Retrofit使用与解析
Retrofit是一个基于OkHttp的网络请求工具，其功能与Volley很相似，但是使用起来就很不一样。
Retrofit不用你去创建Request对象，每次指定Get还是Post等，它请求一个api只需你去调用一个Java方法。
使用
1.首先需要创建一个Retrofit对象,并指定域名：
//使用Builder模式构建对象
private static final String BASE_URL = "http://xxx";//域名
Retrofit retrofit = new Retrofit.Builder()
.baseUrl(BASE_URL)
.addConverterFactory(GsonConverterFactory.create())
.build();Retrofit中Bulider类的build方法
public Retrofit build() {
if (baseUrl == null) {
throw new IllegalStateException("Base URL required.");
}
okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) {
callFactory = new OkHttpClient();//默认使用OkHttp
}
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);
}
}2.之后，你只要根据api新建一个Java接口，并使用注解进行相关描述：
//Get请求
public interface ElectricAPI {
@GET("/api/v1/get/power/{lou}/{hao}")
Call<Electric> electricData(@Path("lou") String lou,@Path("hao") String hao);
}
//Post请求
public interface FeedBackAPI {
@FormUrlEncoded
@POST("/home/msg/0")
Call<ResponseBody> feed(@Field("email") String email,
@Field("content") String content);
}
//文件上传
public interface FileUploadAPI {
@Multipart
@POST("api/v1/stuff/upload")
Call<HttpResult<String>> uploadImage(@Part MultipartBody.Part file);
}3.最后只要通过retrofit对象创建Api对象了,并使用call对象进行请求和处理：
ElectricAPI api = retrofit.create(ElectricAPI.class);
Call<Electric> call = api.electricData("12","122");
// 请求数据，并且处理response
call.enqueue(new Callback<Electric>() {
@Override
public void onResponse(Response<Electric> r) {
System.out.println("请求成功");
}
@Override
public void onFailure(Throwable t) {
}
});原理
从使用上可以看出，Retrofit其实是把一个Java接口对象翻译成一个Http请求，然后使用OkHttp请求。Retrofit使用的技术就是动态代理。
先看create方法：
public <T> T create(final Class<T> service) {
Utils.validateServiceInterface(service);//检查是否是接口
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, 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)) {//默认返回false
return platform.invokeDefaultMethod(method, service, proxy, args);
}
ServiceMethod serviceMethod = loadServiceMethod(method);//处理注解
OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
});
}接着是create方法的使用：
ElectricAPI api = retrofit.create(ElectricAPI.class);
Call<Electric> call = api.electricData("12","122");这里的api对象其实是一个动态代理，并不是实现了相关接口的对象。当api对象调用eletricData方法时，会被动态代理拦截，然后调用Proxy.newProxyInstance方法中的InvocationHandler对象的invoke方法。这里invoke方法传入三个对象，依次为代理对象（不用管）、method（这里是electricData方法），方法的参数（这里是”12”，”122”）。
这里Retrofit关心的就是method和它的参数args，接下去Retrofit就会用Java反射获取到electricData方法的注解信息，配合args参数，创建一个ServiceMethod对象。
ServiceMethod就像是一个处理器，传入Retrofit对象和Method对象，调用各个接口和解析器，最终生成一个Request，包含api 的域名、path、http请求方法、请求头、是否有body、是否是multipart等等。最后返回一个Call对象，Retrofit2中Call接口的默认实现是OkHttpCall，它默认使用OkHttp3作为底层http请求client。
接着看下上面的loadServiceMethod(Method method)方法
ServiceMethod loadServiceMethod(Method method) {
ServiceMethod result;
synchronized (serviceMethodCache) {
result = serviceMethodCache.get(method);
if (result == null) {
result = new ServiceMethod.Builder(this, method).build();
serviceMethodCache.put(method, result);
}
}
return result;
}
private final Map<Method, ServiceMethod> serviceMethodCache = new LinkedHashMap<>();这里对解析请求进行了缓存，用于避免解析过慢。这里同样使用Builder模式构建ServiceMethod方法：
public ServiceMethod build() {
callAdapter = createCallAdapter();
responseType = callAdapter.responseType();
if (responseType == Response.class || responseType == okhttp3.Response.class) {
throw methodError("'"
+ Utils.getRawType(responseType).getName()
+ "' is not a valid response body type. Did you mean ResponseBody?");
}
responseConverter = createResponseConverter();
//处理注解
for (Annotation annotation : methodAnnotations) {
parseMethodAnnotation(annotation);
}
if (httpMethod == null) {
throw methodError("HTTP method annotation is required (e.g., @GET, @POST, etc.).");
}
//检查错误
if (!hasBody) {
if (isMultipart) {
throw methodError(
"Multipart can only be specified on HTTP methods with request body (e.g., @POST).");
}
if (isFormEncoded) {
throw methodError("FormUrlEncoded can only be specified on HTTP methods with "
+ "request body (e.g., @POST).");
}
}
//处理api中的参数{ }占位符，真实参数在Java方法中传入，这里Retrofit会使用一个ParameterHandler来进行替换
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);
}
if (relativeUrl == null && !gotUrl) {
throw methodError("Missing either @%s URL or @Url parameter.", httpMethod);
}
if (!isFormEncoded && !isMultipart && !hasBody && gotBody) {
throw methodError("Non-body HTTP method cannot contain @Body.");
}
if (isFormEncoded && !gotField) {
throw methodError("Form-encoded method must contain at least one @Field.");
}
if (isMultipart && !gotPart) {
throw methodError("Multipart method must contain at least one @Part.");
}
return new ServiceMethod<>(this);
}执行Http请求
之前讲到，OkHttpCall是实现了Call接口的，并且是真正调用OkHttp3发送Http请求的类。OkHttp3发送一个Http请求需要一个Request对象，而这个Request对象就是从ServiceMethod的toRequest返回的。
Request toRequest(Object... args) throws IOException {
RequestBuilder requestBuilder = new RequestBuilder(httpMethod, baseUrl, relativeUrl, headers,
contentType, hasBody, isFormEncoded, isMultipart);
@SuppressWarnings("unchecked") // It is an error to invoke a method with the wrong arg types.
ParameterHandler<Object>[] handlers = (ParameterHandler<Object>[]) parameterHandlers;
//参数替换
int argumentCount = args != null ? args.length : 0;
if (argumentCount != handlers.length) {
throw new IllegalArgumentException("Argument count (" + argumentCount
+ ") doesn't match expected count (" + handlers.length + ")");
}
for (int p = 0; p < argumentCount; p++) {
handlers[p].apply(requestBuilder, args[p]);
}
return requestBuilder.build();
}总的来说，OkHttpCall就是调用ServiceMethod获得一个可以执行的Request对象，然后等到Http请求返回后，再将response body传入ServiceMethod中，ServiceMethod就可以调用Converter接口将response body转成一个Java对象。
结合上面说的就可以看出，ServiceMethod中几乎保存了一个api请求所有需要的数据，OkHttpCall需要从ServiceMethod中获得一个Request对象，然后得到response后，还需要传入ServiceMethod用Converter转换成Java对象。