?>?>?

val listRepos = retrofit.create(ApiService::class.java).listRepos(User())

上传单个文件

 * @param photo 上传的文件
 * @param desc 文件描述（简单的键值对）
 */
@Multipart //表示允许多个part
@POST("user/photo")
fun uploadPhoto(@Part photo: MultipartBody.Part,
                @Part("desc") desc:

val file = File("文件路径")
val create = file.asRequestBody("image".toMediaTypeOrNull())
val photo = MultipartBody.Part.createFormData("photos", "aaa", create)
val requestBodyDesc = "picture".toRequestBody(null)
val listRepos = retrofit.create(ApiService::class.java).uploadPhoto(photo,requestBodyDesc)

上传多个文件
与上传单个文件类似，使用Map封装上传的文件，用@PartMap注解标注

更多用法可参考Carson带你学Android：网络请求库Retrofit使用教程(含实例讲解)

Retrofit原理解析

Retrofit最终的目的是通过使用大量的设计模式进行功能模块的解耦，使得网络请求过程变得更加简单流畅。

下图可以直观对比出一般网络请求流程与Retrofit请求的区别

Retrofit流程

详细步骤如下：

1. 通过解析网络请求接口的注解配置网络请求参数
2. 通过动态代理生成网络请求对象
3. 通过网络请求适配器将网络请求对象进行平台适配（平台包括：Android、Rxjava、Guava和java8）
4. 通过网络请求执行器发送网络请求
5. 通过数据转换器解析服务器返回的数据
6. 通过回调执行器切换线程（子线程 ->>主线程）
7. 用户在主线程处理返回结果

主要角色说明

源码分析（基于Retrofit 2.9.0）

我们从Retrofit最基本的使用方式作为切入口，一步步深入源码探索Retrofit实现原理。

Retrofit使用主要分为以下几步：

1. 创建Retrofit对象（建造者模式）
2. 实例化接口类并调用请求方法（动态代理）
3. 使用第2步返回的Call发起网络请求，Callback回调会运行在UI线程

接下来展开分析：

1. Retrofit.Builder()

因为Retrofit可配置的参数较多，所以采用建造者模式创建Retrofit实例。

Builder类主要代码如下：

  public static final class Builder {
    private final Platform platform;
    private @Nullable okhttp3.Call.Factory callFactory;
    private @Nullable HttpUrl baseUrl;  //网络请求基本地址
    private final List<Converter.Factory> converterFactories = new ArrayList<>();   //用于创建Converter.Factory
    private final List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>();   //用于创建CallAdapter.Factory
    private @Nullable Executor callbackExecutor;    //切换线程执行回调任务（默认是主线程）
    private boolean validateEagerly;

    //指定OkHttpClient
    public Builder client(OkHttpClient client) {  
      return callFactory(Objects.requireNonNull(client, "client == null"));
    }

    //设置baseUrl
    public Builder baseUrl(URL baseUrl) {  
      Objects.requireNonNull(baseUrl, "baseUrl == null");
      return baseUrl(HttpUrl.get(baseUrl.toString()));
    }

    /** 添加自定义的Converter Factory：用于创建数据转换器Converter  */
    public Builder addConverterFactory(Converter.Factory factory) {
      converterFactories.add(Objects.requireNonNull(factory, "factory == null"));
      return this;
    }

    /**
     * Add a call adapter factory for supporting service method return types other than {@link
     * Call}.  添加自定义的CallAdapter Factory：用于创建网络请求适配器CallAdapter
     */
    public Builder addCallAdapterFactory(CallAdapter.Factory factory) {
      callAdapterFactories.add(Objects.requireNonNull(factory, "factory == null"));
      return this;
    }

    //指定执行回调任务的Executor
    public Builder callbackExecutor(Executor executor) {
      this.callbackExecutor = Objects.requireNonNull(executor, "executor == null");
      return this;
    }

    public Retrofit build() {
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }

      //设置默认OkHttpClient
      okhttp3.Call.Factory callFactory = this.callFactory;  
      if (callFactory == null) {
        callFactory = new OkHttpClient(); 
      }
     //android平台默认使用MainThreadExecutor，用于切换到主线程执行任务
      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {  
        callbackExecutor = platform.defaultCallbackExecutor();  
      }

      //添加平台默认CallAdapter.Factory到CallAdapterFactories
      List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);  
      callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));

