public interface GitHubService {
 @GET("/users/{user}/repos")
 List<Repo> listRepos(@Path("user") String user);
}

RestAdapter restAdapter = new RestAdapter.Builder()
  .setEndpoint("https://api.github.com")
  .build();

 
GitHubService service = restAdapter.create(GitHubService.class);

List<Repo> repos = service.listRepos("octocat");

@GET("/group/{id}/users")
List<User> groupList(@Path("id") int groupId);

/** Make a GET request to a REST path relative to base URL. */
@Documented
@Target(METHOD)
@Retention(RUNTIME)
@RestMethod("GET")
public @interface GET {
 String value();
}

@Documented
@Target(ANNOTATION_TYPE)
@Retention(RUNTIME)
public @interface RestMethod {
 String value();
 boolean hasBody() default false;
}

@Documented
@Target(METHOD)
@Retention(RUNTIME)
@RestMethod(value = "POST", hasBody = true)
public @interface POST {
 String value();
}

RestAdapter restAdapter = new RestAdapter.Builder()
  .setEndpoint("https://api.github.com")
  .build();

GitHubService service = restAdapter.create(GitHubService.class);

public RestAdapter build() {
 if (endpoint == null) {
  throw new IllegalArgumentException("Endpoint may not be null.");
 }
 
 ensureSaneDefaults();
 
 return new RestAdapter(endpoint, clientProvider, httpExecutor, callbackExecutor,
   requestInterceptor, converter, profiler, errorHandler, log, logLevel);
}

private void ensureSaneDefaults() {
 if (converter == null) { converter = Platform.get().defaultConverter(); }
 if (clientProvider == null) { clientProvider = Platform.get().defaultClient(); }
 if (httpExecutor == null) { httpExecutor = Platform.get().defaultHttpExecutor(); }
 if (callbackExecutor == null) { callbackExecutor = Platform.get().defaultCallbackExecutor(); }
 if (errorHandler == null) { errorHandler = ErrorHandler.DEFAULT; }
 if (log == null) { log = Platform.get().defaultLog(); }
 if (requestInterceptor == null) { requestInterceptor = RequestInterceptor.NONE; }
}

private static final Platform PLATFORM = findPlatform();
 static final boolean HAS_RX_JAVA = hasRxJavaOnClasspath();

 static Platform get() {
  return PLATFORM;
 }

 private static Platform findPlatform() {
  try {
   Class.forName("android.os.Build");
   if (Build.VERSION.SDK_INT != 0) {
    return new Android();
   }
  } catch (ClassNotFoundException ignored) {
  }

  if (System.getProperty("com.google.appengine.runtime.version") != null) {
   return new AppEngine();
  }

  return new Base();
 }

abstract Converter defaultConverter(); // 默认的Converter，用于将请求结果转化成需要的数据，如GsonConverter将JSON请求结果用Gson解析成Java对象
 abstract Client.Provider defaultClient(); // Http请求类，如果是AppEngine就使用`UrlFetchClient`，否则如果有OKHttp就使用OKHttp，如果是Android，2.3以后使用HttpURLConnection，2.3以前使用HttpClient
 abstract Executor defaultHttpExecutor(); // 用于执行Http请求的Executor
 abstract Executor defaultCallbackExecutor(); // Callback调用中用于执行Callback的Executor（可能是同步的）
 abstract RestAdapter.Log defaultLog(); // Log接口，用于输出Log

@Override Client.Provider defaultClient() {
 final Client client;
 if (hasOkHttpOnClasspath()) {
  client = OkClientInstantiator.instantiate();
 } else if (Build.VERSION.SDK_INT < Build.VERSION_CODES.GINGERBREAD) {
  client = new AndroidApacheClient();
 } else {
  client = new UrlConnectionClient();
 }
 return new Client.Provider() {
  @Override public Client get() {
   return client;
  }
 };
}

@Override Client.Provider defaultClient() {
 final Client client;
 if (hasOkHttpOnClasspath()) {
  client = OkClientInstantiator.instantiate();
 } else {
  client = new UrlConnectionClient();
 }
 return new Client.Provider() {
  @Override public Client get() {
   return client;
  }
 };
}

@Override Client.Provider defaultClient() {
 final UrlFetchClient client = new UrlFetchClient();
 return new Client.Provider() {
  @Override public Client get() {
   return client;
  }
 };
}

