Retrofit源码阅读

绝地灬酷狼 2023-10-15 09:36 56阅读 0赞

## 1. Retrofit的简介与使用 ##

### 1.1 Retrofit的简介 ###

Retrofit主要是对OkHttp做了一层封装，利用注解的方式使得接口请求更简单，灵活的使用Converter可使得请求数据与解析一体化更方便，Android端还会自动将回调执行在主线程可以直接修改UI，新版的Retrofit还集成了Kotlin协程，更是利用协程解决了接口请求回调地狱的问题。

### 1.2 Retrofit的使用 ###

1.  添加依赖

Groovy

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`implementation 'com.squareup.retrofit2:retrofit:2.9.0' implementation 'com.squareup.retrofit2:converter-gson:2.9.0'`

1.  定义接口请求和解析的实体类

kotlin

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`/** * 接口服务 * @author LTP 2023/7/18 */ interface Api { /** 获取首页banner数据（协程方式返回无包装的实体） */ @GET("banner/json") suspend fun getBanner(): ApiResponse<List<Banner>> /** 获取首页banner数据（使用协程返回Response包装的实体） */ @GET("banner/json") suspend fun getBanner2(): Response<ApiResponse<List<Banner>>> /** 获取首页banner数据（普通方式返回Call包装的实体） */ @GET("banner/json") fun getBanner3(): Call<ApiResponse<List<Banner>>> }`

kotlin

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`/** * 接口返回外层封装实体 * * @author LTP 2022/3/22 */ data class ApiResponse<T>( val data: T, val errorCode: Int, val errorMsg: String )`

kotlin

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`/** * 轮播图实体 * * @author LTP 2022/3/22 */ data class Banner( var desc: String = "", var id: Int = 0, var imagePath: String = "", var isVisible: Int = 0, var order: Int = 0, var title: String = "", var type: Int = 0, var url: String = "" )`

1.  创建Retrofit对象，开启请求

kotlin

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`/** * Retrofit请求网络测试类 * @author LTP 2023/7/17 */ class RetrofitUnitTest { /**创建retrofit对象 */ private val retrofit = Retrofit.Builder().baseUrl("https://www.wanandroid.com") .addConverterFactory(GsonConverterFactory.create()) .build() @Test fun getBanner() { val api = retrofit.create(Api::class.java) // runBlocking启动的协程会阻塞线程 runBlocking { val response = api.getBanner() println(response) val response2 = api.getBanner2() println(response2.body()) } // Retrofit异步请求 api.getBanner3().enqueue(object : Callback<ApiResponse<List<Banner>>> { override fun onResponse( call: Call<ApiResponse<List<Banner>>>, response: Response<ApiResponse<List<Banner>>> ) { println(response) } override fun onFailure(call: Call<ApiResponse<List<Banner>>>, t: Throwable) { } }) // 异步请求需要阻塞住主线程以等待请求完毕 Thread.sleep(5000) // Retrofit同步请求 val response = api.getBanner3().execute() println(response) } }`

*  整个过程非常简单舒适，当然，Retrofit还可以进行更多的复杂操作，如配置请求头、请求提、表单提交等，可以直接看[官网][Link 1]，这里就不再赘述了，直接看下整个请求过程

## 2. 源码解析 ##

### 2.1 Retrofit对象的创建 ###

*  创建方式为：Retrofit.Builder().build()，一看就是Builder设计模式了，先看下Retrofit中的几个成员变量

java

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`public final class Retrofit { /** 主要用来缓存请求方法 */ private final Map<Method, ServiceMethod<?>> serviceMethodCache = new ConcurrentHashMap<>(); /** okhttp3.Call.Factory接口的实现类（OkHttpClient是唯一实现类） */ final okhttp3.Call.Factory callFactory; /** 服务器根路径（当包含路径时必须以/结尾） */ final HttpUrl baseUrl; /** 请求响应数据解析器集合（如GsonConverterFactory.create()） */ final List<Converter.Factory> converterFactories; /** 网络请求适配器工厂集合，用于放置我们的网络请求适配器工厂（网络请求适配器：把我们的call对象转换成其他类型） * 如RxJavaCallAdapterFactory */ final List<CallAdapter.Factory> callAdapterFactories; /** 用于回调执行（如Android中是MainThreadExecutor即主线程执行） */ final @Nullable Executor callbackExecutor; /** 标志位，控制 Retrofit 是否立即验证 HTTP 请求和响应，以确保它们的正确性和完整性 */ final boolean validateEagerly; ...`

