Retrofit2是针对于Android/Java的、基于okHttp的、一种轻量级且安全的、并使用注解方式的网络请求框架。简单的说它是一个基于OkHttp的RESTFUL Api请求工具，它通过接口注解的方式，把注解信息封装成一个Http请求，然后用OkHttp去发送这个请求。
以下是Retrofit的基本使用方法（代码来自网络）：
public interface ZhuanLanApi {
@GET("/api/columns/{user} ")
Call<ZhuanLanAuthor> getAuthor(@Path("user") String user)
}
public static final String API_URL = "https://zhuanlan.zhihu.com";
Create a very simple REST adapter which points the Zhuanlan API.
Retrofit retrofit = new Retrofit.Builder()
.baseUrl(API_URL)
.addConverterFactory(GsonConverterFactory.create())
.build();
ZhuanLanApi api = retrofit.create(ZhuanLanApi.class);
Call<ZhuanLanAuthor> call = api.getAuthor("qinchao");
// 请求数据，并且处理response
call.enqueue(new Callback<ZhuanLanAuthor>() {
@Override
public void onResponse(Response<ZhuanLanAuthor> author) {
System.out.println("name： " + author.getName());
}
@Override
public void onFailure(Throwable t) {
}
});
一、Retrofit的构建
Retrofit通过Builder模式创建，我们先看它的成员变量：
private Platform platform;//平台：安卓、java等
private okhttp3.Call.Factory callFactory; //okhttp的Call工厂类,自定义newCall将Request转为Call
private HttpUrl baseUrl;//okhttp中的类，保存解析过的url
private List<Converter.Factory> converterFactories = new ArrayList<>();//类型转换工厂列表。
private List<CallAdapter.Factory> adapterFactories = new ArrayList<>();//CallAdapter工厂列表。
private Executor callbackExecutor;//回调线程池
Platform会根据不同平台创建不同的Platform：
private static Platform findPlatform() {
try {
Class.forName("android.os.Build");
if (Build.VERSION.SDK_INT != 0) {
return new Android();
}
} catch (ClassNotFoundException ignored) {
}
try {
Class.forName("java.util.Optional");
return new Java8();
} catch (ClassNotFoundException ignored) {
}
return new Platform();
}
static class Android extends Platform {
@Override public Executor defaultCallbackExecutor() {
return new MainThreadExecutor();
}
@Override CallAdapter.Factory defaultCallAdapterFactory(@Nullable Executor callbackExecutor) {
if (callbackExecutor == null) throw new AssertionError();
return new ExecutorCallAdapterFactory(callbackExecutor);
}
static class MainThreadExecutor implements Executor {
private final Handler handler = new Handler(Looper.getMainLooper());
@Override public void execute(Runnable r) {
handler.post(r);
}
}
}
MainThreadExecutor是主线程回调线程池，ExecutorCallAdapterFactory是默认的类型转换工厂。
build方法中，初始化了网络请求Call、回调线程池callbackExecutor，以及Call适配器CallAdapter和类型转换器converterFactories：
public Retrofit build() {
if (baseUrl == null) {
throw new IllegalStateException("Base URL required.");
}
okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) {
callFactory = new OkHttpClient();
}
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);
}
}
二、ServiceMethod类：把接口注解数据转出请求数据
当我们要发起一个网络请求时，我们会先定义一个请求接口，通过create方法建构一个Call，那么接口里设置的参数、信息是如果传递到网络请求的呢？或者说，为什么一个接口就可以决定网络请求的方式、网络请求的请求数据呢？看create方法：
public <T> T create(final Class<T> service) {
Utils.validateServiceInterface(service);
if (validateEagerly) {
eagerlyValidateMethods(service);
}
return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
new InvocationHandler() {
private final Platform platform = Platform.get();
@Override public 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);
}
ServiceMethod<Object, Object> serviceMethod =
(ServiceMethod<Object, Object>) loadServiceMethod(method);
OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
});
}
在create方法里面，读取接口，并根据接口的名称、注解、参数，调用Java的动态代理类生成请求。ServiceMethod类根据方法非返回值类型构建请求适配器CallAdapter<T, R>和网络结果转换器Converter<ResponseBody, T>，根据方法注解和方法参数注解生成网络请求。
首先会通过loadServiceMethod方法，调用serviceMethodCache是否已经解析过这个方法，缓存击中则返回缓存中的ServiceMethod。没有击中，则调用ServiceMethod的build方法构建新的ServiceMethod。
ServiceMethod<?, ?> loadServiceMethod(Method method) {
ServiceMethod<?, ?> result = serviceMethodCache.get(method);
if (result != null) return result;
synchronized (serviceMethodCache) {
result = serviceMethodCache.get(method);
if (result == null) {
result = new ServiceMethod.Builder<>(this, method).build();
serviceMethodCache.put(method, result);
}
}
return result;
}
ServiceMethod的构建由build生成，ServiceMethod的build方法拆分下面几段来解析：
1、创建请求适配器CallAdapter<T, R> CallAdapter：
callAdapter = createCallAdapter();
private CallAdapter<T, R> createCallAdapter() {
Type returnType = method.getGenericReturnType();
if (Utils.hasUnresolvableType(returnType)) {
throw methodError(
"Method return type must not include a type variable or wildcard: %s", returnType);
}
if (returnType == void.class) {
throw methodError("Service methods cannot return void.");
}
Annotation[] annotations = method.getAnnotations();
try {
//noinspection unchecked
return (CallAdapter<T, R>) retrofit.callAdapter(returnType, annotations);
} catch (RuntimeException e) { // Wide exception range because factories are user code.
