前言

Retrofit是Square公司開發的一款Android網絡請求庫，Retrofit僅負責網絡請求接口的封裝，它使用運行時注解的方式提供請求參數的配置功能，底層基于OkHttp實現真正的網絡請求。

如果還不了解OkHttp框架，強烈建議先閱讀Android 網絡請求庫OkHttp基本原理與實現再來閱讀本篇文章，會更加酣暢淋漓哦~

本文所提及的網絡請求如無特殊說明，都是異步網絡請求

為什么選擇Retrofit（與其他主流框架對比）

現在市場上比較主流的網絡請求框架有：

• android-async-http（作者停止維護、不推薦使用）
• Volley（谷歌）
• OkHttp（Square）
• Retrofit（Square）

一圖讓你了解全部的網絡請求庫和他們之間的區別！

Retrofit基本用法

1. 導入庫

    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") //支持返回值為Gson類型的數據

默認情況下，Retrofit只能將數據反序列化為OkHttp的ResponseBody類型，并且它只能接受@Body注解的RequestBody類型。

除了默認情況，我們也可以添加轉換器以支持其他類型。

• 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. 注解分類

Retrofit注解分為三大類，分別是請求方法注解、標記類注解和參數類注解。

請求方法注解有8種：GET/POST/PUT/DELETE/HEAD/PATCH/OPTIONS/HTTP，前7種與HTTP請求方法是對應的，最后的HTTP可以替換前面七種，也可以擴展請求方法。

標記類注解有3種：FormUrlEncoded、Multipart、Streaming。

參數類注解有Header、Headers、Body、Path、Field、FieldMap、Query、QueryMap、Part、PartMap等。

3. GET網絡請求

首先編寫請求網絡接口

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

@GET 注解指定當前網絡請求方式，注解參數是訪問地址，方法返回Call<List<Repo?>?>類型的數據。@Path注解用來動態配置Url地址，@Path("user")對應@GET注解中的{user}，即使用傳入的 user: String?值動態替換{user}

接下來創建Retrofit對象，創建接口類（動態代理）調用網絡請求方法，得到Call并發起異步請求，回調的Callback運行在UI線程。

val retrofit = Retrofit.Builder()
        .baseUrl("https://api.github.com/")
        .client(okHttpClient)
        .addConverterFactory(GsonConverterFactory.create())  //數據轉換器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) {

    }
})

請求的Url是由baseUrl和@GET注解參數拼接出來的：https://api.github.com/users/octocat/repos

一般情況下為了更準確地去查找網絡數據，我們需要在Url中添加查詢參數，這種情況可以使用Query指定查詢條件。例如根據id查找數據，我們修改代碼如下：

@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)

這樣請求Url變為：https://api.github.com/users/octocat/repos?id=132935648

如果有多個查詢參數使用@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網絡請求

POST請求最常見的情況是傳輸鍵值對的數據類型，使用@Field注解就可以為POST請求添加鍵值對

@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("張", "三")

@FormUrlEncoded表明這是一個表單請求，使用@Field注解標注對應的鍵值對。

POST請求除了傳輸鍵值對，還可以傳輸JSON字符串，對應注解@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())

上傳單個文件

 * @param photo 上傳的文件
 * @param desc 文件描述（簡單的鍵值對）
 */
@Multipart //表示允許多個part
@POST("user/photo")
fun uploadPhoto(@Part photo: MultipartBody.Part,
                @Part("desc") desc:

val file = File("文件路徑")
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)

上傳多個文件
與上傳單個文件類似，使用Map封裝上傳的文件，用@PartMap注解標注

更多用法可參考Carson帶你學Android：網絡請求庫Retrofit使用教程(含實例講解)

Retrofit原理解析

Retrofit最終的目的是通過使用大量的設計模式進行功能模塊的解耦，使得網絡請求過程變得更加簡單流暢。

下圖可以直觀對比出一般網絡請求流程與Retrofit請求的區別

Retrofit流程

詳細步驟如下：

1. 通過解析網絡請求接口的注解配置網絡請求參數
2. 通過動態代理生成網絡請求對象
3. 通過網絡請求適配器將網絡請求對象進行平臺適配（平臺包括：Android、Rxjava、Guava和java8）
4. 通過網絡請求執行器發送網絡請求
5. 通過數據轉換器解析服務器返回的數據
6. 通過回調執行器切換線程（子線程 ->>主線程）
7. 用戶在主線程處理返回結果

主要角色說明

源碼分析（基于Retrofit 2.9.0）

我們從Retrofit最基本的使用方式作為切入口，一步步深入源碼探索Retrofit實現原理。

Retrofit使用主要分為以下幾步：

1. 創建Retrofit對象（建造者模式）
2. 實例化接口類并調用請求方法（動態代理）
3. 使用第2步返回的Call發起網絡請求，Callback回調會運行在UI線程

