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Retrofit 是一种基于 Java 的 RESTful Web Service 客户端库#xff0c;它可以将网络请求抽象出来并支持多种转换器#xff0c;可以将 JSON、XML 和其他格式的响应数据自动转换为 Java 对象。Retrofit 通过注解的方式来描述 REST API 调用#xff0c;使开发人员能够…1、简介
Retrofit 是一种基于 Java 的 RESTful Web Service 客户端库它可以将网络请求抽象出来并支持多种转换器可以将 JSON、XML 和其他格式的响应数据自动转换为 Java 对象。Retrofit 通过注解的方式来描述 REST API 调用使开发人员能够以极少的代码量和样板代码来实现网络请求同时具备灵活的自定义功能和强大的扩展性。
Retrofit 的主要特点包括
直观的注解通过注解将 REST 操作和 Java 方法联系起来完全避免了无意义的样板代码。大量的拓展器支持转换器、回调适配器、Call 自定义及拦截器等。可在 Android 和Java 等平台使用它支持各种 HTTP 方法包括 GET、POST 和 PUT 等。高度灵活的配置可以轻松配置超时、线程池、回退等行为。
Retrofit 是一种方便快捷的 RESTful Web Service 客户端库它简化了 Android 和 Java 应用程序中的网络请求并且可以根据需要进行定制化配置是 Android 和 Java 开发领域最受欢迎的网络请求库之一。
2、入门
下面写一个小例子接口用的是 玩安卓 的接口请求项目的分类。
//1.定义 API用来描述请求的接口
public interface WanAndroidService {GET(project/tree/json)CallCategoryEntity getCategory();
}
//2.创建 Retrofit
val retrofit Retrofit.Builder().baseUrl(https://www.wanandroid.com/).addConverterFactory(GsonConverterFactory.create()).build()//3.创建网络请求实例
val service retrofit.create(WanAndroidService::class.java)//4.调用网络请求 API生成 Call执行请求
val call service.getCategory()
call.enqueue(object : retrofit2.CallbackCategoryEntity {override fun onFailure(call: retrofit2.CallCategoryEntity, t: Throwable) {}override fun onResponse(call: retrofit2.CallCategoryEntity,response: retrofit2.ResponseCategoryEntity) {val result response.body()Log.d(result, result.toString())}})由上面代码我们可以看出Retrofit 使用流程非常简洁但是本文不是讲 Retrofit 的使用我们在学习中不仅要看表象更要看本质才能不断进步。
3、源码解析
3.1.Retrofit 的创建
在上面使用代码中有一个非常重要的关键点就是 Retrofit 的创建我们来看 Retrofit 是怎样构建的。
3.1.1. Builder 的创建
//采用建造者模式构建 Retrofit
val retrofit Retrofit.Builder().baseUrl(https://www.wanandroid.com/).addConverterFactory(GsonConverterFactory.create()).build()接下来我看看看 BuilderBuilder 是 Retrofit 一个内部类
public static final class Builder {//平台类型private final Platform platform;//请求工厂默认为 Okhttpprivate Nullable okhttp3.Call.Factory callFactory;//请求的 url 的地址private Nullable HttpUrl baseUrl;//数据转换的工厂集合private final ListConverter.Factory converterFactories new ArrayList();//适配器工厂的集合默认 ExecutorCallAdapterFactoryprivate final ListCallAdapter.Factory callAdapterFactories new ArrayList();//回调执行器把子线程切换成主线程Android 上的是封装了 handler 的 MainThreadExecutorprivate Nullable Executor callbackExecutor;//缓存为 true 会创建 ServiceMethodprivate boolean validateEagerly;} 我们在看看 Builder 默认初始化
public static final class Builder {Builder(Platform platform) {this.platform platform;}public Builder() {this(Platform.get());}.....