      //添加平台默认Converter.Factory到converterFactories 
      List<Converter.Factory> converterFactories =
          new ArrayList<>(
              1 + this.converterFactories.size() + platform.defaultConverterFactoriesSize());
      converterFactories.add(new BuiltInConverters());
      converterFactories.addAll(this.converterFactories);
      converterFactories.addAll(platform.defaultConverterFactories());

      return new Retrofit(
          callFactory,
          baseUrl,
          unmodifiableList(converterFactories),
          unmodifiableList(callAdapterFactories),
          callbackExecutor,
          validateEagerly);
    }
}

如果未配置参数，build()方法会为我们提供大多数的默认配置，例如callAdapterFactories（创建网络请求适配器）、converterFactories（创建数据转换器）、callbackExecutor（执行回调任务）等待。

2. retrofit.create(ApiService::class.java).listRepos("octocat")

第一步：采取动态代理的方式创建请求接口的实现类

  public <T> T create(final Class<T> service) {
    validateServiceInterface(service);
    return (T)
        Proxy.newProxyInstance(
            service.getClassLoader(),
            new Class<?>[] {service},
            new InvocationHandler() {
              private final Platform platform = Platform.get();
              private final Object[] emptyArgs = new Object[0];

              @Override
              public @Nullable Object invoke(Object proxy, Method method, @Nullable Object[] args)
                  throws Throwable {
                // If the method is a method from Object then defer to normal invocation.
            //如果是Object的成员函数，直接使用反射调用方法
                if (method.getDeclaringClass() == Object.class) {
                  return method.invoke(this, args);
                }
                args = args != null ? args : emptyArgs;
                //platform.isDefaultMethod(method)返回true表示方法是公共的非抽象实例方法
                return platform.isDefaultMethod(method)
                    ? platform.invokeDefaultMethod(method, service, proxy, args)
                    : loadServiceMethod(method).invoke(args);   //接口方法会执行到此处
              }
            });
  }

ApiService.kt是一个接口类，当我们调用接口内的方法时会执行代码：InvocationHandler# invoke(Object proxy, Method method, @Nullable Object[] args) -> loadServiceMethod(method).invoke(args)，先分析loadServiceMethod(method)方法：

  ServiceMethod<?> loadServiceMethod(Method method) {
    ServiceMethod<?> result = serviceMethodCache.get(method);
    if (result != null) return result;

    synchronized (serviceMethodCache) {
      result = serviceMethodCache.get(method);
      if (result == null) {
        result = ServiceMethod.parseAnnotations(this, method);
        serviceMethodCache.put(method, result);
      }
    }
    return result;
  }

代码很简单，调用ServiceMethod.parseAnnotations(this, method)解析接口中的注解方法并封装成ServiceMethod。并且利用serviceMethodCache缓存解析结果，这样可以防止重复解析，使用缓存是因为解析的过程用到了比较耗费性能的反射机制。

abstract class ServiceMethod<T> {
  static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
    //注释2.1：解析方法注解和参数注解，把注解中的网络请求参数封装到RequestFactory中
    RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method); 

    //注释2.2：保证方法返回类型不是Unresolvable和void.class
    Type returnType = method.getGenericReturnType();
    if (Utils.hasUnresolvableType(returnType)) {
      throw methodError(
          method,
          "Method return type must not include a type variable or wildcard: %s",
          returnType);
    }
    if (returnType == void.class) {
      throw methodError(method, "Service methods cannot return void.");
    }

    //注释2.3
    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
  }

  abstract @Nullable T invoke(Object[] args);
}

注释2.1：RequestFactory.parseAnnotations(retrofit, method)解析注解信息，把信息封装到RequestFactory中
注释2.2：保证方法返回类型不是Unresolvable和void.class
注释2.3：调用HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory)获取ServiceMethod

#HttpServiceMethod
//这里只贴出部分代码
  static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
      Retrofit retrofit, Method method, RequestFactory requestFactory) {
    boolean isKotlinSuspendFunction = requestFactory.isKotlinSuspendFunction;
    boolean continuationWantsResponse = false;
    boolean continuationBodyNullable = false;

