Retrofit2 源码解析
1. Retrofit是什么
来自Retrofit官网的介绍：
A type-safe HTTP client for Android and Java
一个基于OkHttp的RESTFUL Api请求工具
2. Retrofit的用法
- 创建Retrofit对象
public static final String API_URL = "https://api.github.com";
Retrofit retrofit = new Retrofit.Builder()
.baseUrl(API_URL)
.addConverterFactory(GsonConverterFactory.create())
.build();
- api新建一个Java接口，用Java注解来描述这个api
public interface GitHubApiService {
@GET("/users/{user}/repos")
Call<List<Repo>> listRepos(@Path("user") String user);
// you can add some other meathod
}
- 这个retrofit对象创建一个GitHubService对象：
GitHubApiService gitHubApiService=retrofit.create(GitHubApiService.class);//获取API接口的实现类的实例对象
Call<List<Repo>> call = gitHubApiService.listRepos("octocat");
- 异步回调结果
// 请求数据，并且处理response
call.enqueue(new Callback<List<Repo>>() {
@Override
public void onResponse(Response<List<Repo>> list) {
Log.d("list： " + list);
}
@Override
public void onFailure(Throwable t) {
}
});
- 同步回调结果
List<Repo> list = call.execute();
3. Retrofit的原理
Retrofit整个工作流程：将java接口，转换成一个个Http请求，交由Okhttp进行发送。
Retrofit库依赖了Okhttp库和okio库,核心库分为Http包和其他核心类。
Http包里面包含各种定义的注解，帮忙开发者定义请求方式和请求参数。
如何将一个请求放到Okhttp里面去?
答案是： Retrofit利用了java的动态代理技术。
动态代理代码如下：
/** Create an implementation of the API defined by the {@code service} interface. */
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, 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 serviceMethod = loadServiceMethod(method);
OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
});
首先看动态代理方法，这里是j接口动态代理的标准写法
Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
new InvocationHandler(){});
InvocationHandler类中的插入方法
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)) {
return platform.invokeDefaultMethod(method, service, proxy, args);
}
ServiceMethod serviceMethod = loadServiceMethod(method);
OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
api对象其实是一个动态代理对象,类型是接口类型，真正的对象是被动态代理拦截，然后调用Proxy.newProxyInstance方法中的InvocationHandler对象。
其中invoke方法：
Object proxy: 代理对象
Method method：调用的方法
Object... args：方法的参数
然后Retrofit就会用Java反射获取到方法的注解信息，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
翻译请求方式和参数，同时构建出ServiceMethod对象
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();
for (Annotation annotation : methodAnnotations) {
parseMethodAnnotation(annotation);
}
if (httpMethod == null) {
throw methodError("HTTP method annotation is required (e.g., @GET, @POST, etc.).");
}
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).");
}
}
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);
}
总结来说：使用Java动态代理的目的就要拦截被调用的Java方法，然后解析这个Java方法的注解，最后生成Request由OkHttp发送。