*  直接看Builder模式最关键的方法build()，主要是针对成员进行组装判断，最终构造Retrofit对象

java

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`// Retrofit.java public Retrofit build() { if (baseUrl == null) { throw new IllegalStateException("Base URL required."); } // 设置OkHttp中Call的工厂类（OkHttpClient是唯一实现类） okhttp3.Call.Factory callFactory = this.callFactory; if (callFactory == null) { // 没有设置的话使用默认的OkHttpClient对象 callFactory = new OkHttpClient(); } // 执行请求回调的对象，Android中defaultCallbackExecutor是MainThreadExecutor即主线程 Executor callbackExecutor = this.callbackExecutor; if (callbackExecutor == null) { callbackExecutor = platform.defaultCallbackExecutor(); } // 网络请求适配器工厂集合 List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories); callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor)); // 数据转换器工厂集合（网络请求返回数据解析） List<Converter.Factory> converterFactories = new ArrayList<>( 1 + this.converterFactories.size() + platform.defaultConverterFactoriesSize()); // 先添加内置转换器工厂BuiltInConverters，主要包含一些基本类型（如 // String：将响应数据转换为 String 类型、Integer、Long、Float、Double、Boolean、Character）的转换 converterFactories.add(new BuiltInConverters()); converterFactories.addAll(this.converterFactories); converterFactories.addAll(platform.defaultConverterFactories()); return new Retrofit( callFactory, baseUrl, unmodifiableList(converterFactories), unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly); }`

*  着重看下几个注意的点
 *  `baseUrl中包含路径时必须以/结尾`

java

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`public Builder baseUrl(URL baseUrl) { Objects.requireNonNull(baseUrl, "baseUrl == null"); return baseUrl(HttpUrl.get(baseUrl.toString())); } public Builder baseUrl(String baseUrl) { Objects.requireNonNull(baseUrl, "baseUrl == null"); return baseUrl(HttpUrl.get(baseUrl)); } public Builder baseUrl(HttpUrl baseUrl) { Objects.requireNonNull(baseUrl, "baseUrl == null"); List<String> pathSegments = baseUrl.pathSegments(); // 当baseUrl是一个域名加路径但路径最后一个字符不是以'/'结尾抛异常 // 如https://www.wanandroid.com/api不行，必须为https://www.wanandroid.com/api/ if (!"".equals(pathSegments.get(pathSegments.size() - 1))) { throw new IllegalArgumentException("baseUrl must end in /: " + baseUrl); } this.baseUrl = baseUrl; return this; }`

*  `platform.defaultCallbackExecutor()`中platform是在Builder构造中通过Platform.get()获取的，看下这个`Platform`类

java

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`/** 平台类（Android平台和Java平台） */ class Platform { private static final Platform PLATFORM = findPlatform(); static Platform get() { return PLATFORM; } /** * 判断当前平台是Android还是Java * @return 平台类型 */ private static Platform findPlatform() { return "Dalvik".equals(System.getProperty("java.vm.name")) ? new Android() // : new Platform(true); } ... /** Android平台 */ static final class Android extends Platform { Android() { super(Build.VERSION.SDK_INT >= 24); } /** Android平台下默认的callbackExecutor是MainThreadExecutor，即主线程 */ @Override public Executor defaultCallbackExecutor() { return new MainThreadExecutor(); } ... static final class MainThreadExecutor implements Executor { // 子线程到主线程还是通过Handler来切换线程的 private final Handler handler = new Handler(Looper.getMainLooper()); @Override public void execute(Runnable r) { handler.post(r); } } } }`

*  **addConverterFactory**：添加响应数据转换工厂，在build()方法中已添加默认的`BuiltInConverters`转换器，可针对一些常见类型（如服务器接口返回String、Integer、Long、Float、Double、Boolean、Character）进行转换，但通常服务器返回的是更复杂的数据类型，譬如Json数据、xml数据，而针对这种数据，Retrofit也提供了通用的一些转换插件（如converter-gson、converter-simplexml、converter-moshi等），这里以converter-gson为例

kotlin

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`private val retrofit = Retrofit.Builder().baseUrl("https://www.wanandroid.com") .addConverterFactory(GsonConverterFactory.create()) // 添加GsonConverter .build()`

*  在covert-gson插件中看下GsonConverterFactory.create()方法

java

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`public final class GsonConverterFactory extends Converter.Factory { public static GsonConverterFactory create() { return create(new Gson()); } @SuppressWarnings("ConstantConditions") 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); } }`

*  主要是`继承了Converter.Factory抽象类`，实现请求和响应转换方法，当然你也可以通过继承Converter.Factory抽象类来实现自定义请求响应转换器

### 2.2 网络请求 ###

#### 2.2.1 retrofit.create() ####

*  从关键代码retrofit.create(Api::class.java)方法看起

java

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`// Retrofit.java 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 { // 如果方法是Object类中的方法，直接正常调用即可，无需代理 if (method.getDeclaringClass() == Object.class) { return method.invoke(this, args); } args = args != null ? args : emptyArgs; // 这里判断了是否是默认方法（java8接口新增了默认方法），不是就调用loadServiceMethod return platform.isDefaultMethod(method) ? platform.invokeDefaultMethod(method, service, proxy, args) : loadServiceMethod(method).invoke(args); } }); }`

