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简单使用
定义HTTP API
public interface GitHubService {@GET("users/{user}/repos")Call<List<Repo>> listRepos(@Path("user") String user);
}
复制代码创建Retrofit并生成API的实现
Retrofit retrofit = new Retrofit.Builder().baseUrl("https://api.github.com/").build();
GitHubService service = retrofit.create(GitHubService.class);
复制代码调用API方法，生成Call
Call<List<Repo>> repos = service.listRepos("octocat");
复制代码Retrofit的创建
retrofit实例的创建，使用了builder模式，从下面的源码中可以看出
public static final class Builder {Builder(Platform platform) {this.platform = platform;converterFactories.add(new BuiltInConverters());}public Builder() {// Platform.get()方法可以用于判断当前的环境this(Platform.get());}public Builder baseUrl(String baseUrl) {checkNotNull(baseUrl, "baseUrl == null");HttpUrl httpUrl = HttpUrl.parse(baseUrl);if (httpUrl == null) {throw new IllegalArgumentException("Illegal URL: " + baseUrl);}return baseUrl(httpUrl);}public Retrofit build() {if (baseUrl == null) {throw new IllegalStateException("Base URL required.");}okhttp3.Call.Factory callFactory = this.callFactory;if (callFactory == null) {callFactory = new OkHttpClient();// 新建Client，留到之后newCall什么的}Executor callbackExecutor = this.callbackExecutor;if (callbackExecutor == null) {callbackExecutor = platform.defaultCallbackExecutor();}// Make a defensive copy of the adapters and add the default Call adapter.List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));// Make a defensive copy of the converters.List<Converter.Factory> converterFactories = new ArrayList<>(this.converterFactories);return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,callbackExecutor, validateEagerly);}
}
复制代码retrofit.create
好玩的地方开始了，我们先来看看这个方法
public <T> T create(final Class<T> service) {Utils.validateServiceInterface(service);if (validateEagerly) {eagerlyValidateMethods(service);}// 动态代理，啦啦啦return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },new InvocationHandler() {// platform 可以分辨出你是在android，还是java8，又或者别的private final Platform platform = Platform.get();@Override public Object invoke(Object proxy, Method method, Object[] args)throws Throwable {// If the method is a method from Object then defer to normal invocation.// 这里的invoke，Object方法都走这里，比如equals、toString、hashCode什么的if (method.getDeclaringClass() == Object.class) {return method.invoke(this, args);}// java8默认方法，1.8的新特性if (platform.isDefaultMethod(method)) {return platform.invokeDefaultMethod(method, service, proxy, args);}// 这里是核心代码了ServiceMethod<Object, Object> serviceMethod =(ServiceMethod<Object, Object>) loadServiceMethod(method);OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);return serviceMethod.callAdapter.adapt(okHttpCall);}});
}
复制代码可以看出创建API使用了动态代理，根据接口动态生成的代理类，将接口的都转发给了负责连接代理类和委托类的InvocationHandler实例，接口方法也都通过其invoke方法来处理。 在invoke方法中，首先会通过Platform.get()方法判断出当前代码的执行环境，之后会先把Object和Java8的默认方法进行一个处理，也是在进行后续处理之前进行去噪。其中的关键代码其实就是最后三句，这也是这篇文章将要分析的
创建ServiceMethod
erviceMethod<?, ?> loadServiceMethod(Method method) {// 从缓存里面取出，如果有的话，直接返回好了ServiceMethod<?, ?> result = serviceMethodCache.get(method);if (result != null) return result;synchronized (serviceMethodCache) {result = serviceMethodCache.get(method);if (result == null) {// 为null的话，解析方法的注解和返回类型、参数的注解he参数类型，新建一个ServiceMethodresult = new ServiceMethod.Builder<>(this, method).build();// ->// 新建的ServiceMethod加到缓存列表里面serviceMethodCache.put(method, result);}}return result;
}
复制代码注解的解析
CallAdapter和Converter等到后面再分析，这里先看看parseMethodAnnotation(annotation)，功能和其名字一样，其对方法注解进行了解析
/*** 解析方法注解，呜啦啦* 通过判断注解类型来解析* @param annotation*/
private void parseMethodAnnotation(Annotation annotation) {if (annotation instanceof DELETE) {parseHttpMethodAndPath("DELETE", ((DELETE) annotation).value(), false);} else if (annotation instanceof GET) {parseHttpMethodAndPath("GET", ((GET) annotation).value(), false);} // 其他的一些方法注解的解析...