    Annotation[] annotations = method.getAnnotations();
    Type adapterType;  //方法返回值类型
    //isKotlinSuspendFunction默认为false，进入else语句获取adapterType 
    if (isKotlinSuspendFunction) {  
      ……
    } else {
      adapterType = method.getGenericReturnType(); 
    }
    //注释2.3.1：从callAdapterFactories列表中取出CallAdapter.Factory创建CallAdapter
    //我们可以使用Retrofit.Builder#addCallAdapterFactory(Factory)添加CallAdapter.Factory到列表callAdapterFactories
    //CallAdapter负责执行网络请求Call的适配工作
    CallAdapter<ResponseT, ReturnT> callAdapter =
        createCallAdapter(retrofit, method, adapterType, annotations);  
    Type responseType = callAdapter.responseType();

    ……

    //注释2.3.2：从converterFactories列表中取出Converter.Factory创建Converter
    //我们可以使用Retrofit.Builder#addConverterFactory(Factory)添加Converter .Factory到converterFactories列表
    //Converter负责将服务端返回的数据转换成对象实例
    Converter<ResponseBody, ResponseT> responseConverter =
        createResponseConverter(retrofit, method, responseType);

    okhttp3.Call.Factory callFactory = retrofit.callFactory;
    //注释2.3.3：把requestFactory, callFactory, responseConverter, callAdapter封装进最终的HttpServiceMethod
    if (!isKotlinSuspendFunction) {
      return new CallAdapted<>(requestFactory, callFactory, responseConverter, callAdapter);
    } else if (continuationWantsResponse) {
      //noinspection unchecked Kotlin compiler guarantees ReturnT to be Object.
      return (HttpServiceMethod<ResponseT, ReturnT>)
          new SuspendForResponse<>(
              requestFactory,
              callFactory,
              responseConverter,
              (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter);
    } else {
      //noinspection unchecked Kotlin compiler guarantees ReturnT to be Object.
      return (HttpServiceMethod<ResponseT, ReturnT>)
          new SuspendForBody<>(
              requestFactory,
              callFactory,
              responseConverter,
              (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter,
              continuationBodyNullable);
    }
  }

注释2.3.1：从callAdapterFactories列表中取出CallAdapter.Factory创建CallAdapter，具体流程如下：

//HttpServiceMethod.java
  private static <ResponseT, ReturnT> CallAdapter<ResponseT, ReturnT> createCallAdapter(
      Retrofit retrofit, Method method, Type returnType, Annotation[] annotations) {
    try {
      //noinspection unchecked
      return (CallAdapter<ResponseT, ReturnT>) retrofit.callAdapter(returnType, annotations);
    } catch (RuntimeException e) { // Wide exception range because factories are user code.
      throw methodError(method, e, "Unable to create call adapter for %s", returnType);
    }
  }

调用retrofit.callAdapter(returnType, annotations)

#Retrofit.java
  public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) {
    return nextCallAdapter(null, returnType, annotations);
  }

  public CallAdapter<?, ?> nextCallAdapter(
      @Nullable CallAdapter.Factory skipPast, Type returnType, Annotation[] annotations) {
    Objects.requireNonNull(returnType, "returnType == null");
    Objects.requireNonNull(annotations, "annotations == null");

    int start = callAdapterFactories.indexOf(skipPast) + 1;
    //遍历callAdapterFactories列表获取CallAdapter.Factory来创建CallAdapter
    for (int i = start, count = callAdapterFactories.size(); i < count; i++) {  
      CallAdapter<?, ?> adapter = callAdapterFactories.get(i).get(returnType, annotations, this);
      if (adapter != null) {
        return adapter;
      }
    }

    //获取失败
  }

最终遍历callAdapterFactories列表获取CallAdapterFactory来创建我们需要的网络请求适配器CallAdapter，在构建Retrofit时已经添加Android平台默认的platform.defaultCallAdapterFactories到callAdapterFactories

//Retrofit.Builder#build()
//添加默认Call adapter.
List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));

除了使用默认的CallAdapterFactory，我们也可以使用Retrofit.Builder#addCallAdapterFactory(Factory)主动添加CallAdapter.Factory到列表callAdapterFactories

