Android网络编程(十一)源码解析Retrofit

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Android网络编程系列

前言

最近博客的产出确实很少,因为博主我正在写一本Android进阶书籍,两头很难兼顾,但是每个月也得至少发一篇博客。上一篇我们介绍了Retrofit的使用方法,这一篇我们照例来学习Retrofit的源码。

1.Retrofit的创建过程

当我们使用Retrofit请求网络时,首先要写请求接口:

public interface IpService {
    @GET("getIpInfo.php?ip=59.108.54.37")
      Call<IpModel> getIpMsg();

接着我们通过调用如下代码来创建Retrofit:

Retrofit retrofit = new Retrofit.Builder()
             .baseUrl(url)
             .addConverterFactory(GsonConverterFactory.create())
             .build();

Retrofit 是通过建造者模式构建出来的,接下来查看Builder方法做了什么:
java

public Builder() {
    this(Platform.get());
  }

很简短,查看Platform的get方法,如下所示。

private static final Platform PLATFORM = findPlatform();
 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) {
   }
   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();
 }

Platform的get方法最终调用的是findPlatform方法,根据不同的运行平台来提供不同的线程池。接下来查看build方法,代码如下所示。

public Retrofit build() {
   if (baseUrl == null) {//1
     throw new IllegalStateException("Base URL required.");
   }
   okhttp3.Call.Factory callFactory = this.callFactory;//2
   if (callFactory == null) {
     callFactory = new OkHttpClient();//3
   }
   Executor callbackExecutor = this.callbackExecutor;
   if (callbackExecutor == null) {
     callbackExecutor = platform.defaultCallbackExecutor();//4
   }
   List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);//5
   adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));
   List<Converter.Factory> converterFactories = new ArrayList<>(this.converterFactories);//6
   return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,
       callbackExecutor, validateEagerly);
 }

从注释1处可以看出baseUrl 是必须指定的。注释2处callFactory默认为this.callFactory,this.callFactory就是我们在构建Retrofit时调用callFactory方法所传进来的,如下所示。

public Builder callFactory(okhttp3.Call.Factory factory) {
   this.callFactory = checkNotNull(factory, "factory == null");
   return this;
 }

因此,如果需要对OkHttpClient进行设置,则可以构建OkHttpClient对象,然后调用callFactory方法将设置好的OkHttpClient传进去。注释3处,如果没有设置callFactory则直接创建OkHttpClient。注释4的callbackExecutor用来将回调传递到UI线程。注释5的adapterFactories主要用于存储对Call进行转化的对象,后面在Call的创建过程会再次提到它。注释6处的converterFactories主要用于存储转化数据对象,后面也会提及到。此前在例子中调用的addConverterFactory(GsonConverterFactory.create()),就是设置返回的数据支持转换为Gson对象。最终会返回配置好的Retrofit类。

2.Call的创建过程

紧接着我们创建Retrofit实例并调用如下代码来生成接口的动态代理对象:

IpService ipService = retrofit.create(IpService.class);

接下来看Retrofit的create方法做了什么,代码如下所示。

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);//1
          OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
          return serviceMethod.callAdapter.adapt(okHttpCall);
        }
      });
}

可以看到create方法返回了一个Proxy.newProxyInstance动态代理对象,当我们调用IpService的getIpMsg方法最终会调用InvocationHandler的invoke 方法,它有3个参数,第一个是代理对象,第二个是调用的方法,第三个是方法的参数。注释1处的loadServiceMethod(method)中的method就是我们定义的getIpMsg方法。接下来查看loadServiceMethod方法里做了什么:

private final Map<Method, ServiceMethod> serviceMethodCache = new LinkedHashMap<>();
ServiceMethod loadServiceMethod(Method method) {
   ServiceMethod result;
   synchronized (serviceMethodCache) {
     result = serviceMethodCache.get(method);
     if (result == null) {
       result = new ServiceMethod.Builder(this, method).build();
       serviceMethodCache.put(method, result);
     }
   }
   return result;
 }

首先会从serviceMethodCache查询传入的方法是否有缓存,如果有就用缓存的ServiceMethod,如果没有就创建一个,并加入serviceMethodCache缓存起来。接下来看ServiceMethod是如何构建的,代码如下所示。

public ServiceMethod build() {
   callAdapter = createCallAdapter();//1
   responseType = callAdapter.responseType();//2
   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();//3
   for (Annotation annotation : methodAnnotations) {
     parseMethodAnnotation(annotation);//4
   }
  ...
   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];//5
     if (parameterAnnotations == null) {
       throw parameterError(p, "No Retrofit annotation found.");
     }
     parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
   }
   ...
   return new ServiceMethod<>(this);
 }

注释1处调用了createCallAdapter方法,它最终会得到我们在构建Retrofit调用build方法时adapterFactories添加的对象的get方法,Retrofit的build方法部分代码:

