Retrofit是一个基于AOP思想，对RestfulApi注解进行动态代理的网络框架;今天我们就来探讨下实现原理，一起进步。
前言
Retrofit是一个基于AOP思想，对RestfulApi注解进行动态代理的网络框架;
今天我们就来探讨下实现原理，一起进步
一、使用Retrofit
1、包引用
在gradle文件中引用retrofit
compile 'com.squareup.retrofit2:retrofit:2.3.0' compile 'com.squareup.retrofit2:retrofit-converters:2.3.0' compile 'com.squareup.retrofit2:retrofit-adapters:2.3.0'
如果需要使用更多扩展功能，比如gson转换，rxjava适配等，可以视自己需要继续添加引用;
compile 'com.squareup.retrofit2:converter-gson:2.3.0' compile 'com.squareup.retrofit2:adapter-rxjava2:2.3.0'
如果现有的扩展包不能满足需要，还可以自己扩展converter，adapter等;
2、定义接口
Retrofit要求定义一个网络请求的接口，接口函数里要定义url路径、请求参数、返回类型;
public interface INetApiService {
@GET("/demobiz/api.php")
Call<BizEntity> getBizInfo(@Query("id") String id);
}在这个接口定义中，用注解@GET("/demobiz/api.php")声明了url路径，用注解@Query("id") 声明了请求参数;
最重要的是，用Call声明了返回值是一个Retrofit的Call对象，并且声明了这个对象处理的数据类型为BizEntity，BizEntity是我们自定义的数据模型;
3、依次获得Retrofit对象、接口实例对象、网络工作对象
首先，需要新建一个retrofit对象;
然后，根据上一步的接口，实现一个retrofit加工过的接口对象;
最后，调用接口函数，得到一个可以执行网络访问的网络工作对象;
//新建一个Retrofit对象 Retrofit retrofit=new Retrofit.Builder() .baseUrl(Config.DOMAIN)//要访问的网络地址域名，如http://www.zhihu.com .addConverterFactory(GsonConverterFactory.create()) .build(); ... //用retrofit加工出对应的接口实例对象 INetApiService netApiService= retrofit.create(INetApiService.class); //可以继续加工出其他接口实例对象 IOtherService otherService= retrofit.create(IOtherService.class); ··· //调用接口函数，获得网络工作对象 Call<BizEntity> callWorker= netApiService.getBizInfo("id001");
这个复杂的过程下来，最终得到的callWorker对象，才可以执行网络访问。
4、访问网络数据
用上一步获取的worker对象，执行网络请求
callWorker.enqueue(new Callback<BizEntity>() {
@Override
public void onResponse(Call<BizEntity> call, Response<BizEntity> response) {...}
@Override
public void onFailure(Call<BizEntity> call, Throwable t) {...}
});在回调函数里，取得我们需要的BizEntity数据对象
二、Retrofit实现原理
1、Retrofit对象的构建就是简单的builder模式，直接看create
//Retrofit.java
public <T> T create(final Class<T> service) {
//验证
validateServiceInterface(service);
return (T)
//动态代理
Proxy.newProxyInstance(
service.getClassLoader(), //类加载器
new Class<?>[] {service}, //一组接口
new InvocationHandler() {
//判断android和jvm平台及其版本
private final Platform platform = Platform.get();
@Override
public Object invoke(Object proxy, Method method, Object[] args){
//如果该方法是Object的方法，直接执行不用管
if (method.getDeclaringClass() == Object.class) {
return method.invoke(this, args);
}
//isDefaultMethod：检查是否是java8开始支持的接口默认方法
return platform.isDefaultMethod(method)
? platform.invokeDefaultMethod(method, service, proxy, args)
: loadServiceMethod(method).invoke(args); //我们关注这里
}
});
}Proxy.newProxyInstance动态代理，运行期会生成一个类(字节码)如$ProxyN，实现传入的接口即WanApi，重写接口方法然后转发给InvocationHandler的invoke
class $ProxyN extends Proxy implements WanApi{
Call<WanArticleBean> articleList(@Path("page") int page){
//转发给invocationHandler
invocationHandler.invoke(this,method,args);
}
}2、validateServiceInterface验证逻辑
//Retrofit.java
private void validateServiceInterface(Class<?> service) {
//检查：WanApi不是接口就抛异常...
//检查：WanApi不能有泛型参数，不能实现其他接口...
if (validateEagerly) { //是否进行严格检查，默认关闭
Platform platform = Platform.get();
for (Method method : service.getDeclaredMethods()) { //遍历WanApi方法
//不是默认方法，并且不是静态方法
if (!platform.isDefaultMethod(method) && !Modifier.isStatic(method.getModifiers())) {
//把方法提前加载进来（检查下有没有问题）
loadServiceMethod(method);
}
}
}
}如果开了validateEagerly，会一次性把接口WanApi的所有方法都检查一遍并加载进来，可以在debug模式下开启，提前发现错误写法，比如在@GET请求设置了@Body这种错误就会抛出异常：
java.lang.IllegalArgumentException: Non-body HTTP method cannot contain @Body.
3、loadServiceMethod
然后是loadServiceMethod(method).invoke(args)，看名字可知是先找方法，然后执行
//Retrofit.java //缓存，用了线程安全ConcurrentHashMap final Map<Method, ServiceMethod<?>> serviceMethodCache = new ConcurrentHashMap<>(); ServiceMethod<?> loadServiceMethod(Method method) { ServiceMethod<?> result = serviceMethodCache.get(method); //WanApi的articleList方法已缓存，直接返回 if (result != null) return result; synchronized (serviceMethodCache) { result = serviceMethodCache.get(method); if (result == null) { //解析articleList的注解，创建ServiceMethod并缓存起来 result = ServiceMethod.parseAnnotations(this, method); serviceMethodCache.put(method, result); } } return result; }
跟进ServiceMethod.parseAnnotations
//ServiceMethod.java
static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
//1.
RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);
//检查：articleList方法返回类型不能用通配符和void...
//2.
return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
}4、 RequestFactory.parseAnnotations
//RequestFactory.java
static RequestFactory parseAnnotations(Retrofit retrofit, Method method) {
return new Builder(retrofit, method).build();
}
class Builder {
RequestFactory build() {
//解析方法注解如GET
for (Annotation annotation : methodAnnotations) {
parseMethodAnnotation(annotation);
}
//省略各种检查...
//解析参数注解如Path
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);
}
//省略各种检查...
return new RequestFactory(this);
}
}得到RequestFactory后， HttpServiceMethod.parseAnnotations，HttpServiceMethod负责适配和转换处理，将接口方法的调用调整为HTTP调用
//HttpServiceMethod.java
//ResponseT响应类型如WanArticleBean，ReturnT返回类型如Call
static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
Retrofit retrofit, Method method, RequestFactory requestFactory) {
//省略kotlin协程逻辑...
Annotation[] annotations = method.getAnnotations();
//遍历找到合适的适配器
CallAdapter<ResponseT, ReturnT> callAdapter =
createCallAdapter(retrofit, method, adapterType, annotations);
//得到响应类型，如WanArticleBean
Type responseType = callAdapter.responseType();
//遍历找到合适的转换器
Converter<ResponseBody, ResponseT> responseConverter =
createResponseConverter(retrofit, method, responseType);
okhttp3.Call.Factory callFactory = retrofit.callFactory;
return new CallAdapted<>(requestFactory, callFactory, responseConverter, callAdapter);
}5、最终返回了一个CallAdapted，看到CallAdapted
//CallAdapted extends HttpServiceMethod extends ServiceMethod
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) {
//适配器
return callAdapter.adapt(call);
}
}回到Retrofit.Builder
//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<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
//添加默认的DefaultCallAdapterFactory
callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));
}DefaultCallAdapterFactory这个工厂创建具体的CallAdapter实例
//DefaultCallAdapterFactory.java
public CallAdapter<?, ?> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
final Type responseType = Utils.getParameterUpperBound(0, (ParameterizedType) returnType);
//如果指定了SkipCallbackExecutor注解，就表示不需要切回主线程
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) {
//默认情况下，返回用主线程池包装的Call，他的enqueue会使用主线程池的execute
return executor == null ? call : new ExecutorCallbackCall<>(executor, call);
}
};
}6、invoke
前边loadServiceMethod得到了CallAdapted，然后执行invoke，实现在父类HttpServiceMethod里，
//HttpServiceMethod.java
final ReturnT invoke(Object[] args) {
//终于见到okhttp了！
Call<ResponseT> call = new OkHttpCall<>(requestFactory, args, callFactory, responseConverter);
return adapt(call, args);
}
class CallAdapted<ResponseT, ReturnT> extends HttpServiceMethod<ResponseT, ReturnT> {
private final CallAdapter<ResponseT, ReturnT> callAdapter;
@Override
protected ReturnT adapt(Call<ResponseT> call, Object[] args) {
//用前边得到的适配器，把OkHttpCall包成ExecutorCallbackCall
return callAdapter.adapt(call);
}
}然后是请求入队，ExecutorCallbackCall.enqueue -> OkHttpCall.enqueue，
//ExecutorCallbackCall.java
void enqueue(final Callback<T> callback) {
delegate.enqueue(
new Callback<T>() {
@Override
public void onResponse(Call<T> call, final Response<T> response) {
//将回调切回主线程
callbackExecutor.execute(
() -> {
callback.onResponse(ExecutorCallbackCall.this, response);
});
//...
}
@Override
public void onFailure(Call<T> call, final Throwable t) {}
});
}
//OkHttpCall.java
void enqueue(final Callback<T> callback) {
//okhttp逻辑
okhttp3.Call call;
call.enqueue(new okhttp3.Callback() {
void onResponse(okhttp3.Call call, okhttp3.Response rawResponse) {
callback.onResponse(OkHttpCall.this, response);
}
})
}到此整个流程就通了
二、功能扩展
1、OkHttpClient
Retrofit使用OkHttpClient来实现网络请求，这个OkHttpClient虽然不能替换为其他的网络执行框架比如Volley，但是Retrofit允许我们使用自己扩展OkHttpClient，一般最常扩展的就是Interceptor拦截器了;
OkHttpClient mClient = new OkHttpClient.Builder()
.addInterceptor(new Interceptor() {
@Override
public Response intercept(Chain chain) throws IOException {
try {
Request.Builder builder = chain.request().newBuilder();
builder.addHeader("Accept-Charset", "UTF-8");
builder.addHeader("Accept", " application/json");
builder.addHeader("Content-type", "application/json");
Request request = builder.build();
return chain.proceed(request);
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
}).build();
Retrofit retrofit = new Retrofit.Builder()
.baseUrl(Config.DOMAIN)
.addConverterFactory(GsonConverterFactory.create())
.client(mClient)
.build();2、addConverterFactory
扩展的是对返回的数据类型的自动转换，把一种数据对象转换为另一种数据对象;
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