}
private void parseHttpMethodAndPath(String httpMethod, String value, boolean hasBody) {if (this.httpMethod != null) {// 已经赋值过了throw methodError("Only one HTTP method is allowed. Found: %s and %s.",this.httpMethod, httpMethod);}this.httpMethod = httpMethod;this.hasBody = hasBody;// value为设置注解方法时候，设置的值，官方例子中的users/{user}/repos or userif (value.isEmpty()) {return;}// 查询条件的一些判断...this.relativeUrl = value;this.relativeUrlParamNames = parsePathParameters(value);
}
`
复制代码在解析注解时，先通过instanceof判断出注解的类型，之后调用parseHttpMethodAndPath方法解析注解参数值，并设置httpMethod、relativeUrl、relativeUrlParamNames等属性。 上面说了API中方法注解的解析，现在来看看方法参数注解的解析，这是通过调用parseParameterAnnotation方法生成ParameterHandler实例来实现的，代码比较多，这里挑选@Query来看看。
else if (annotation instanceof Query) {
Query query = (Query) annotation;
String name = query.value();
boolean encoded = query.encoded();
Class<?> rawParameterType = Utils.getRawType(type);// 返回基础的类
gotQuery = true;
// 可以迭代，Collection
if (Iterable.class.isAssignableFrom(rawParameterType)) {if (!(type instanceof ParameterizedType)) {throw parameterError(p, rawParameterType.getSimpleName()+ " must include generic type (e.g., "+ rawParameterType.getSimpleName()+ "<String>)");}ParameterizedType parameterizedType = (ParameterizedType) type;Type iterableType = Utils.getParameterUpperBound(0, parameterizedType);// 返回基本类型Converter<?, String> converter =retrofit.stringConverter(iterableType, annotations);return new ParameterHandler.Query<>(name, converter, encoded).iterable();
} else if (rawParameterType.isArray()) {// ArrayClass<?> arrayComponentType = boxIfPrimitive(rawParameterType.getComponentType());// 如果是基本类型，自动装箱Converter<?, String> converter =retrofit.stringConverter(arrayComponentType, annotations);return new ParameterHandler.Query<>(name, converter, encoded).array();
} else {// OtherConverter<?, String> converter =retrofit.stringConverter(type, annotations);return new ParameterHandler.Query<>(name, converter, encoded);
}
复制代码在@Query中，将分成Collection、array、other三种情况处理参数，之后根据这些参数，调用ParameterHandler中的Query静态类，创建出一个ParameterHandler实例。这样循环直到解析了所有的参数注解，组合成为全局变量parameterHandlers，之后构建请求时会用到
OkHttpCall
ServiceMethod创建完成之后，我们来看看下一行代码中的OkHttpCall类，里面的包含了请求的执行和响应处理，我们来看看异步请求的做法
OkHttpCall(ServiceMethod<T, ?> serviceMethod, Object[] args) {this.serviceMethod = serviceMethod;this.args = args;
}
@Override public void enqueue(final Callback<T> callback) {
checkNotNull(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();// 创建OkHttp3.Call} catch (Throwable 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)throws IOException {Response<T> response;try {response = parseResponse(rawResponse);// ->} catch (Throwable e) {callFailure(e);return;}callSuccess(response);}@Override public void onFailure(okhttp3.Call call, IOException e) {try {callback.onFailure(OkHttpCall.this, e);} catch (Throwable t) {t.printStackTrace();}}private void callFailure(Throwable e) {try {callback.onFailure(OkHttpCall.this, e);} catch (Throwable t) {t.printStackTrace();}}private void callSuccess(Response<T> response) {try {callback.onResponse(OkHttpCall.this, response);} catch (Throwable t) {t.printStackTrace();}}