*  这是一个典型的[动态代理设计模式][Link 2]，代理返回一个Api实现类，调用api.getBanner就会去执行`invoke`方法，直接看invoke方法，忽略掉特殊情况（Object类的方法、java8接口的默认方法），最后调用的是`loadServiceMethod(method).invoke(args)`，其中method表示getBanner接口方法

java

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`// Retrofit.java /** * 加载数据请求服务的方法（由于解析会用到反射，加载后会存入缓存提升性能） * @param method Method * @return 请求服务的方法 */ ServiceMethod<?> loadServiceMethod(Method method) { // 从缓存获取有就直接返回 ServiceMethod<?> result = serviceMethodCache.get(method); if (result != null) return result; synchronized (serviceMethodCache) { result = serviceMethodCache.get(method); if (result == null) { // 缓存中没有，进行解析，返回的同时插入缓存 result = ServiceMethod.parseAnnotations(this, method); serviceMethodCache.put(method, result); } } return result; }`

*  主要使用了缓存策略，避免请求方法的多次解析，提高效率，直接看解析的方法`ServiceMethod.parseAnnotations(this, method)`

java

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`abstract class ServiceMethod<T> { static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) { // 这里会创建一个封装了OkHttp.Request的请求对象 RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method); // 获取请求服务的返回值,校验合法性 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."); } // 请求方法的解析（解析注解） return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory); } abstract @Nullable T invoke(Object[] args); }`

*  `RequestFactory`是对OkHttp.Request请求的封装，后面再深入源码，先一路深入，看下HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory)这个方法

java

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`abstract class HttpServiceMethod<ResponseT, ReturnT> extends ServiceMethod<ReturnT> { /** 检查接口方法上的注释，以构造一个可重复使用的HTTP服务方法。需要通过反射，所以需要使用缓存进行重用 */ static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations( Retrofit retrofit, Method method, RequestFactory requestFactory) { /** 是否是协程调用 */ boolean isKotlinSuspendFunction = requestFactory.isKotlinSuspendFunction; /** 协程返回是否是Response包装实体 */ boolean continuationWantsResponse = false; /** 协程返回是否是null */ boolean continuationBodyNullable = false; // 解析方法上的所有注解 Annotation[] annotations = method.getAnnotations(); Type adapterType; if (isKotlinSuspendFunction) { Type[] parameterTypes = method.getGenericParameterTypes(); Type responseType = Utils.getParameterLowerBound( 0, (ParameterizedType) parameterTypes[parameterTypes.length - 1]); if (getRawType(responseType) == Response.class && responseType instanceof ParameterizedType) { // 这里判断协程的返回是否是一个Response，即判断是一个Response包装实体 responseType = Utils.getParameterUpperBound(0, (ParameterizedType) responseType); continuationWantsResponse = true; } else { } adapterType = new Utils.ParameterizedTypeImpl(null, Call.class, responseType); annotations = SkipCallbackExecutorImpl.ensurePresent(annotations); } else { // 获取方法返回类型 adapterType = method.getGenericReturnType(); } // 获取CallAdapter,分析可发现这里最后返回的是DefaultCallAdapterFactory CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method, adapterType, annotations); Type responseType = callAdapter.responseType(); // 返回类型的校验 // 返回类型不能为okhttp3.Response类型 if (responseType == okhttp3.Response.class) { throw methodError( method, "'" + getRawType(responseType).getName() + "' is not a valid response body type. Did you mean ResponseBody?"); } // 返回类型不能是Response，必须要包含泛型才行,类似于Response<String> if (responseType == Response.class) { throw methodError(method, "Response must include generic type (e.g., Response<String>)"); } // TODO support Unit for Kotlin? if (requestFactory.httpMethod.equals("HEAD") && !Void.class.equals(responseType)) { throw methodError(method, "HEAD method must use Void as response type."); } // 获取Convert对象，自带的是BuiltInConverters，我们传入了GsonConverterFactory Converter<ResponseBody, ResponseT> responseConverter = createResponseConverter(retrofit, method, responseType); // 获取一个okhttp3.Call.Factory对象，其实就是OkHttpClient对象 okhttp3.Call.Factory callFactory = retrofit.callFactory; if (!isKotlinSuspendFunction) { // 不是Kotlin协程的suspend方法 // 返回一个新建的CallAdapted对象，传入requestFactory, callFactory, responseConverter, callAdapter return new CallAdapted<>(requestFactory, callFactory, responseConverter, callAdapter); } else if (continuationWantsResponse) { // 协程返回类型是Response<T> return (HttpServiceMethod<ResponseT, ReturnT>) new SuspendForResponse<>( requestFactory, callFactory, responseConverter, (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter); } else { // 协程返回类型是 T return (HttpServiceMethod<ResponseT, ReturnT>) new SuspendForBody<>( requestFactory, callFactory, responseConverter, (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter, continuationBodyNullable); } }`