接下來展開分析：

1. Retrofit.Builder()

因為Retrofit可配置的參數較多，所以采用建造者模式創建Retrofit實例。

Builder類主要代碼如下：

  public static final class Builder {
    private final Platform platform;
    private @Nullable okhttp3.Call.Factory callFactory;
    private @Nullable HttpUrl baseUrl;  //網絡請求基本地址
    private final List<Converter.Factory> converterFactories = new ArrayList<>();   //用于創建Converter.Factory
    private final List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>();   //用于創建CallAdapter.Factory
    private @Nullable Executor callbackExecutor;    //切換線程執行回調任務（默認是主線程）
    private boolean validateEagerly;

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

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

    /** 添加自定義的Converter Factory：用于創建數據轉換器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}.  添加自定義的CallAdapter Factory：用于創建網絡請求適配器CallAdapter
     */
    public Builder addCallAdapterFactory(CallAdapter.Factory factory) {
      callAdapterFactories.add(Objects.requireNonNull(factory, "factory == null"));
      return this;
    }

    //指定執行回調任務的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.");
      }

      //設置默認OkHttpClient
      okhttp3.Call.Factory callFactory = this.callFactory;  
      if (callFactory == null) {
        callFactory = new OkHttpClient(); 
      }
     //android平臺默認使用MainThreadExecutor，用于切換到主線程執行任務
      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {  
        callbackExecutor = platform.defaultCallbackExecutor();  
      }

      //添加平臺默認CallAdapter.Factory到CallAdapterFactories
      List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);  
      callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));

      //添加平臺默認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);
    }
}

如果未配置參數，build()方法會為我們提供大多數的默認配置，例如callAdapterFactories（創建網絡請求適配器）、converterFactories（創建數據轉換器）、callbackExecutor（執行回調任務）等待。

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

第一步：采取動態代理的方式創建請求接口的實現類

  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的成員函數，直接使用反射調用方法
                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實例方法
                return platform.isDefaultMethod(method)
                    ? platform.invokeDefaultMethod(method, service, proxy, args)
                    : loadServiceMethod(method).invoke(args);   //接口方法會執行到此處
              }
            });
  }

ApiService.kt是一個接口類，當我們調用接口內的方法時會執行代碼：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;
  }

代碼很簡單，調用ServiceMethod.parseAnnotations(this, method)解析接口中的注解方法并封裝成ServiceMethod。并且利用serviceMethodCache緩存解析結果，這樣可以防止重復解析，使用緩存是因為解析的過程用到了比較耗費性能的反射機制。

abstract class ServiceMethod<T> {
  static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
    //注釋2.1：解析方法注解和參數注解，把注解中的網絡請求參數封裝到RequestFactory中
    RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method); 

    //注釋2.2：保證方法返回類型不是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.");
    }

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

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

注釋2.1：RequestFactory.parseAnnotations(retrofit, method)解析注解信息，把信息封裝到RequestFactory中
注釋2.2：保證方法返回類型不是Unresolvable和void.class
注釋2.3：調用HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory)獲取ServiceMethod

#HttpServiceMethod
//這里只貼出部分代碼
  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;  //方法返回值類型
    //isKotlinSuspendFunction默認為false，進入else語句獲取adapterType 
    if (isKotlinSuspendFunction) {  
      ……
    } else {
      adapterType = method.getGenericReturnType(); 
    }
    //注釋2.3.1：從callAdapterFactories列表中取出CallAdapter.Factory創建CallAdapter
    //我們可以使用Retrofit.Builder#addCallAdapterFactory(Factory)添加CallAdapter.Factory到列表callAdapterFactories
    //CallAdapter負責執行網絡請求Call的適配工作
    CallAdapter<ResponseT, ReturnT> callAdapter =
        createCallAdapter(retrofit, method, adapterType, annotations);  
    Type responseType = callAdapter.responseType();

    ……

    //注釋2.3.2：從converterFactories列表中取出Converter.Factory創建Converter
    //我們可以使用Retrofit.Builder#addConverterFactory(Factory)添加Converter .Factory到converterFactories列表
    //Converter負責將服務端返回的數據轉換成對象實例
    Converter<ResponseBody, ResponseT> responseConverter =
        createResponseConverter(retrofit, method, responseType);

    okhttp3.Call.Factory callFactory = retrofit.callFactory;
    //注釋2.3.3：把requestFactory, callFactory, responseConverter, callAdapter封裝進最終的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);
    }
  }

注釋2.3.1：從callAdapterFactories列表中取出CallAdapter.Factory創建CallAdapter，具體流程如下：

//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);
    }
  }

調用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;
    //遍歷callAdapterFactories列表獲取CallAdapter.Factory來創建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;
      }
    }

    //獲取失敗
  }

最終遍歷callAdapterFactories列表獲取CallAdapterFactory來創建我們需要的網絡請求適配器CallAdapter，在構建Retrofit時已經添加Android平臺默認的platform.defaultCallAdapterFactories到callAdapterFactories

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

除了使用默認的CallAdapterFactory，我們也可以使用Retrofit.Builder#addCallAdapterFactory(Factory)主動添加CallAdapter.Factory到列表callAdapterFactories

CallAdapter主要負責執行網絡請求Call的適配工作，注釋2.3.2會詳細分析

注釋2.3.2：從converterFactories列表中取出Converter.Factory創建Converter，具體流程如下：

//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);
    }
  }

調用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;
    //遍歷converterFactories列表獲取Converter.Factory來創建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;
      }
    }