@Override Executor defaultCallbackExecutor() {
  return new Utils.SynchronousExecutor();
}
Platform.Android

@Override Executor defaultCallbackExecutor() {
  return new MainThreadExecutor();
}

static class SynchronousExecutor implements Executor {
  @Override public void execute(Runnable runnable) {
   runnable.run();
  }
}

public final class MainThreadExecutor implements Executor {
 private final Handler handler = new Handler(Looper.getMainLooper());

 @Override public void execute(Runnable r) {
  handler.post(r);
 }
}

 public <T> T create(Class<T> service) {
  Utils.validateServiceClass(service);
  return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
    new RestHandler(getMethodInfoCache(service)));
 }
 
 Map<Method, RestMethodInfo> getMethodInfoCache(Class<?> service) {
  synchronized (serviceMethodInfoCache) {
   Map<Method, RestMethodInfo> methodInfoCache = serviceMethodInfoCache.get(service);
   if (methodInfoCache == null) {
    methodInfoCache = new LinkedHashMap<Method, RestMethodInfo>();
    serviceMethodInfoCache.put(service, methodInfoCache);
   }
   return methodInfoCache;
  }
 }

@Override public Object invoke(Object proxy, Method method, final Object[] args)
  throws Throwable {
 // If the method is a method from Object then defer to normal invocation.
 if (method.getDeclaringClass() == Object.class) { // 1
  return method.invoke(this, args);
 }
 
 // Load or create the details cache for the current method.
 final RestMethodInfo methodInfo = getMethodInfo(methodDetailsCache, method); // 2
 
 if (methodInfo.isSynchronous) { // 3
  try {
   return invokeRequest(requestInterceptor, methodInfo, args);
  } catch (RetrofitError error) {
   Throwable newError = errorHandler.handleError(error);
   if (newError == null) {
    throw new IllegalStateException("Error handler returned null for wrapped exception.",
      error);
   }
   throw newError;
  }
 }
 
 if (httpExecutor == null || callbackExecutor == null) {
  throw new IllegalStateException("Asynchronous invocation requires calling setExecutors.");
 }
 
 // Apply the interceptor synchronously, recording the interception so we can replay it later.
 // This way we still defer argument serialization to the background thread.
 final RequestInterceptorTape interceptorTape = new RequestInterceptorTape();
 requestInterceptor.intercept(interceptorTape); // 4
 
 if (methodInfo.isObservable) { // 5
  if (rxSupport == null) {
   if (Platform.HAS_RX_JAVA) {
    rxSupport = new RxSupport(httpExecutor, errorHandler);
   } else {
    throw new IllegalStateException("Observable method found but no RxJava on classpath");
   }
  }
  
  return rxSupport.createRequestObservable(new Callable<ResponseWrapper>() {
   @Override public ResponseWrapper call() throws Exception {
    return (ResponseWrapper) invokeRequest(interceptorTape, methodInfo, args);
   }
  });
 }
 
 Callback<?> callback = (Callback<?>) args[args.length - 1]; // 6
 httpExecutor.execute(new CallbackRunnable(callback, callbackExecutor, errorHandler) {
  @Override public ResponseWrapper obtainResponse() {
   return (ResponseWrapper) invokeRequest(interceptorTape, methodInfo, args);
  }
 });
 
 return null; // Asynchronous methods should have return type of void.
}

 static RestMethodInfo getMethodInfo(Map<Method, RestMethodInfo> cache, Method method) {
  synchronized (cache) {
   RestMethodInfo methodInfo = cache.get(method);
   if (methodInfo == null) {
    methodInfo = new RestMethodInfo(method);
    cache.put(method, methodInfo);
   }
   return methodInfo;
  }

private enum ResponseType {
  VOID,
  OBSERVABLE,
  OBJECT
}
RestMethodInfo(Method method) {
  this.method = method;
  responseType = parseResponseType();
  isSynchronous = (responseType == ResponseType.OBJECT);
  isObservable = (responseType == ResponseType.OBSERVABLE);
}

 synchronized void init() {
  if (loaded) return;

  parseMethodAnnotations();
  parseParameters();

  loaded = true;
 }

for (Annotation methodAnnotation : method.getAnnotations()) {
 Class<? extends Annotation> annotationType = methodAnnotation.annotationType();
 RestMethod methodInfo = null;
 // Look for a @RestMethod annotation on the parameter annotation indicating request method.
 for (Annotation innerAnnotation : annotationType.getAnnotations()) {
  if (RestMethod.class == innerAnnotation.annotationType()) {
   methodInfo = (RestMethod) innerAnnotation;
   break;
  }
 }
 ...
}