*  直接看最后的返回，不是协程时返回的是`CallAdapted`对象，协程时当请求suspend函数返回的是Response时，返回`SuspendForResponse`对象，协程直接返回实体T时返回`SuspendForBody`
 *  参数中`requestFactory`, `callFactory`, `responseConverter`, `callAdapter`均比较重要后面再逐个分析
 *  追了这么久的返回值，最开始是从loadServiceMethod(method).invoke(args)调用的，loadServiceMethod(method)返回的是CallAdapted/SuspendForResponse/SuspendForBody,所以最后分别是这三个对象调用了invoke()，先看不是协程时callAdapted.invoke()

java

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`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); } }`

*  没看到有invoke()方法，向他的父类继续寻找

java

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`abstract class HttpServiceMethod<ResponseT, ReturnT> extends ServiceMethod<ReturnT> { ... @Override final @Nullable ReturnT invoke(Object[] args) { Call<ResponseT> call = new OkHttpCall<>(requestFactory, args, callFactory, responseConverter); return adapt(call, args); } protected abstract @Nullable ReturnT adapt(Call<ResponseT> call, Object[] args); ... }`

*  父类的invoke()调用了抽象方法adapt(call,args)，这个call是一个`OkHttpCall`本质上是okhttp3.Call的扩展，先放着后面再探讨，adapt这个抽象方法再在子类CallAdapted中被重写，所以最终会执行CallAdapted自身的adapt(),而上面CallAdapted类的adapt方法最终执行的是callAdapter.adapt(call),接下来就要分析下之前遗留的四个参数中的callAdapter了

#### 2.2.2 CallAdapter ####

*  CallAdapter是一个接口，主要用于创建 `Call` 对象，并指定如何处理异步请求和响应，例如自身默认的DefaultCallAdapterFactory和插件RxJavaCallAdapter都是它的实现

java

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`public interface CallAdapter<R, T> { Type responseType(); T adapt(Call<R> call); abstract class Factory { /** 获取CallAdapter实例 */ public abstract @Nullable CallAdapter<?, ?> get( Type returnType, Annotation[] annotations, Retrofit retrofit); protected static Type getParameterUpperBound(int index, ParameterizedType type) { return Utils.getParameterUpperBound(index, type); } protected static Class<?> getRawType(Type type) { return Utils.getRawType(type); } } }`

*  追一下刚刚CallAdapted类中的callAdapter，是在HttpServiceMethod.parseAnnotations()方法中获取并通过CallAdapted构造传入的

java

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`abstract class HttpServiceMethod<ResponseT, ReturnT> extends ServiceMethod<ReturnT> { /** * 检查接口方法上的注释，以构造一个可重复使用的HTTP服务方法。需要通过反射，所以需要使用缓存进行重用 */ static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations( Retrofit retrofit, Method method, RequestFactory requestFactory) { ... // 注释1 // 获取CallAdapter,分析可发现这里最后返回的是DefaultCallAdapterFactory CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method, adapterType, annotations); ... if (!isKotlinSuspendFunction) { // 不是Kotlin协程的suspend方法 // 返回一个新建的CallAdapted对象，传入requestFactory, callFactory, responseConverter, callAdapter return new CallAdapted<>(requestFactory, callFactory, responseConverter, callAdapter); } else if (continuationWantsResponse) { return (HttpServiceMethod<ResponseT, ReturnT>) new SuspendForResponse<>( requestFactory, callFactory, responseConverter, (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter); } else { return (HttpServiceMethod<ResponseT, ReturnT>) new SuspendForBody<>( requestFactory, callFactory, responseConverter, (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter, continuationBodyNullable); } }`

*  从上面的注释1得知callAdapter是由createCallAdapter方法返回的

java

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

*  最终从retrofit.callAdapter()获取

java

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`public CallAdapter<?, ?> callAdapter(Type returnType, Annotation[] annotations) { return nextCallAdapter(null, returnType, annotations); } public CallAdapter<?, ?> nextCallAdapter( @Nullable CallAdapter.Factory skipPast, Type returnType, Annotation[] annotations) { ... int start = callAdapterFactories.indexOf(skipPast) + 1; for (int i = start, count = callAdapterFactories.size(); i < count; i++) { CallAdapter<?, ?> adapter = callAdapterFactories.get(i).get(returnType, annotations, this); if (adapter != null) { return adapter; } } ... }`