CallAdapter主要负责执行网络请求Call的适配工作，注释2.3.2会详细分析

注释2.3.2：从converterFactories列表中取出Converter.Factory创建Converter，具体流程如下：

//HttpServiceMethod.java
  private static <ResponseT> Converter<ResponseBody, ResponseT> createResponseConverter(
      Retrofit retrofit, Method method, Type responseType) {
    Annotation[] annotations = method.getAnnotations();
    try {
      return retrofit.responseBodyConverter(responseType, annotations);
    } catch (RuntimeException e) { // Wide exception range because factories are user code.
      throw methodError(method, e, "Unable to create converter for %s", responseType);
    }
  }

调用retrofit.responseBodyConverter(responseType, annotations)

  public <T> Converter<ResponseBody, T> responseBodyConverter(Type type, Annotation[] annotations) {
    return nextResponseBodyConverter(null, type, annotations);
  }

  public <T> Converter<ResponseBody, T> nextResponseBodyConverter(
      @Nullable Converter.Factory skipPast, Type type, Annotation[] annotations) {
    Objects.requireNonNull(type, "type == null");
    Objects.requireNonNull(annotations, "annotations == null");

    int start = converterFactories.indexOf(skipPast) + 1;
    //遍历converterFactories列表获取Converter.Factory来创建Converter
    for (int i = start, count = converterFactories.size(); i < count; i++) {  
      Converter<ResponseBody, ?> converter =
          converterFactories.get(i).responseBodyConverter(type, annotations, this);
      if (converter != null) {
        //noinspection unchecked
        return (Converter<ResponseBody, T>) converter;
      }
    }

    //获取失败
  }

最终遍历converterFactories列表获取Converter.Factory来创建我们需要的Converter，在构建Retrofit时已经添加Android平台默认的platform.defaultConverterFactories()到defaultConverterFactories

//Retrofit.Builder#build()
List<Converter.Factory> converterFactories = new ArrayList<>(
     1 + this.converterFactories.size() + platform.defaultConverterFactoriesSize());
converterFactories.add(new BuiltInConverters());
converterFactories.addAll(this.converterFactories);
converterFactories.addAll(platform.defaultConverterFactories());

除了使用默认的ConverterFactory，我们也可以使用Retrofit.Builder#addConverterFactory(Factory)添加Converter .Factory到converterFactories列表；Converter负责将服务端返回的数据转换成对象实例

注释2.3.3：把requestFactory, callFactory, responseConverter, callAdapter封装进最终的HttpServiceMethod

如果未使用Kotlin的Suspend功能，注释2.3.3处会直接执行if语句，即创建一个CallAdapted，它继承自HttpServiceMethod，代码如下：

  static final class CallAdapted<ResponseT, ReturnT> extends HttpServiceMethod<ResponseT, ReturnT> {
    private final CallAdapter<ResponseT, ReturnT> callAdapter;

    CallAdapted(
        RequestFactory requestFactory,
        okhttp3.Call.Factory callFactory,
        Converter<ResponseBody, ResponseT> responseConverter,
        CallAdapter<ResponseT, ReturnT> callAdapter) {
      super(requestFactory, callFactory, responseConverter);
      this.callAdapter = callAdapter;
    }

    @Override
    protected ReturnT adapt(Call<ResponseT> call, Object[] args) {
      return callAdapter.adapt(call);
    }
  }

CallAdapted主要是为了实现HttpServiceMethod的抽象方法ReturnT adapt(Call<ResponseT> call, Object[] args)，而具体工作交由callAdapter.adapt(call)方法执行。

我们再回到retrofit.create(ApiService::class.java)方法，重新贴一下代码：

  public <T> T create(final Class<T> service) {
    validateServiceInterface(service);
    return (T)
        Proxy.newProxyInstance(
            service.getClassLoader(),
            new Class<?>[] {service},
            new InvocationHandler() {
              private final Platform platform = Platform.get();
              private final Object[] emptyArgs = new Object[0];

              @Override
              public @Nullable Object invoke(Object proxy, Method method, @Nullable Object[] args)
                  throws Throwable {
                // If the method is a method from Object then defer to normal invocation.
                if (method.getDeclaringClass() == Object.class) {
                  return method.invoke(this, args);
                }
                args = args != null ? args : emptyArgs;
                return platform.isDefaultMethod(method)
                    ? platform.invokeDefaultMethod(method, service, proxy, args)
                    : loadServiceMethod(method).invoke(args);
              }
            });
  }