}
//涉及到 Platform 这个类
class Platform {private static final Platform PLATFORM findPlatform();static Platform get() {return PLATFORM;}private static Platform findPlatform() {try {//判断是否是 Android 平台Class.forName(android.os.Build);if (Build.VERSION.SDK_INT ! 0) {//创建一个 Android 类return new Android();}} catch (ClassNotFoundException ignored) {}return new Platform(true);}//创建默认网络请求适配器工厂List? extends CallAdapter.Factory defaultCallAdapterFactories(Nullable Executor callbackExecutor) {//默认网络适配器 DefaultCallAdapterFactory executorFactory new DefaultCallAdapterFactory(callbackExecutor);return hasJava8Types? asList(CompletableFutureCallAdapterFactory.INSTANCE, executorFactory): singletonList(executorFactory);}int defaultCallAdapterFactoriesSize() {return hasJava8Types ? 2 : 1;}List? extends Converter.Factory defaultConverterFactories() {return hasJava8Types? singletonList(OptionalConverterFactory.INSTANCE): emptyList();}//继承 Platform
static final class Android extends Platform {Android() {super(Build.VERSION.SDK_INT 24);}Override public Executor defaultCallbackExecutor() {//切换线程子线程切换成主线程return new MainThreadExecutor();}// Handler 机制子线程切换成主线程static class MainThreadExecutor implements Executor {private final Handler handler new Handler(Looper.getMainLooper());Override public void execute(Runnable r) {handler.post(r);}}}3.1.2.添加 baseUrl
//Retrofit.java
public Builder baseUrl(String baseUrl) {Objects.requireNonNull(baseUrl, baseUrl null);//将字符串转换成 HttpUrlreturn baseUrl(HttpUrl.get(baseUrl));
}public Builder baseUrl(HttpUrl baseUrl) {Objects.requireNonNull(baseUrl, baseUrl null);ListString pathSegments baseUrl.pathSegments();if (!.equals(pathSegments.get(pathSegments.size() - 1))) {throw new IllegalArgumentException(baseUrl must end in /: baseUrl);}this.baseUrl baseUrl;return this;}3.1.3.添加 GsonConverterFactory
//1.GsonConverterFactory 的 createpublic static GsonConverterFactory create() {return create(new Gson());}//2.调用 create
public static GsonConverterFactory create(Gson gson) {if (gson null) throw new NullPointerException(gson null);return new GsonConverterFactory(gson);}private final Gson gson;//3.创建含有 Gson 对象的 GsonConverterFactory private GsonConverterFactory(Gson gson) {this.gson gson;
}
//4.添加 addGsonConverFactory说白了就是将含有 Gson 对象 GsonConverterFactory 添加到 数据转换工厂 converterFactories 中
public Builder addConverterFactory(Converter.Factory factory) {converterFactories.add(Objects.requireNonNull(factory, factory null));return this;
}3.1.4.build()
接下来看看 build() 方法里面做了什么。 public Retrofit build() {if (baseUrl null) {throw new IllegalStateException(Base URL required.);}okhttp3.Call.Factory callFactory this.callFactory;if (callFactory null) {//默认请求工厂使用 OkHttpClientcallFactory new OkHttpClient();}Executor callbackExecutor this.callbackExecutor;if (callbackExecutor null) {//回调callbackExecutor platform.defaultCallbackExecutor();}// Make a defensive copy of the adapters and add the default Call adapter.ListCallAdapter.Factory callAdapterFactories new ArrayList(this.callAdapterFactories);callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));//添加默认适配器// Make a defensive copy of the converters.ListConverter.Factory converterFactories new ArrayList(1 this.converterFactories.size() platform.defaultConverterFactoriesSize());// Add the built-in converter factory first. This prevents overriding its behavior but also// ensures correct behavior when using converters that consume all types.converterFactories.add(new BuiltInConverters());converterFactories.addAll(this.converterFactories);converterFactories.addAll(platform.defaultConverterFactories());return new Retrofit(callFactory, baseUrl, unmodifiableList(converterFactories),unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly);}}3.2.创建网络请求
接下来分析 retrofit.create(流程这里采用外观模式和代理模式。 public T T create(final ClassT service) {//检验是否是接口validateServiceInterface(service);//使用动态代理获取请求接口的所有接口注解配置并且创建网络请求接口实例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);}if (platform.isDefaultMethod(method)) {return platform.invokeDefaultMethod(method, service, proxy, args);}return loadServiceMethod(method).invoke(args ! null ? args : emptyArgs);}});}private void validateServiceInterface(Class? service) {if (!service.isInterface()) {throw new IllegalArgumentException(API declarations must be interfaces.);}DequeClass? 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());}if (validateEagerly) {Platform platform Platform.get();for (Method method : service.getDeclaredMethods()) {if (!platform.isDefaultMethod(method) !Modifier.isStatic(method.getModifiers())) {loadServiceMethod(method);}}}}接下来看 loadServiceMethod