*  最终是获取的callAdapterFactories中配置的callAdapterFactory，在2.2.1 Retrofit对象的创建中 得知，默认会添加DefaultCallAdapterFactory

java

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`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) { if (getRawType(returnType) != Call.class) { return null; } if (!(returnType instanceof ParameterizedType)) { throw new IllegalArgumentException( "Call return type must be parameterized as Call<Foo> or Call<? extends Foo>"); } final Type responseType = Utils.getParameterUpperBound(0, (ParameterizedType) returnType); // 注释1 // 判断请求方法上是否有@SkipCallbackExecutor注解，有返回null，没有返回callbackExecutor // 注意默认情况下callbackExecutor在Java平台也是null，而在Android平台返回MainThreadExecutor final Executor executor = Utils.isAnnotationPresent(annotations, SkipCallbackExecutor.class) ? null : callbackExecutor; // 返回CallAdapter的匿名实现类 注释2 return new CallAdapter<Object, Call<?>>() { @Override public Type responseType() { return responseType; } @Override public Call<Object> adapt(Call<Object> call) { // 最终会调用这里 注释3 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()) { 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)); } }); } ... } }`

*  从上面得之CallAdapter是从DefaultCallAdapterFactory.get()返回的，从上面代码的注释1可得知返回的是一个匿名CallAdapter实现类，从2.2.1得知最后走的是CallAdapter.adapt()方法，所以这里最终会执行这个匿名CallAdapter实现类的重写方法adapt()
 *  在adapt()方法里（注释3）中，会对executor进行判断，如果为null直接返回call,即上面分析的`OkHttpCall`,否则返回`ExecutorCallbackCall`；至于这个excutor则是在注释1处进行判断的，通过判断请求方法中是否包含@SkipCallbackExecutor注解，如果有就返回null,没有就返回callbackExecutorcallbackExecutor默认情况下在Java平台返回null而Android平台会返回`MainThreadExecutor`

java

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java

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`class Platform { ... @Nullable Executor defaultCallbackExecutor() { return null; } List<? extends CallAdapter.Factory> defaultCallAdapterFactories( @Nullable Executor callbackExecutor) { // callbackExecutor是从这里传入的 DefaultCallAdapterFactory executorFactory = new DefaultCallAdapterFactory(callbackExecutor); return hasJava8Types ? asList(CompletableFutureCallAdapterFactory.INSTANCE, executorFactory) : singletonList(executorFactory); } ... static final class Android extends Platform { @Override public Executor defaultCallbackExecutor() { return new MainThreadExecutor(); } ... }`

*  callbackExecutor是从PlatForm.defaultCallAdapterFactories()传入的，继续追这个方法的调用

java

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`public final class Retrofit { ... public static final class Builder { ... public Retrofit build() { ... // 执行请求回调的对象，Android中defaultCallbackExecutor是MainThreadExecutor即主线程 Executor callbackExecutor = this.callbackExecutor; if (callbackExecutor == null) { // 注释1 callbackExecutor = platform.defaultCallbackExecutor(); } // 网络请求适配器工厂集合 List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories); callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor)); ... } } }`

*  由上面的注释1可得最终是获取得platform.defaultCallbackExecutor()，这就证明了上面得结论：默认情况下在Java平台返回null而Android平台会返回`MainThreadExecutor`，也就是`Java平台最终会返回OkhttpCall，Android平台返回ExecutorCallbackCall`