第二步：当我们调用接口ApiService中的listRepos("octocat")方法，动态代理机制会调用InvocationHandler内的invoke(Object proxy, Method method, @Nullable Object[] args)方法，然后会执行loadServiceMethod(method).invoke(args)。

通过前面的分析我们知道，loadServiceMethod(method)方法返回的是一个ServiceMethod，查看它的invoke()方法：

//ServiceMethod.java
abstract class ServiceMethod<T> {
  static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
    RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);

    Type returnType = method.getGenericReturnType();
    if (Utils.hasUnresolvableType(returnType)) {
      throw methodError(
          method,
          "Method return type must not include a type variable or wildcard: %s",
          returnType);
    }
    if (returnType == void.class) {
      throw methodError(method, "Service methods cannot return void.");
    }

    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
  }

  abstract @Nullable T invoke(Object[] args);
}

invoke(Object[] args)方法在HttpServiceMethod中实现：

//HttpServiceMethod.java
  @Override
  final @Nullable ReturnT invoke(Object[] args) {
    Call<ResponseT> call = new OkHttpCall<>(requestFactory, args, callFactory, responseConverter);
    return adapt(call, args);
  }

创建一个OkHttpCall对象作为adapt(call, args)方法的参数，前面已经分析过CallAdapted中实现了adapt(call, args)方法，call最终传递给callAdapter.adapt(call)方法适配出新call。（适配器模式）

如果没有手动指定callAdapter，默认使用DefaultCallAdapterFactory创建callAdapter：

//精简后的代码
final class DefaultCallAdapterFactory extends CallAdapter.Factory {
  private final @Nullable Executor callbackExecutor;

  DefaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
    this.callbackExecutor = callbackExecutor;
  }

  @Override
  public @Nullable CallAdapter<?, ?> get(
      Type returnType, Annotation[] annotations, Retrofit retrofit) {

    final Executor executor =
        Utils.isAnnotationPresent(annotations, SkipCallbackExecutor.class)
            ? null
            : callbackExecutor;

    return new CallAdapter<Object, Call<?>>() {
      @Override
      public Type responseType() {
        return responseType;
      }

      @Override
      public Call<Object> adapt(Call<Object> call) {
        return executor == null ? call : new ExecutorCallbackCall<>(executor, call);
      }
    };
  }

  static final class ExecutorCallbackCall<T> implements Call<T> {
    final Executor callbackExecutor;
    final Call<T> delegate;

    ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
      this.callbackExecutor = callbackExecutor;
      this.delegate = delegate;
    }

    @Override
    public void enqueue(final Callback<T> callback) {
      Objects.requireNonNull(callback, "callback == null");

      delegate.enqueue(
          new Callback<T>() {
            @Override
            public void onResponse(Call<T> call, final Response<T> response) {
              callbackExecutor.execute(
                  () -> {
                    if (delegate.isCanceled()) {
                      // Emulate OkHttp's behavior of throwing/delivering an IOException on
                      // cancellation.
                      callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
                    } else {
                      callback.onResponse(ExecutorCallbackCall.this, response);
                    }
                  });
            }

            @Override
            public void onFailure(Call<T> call, final Throwable t) {
              callbackExecutor.execute(() -> callback.onFailure(ExecutorCallbackCall.this, t));
            }
          });
    }

  }
}

默认的网络请求适配器主要做了两件事：

1. 在CallAdapter#adapt(Call<Object> call) 方法中将OkHttpCall适配成ExecutorCallbackCall来执行网络请求（适配器模式），真正的网络请求依然由OkHttpCall自己发起（代理模式）
2. 请求完成后将Callback回调方法切换到callbackExecutor中执行，Retrofit提供的默认callbackExecutor负责将任务提交到主线程

3. OkHttpCall发起网络请求

我们已经知道真正的网络请求由OkHttpCall发起，那还等什么快去看看吧

  @Override
  public void enqueue(final Callback<T> callback) {
    Objects.requireNonNull(callback, "callback == null");

    okhttp3.Call call;
    Throwable failure;

    synchronized (this) {
      if (executed) throw new IllegalStateException("Already executed.");
      executed = true;

      call = rawCall;
      failure = creationFailure;
      if (call == null && failure == null) {
        try {
          call = rawCall = createRawCall();  //注释3.1： 使用OkHttpClient创建okhttp3.Call
        } catch (Throwable t) {
          throwIfFatal(t);
          failure = creationFailure = t;
        }
      }
    }

