[置顶] 【Android实战】----从Retrofit源码分析到Java网络编程以及HTTP权威指南想到的

一、简介

接上一篇【Android实战】----基于Retrofit实现多图片/文件、图文上传中曾说非常想搞明白为什么Retrofit那么屌。最近也看了一些其源码分析的文章以及亲自查看了源码，发现其对Java网络编程及HTTP权威指南有了一个很好的诠释。一直以来，都信奉一个原则，在这个新技术日新月异的时代，如何在Java界立足，凭借的就两点：

1、基本功，包括：Java基本知识，（Java编程思想、Effective Java），Java进阶（Java虚拟机、Java设计模式）、网络相关（这个时代没有网络就没有一切，Java网络编程、HTTP权威指南、TCP/IP协议），计算机系统相关（编译原理、深入理解计算机系统等）这些都是根本，所谓万变不离其宗，在掌握这些基本功的基础上，再学习新技术，面对日新月异的新技术时就会游刃有余。

2、将琐碎知识串联起来的能力，也就是归纳总结能力。无论是在工作中还是学习中，起初用到的知识是东一块、西一块，其实许多东西都是关联的，通过系统的梳理形成自己的知识体系，打造属于自己的专属领域，这也是在Java界立足的根本。

以上这些纯属自己这段时间的瞎想胡扯，大家共勉。

二、Retrofit简介

可参见Jake Wharton的演讲https://realm.io/cn/news/droidcon-jake-wharton-simple-http-retrofit-2/

retrofit是由square公司开发的。square在github上发布了很多优秀的Android开源项目。例如:otto(事件总线),leakcanary(排查内存泄露),android-times-square(日历控件),dagger(依赖注入),picasso(异步加载图片),okhttp(网络请求),retrofit(网络请求)等等。更多square上的开源项目我们可以去square的GitHub进行查看。

根据Retrofit的官方文档（https://github.com/square/retrofit及http://square.github.io/retrofit/）看Retrofit是

A type-safe HTTP client for Android and Java

可见Retrofit可以应用到Android平台和Java平台中，其源码中也可看出retrofit2\Platform.java

private static Platform findPlatform() {
    try {
      Class.forName("android.os.Build");
      if (Build.VERSION.SDK_INT != 0) {
        return new Android();
      }
    } catch (ClassNotFoundException ignored) {
    }
    try {
      Class.forName("java.util.Optional");
      return new Java8();
    } catch (ClassNotFoundException ignored) {
    }
    try {
      Class.forName("org.robovm.apple.foundation.NSObject");
      return new IOS();
    } catch (ClassNotFoundException ignored) {
    }
    return new Platform();
  }

可见可以应用在Android、Java8及IOS平台中，当然在IOS中要基于RoboVM。RoboVM它是一种可以在iOS设备上运行Java应用程序的技术，这种技术主要还是用于在游戏开发中。

Retrofit简化了从Web API下载数据，解析成普通的Java对象（POJO）。

Retrofit重要的一点是应用了Java中的动态代理机制http://blog.csdn.net/honghailiang888/article/details/50619545，有必要好好研究下：

/**
   * Create an implementation of the API endpoints defined by the {@code service} interface.
   * <p>
   * The relative path for a given method is obtained from an annotation on the method describing
   * the request type. The built-in methods are {@link retrofit2.http.GET GET},
   * {@link retrofit2.http.PUT PUT}, {@link retrofit2.http.POST POST}, {@link retrofit2.http.PATCH
   * PATCH}, {@link retrofit2.http.HEAD HEAD}, {@link retrofit2.http.DELETE DELETE} and
   * {@link retrofit2.http.OPTIONS OPTIONS}. You can use a custom HTTP method with
   * {@link HTTP @HTTP}. For a dynamic URL, omit the path on the annotation and annotate the first
   * parameter with {@link Url @Url}.
   * <p>
   * Method parameters can be used to replace parts of the URL by annotating them with
   * {@link retrofit2.http.Path @Path}. Replacement sections are denoted by an identifier
   * surrounded by curly braces (e.g., "{foo}"). To add items to the query string of a URL use
   * {@link retrofit2.http.Query @Query}.
   * <p>
   * The body of a request is denoted by the {@link retrofit2.http.Body @Body} annotation. The
   * object will be converted to request representation by one of the {@link Converter.Factory}
   * instances. A {@link RequestBody} can also be used for a raw representation.
   * <p>
   * Alternative request body formats are supported by method annotations and corresponding
   * parameter annotations:
   * <ul>
   * <li>{@link retrofit2.http.FormUrlEncoded @FormUrlEncoded} - Form-encoded data with key-value
   * pairs specified by the {@link retrofit2.http.Field @Field} parameter annotation.
   * <li>{@link retrofit2.http.Multipart @Multipart} - RFC 2388-compliant multipart data with
   * parts specified by the {@link retrofit2.http.Part @Part} parameter annotation.
   * </ul>
   * <p>
   * Additional static headers can be added for an endpoint using the
   * {@link retrofit2.http.Headers @Headers} method annotation. For per-request control over a
   * header annotate a parameter with {@link Header @Header}.
   * <p>
   * By default, methods return a {@link Call} which represents the HTTP request. The generic
   * parameter of the call is the response body type and will be converted by one of the
   * {@link Converter.Factory} instances. {@link ResponseBody} can also be used for a raw
   * representation. {@link Void} can be used if you do not care about the body contents.
   * <p>
   * For example:
   * <pre>
   * public interface CategoryService {
   *   @POST("category/{cat}/")
   *   Call<List<Item>> categoryList(@Path("cat") String a, @Query("page") int b);
   * }
   * </pre>
   */
  @SuppressWarnings("unchecked") // Single-interface proxy creation guarded by parameter safety.
  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);
          }
        });
  }