#### 2.2.3 OkhttpCall & ExecutorCallbackCall ####

*  先看下这个OkhttpCall,它可以理解为okhttp3.Call的封装

java

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`/** 对okhttp3.Call的封装 */ final class OkHttpCall<T> implements Call<T> { /** OkHttp的请求Request的封装 */ private final RequestFactory requestFactory; private final Object[] args; /** okhttp3.Call.Factory可以理解为okhttpClient */ private final okhttp3.Call.Factory callFactory; private final Converter<ResponseBody, T> responseConverter; private volatile boolean canceled; /** 持有对okhttp3.Call的引用 */ @GuardedBy("this") private @Nullable okhttp3.Call rawCall; @GuardedBy("this") private @Nullable Throwable creationFailure; /** 是否已经执行execute的标志位（是否已经在请求队列） */ @GuardedBy("this") private boolean executed; OkHttpCall( RequestFactory requestFactory, Object[] args, okhttp3.Call.Factory callFactory, Converter<ResponseBody, T> responseConverter) { this.requestFactory = requestFactory; this.args = args; this.callFactory = callFactory; this.responseConverter = responseConverter; } ... /** 返回okhttp3.Call，有异常抛出 */ @GuardedBy("this") private okhttp3.Call getRawCall() throws IOException { okhttp3.Call call = rawCall; if (call != null) return call; ... try { return rawCall = createRawCall(); } catch (RuntimeException | Error | IOException e) { throwIfFatal(e); creationFailure = e; throw e; } } /** 异步请求 */ @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 { // 构建okhttp.call 注释1 call = rawCall = createRawCall(); } catch (Throwable t) { throwIfFatal(t); failure = creationFailure = t; } } } if (failure != null) { callback.onFailure(this, failure); return; } if (canceled) { call.cancel(); } call.enqueue( new okhttp3.Callback() { @Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse) { Response<T> response; try { // 解析响应数据 response = parseResponse(rawResponse); } 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 } } }); } /** 同步请求 */ @Override public Response<T> execute() throws IOException { okhttp3.Call call; synchronized (this) { if (executed) throw new IllegalStateException("Already executed."); executed = true; call = getRawCall(); } if (canceled) { call.cancel(); } return parseResponse(call.execute()); } /** 利用okhttpClient创建okhttp3.Call对象 */ private okhttp3.Call createRawCall() throws IOException { // 注释2 okhttp3.Call call = callFactory.newCall(requestFactory.create(args)); if (call == null) { throw new NullPointerException("Call.Factory returned null."); } return call; } /** 负责请求响应的解析 */ Response<T> parseResponse(okhttp3.Response rawResponse) throws IOException { ResponseBody rawBody = rawResponse.body(); ... ExceptionCatchingResponseBody catchingBody = new ExceptionCatchingResponseBody(rawBody); try { // 解析请求响应 T body = responseConverter.convert(catchingBody); return Response.success(body, rawResponse); ... } }`

*  回忆下Retrofit的请求步骤，从上文分析api.getBanner()返回的是一个OkhttpCall(Android平台是ExecutorCallbackCall，这个稍后分析)，接下来的步骤就是通过异步调用okhttpCall.enqueue()或同步调用okhttpCall.execute()，我们直接看这两个方法
 *  enqueue()中先在注释1处通过`createRawCall()`拿到okhttp.call，具体的方法就一行 okhttp3.Call call = callFactory.newCall(requestFactory.create(args))，这个callFactory就是okHttpClient,这也符合[okHttp][]创建Call的步骤，所以参数中requestFactory.create(args)必然返回的是okhttp.Request，这个后面分析`RequestFactory`时再细讲
 *  按照okhttp的请求步骤，接着就是接管okhttp中call.enqueue()的请求回调，在成功回调中对响应数据进行解析后用自己的回调去响应即可，失败就原样回调接管返回，而同步请求execute()就直接解析就可以了
 *  接着看下ExecutorCallbackCall是个啥

java

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`static final class ExecutorCallbackCall<T> implements Call<T> { final Executor callbackExecutor; /** Call代理，这里传入的其实就是okHttpCall */ 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()) { 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 boolean isExecuted() { return delegate.isExecuted(); } @Override public Response<T> execute() throws IOException { return delegate.execute(); } @Override public void cancel() { delegate.cancel(); } @Override public boolean isCanceled() { return delegate.isCanceled(); } @SuppressWarnings("CloneDoesntCallSuperClone") @Override public Call<T> clone() { return new ExecutorCallbackCall<>(callbackExecutor, delegate.clone()); } @Override public Request request() { return delegate.request(); } @Override public Timeout timeout() { return delegate.timeout(); } }`

*  其实就是用到了[装饰器模式][Link 3]和[静态代理][Link 2],在不改变OkhttpCall的情况下，其他方法均由okhttpCall这个代理对象进行操作，只对enqueue()方法使用callbackExecutor进行装饰，之前分析得到在Android平台callbackExecutor是一个MainThreadExecutor对象

java

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`// Platform.java static final class MainThreadExecutor implements Executor { private final Handler handler = new Handler(Looper.getMainLooper()); @Override public void execute(Runnable r) { handler.post(r); } }`

*  其实就是在接口返回中使用Handler包装一层实现线程的切换（okhttp请求是单独开启线程的，回调也是发生在子线程的）这样更适合Android平台，方便请求后直接操作UI

#### 2.2.4 RequestFactory ####

*  刚刚上面说到会使用requestFactory.create(args)构造一个okhttp.Request对象，现在就来看下里面的魔法

java

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`/** * 请求封装，主要是将Retrofit的请求转换为OkHttp的请求 */ final class RequestFactory { static RequestFactory parseAnnotations(Retrofit retrofit, Method method) { return new Builder(retrofit, method).build(); } private final Method method; private final HttpUrl baseUrl; final String httpMethod; private final @Nullable String relativeUrl; private final @Nullable Headers headers; private final @Nullable MediaType contentType; private final boolean hasBody; private final boolean isFormEncoded; private final boolean isMultipart; private final ParameterHandler<?>[] parameterHandlers; /** 请求是否是一个Kotlin协程中的挂载函数 */ final boolean isKotlinSuspendFunction; ... }`