    //獲取失敗
  }

最終遍歷converterFactories列表獲取Converter.Factory來創建我們需要的Converter，在構建Retrofit時已經添加Android平臺默認的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());

除了使用默認的ConverterFactory，我們也可以使用Retrofit.Builder#addConverterFactory(Factory)添加Converter .Factory到converterFactories列表；Converter負責將服務端返回的數據轉換成對象實例

注釋2.3.3：把requestFactory, callFactory, responseConverter, callAdapter封裝進最終的HttpServiceMethod

如果未使用Kotlin的Suspend功能，注釋2.3.3處會直接執行if語句，即創建一個CallAdapted，它繼承自HttpServiceMethod，代碼如下：

  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主要是為了實現HttpServiceMethod的抽象方法ReturnT adapt(Call<ResponseT> call, Object[] args)，而具體工作交由callAdapter.adapt(call)方法執行。

我們再回到retrofit.create(ApiService::class.java)方法，重新貼一下代碼：

  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：當我們調用接口ApiService中的listRepos("octocat")方法，動態代理機制會調用InvocationHandler內的invoke(Object proxy, Method method, @Nullable Object[] args)方法，然后會執行loadServiceMethod(method).invoke(args)。

通過前面的分析我們知道，loadServiceMethod(method)方法返回的是一個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中實現：

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

創建一個OkHttpCall對象作為adapt(call, args)方法的參數，前面已經分析過CallAdapted中實現了adapt(call, args)方法，call最終傳遞給callAdapter.adapt(call)方法適配出新call。（適配器模式）

如果沒有手動指定callAdapter，默認使用DefaultCallAdapterFactory創建callAdapter：

//精簡后的代碼
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));
            }
          });
    }

  }
}

默認的網絡請求適配器主要做了兩件事：

1. 在CallAdapter#adapt(Call<Object> call) 方法中將OkHttpCall適配成ExecutorCallbackCall來執行網絡請求（適配器模式），真正的網絡請求依然由OkHttpCall自己發起（代理模式）
2. 請求完成后將Callback回調方法切換到callbackExecutor中執行，Retrofit提供的默認callbackExecutor負責將任務提交到主線程

3. OkHttpCall發起網絡請求

我們已經知道真正的網絡請求由OkHttpCall發起，那還等什么快去看看吧

  @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();  //注釋3.1： 使用OkHttpClient創建okhttp3.Call
        } catch (Throwable t) {
          throwIfFatal(t);
          failure = creationFailure = t;
        }
      }
    }

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

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

    //注釋3.2：使用創建好的okhttp3.Call發起異步請求
    call.enqueue(
        new okhttp3.Callback() {
          @Override
          public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse) {
            Response<T> response;
            try {
              response = parseResponse(rawResponse);  //注釋3.3：解析服務端返回的數據
            } 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
            }
          }
        });
  }

注釋3.1： 使用OkHttpClient創建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;
  }

注釋3.2：使用創建好的okhttp3.Call發起異步請求

注釋3.3：處理服務端返回的數據

  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();

    //返回碼處理
    int code = rawResponse.code();
    if (code < 200 || code >= 300) {  //網絡請求出現異常
      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為null的Response
      rawBody.close();
      return Response.success(null, rawResponse);  
    }

    ExceptionCatchingResponseBody catchingBody = new ExceptionCatchingResponseBody(rawBody);
    try {
      //responseConverter開始轉換網絡數據
      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;
    }
  }

執行responseConverter.convert(catchingBody)方法轉換Response數據，那么responseConverter具體類型是什么呢？

我們在初始化Retrofit時調用了Retrofit.Builder()#addConverterFactory(GsonConverterFactory.create())方法

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

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

也就是將GsonConverterFactory添加到列表converterFactories，看一下GsonConverterFactory源碼：

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);
  }
}

通過注釋2.3.2可知，我們要尋找的responseConverter就是GsonResponseBodyConverter，重新回到注釋3.3中的Response轉換過程responseConverter.convert，本質上是執行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)方法負責把ResponseBody轉換成指定的數據類型

到此為止，還剩最后一個任務：異步請求任務完成之后會以回調的方式通知發起方請求已經結束，為了方便將請求結果展示到UI上，需要把回調操作切換到主線程執行，這個任務由ExecutorCallbackCall負責完成

  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();
    }
    
    ……
  }

具體方法是將callback.onXxxxx()回調方法丟到callbackExecutor執行，可以在Retrofit中使用方法callbackExecutor(Executor executor)配置callbackExecutor。

如果未配置callbackExecutor，Retrofit會提供默認callbackExecutor，默認配置相關的實現依然是在Retrofit.Builder#build()方法中

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

//Android平臺
  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默認是MainThreadExecutor，MainThreadExecutor負責將任務提交到主線程執行

源碼分析結束！

Retrofit源碼真是太牛逼了，本人水平有限，如有錯誤希望大家批評指正！

參考資料

• Carson帶你學Android：手把手帶你深入讀懂Retrofit 2.0源碼