    if (failure != null) {
      callback.onFailure(this, failure);
      return;
    }

    if (canceled) {
      call.cancel();
    }

    //注释3.2：使用创建好的okhttp3.Call发起异步请求
    call.enqueue(
        new okhttp3.Callback() {
          @Override
          public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse) {
            Response<T> response;
            try {
              response = parseResponse(rawResponse);  //注释3.3：解析服务端返回的数据
            } catch (Throwable e) {
              throwIfFatal(e);
              callFailure(e);
              return;
            }

            try {
              callback.onResponse(OkHttpCall.this, response); 
            } catch (Throwable t) {
              throwIfFatal(t);
              t.printStackTrace(); // TODO this is not great
            }
          }

          @Override
          public void onFailure(okhttp3.Call call, IOException e) {
            callFailure(e);
          }

          private void callFailure(Throwable e) {
            try {
              callback.onFailure(OkHttpCall.this, e);
            } catch (Throwable t) {
              throwIfFatal(t);
              t.printStackTrace(); // TODO this is not great
            }
          }
        });
  }

注释3.1： 使用OkHttpClient创建okhttp3.Call

  private okhttp3.Call createRawCall() throws IOException {
    okhttp3.Call call = callFactory.newCall(requestFactory.create(args));
    if (call == null) {
      throw new NullPointerException("Call.Factory returned null.");
    }
    return call;
  }

注释3.2：使用创建好的okhttp3.Call发起异步请求

注释3.3：处理服务端返回的数据

  Response<T> parseResponse(okhttp3.Response rawResponse) throws IOException {
    ResponseBody rawBody = rawResponse.body();

    // Remove the body's source (the only stateful object) so we can pass the response along.
    rawResponse =
        rawResponse
            .newBuilder()
            .body(new NoContentResponseBody(rawBody.contentType(), rawBody.contentLength()))
            .build();

    //返回码处理
    int code = rawResponse.code();
    if (code < 200 || code >= 300) {  //网络请求出现异常
      try {
        // Buffer the entire body to avoid future I/O.
        ResponseBody bufferedBody = Utils.buffer(rawBody);
        return Response.error(bufferedBody, rawResponse);
      } finally {
        rawBody.close();
      }
    }

    if (code == 204 || code == 205) {  //返回Body为null的Response
      rawBody.close();
      return Response.success(null, rawResponse);  
    }

    ExceptionCatchingResponseBody catchingBody = new ExceptionCatchingResponseBody(rawBody);
    try {
      //responseConverter开始转换网络数据
      T body = responseConverter.convert(catchingBody);  
      return Response.success(body, rawResponse);
    } catch (RuntimeException e) {
      // If the underlying source threw an exception, propagate that rather than indicating it was
      // a runtime exception.
      catchingBody.throwIfCaught();
      throw e;
    }
  }

执行responseConverter.convert(catchingBody)方法转换Response数据，那么responseConverter具体类型是什么呢？

我们在初始化Retrofit时调用了Retrofit.Builder()#addConverterFactory(GsonConverterFactory.create())方法

val retrofit = Retrofit.Builder()
        .baseUrl("https://api.github.com/")
        .client(okHttpClient)
        .addConverterFactory(GsonConverterFactory.create())  //添加Gson转换器
        .build()

#Retrofit.java
    public Builder addConverterFactory(Converter.Factory factory) {
      converterFactories.add(Objects.requireNonNull(factory, "factory == null"));
      return this;
    }

也就是将GsonConverterFactory添加到列表converterFactories，看一下GsonConverterFactory源码：

public final class GsonConverterFactory extends Converter.Factory {

  public static GsonConverterFactory create() {
    return create(new Gson());
  }

  @SuppressWarnings("ConstantConditions") // Guarding public API nullability.
  public static GsonConverterFactory create(Gson gson) {
    if (gson == null) throw new NullPointerException("gson == null");
    return new GsonConverterFactory(gson);
  }

  private final Gson gson;

  private GsonConverterFactory(Gson gson) {
    this.gson = gson;
  }

  @Override
  public Converter<ResponseBody, ?> responseBodyConverter(
      Type type, Annotation[] annotations, Retrofit retrofit) {
    TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
    return new GsonResponseBodyConverter<>(gson, adapter);
  }