RequestBuilder requestBuilder = new RequestBuilder(serverUrl, methodInfo, converter);
requestBuilder.setArguments(args);
requestInterceptor.intercept(requestBuilder);
Request request = requestBuilder.build();

/** Intercept every request before it is executed in order to add additional data. */
public interface RequestInterceptor {
 /** Called for every request. Add data using methods on the supplied {@link RequestFacade}. */
 void intercept(RequestFacade request);

 interface RequestFacade {
  void addHeader(String name, String value);
  void addPathParam(String name, String value);
  void addEncodedPathParam(String name, String value);
  void addQueryParam(String name, String value);
  void addEncodedQueryParam(String name, String value);
 }

 /** A {@link RequestInterceptor} which does no modification of requests. */
 RequestInterceptor NONE = new RequestInterceptor() {
  @Override public void intercept(RequestFacade request) {
   // Do nothing.
  }
 };
}

// Apply the interceptor synchronously, recording the interception so we can replay it later.
// This way we still defer argument serialization to the background thread.
final RequestInterceptorTape interceptorTape = new RequestInterceptorTape();
requestInterceptor.intercept(interceptorTape);

Callback<?> callback = (Callback<?>) args[args.length - 1];
httpExecutor.execute(new CallbackRunnable(callback, callbackExecutor, errorHandler) {
  @Override public ResponseWrapper obtainResponse() {
   return (ResponseWrapper) invokeRequest(interceptorTape, methodInfo, args);
  }
});

abstract class CallbackRunnable<T> implements Runnable {
 private final Callback<T> callback;
 private final Executor callbackExecutor;
 private final ErrorHandler errorHandler;

 CallbackRunnable(Callback<T> callback, Executor callbackExecutor, ErrorHandler errorHandler) {
  this.callback = callback;
  this.callbackExecutor = callbackExecutor;
  this.errorHandler = errorHandler;
 }

 @SuppressWarnings("unchecked")
 @Override public final void run() {
  try {
   final ResponseWrapper wrapper = obtainResponse();
   callbackExecutor.execute(new Runnable() {
    @Override public void run() {
     callback.success((T) wrapper.responseBody, wrapper.response);
    }
   });
  } catch (RetrofitError e) {
   Throwable cause = errorHandler.handleError(e);
   final RetrofitError handled = cause == e ? e : unexpectedError(e.getUrl(), cause);
   callbackExecutor.execute(new Runnable() {
    @Override public void run() {
     callback.failure(handled);
    }
   });
  }
 }

 public abstract ResponseWrapper obtainResponse();
} 

/**
 * Binary data with an associated mime type.
 *
 * @author Jake Wharton (jw@squareup.com)
 */
public interface TypedInput {

 /** Returns the mime type. */
 String mimeType();

 /** Length in bytes. Returns {@code -1} if length is unknown. */
 long length();

 /**
  * Read bytes as stream. Unless otherwise specified, this method may only be called once. It is
  * the responsibility of the caller to close the stream.
  */
 InputStream in() throws IOException;
}

/**
 * Binary data with an associated mime type.
 *
 * @author Bob Lee (bob@squareup.com)
 */
public interface TypedOutput {
 /** Original filename.
  *
  * Used only for multipart requests, may be null. */
 String fileName();

 /** Returns the mime type. */
 String mimeType();

 /** Length in bytes or -1 if unknown. */
 long length();

 /** Writes these bytes to the given output stream. */
 void writeTo(OutputStream out) throws IOException;
}

TypedByteArray，内部数据是一个Byte数组

 private final byte[] bytes;

 @Override public long length() {
  return bytes.length;
 }

 @Override public void writeTo(OutputStream out) throws IOException {
  out.write(bytes);
 }

 @Override public InputStream in() throws IOException {
  return new ByteArrayInputStream(bytes);
 }

TypedString，继承自TypedByteArray，内部表示是一样的

public TypedString(String string) {
  super("text/plain; charset=UTF-8", convertToBytes(string));
 }

 private static byte[] convertToBytes(String string) {
  try {
   return string.getBytes("UTF-8");
  } catch (UnsupportedEncodingException e) {
   throw new RuntimeException(e);
  }
 }