从OkHttp到Retrofit

• 简单使用
• 实现原理
• • loadServiceMethod
  • Converter

简单使用

class RetrofitActivity : AppCompatActivity() {companion object {const val SERVER = "https://www.xxx.com/"}var disposable:Disposable? = null;override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_retrofit)val retrofit = Retrofit.Builder().baseUrl(SERVER).addConverterFactory(GsonConverterFactory.create()).addCallAdapterFactory(RxJava2CallAdapterFactory.create()).build()disposable = retrofit.create(TestApi::class.java).getIcons(1).subscribe({Log.d("api", it.isSuccessful.toString())}, {Log.d("api", it.toString())})}override fun onStop() {super.onStop()disposable.dispose()}
}//RESTFUL 风格接口
interface TestApi {@GET("/test/icon")fun getIcons(@Query("icon_id") icon_id: Long): Observable>
}class IconBean{}

实现原理

Retrofit的代码逻辑比较复杂，但是实际上Retrofit的主要功能就是给OkHttp做一层封装，这里我把主要流程进行了总结，简单来说当调用Retrofit.create(TestApi.classjava)方法之后，Retrofit首先会判断TestApi这个类是否符合条件，如果开启了严格检查的话，还会提前加载HttpServiceMethod。

//Retrofit.java
public  T create(final Class service) {//验证validateServiceInterface(service);return (T)//动态代理Proxy.newProxyInstance(service.getClassLoader(), new Class[] {service}, new InvocationHandler() {//判断android和jvm平台及其版本private final Platform platform = Platform.get();@Overridepublic Object invoke(Object proxy, Method method, Object[] args){//如果是静态方法则直接执行if (method.getDeclaringClass() == Object.class) {return method.invoke(this, args);}//isDefaultMethod：是否是默认方法return platform.isDefaultMethod(method)? platform.invokeDefaultMethod(method, service, proxy, args): loadServiceMethod(method).invoke(args); }});
}

private void validateServiceInterface(Class service) {//判断是不是接口，不是就抛出异常if (!service.isInterface()) {throw new IllegalArgumentException("API declarations must be interfaces.");}//判断存不存在泛型，存在则抛出异常Deque> check = new ArrayDeque<>(1);check.add(service);while (!check.isEmpty()) {Class candidate = check.removeFirst();if (candidate.getTypeParameters().length != 0) {StringBuilder message =new StringBuilder("Type parameters are unsupported on ").append(candidate.getName());if (candidate != service) {message.append(" which is an interface of ").append(service.getName());}throw new IllegalArgumentException(message.toString());}Collections.addAll(check, candidate.getInterfaces());}//提前加载if (validateEagerly) {Platform platform = Platform.get();for (Method method : service.getDeclaredMethods()) {if (!platform.isDefaultMethod(method) && !Modifier.isStatic(method.getModifiers())) {loadServiceMethod(method);}}}}

loadServiceMethod

当检查完成之后，Retrofit就会创建出一个代理类，因此调用retrofit.create(TestApi::class.java).getIcons(1)事实上是调用了代理类的invoke方法，这个invoke方法会首先根据注解进行解析，添加参数，生成一个okHttp的Call请求，然后通过适配器模式将Call的返回结果进行适配。例如我希望使用RxJava的Observable作为返回类型，那么这里可以使用RxJavaCallAdapterFactory将OkHttp返回的Call类型转换为Observable。
基于这个流程，来阅读一下代码。

ServiceMethod loadServiceMethod(Method method) {//判断是否有缓存ServiceMethod result = serviceMethodCache.get(method);if (result != null) return result;synchronized (serviceMethodCache) {result = serviceMethodCache.get(method);if (result == null) {//没有缓存的话根据注解解析一个//这里的返回是一个HttpServiceMethod，里面包含了一个适配好的callresult = ServiceMethod.parseAnnotations(this, method);serviceMethodCache.put(method, result);}}return result;}
//ServiceMethod.java
static  ServiceMethod parseAnnotations(Retrofit retrofit, Method method) {RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);//检查：articleList方法返回类型不能用通配符和void...return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
}

