今天我们主要聊一聊 Retrofit 的源码，他是怎样对 OkHttp 包装的。
此次分析源码的版本 Retrofit 2.5.0
首先我们先了解一下 Retrofit 用法。
public interface GitHubService {
@GET("users/{user}/repos")
Call<List<Repo>> listRepos(@Path("user") String user);
}
Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.github.com/")
.build();
GitHubService service = retrofit.create(GitHubService.class);
Call<List<Repo>> call = service.listRepos("octocat");
//发送网络请求(异步)
call.enqueue(new Callback<List<Repo>>() {
@Override
public void onResponse(Call<List<Repo>> call, Response<List<Repo>> response) {
}
@Override
public void onFailure(Call<List<Repo>> call, Throwable t) {
}
});
// 发送网络请求（同步）
Response<List<Repo>> response = call.execute();
Retrofit 的 Builder 建造者模式
首先我们看到 Retrofit 的 Builder 建造者模式，
public Retrofit build() {
// 配置 baseUrl
if (baseUrl == null) {
throw new IllegalStateException("Base URL required.");
}
// 配置 OkHttpClient
okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) {
callFactory = new OkHttpClient();
}
// 执行 callback 的线程
Executor callbackExecutor = this.callbackExecutor;
if (callbackExecutor == null) {
callbackExecutor = platform.defaultCallbackExecutor();
}
// Call 适配器，比如 RxJava、Java8 适配器
List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));
// 数据转换器，比如 Gson、Jackson 等转换器
List<Converter.Factory> converterFactories = new ArrayList<>(
1 + this.converterFactories.size() + platform.defaultConverterFactoriesSize());
converterFactories.add(new BuiltInConverters());
converterFactories.addAll(this.converterFactories);
converterFactories.addAll(platform.defaultConverterFactories());
return new Retrofit(callFactory, baseUrl, unmodifiableList(converterFactories),
unmodifiableList(callAdapterFactories), callbackExecutor, validateEagerly);
}
}
根据接口方法获取 Service
接下来我们再往下看 retrofit.create() 方法
public <T> T create(final Class<T> service) {
...
return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
new InvocationHandler() {
private final Platform platform = Platform.get();
private final Object[] emptyArgs = new Object[0];
@Override public Object invoke(Object proxy, Method method, @Nullable Object[] args)
throws Throwable {
// 处理默认方法
if (method.getDeclaringClass() == Object.class) {
return method.invoke(this, args);
}
if (platform.isDefaultMethod(method)) {
return platform.invokeDefaultMethod(method, service, proxy, args);
}
// 处理接口方法
return loadServiceMethod(method).invoke(args != null ? args : emptyArgs);
}
});
}
create() 本身就是一个动态代理。
调用 Service 的方法就会走动态代理
先看 loadServiceMethod() 方法，等会再看 invoke();
ServiceMethod<?> loadServiceMethod(Method method) {
// 查询方法缓存
ServiceMethod<?> result = serviceMethodCache.get(method);
if (result != null) return result;
synchronized (serviceMethodCache) {
result = serviceMethodCache.get(method);
if (result == null) {
// 解析方法的注解以及返回类型确定各项参数以及 Call 适配器、数据转换器。
result = ServiceMethod.parseAnnotations(this, method);
serviceMethodCache.put(method, result);
}
}
return result;
}
继续往下看 ServiceMethod.parseAnnotations() 方法。
static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
// 解析方法的注解、参数注解、请求头等等，具体解析方式自己看 parseAnnotations() 方法的代码
RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);
Type returnType = method.getGenericReturnType();
if (Utils.hasUnresolvableType(returnType)) {
throw methodError(method,
"Method return type must not include a type variable or wildcard: %s", returnType);
}
if (returnType == void.class) {
throw methodError(method, "Service methods cannot return void.");
}
// 解析并创建 Call 适配器、数据转换器
return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
}
继续查看 HttpServiceMethod.parseAnnotations() 方法。
static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
Retrofit retrofit, Method method, RequestFactory requestFactory) {
// 根据返回类型创建并获取 CallAdapter
CallAdapter<ResponseT, ReturnT> callAdapter = createCallAdapter(retrofit, method);
Type responseType = callAdapter.responseType();
if (responseType == Response.class || responseType == okhttp3.Response.class) {
throw methodError(method, "'"
+ Utils.getRawType(responseType).getName()
+ "' is not a valid response body type. Did you mean ResponseBody?");
}
if (requestFactory.httpMethod.equals("HEAD") && !Void.class.equals(responseType)) {
throw methodError(method, "HEAD method must use Void as response type.");
}
// 根据 Response 类型创建并获取 Converter
Converter<ResponseBody, ResponseT> responseConverter =
createResponseConverter(retrofit, method, responseType);
okhttp3.Call.Factory callFactory = retrofit.callFactory;
return new HttpServiceMethod<>(requestFactory, callFactory, callAdapter, responseConverter);
}
继续回去看 invoke() 方法
@Override ReturnT invoke(Object[] args) {
// 转换成相应的类型
return callAdapter.adapt(
new OkHttpCall<>(requestFactory, args, callFactory, responseConverter));
}
// 以 Retrofit 默认适配器为例，假设 SDK version 小于 24
@Override public Call<Object> adapt(Call<Object> call) {
return new ExecutorCallbackCall<>(callbackExecutor, call);
}
发送异步网络请求
@Override public void enqueue(final Callback<T> callback) {
checkNotNull(callback, "callback == null");
// 这里 delegate 是 OkHttpCall 对象
// 大家可以自己看一看 OkHttpCall 的 enqueue() 方法，它本身就是对 okhttp3.Call 的包装。
delegate.enqueue(new Callback<T>() {
@Override public void onResponse(Call<T> call, final Response<T> response) {
// callbackExecutor 最初配置的线程运行环境
callbackExecutor.execute(new Runnable() {
@Override public void run() {
if (delegate.isCanceled()) {
callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
} else {
callback.onResponse(ExecutorCallbackCall.this, response);
}
}
});
}
@Override public void onFailure(Call<T> call, final Throwable t) {
callbackExecutor.execute(new Runnable() {
@Override public void run() {
callback.onFailure(ExecutorCallbackCall.this, t);
}
});
}
});
}
发送同步网络请求
@Override public Response<T> execute() throws IOException {
// 这里 delegate 是 OkHttpCall 对象
return delegate.execute();
}
