一：使用注解

1.1 添加一个Header参数

public interface UserService {  
    @Headers("Cache-Control: max-age=640000")
    @GET("/tasks")
    Call<List<Task>> getTasks();
}

1.2 添加多个Header参数

public interface UserService {  
    @Headers({
        "Accept: application/vnd.yourapi.v1.full+json",
        "User-Agent: Your-App-Name"
    })
    @GET("/tasks/{task_id}")
    Call<Task> getTask(@Path("task_id") long taskId);
}

二：使用注解动态Header

使用注解的方式，但是Header参数每次提交的都不同，也就是动态的Header

public interface UserService {  
    @GET("/tasks")
    Call<List<Task>> getTasks(@Header("Content-Range") String contentRange);
}

三：使用代码的方式需要使用拦截器

OkHttpClient.Builder httpClient = new OkHttpClient.Builder();  
httpClient.addInterceptor(new Interceptor() {  
    @Override
    public Response intercept(Interceptor.Chain chain) throws IOException {
        Request original = chain.request();
 
        Request request = original.newBuilder()
            .header("User-Agent", "Your-App-Name")
            .header("Accept", "application/vnd.yourapi.v1.full+json")
            .method(original.method(), original.body())
            .build();
 
        return chain.proceed(request);
    }
}
 
OkHttpClient client = httpClient.build();  
Retrofit retrofit = new Retrofit.Builder()  
    .baseUrl(API_BASE_URL)
    .addConverterFactory(GsonConverterFactory.create())
    .client(client)
    .build();