可以看到ServiceMethod中首先创建了一个RequestFactory，RequestFactory的作用是根据注解来

//RequestFactory.java
static RequestFactory parseAnnotations(Retrofit retrofit, Method method) {return new Builder(retrofit, method).build();
}class Builder {RequestFactory build() {//解析请求类型，methodAnnotations是method中的注解，通过getAnnotations()获取for (Annotation annotation : methodAnnotations) {parseMethodAnnotation(annotation);}//...//解析参数注解如Path，parameterAnnotationsArray是参数注解数组int parameterCount = parameterAnnotationsArray.length;parameterHandlers = new ParameterHandler[parameterCount];for (int p = 0, lastParameter = parameterCount - 1; p < parameterCount; p++) {parameterHandlers[p] =parseParameter(p, parameterTypes[p], parameterAnnotationsArray[p], p == lastParameter);}//... return new RequestFactory(this);}
}

从这里可以看出当build方法调用之后，事实上RequestFactory对传入的方法完成了解析，并且把解析的结果保存在自己成员变量中。

//HttpServiceMethod.java
//ResponseT响应类型如WanArticleBean，ReturnT返回类型如Call
static  HttpServiceMethod parseAnnotations(Retrofit retrofit, Method method, RequestFactory requestFactory) {//...Annotation[] annotations = method.getAnnotations();//遍历找到合适的适配器CallAdapter callAdapter =createCallAdapter(retrofit, method, adapterType, annotations);//得到响应类型Type responseType = callAdapter.responseType();//根据返回类型找到合适的转换器//最终又会回调到retrofit里面的callAdapterFactories//callAdapterFactories在一开始builder时候传入Converter responseConverter =createResponseConverter(retrofit, method, responseType);okhttp3.Call.Factory callFactory = retrofit.callFactory;return new CallAdapted<>(requestFactory, callFactory, responseConverter, callAdapter);
}static final class CallAdapted extends HttpServiceMethod {private final CallAdapter callAdapter;CallAdapted(RequestFactory requestFactory,okhttp3.Call.Factory callFactory,Converter responseConverter,CallAdapter callAdapter) {super(requestFactory, callFactory, responseConverter);this.callAdapter = callAdapter;}@Overrideprotected ReturnT adapt(Call call, Object[] args) {return callAdapter.adapt(call);}//父类HttpServiceMethod中的实现。@Overridefinal @Nullable ReturnT invoke(Object[] args) {Call call = new OkHttpCall<>(requestFactory, args, callFactory, responseConverter);return adapt(call, args);}}

结合之前的说明，可以看到当找到和是的适配器之后，loadServiceMethod会返回一个CallAdapted，并且当调用invoke方法时候，会根据CallAdapted内部的callAdapter，将Call转化为适配类型。
callAdapter又是从retrofit的callAdapterFactories中遍历得到，所以这里再回过来看看retrofit.builder()方法。

//Retrofit.Builder.java
public Retrofit build() {Executor callbackExecutor = this.callbackExecutor;//如果没设置线程池，则给android平台设置一个默认的MainThreadExecutor（用Handler将回调切回主线程）if (callbackExecutor == null) {callbackExecutor = platform.defaultCallbackExecutor();}List callAdapterFactories = new ArrayList<>(this.callAdapterFactories);//添加默认的DefaultCallAdapterFactorycallAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));
}

List defaultCallAdapterFactories(@Nullable Executor callbackExecutor) {DefaultCallAdapterFactory executorFactory = new DefaultCallAdapterFactory(callbackExecutor);return hasJava8Types//默认的只有两个，如果需要使用其他的AdapterFactory需要在创建的时候传入? asList(CompletableFutureCallAdapterFactory.INSTANCE, executorFactory): singletonList(executorFactory);}

Converter

Converter的作用就是转化各种参数类型。

interface WanApi {//Long cur 当前时间@GET("article/list/{page}/json")Call articleList(@Path("page") int page, @Query("cur") Long cur);
}class TimeConverter implements Converter {private SimpleDateFormat mFormat = new SimpleDateFormat("yyyy-MM-dd-HHmmss");@Overridepublic String convert(Long value) throws IOException {if (value > 1_000_000_000_000L) {//毫秒，不是很严谨 - -return mFormat.format(new Date(value));}return String.valueOf(value);}
}class TimeConverterFactory extends Converter.Factory {@Overridepublic Converter stringConverter(Type type, Annotation[] annotations, Retrofit retrofit) {if (type == Long.class) {//使用自定义TimeConverterreturn new TimeConverter();}return super.stringConverter(type, annotations, retrofit);}public static Converter.Factory create() {return new TimeConverterFactory();}
}//addConverterFactory(TimeConverterFactory.create())