Retrofit依赖于OkHttp，最终的请求响应是在OkHttp中做的，性能之所以高，也是因为okHttp的性能高。

三、OkHttp简介

android网络框架之OKhttp是一个处理网络请求的开源项目，是安卓端最火热的轻量级框架,由移动支付Square公司贡献，用于替代HttpUrlConnection和Apache HttpClient(android API23 6.0里已移除HttpClient,现在已经打不出来)。

官网简介：https://github.com/square/okhttp及http://square.github.io/okhttp/

HTTP is the way modern applications network. It’s how we exchange data & media. Doing HTTP efficiently makes your stuff load faster and saves bandwidth.

OkHttp is an HTTP client that’s efficient by default:
•HTTP/2 support allows all requests to the same host to share a socket.
•Connection pooling reduces request latency (if HTTP/2 isn’t available).
•Transparent GZIP shrinks download sizes.
•Response caching avoids the network completely for repeat requests.

OkHttp perseveres when the network is troublesome: it will silently recover from common connection problems. If your service has multiple IP addresses OkHttp will attempt alternate addresses if the first connect fails. This is necessary for IPv4+IPv6 and for services hosted in redundant data centers. OkHttp initiates new connections with modern TLS features (SNI, ALPN), and falls back to TLS 1.0 if the handshake fails.

Using OkHttp is easy. Its request/response API is designed with fluent builders and immutability. It supports both synchronous blocking calls and async calls with callbacks.

OkHttp supports Android 2.3 and above. For Java, the minimum requirement is 1.7.

优势：

允许连接到同一个主机地址的所有请求,提高请求效率

共享Socket,减少对服务器的请求次数

通过连接池,减少了请求延迟

缓存响应数据来减少重复的网络请求

减少了对数据流量的消耗

自动处理GZip压缩

功能：

get,post等请求

文件的上传下载

加载图片(内部会图片大小自动压缩)

支持请求回调，直接返回对象、对象集合

支持session的保持

okHttp是直接封装的Socket,io流是基于Okio的。

四、Okio简介

官方文档：https://github.com/square/okio

Okio is a new library that complements  java.io  and  java.nio  to make it much easier to access, store, and process your data.

Okio库是一个由square公司开发的，它补充了Java.io和java.nio的不足，以便能够更加方便，快速的访问、存储和处理你的数据。OkHttp的底层也使用该库作为支持，而在开发中，使用该库可以大大给你带来方便。

总结：现在第三方开源框架非常之多，以至于我们可以更关注和快速的开发业务。但是这些第三方框架恰恰是我们应该学习的，它们将“简介“”中说的基本功给剥夺了，像Retrofit用到了注解、动态代理等、OkHttp封装了Socket、连接池等、Okio是io操作更快捷，其中这些恰恰也是我们应该掌握的。学习Retrofit源码使我又重温了Java网络编程、HTTP权威指南、Java IO操作等，其囊括的技术之多，令人叹服！就像做Java Web开发需要理解Spring、Hibernate等框架一样，在做业务的同时，其核心技术才应该是我们最该掌握和学习的。做个广告，Spring相关可参考我的Spring实战系列：http://blog.csdn.net/honghailiang888/article/details/53113853。