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;}abstract class ServiceMethodT {static T ServiceMethodT parseAnnotations(Retrofit retrofit, Method method) {//解析请求配置的注解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.);}//通过 HttpServiceMethod 构建的请求方法 return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);}abstract Nullable T invoke(Object[] args);
}接下来看看 HttpServiceMethod#parseAnnotations static ResponseT, ReturnT HttpServiceMethodResponseT, ReturnT parseAnnotations(Retrofit retrofit, Method method, RequestFactory requestFactory) {//1.从Retrofit对象中获取对应的网络请求适配器CallAdapterResponseT, ReturnT callAdapter createCallAdapter(retrofit, method, adapterType, annotations);//2.根据网络请求接口方法的 返回值 和 注解类型 从 Retrofit 对象中获取对应的数据转换器 ConverterResponseBody, ResponseT responseConverter createResponseConverter(retrofit, method, responseType); 接下来看 createResponseConverter private static ResponseT, ReturnT CallAdapterResponseT, ReturnT createCallAdapter(Retrofit retrofit, Method method, Type returnType, Annotation[] annotations) {try {//noinspection uncheckedreturn (CallAdapterResponseT, 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);}}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;//循环获取合适请求工厂for (int i start, count callAdapterFactories.size(); i count; i) {CallAdapter?, ? adapter callAdapterFactories.get(i).get(returnType, annotations, this);if (adapter ! null) {return adapter;}}接下来看 createResponseConverter
//最终都走到这个方法里面public T ConverterResponseBody, 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;//循环获取合适转换工厂for (int i start, count converterFactories.size(); i count; i) {ConverterResponseBody, ? converter converterFactories.get(i).responseBodyConverter(type, annotations, this);if (converter ! null) {//noinspection uncheckedreturn (ConverterResponseBody, T) converter;}}
}最后执行 HttpServiceMethod#invoke Override final Nullable ReturnT invoke(Object[] args) {//负责网络请求的 OkHttpCallCallResponseT call new OkHttpCall(requestFactory, args, callFactory, responseConverter);return adapt(call, args);}3.3.调用网络请求 API生成 Call执行请求
val call service.getCategory()从上面分析得出这个 service 对象其实是动态代理对象 Proxy.newProxyInstance()得到的 Call 对象。
3.3.1.异步请求
异步请求调用的是 enqueue
//DefaultCallAdapterFactory.javaOverride public void enqueue(final CallbackT callback) {Objects.requireNonNull(callback, callback null);//使用静态代理 delegate 进行请求delegate.enqueue(new CallbackT() {Override public void onResponse(CallT call, final ResponseT response) {//线程切换子线程切换成主线程callbackExecutor.execute(() - {if (delegate.isCanceled()) {// Emulate OkHttps 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(CallT call, final Throwable t) {callbackExecutor.execute(() - callback.onFailure(ExecutorCallbackCall.this, t));}});}接下来看看 delegate 中的 enqueue 这个 delegate 实际上是 OkHttpCall //OkHttpCall.javaOverride public void enqueue(final CallbackT 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 {//其实就是创建 Okhttp的 Request 对象调用的是 OkHttp.call。call rawCall createRawCall();} catch (Throwable t) {throwIfFatal(t);failure creationFailure t;}}}3.3.2.同步请求
val response category.execute()调用的还是是 OkhttpCall Override public ResponseT execute() throws IOException {okhttp3.Call call;synchronized (this) {if (executed) throw new IllegalStateException(Already executed.);executed true;if (creationFailure ! null) {if (creationFailure instanceof IOException) {throw (IOException) creationFailure;} else if (creationFailure instanceof RuntimeException) {throw (RuntimeException) creationFailure;} else {throw (Error) creationFailure;}}call rawCall;if (call null) {try {call rawCall createRawCall();} catch (IOException | RuntimeException | Error e) {throwIfFatal(e); // Do not assign a fatal error to creationFailure.creationFailure e;throw e;}}}if (canceled) {call.cancel();}//调用 OkHttpCall 的 execute() 发送网络请求return parseResponse(call.execute());}ResponseT parseResponse(okhttp3.Response rawResponse) throws IOException {ResponseBody rawBody rawResponse.body();// Remove the bodys source (the only stateful object) so we can pass the response along.rawResponse rawResponse.newBuilder().body(new NoContentResponseBody(rawBody.contentType(), rawBody.contentLength())).build();int code rawResponse.code();if (code 200 || code 300) {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) {rawBody.close();return Response.success(null, rawResponse);}ExceptionCatchingResponseBody catchingBody new ExceptionCatchingResponseBody(rawBody);try {//将响应体转为Java对象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;}}到这里Retrofit 流程已经非常清晰用一句话来概括就是采用动态代理最终将封装的请求交给底层的 OkHttp 来处理。 Android 学习笔录
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