  @Override
  public Converter<?, RequestBody> requestBodyConverter(
      Type type,
      Annotation[] parameterAnnotations,
      Annotation[] methodAnnotations,
      Retrofit retrofit) {
    TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
    return new GsonRequestBodyConverter<>(gson, adapter);
  }
}

通过注释2.3.2可知，我们要寻找的responseConverter就是GsonResponseBodyConverter，重新回到注释3.3中的Response转换过程responseConverter.convert，本质上是执行GsonResponseBodyConverter.convert：

final class GsonResponseBodyConverter<T> implements Converter<ResponseBody, T> {
  private final Gson gson;
  private final TypeAdapter<T> adapter;

  GsonResponseBodyConverter(Gson gson, TypeAdapter<T> adapter) {
    this.gson = gson;
    this.adapter = adapter;
  }

  @Override
  public T convert(ResponseBody value) throws IOException {
    JsonReader jsonReader = gson.newJsonReader(value.charStream());
    try {
      T result = adapter.read(jsonReader);
      if (jsonReader.peek() != JsonToken.END_DOCUMENT) {
        throw new JsonIOException("JSON document was not fully consumed.");
      }
      return result;
    } finally {
      value.close();
    }
  }
}

convert(ResponseBody value)方法负责把ResponseBody转换成指定的数据类型

到此为止，还剩最后一个任务：异步请求任务完成之后会以回调的方式通知发起方请求已经结束，为了方便将请求结果展示到UI上，需要把回调操作切换到主线程执行，这个任务由ExecutorCallbackCall负责完成

  static final class ExecutorCallbackCall<T> implements Call<T> {
    final Executor callbackExecutor;
    final Call<T> delegate;

    ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
      this.callbackExecutor = callbackExecutor;
      this.delegate = delegate;
    }

    @Override
    public void enqueue(final Callback<T> callback) {
      Objects.requireNonNull(callback, "callback == null");

      delegate.enqueue(
          new Callback<T>() {
            @Override
            public void onResponse(Call<T> call, final Response<T> response) {
              callbackExecutor.execute(
                  () -> {
                    if (delegate.isCanceled()) {
                      // Emulate OkHttp's behavior of throwing/delivering an IOException on
                      // cancellation.
                      callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
                    } else {
                      callback.onResponse(ExecutorCallbackCall.this, response);
                    }
                  });
            }

            @Override
            public void onFailure(Call<T> call, final Throwable t) {
              callbackExecutor.execute(() -> callback.onFailure(ExecutorCallbackCall.this, t));
            }
          });
    }

    @Override
    public Response<T> execute() throws IOException {
      return delegate.execute();
    }
    
    ……
  }

具体方法是将callback.onXxxxx()回调方法丢到callbackExecutor执行，可以在Retrofit中使用方法callbackExecutor(Executor executor)配置callbackExecutor。

如果未配置callbackExecutor，Retrofit会提供默认callbackExecutor，默认配置相关的实现依然是在Retrofit.Builder#build()方法中

//Retrofit.Builder#build()
  Executor callbackExecutor = this.callbackExecutor;
  if (callbackExecutor == null) {
    callbackExecutor = platform.defaultCallbackExecutor();
  }

//Android平台
  static final class Android extends Platform {
    Android() {
      super(Build.VERSION.SDK_INT >= 24);
    }

    @Override
    public Executor defaultCallbackExecutor() {
      return new MainThreadExecutor();
    }

    @Nullable
    @Override
    Object invokeDefaultMethod(
        Method method, Class<?> declaringClass, Object object, Object... args) throws Throwable {
      if (Build.VERSION.SDK_INT < 26) {
        throw new UnsupportedOperationException(
            "Calling default methods on API 24 and 25 is not supported");
      }
      return super.invokeDefaultMethod(method, declaringClass, object, args);
    }

    static final class MainThreadExecutor implements Executor {
      private final Handler handler = new Handler(Looper.getMainLooper());

      @Override
      public void execute(Runnable r) {
        handler.post(r);
      }
    }
  }

callbackExecutor默认是MainThreadExecutor，MainThreadExecutor负责将任务提交到主线程执行

源码分析结束！

Retrofit源码真是太牛逼了，本人水平有限，如有错误希望大家批评指正！

参考资料

• Carson带你学Android：手把手带你深入读懂Retrofit 2.0源码