});
}
private okhttp3.Call createRawCall() throws IOException {Request request = serviceMethod.toRequest(args);// 根据ParameterHandler组装Request.Builder，生成Requestokhttp3.Call call = serviceMethod.callFactory.newCall(request);// Retrofit中创建的new OkHttpClient().newCall(request)...return call;
}
复制代码CallAdapter 现在来看看enqueue传入的参数callback,这个参数可能和很多人心中想的并不一样，它并不是用户在使用时传入的那个Callback对象。那么他是从哪里来的呢？不知道你还记不记得我之前在Retrofit.Builder.build()方法中提到过一句代码Platform.get()。在不使用addCallAdapterFactory的情况下。将会使用Platform的一种内部类，在Android环境下将会使用到Android类（这其实是个策略模式）
static class Android extends Platform {@Override public Executor defaultCallbackExecutor() {return new MainThreadExecutor();}@Override CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {return new ExecutorCallAdapterFactory(callbackExecutor);}static class MainThreadExecutor implements Executor {// Looper.getMainLooper()就是为嘛响应会在主线程的原因private final Handler handler = new Handler(Looper.getMainLooper());@Override public void execute(Runnable r) {handler.post(r);}}
}
复制代码上面的代码先稍微放一下，我们继续看retrofit.Bulider.build,其中有几句比较关键的代码
callFactory = new OkHttpClient();
callbackExecutor = platform.defaultCallbackExecutor();
adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));
复制代码结合Android类中的代码可以看出，其最后生成了ExecutorCallAdapterFactory类。虽然看到了CallAdapter.Factory，但是到底是哪里执行了enqueue方法呢？现在我们来看看retrofit.create的最后一句代码serviceMethod.callAdapter.adapt(okHttpCall)
Converter
现在回到OkhttpCall.enqueue方法中，在其中还有一句重要的代码没有看，那就是response = parseResponse(rawResponse);,我们来看看这其中做了什么。
Response<T> parseResponse(okhttp3.Response rawResponse) throws IOExceptionResponseBody rawBody = rawResponse.body();// Remove the body's source (the only stateful object) so we can pass thrawResponse = rawResponse.newBuilder().body(new NoContentResponseBody(rawBody.contentType(), rawBody.conte.build();...ExceptionCatchingRequestBody catchingBody = new ExceptionCatchingRequestBody(rawBody);try {T body = serviceMethod.toResponse(catchingBody);// 解析body，比如Gson解析return Response.success(body, rawResponse);} catch (RuntimeException e) {// If the underlying source threw an exception, propagate that rather // a runtime exception.catchingBody.throwIfCaught();throw e;}
}
### ServiceMethod
R toResponse(ResponseBody body) throws IOException {return responseConverter.convert(body);
}
复制代码可以看出parseResponse最终调用了Converter.convert方法。这里以常用的GsonConverterFactory为例。
# GsonConverterFactory
@Override
public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,Retrofit retrofit) {TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));return new GsonResponseBodyConverter<>(gson, adapter);
}
# GsonResponseBodyConverter
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 {return adapter.read(jsonReader);} finally {value.close();}}
}
复制代码responseBodyConverter方法中用到的type参数就是之前我在CallAdapter中提到的responseType方法的返回值。生成adapter方法，用于convert方法使用。OkHttpCall在这之后的代码就比较简单了，通过回调将转换后得响应数据发送出去即可 本文分析了Retrofit的执行流程，其实包含了Retrofit、ServiceMethod、OkHttpCall、CallAdapter、Converter等方面。Retrofit的代码相对是比较少，也比较容易理解的，不过却是很好的架构实例。
Android技术交流群653583088，欢迎大家加入交流，畅谈！本群有免费学习资料视频’