List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
   adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

adapterFactories列表默认会添加defaultCallAdapterFactory,defaultCallAdapterFactory指的是ExecutorCallAdapterFactory,ExecutorCallAdapterFactory的get方法如下所示。

public CallAdapter<Call<?>> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
   if (getRawType(returnType) != Call.class) {
     return null;
   }
   final Type responseType = Utils.getCallResponseType(returnType);
   return new CallAdapter<Call<?>>() {
     @Override public Type responseType() {
       return responseType;
     }
     @Override public <R> Call<R> adapt(Call<R> call) {
       return new ExecutorCallbackCall<>(callbackExecutor, call);
     }
   };
 }

get方法会得到CallAdapter对象,它的responseType方法会返回数据的真实类型,比如 Call<IpModel>,它就会返回IpModel。adapt方法会创建ExecutorCallbackCall,它会将call的回调转发至UI线程。
接着回到ServiceMethod的 build方法,注释2处调用CallAdapter的responseType得到的是返回数据的真实类型。
注释3处调用createResponseConverter方法来遍历converterFactories列表中存储的Converter.Factory,并返回一个合适的Converter用来转换对象。此前我们在构建Retrofit 调用了addConverterFactory(GsonConverterFactory.create())将GsonConverterFactory(Converter.Factory的子类)添加到converterFactories列表中,表示返回的数据支持转换为Json对象。
注释4处遍历parseMethodAnnotation方法来对请求方式(比如GET、POST)和请求地址进行解析。注释5处对方法中的参数注解进行解析(比如@Query、@Part)。最后创建ServiceMethod类并返回。
接下来回过头来查看Retrofit的create方法,在调用了loadServiceMethod方法后会创建OkHttpCall,OkHttpCall的构造函数只是进行了赋值操作。紧接着调用serviceMethod.callAdapter.adapt(okHttpCall),callAdapter的adapt方法前面讲过,它会创建ExecutorCallbackCall,ExecutorCallbackCall的部分代码如下所示。

ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
     this.callbackExecutor = callbackExecutor;
     this.delegate = delegate;
   }
   @Override public void enqueue(final Callback<T> callback) {
     if (callback == null) throw new NullPointerException("callback == null");
     delegate.enqueue(new Callback<T>() {//1
       @Override public void onResponse(Call<T> call, final Response<T> response) {
         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);
           }
         });
       }
     });
   }

可以看出ExecutorCallbackCall是对Call的封装,它主要添加了通过callbackExecutor将请求回调到UI线程。
当我们得到Call对象后会调用它的enqueue方法,其实调用的是ExecutorCallbackCall的enqueue方法,而从注释1处可以看出ExecutorCallbackCall的enqueue方法最终调用的是delegate的enqueue方法。delegate从Retrofit的create方法的代码中我们知道它其实就是OkHttpCall。

3.Call的enqueue方法

接下来我们就来查看OkHttpCall的enqueue方法,代码如下所示。

public void enqueue(final Callback<T> callback) {
  if (callback == null) throw new NullPointerException("callback == null");
  okhttp3.Call call;
 ...
  call.enqueue(new okhttp3.Callback() {//1
    @Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse)
        throws IOException {
      Response<T> response;
      try {
        response = parseResponse(rawResponse);//2
      } catch (Throwable e) {
        callFailure(e);
        return;
      }
      callSuccess(response);
    }
    ...
}

注释1处调用了okhttp3.Call的enqueue方法。注释2处调用parseResponse方法:

Response<T> parseResponse(okhttp3.Response rawResponse) throws IOException {
  ResponseBody rawBody = rawResponse.body();
 ...
  int code = rawResponse.code();
  if (code < 200 || code >= 300) {
    try {
      ResponseBody bufferedBody = Utils.buffer(rawBody);
      return Response.error(bufferedBody, rawResponse);
    } finally {
      rawBody.close();
    }
  }
  if (code == 204 || code == 205) {
    return Response.success(null, rawResponse);
  }
  ExceptionCatchingRequestBody catchingBody = new ExceptionCatchingRequestBody(rawBody);
  try {
    T body = serviceMethod.toResponse(catchingBody);//2
    return Response.success(body, rawResponse);
  } catch (RuntimeException e) {
    catchingBody.throwIfCaught();
    throw e;
  }
}

根据返回的不同的状态码code值来做不同的操作,如果顺利则会调用注释2处的代码,接下来看toResponse方法里做了什么:

T toResponse(ResponseBody body) throws IOException {
   return responseConverter.convert(body);
 }

这个responseConverter就是此前讲过在ServiceMethod的build方法调用createResponseConverter方法返回的Converter,在此前的例子中我们传入的是GsonConverterFactory,因此可以查看GsonConverterFactory的代码,如下所示。

public final class GsonConverterFactory extends Converter.Factory {
...
  @Override
  public Converter<ResponseBody, ?> responseBodyConverter(Type type, Annotation[] annotations,
      Retrofit retrofit) {
    TypeAdapter<?> adapter = gson.getAdapter(TypeToken.get(type));
    return new GsonResponseBodyConverter<>(gson, adapter);
  }
...  
}

在GsonConverterFactory 中有一个方法responseBodyConverter,它最终会创建GsonResponseBodyConverter:

final class GsonResponseBodyConverter<T> implements Converter<ResponseBody, T> {
  private final Gson gson;
  private final TypeAdapter<T> adapter;
  GsonResponseBodyConverter(Gson gson, TypeAdapter<T> adapter) {
    this.gson = gson;
    this.adapter = adapter;
  }
  @Override public T convert(ResponseBody value) throws IOException {
    JsonReader jsonReader = gson.newJsonReader(value.charStream());
    try {
      return adapter.read(jsonReader);
    } finally {
      value.close();
    }
  }
}

在GsonResponseBodyConverter的convert方法里会将回调的数据转换为Json格式。因此我们也知道了此前调用responseConverter.convert是为了转换为特定的数据格式。
Call的enqueue方法主要做的就是用OKHttp来请求网络并将返回的Response进行数据转换并回调给UI线程。
至此,Retrofit的源码就讲到这里。

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