Android网络请求库【Retrofit 2.9.0】基本用法与原理分析

HuldaGnodim · 发表于 2022-6-8 14:45

Retrofit官网：https://square.github.io/retrofit/
Github地址：https://github.com/square/retrofit

<hr>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：

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。

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.retrofitdata 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：

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：

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.             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.javaabstract 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：

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！
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Android网络请求库【Retrofit 2.9.0】基本用法与原理分析

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