*  第一个方法就是parseAnnotations()方法，就是在ServiceMethod.parseAnnotations()中调用的，主要还是用到了Builder模式来构建RequestFactory对象
 *  RequestFactory中还包含一些请求相关的成员变量，例如请求地址、请求方法、请求头等，这里有一个isKotlinSuspendFunction变量主要是判断请求方法是否是一个Kotlin中的挂载函数，具体后面分析Kotlin协程返回再具体分析
 *  直接看requestFactory.create(args)看下它是如何构造出一个okhttp.Request对象的

java

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`/** * 创建okhttp3.Request对象 * @param args * @return * @throws IOException */ okhttp3.Request create(Object[] args) throws IOException { @SuppressWarnings("unchecked") types. ParameterHandler<Object>[] handlers = (ParameterHandler<Object>[]) parameterHandlers; int argumentCount = args.length; if (argumentCount != handlers.length) { throw new IllegalArgumentException( "Argument count (" + argumentCount + ") doesn't match expected count (" + handlers.length + ")"); } RequestBuilder requestBuilder = new RequestBuilder( httpMethod, baseUrl, relativeUrl, headers, contentType, hasBody, isFormEncoded, isMultipart); if (isKotlinSuspendFunction) { // kotlin挂载函数(suspend关键字)编译成class会比java多一个Continuation类型的参数，所以此处需要-1 argumentCount--; } List<Object> argumentList = new ArrayList<>(argumentCount); for (int p = 0; p < argumentCount; p++) { argumentList.add(args[p]); handlers[p].apply(requestBuilder, args[p]); } return requestBuilder.get().tag(Invocation.class, new Invocation(method, argumentList)).build(); }`

*  主要是构造了一个RequestBuilder对象，并调用改对象的get()方法返回一个okHttp的Request.Builder对象，最后使用这个对象调用build()方法就可以构造出okhttp.Request对象了，我们看下这个RequestBuilder对象

java

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`final class RequestBuilder { private final String method; private final HttpUrl baseUrl; private @Nullable String relativeUrl; private @Nullable HttpUrl.Builder urlBuilder; private final Request.Builder requestBuilder; private final Headers.Builder headersBuilder; private @Nullable MediaType contentType; private final boolean hasBody; private @Nullable MultipartBody.Builder multipartBuilder; private @Nullable FormBody.Builder formBuilder; private @Nullable RequestBody body; ... Request.Builder get() { HttpUrl url; HttpUrl.Builder urlBuilder = this.urlBuilder; if (urlBuilder != null) { url = urlBuilder.build(); } else { url = baseUrl.resolve(relativeUrl); if (url == null) { throw new IllegalArgumentException( "Malformed URL. Base: " + baseUrl + ", Relative: " + relativeUrl); } } RequestBody body = this.body; if (body == null) { // Try to pull from one of the builders. if (formBuilder != null) { body = formBuilder.build(); } else if (multipartBuilder != null) { body = multipartBuilder.build(); } else if (hasBody) { // Body is absent, make an empty body. body = RequestBody.create(null, new byte[0]); } } MediaType contentType = this.contentType; if (contentType != null) { if (body != null) { body = new ContentTypeOverridingRequestBody(body, contentType); } else { headersBuilder.add("Content-Type", contentType.toString()); } } // 这里构建OkHttp.Request.Builder对象 return requestBuilder.url(url).headers(headersBuilder.build()).method(method, body); }`

*  主要是挟持了okhttp的诸多对象，通过对请求方法中的注解的解析，最后包装成一个OkHttp.Request对象

#### 2.2.5 Retrofit如何支持协程 ####

*  首先看下它是如何判断请求方法是一个Kotlin挂载（suspend）函数，我们先看下kotlin的suspend函数转变成Java的样子，找到刚刚的Api.kt编译后的class(build目录中)，使用Android Studio自带的Kotlin工具Decompile to Java会编译成

java

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`public interface Api { @GET("banner/json") @Nullable Object getBanner(@NotNull Continuation var1); @GET("banner/json") @Nullable Object getBanner2(@NotNull Continuation var1); @GET("banner/json") @NotNull Call getBanner3(); }`