throw methodError(e, "Unable to create call adapter for %s", returnType);
}
}
实际上这个Call适配器就是在Retrofit类中的adapterFactories，默认情况下是在Retrofit的build方法里面初始化的ExecutorCallAdapterFactory，在retrofit.callAdapter中会调用它的get方法：
@Override
public CallAdapter<?, ?> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
if (getRawType(returnType) != Call.class) {
return null;
}
final Type responseType = Utils.getCallResponseType(returnType);
return new CallAdapter<Object, Call<?>>() {
@Override public Type responseType() {
return responseType;
}
@Override public Call<Object> adapt(Call<Object> call) {
return new ExecutorCallbackCall<>(callbackExecutor, call);
}
};
}
所以，生成的CallAdapter中集成了回调线程池ExecutorCallbackCall，负责调用网络请求enqueue以及分发成功与失败的结果。
2、创建网络结果转换器Converter<ResponseBody, T> responseConverter：
responseConverter = createResponseConverter();
private Converter<ResponseBody, T> createResponseConverter() {
Annotation[] annotations = method.getAnnotations();
try {
return retrofit.responseBodyConverter(responseType, annotations);
} catch (RuntimeException e) { // Wide exception range because factories are user code.
throw methodError(e, "Unable to create converter for %s", responseType);
}
}
调用了Retrofit的responseBodyConverter方法，所以实际上是调用了BuiltInConverters的responseBodyConverter方法生成的：
@Override
public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,
Retrofit retrofit) {
if (type == ResponseBody.class) {
return Utils.isAnnotationPresent(annotations, Streaming.class)
? StreamingResponseBodyConverter.INSTANCE
: BufferingResponseBodyConverter.INSTANCE;
}
if (type == Void.class) {
return VoidResponseBodyConverter.INSTANCE;
}
return null;
}
根据不同类型创建不同的转换器Converter。BufferingResponseBodyConverter用于处理普通的网络返回结果，StreamingResponseBodyConverter用于处理大文件下载的结果。
3、解析方法注解
for (Annotation annotation : methodAnnotations) {
parseMethodAnnotation(annotation);
}
parseMethodAnnotation中会根据注解的类型（如GET、POST等），传入参数调用注解解析器parseHttpMethodAndPath。
parseHttpMethodAndPath里面记录了httpMethod（HTTP请求方式）、hasBody（是否有body）、relativeUrl（基地址之后的请求地址）、relativeUrlParamNames（方法注解中字段如@GET("http://www.ddd.com/{user}")））)
4、解析参数注解
int parameterCount = parameterAnnotationsArray.length;
parameterHandlers = new ParameterHandler<?>[parameterCount];
for (int p = 0; p < parameterCount; p++) {
Type parameterType = parameterTypes[p];
if (Utils.hasUnresolvableType(parameterType)) {
throw parameterError(p, "Parameter type must not include a type variable or wildcard: %s",
parameterType);
}
Annotation[] parameterAnnotations = parameterAnnotationsArray[p];
if (parameterAnnotations == null) {
throw parameterError(p, "No Retrofit annotation found.");
}
parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
}
返回生成的ParameterHandler，生成方法在parseParameterAnnotation里面：
private ParameterHandler<?> parseParameterAnnotation(
int p, Type type, Annotation[] annotations, Annotation annotation) {
...
else if (annotation instanceof Body) {
if (isFormEncoded || isMultipart) {
throw parameterError(p,
"@Body parameters cannot be used with form or multi-part encoding.");
}
if (gotBody) {
throw parameterError(p, "Multiple @Body method annotations found.");
}
Converter<?, RequestBody> converter;
try {
converter = retrofit.requestBodyConverter(type, annotations, methodAnnotations);
} catch (RuntimeException e) {
// Wide exception range because factories are user code.
throw parameterError(e, p, "Unable to create @Body converter for %s", type);
}
gotBody = true;
return new ParameterHandler.Body<>(converter);
}
...
}
最终调用了Retrofit里的requestBodyConverter，也就是调用BuiltInConverters的requestBodyConverter（注意与上面的responseBodyConverter是不同的方法）生成(Converter<T, RequestBody>)
@Override
public Converter<?, RequestBody> requestBodyConverter(Type type,
Annotation[] parameterAnnotations, Annotation[] methodAnnotations, Retrofit retrofit) {
if (RequestBody.class.isAssignableFrom(Utils.getRawType(type))) {
return RequestBodyConverter.INSTANCE;
}
return null;
}
三、OkHttpCall：构建网络请求类
在create方法中，当构建完成ServiceMethod之后，就会出一个OkHttpCall，并设置给Retrofit的callAdapter，默认下即ExecutorCallbackCall的delegate：
OkHttpCall<Object> okHttpCall = new OkHttpCall<>(serviceMethod, args);
那么，我们通过create出来的call的enqueue方法发起网络请求的时候，就是调用了OkHttpCall的enqueue方法，程序中，构建了okhttp3所需的Call，并调用enqueue发起网络请求：
@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();
} 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) {
callFailure(e);
}
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();
}
}
});
}
参考：
http://www.jianshu.com/p/c1a3a881a144
http://blog.csdn.net/guiman/article/details/51480497