*  可见suspend方法比普通的Java方法多出一个Continuation参数（最后一个参数），看下Retrofit中是不是从这点来判断的

java

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````final class RequestFactory { ... /** 请求是否是一个Kotlin协程中的挂载函数 */ final boolean isKotlinSuspendFunction; ``` private @Nullable ParameterHandler<?> parseParameter( int p, Type parameterType, @Nullable Annotation[] annotations, boolean allowContinuation) { ParameterHandler<?> result = null; if (annotations != null) { for (Annotation annotation : annotations) { ParameterHandler<?> annotationAction = parseParameterAnnotation(p, parameterType, annotations, annotation); if (annotationAction == null) { continue; } if (result != null) { throw parameterError( method, p, "Multiple Retrofit annotations found, only one allowed."); } result = annotationAction; } } // 判断请求方法的最后一个参数是否是Continuation类型，是就是一个suspend函数 if (result == null) { if (allowContinuation) { try { if (Utils.getRawType(parameterType) == Continuation.class) { isKotlinSuspendFunction = true; return null; } } catch (NoClassDefFoundError ignored) { } } throw parameterError(method, p, "No Retrofit annotation found."); } return result; } ... }````

*  的确源码就是根据这个参数来判断的，看下parseAnnotations()中协程相关部分

java

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`// HttpServiceMethod.java static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations( Retrofit retrofit, Method method, RequestFactory requestFactory) { /** 是否是协程调用 */ boolean isKotlinSuspendFunction = requestFactory.isKotlinSuspendFunction; /** 协程返回是否是Response包装实体 */ boolean continuationWantsResponse = false; /** 协程返回是否是null */ boolean continuationBodyNullable = false; ... if (!isKotlinSuspendFunction) { // 不是Kotlin协程的suspend方法 // 返回一个新建的CallAdapted对象，传入requestFactory, callFactory, responseConverter, callAdapter return new CallAdapted<>(requestFactory, callFactory, responseConverter, callAdapter); } else if (continuationWantsResponse) { return (HttpServiceMethod<ResponseT, ReturnT>) new SuspendForResponse<>( requestFactory, callFactory, responseConverter, (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter); } else { return (HttpServiceMethod<ResponseT, ReturnT>) new SuspendForBody<>( requestFactory, callFactory, responseConverter, (CallAdapter<ResponseT, Call<ResponseT>>) callAdapter, continuationBodyNullable); } }`

*  这里的`isKotlinSuspendFunction`就是从requestFactory获取的，当为协程时，判断请求返回类型是Response还是T，这里就以T为例，最终返回的是一个SuspendForBody对象

java

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`static final class SuspendForBody<ResponseT> extends HttpServiceMethod<ResponseT, Object> { private final CallAdapter<ResponseT, Call<ResponseT>> callAdapter; private final boolean isNullable; SuspendForBody( RequestFactory requestFactory, okhttp3.Call.Factory callFactory, Converter<ResponseBody, ResponseT> responseConverter, CallAdapter<ResponseT, Call<ResponseT>> callAdapter, boolean isNullable) { super(requestFactory, callFactory, responseConverter); this.callAdapter = callAdapter; this.isNullable = isNullable; } @Override protected Object adapt(Call<ResponseT> call, Object[] args) { call = callAdapter.adapt(call); Continuation<ResponseT> continuation = (Continuation<ResponseT>) args[args.length - 1]; try { return isNullable ? KotlinExtensions.awaitNullable(call, continuation) : KotlinExtensions.await(call, continuation); } catch (Exception e) { return KotlinExtensions.suspendAndThrow(e, continuation); } } }`

*  按照上一篇的分析，最终会执行adapt方法，这个isNullable是从上面parseAnnotations()方法中的continuationBodyNullable传递的，为false,所以最终会执行`KotlinExtensions.await(call, continuation)`

kotlin

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`/** Call的扩展函数 */ suspend fun <T : Any> Call<T>.await(): T { return suspendCancellableCoroutine { continuation -> continuation.invokeOnCancellation { cancel() } enqueue(object : Callback<T> { override fun onResponse(call: Call<T>, response: Response<T>) { if (response.isSuccessful) { val body = response.body() if (body == null) { val invocation = call.request().tag(Invocation::class.java)!! val method = invocation.method() val e = KotlinNullPointerException("Response from " + method.declaringClass.name + '.' + method.name + " was null but response body type was declared as non-null") // 协程返回请求错误 continuation.resumeWithException(e) } else { // 协程返回 continuation.resume(body) } } else { continuation.resumeWithException(HttpException(response)) } } override fun onFailure(call: Call<T>, t: Throwable) { continuation.resumeWithException(t) } }) } }`

*  就是利用kotlin的高阶函数给Call添加一个await扩展函数，再在onResponse回调中，通过协程挂起来执行耗时任务，而成功与失败会分别通过`resume()`与`resumeWithExecption()`来唤起挂起的协程，让它返回之前的挂起点，进行执行

[Link 1]: https://link.juejin.cn?target=https%3A%2F%2Fsquare.github.io%2Fretrofit%2F
[Link 2]: https://juejin.cn/post/7055206363097464846
[okHttp]: https://juejin.cn/post/7256651414067789883
[Link 3]: https://juejin.cn/post/7186487016605876282