Retrofit源码简要分析

一、Retrofit的使用
本文使用的Retrofit 2.1.0版本。
在gradle中加入：

    compile 'com.squareup.retrofit2:retrofit:2.1.0'

如果要是要解析json，还需加入：

    compile 'com.squareup.retrofit2:converter-gson:2.1.0'

如果要结合rxjava
需要加入：

 compile 'io.reactivex:rxjava:1.1.6'
 compile 'io.reactivex:rxandroid:1.2.1'
 compile 'com.squareup.retrofit2:adapter-rxjava:2.1.0'

Retrofit 2.1.0 依赖 okhttp 3发送网络请求。
所以Retrofit的源码其实并不多

http包下的是注解类。

接下来我们看看Retrofit是怎样创建的：

new Retrofit.Builder()
               .baseUrl(Constants.BASE_URL)
               .addConverterFactory(GsonConverterFactory.create())
               .client(okHttpClient)
               .build();  

okHttpClient是okhttp3.0的okHttpClient,Retrofit将会使用它来发送网络请求。
当我们需要调用一个后台接口时，我们只需新建一个接口，然后根据请求类型以及参数写一个抽象方法
例如:

public interface Api {
    @FormUrlEncoded
    @POST("/api/test")
    Call getTest(@Field("test")String test);
}

其中getTest方法就可以向后台发起一个请求，这个请求类型为Post 表单提交，参数名为test.
使用的话，可以这样：

 Api api = new Retrofit.Builder()
                .baseUrl("http://www.test.com/")
                .addConverterFactory(GsonConverterFactory.create())
                .client(okHttpClient)
                .build()
                .create(Api.class);
                Call call = api.getTest("test");

得到 call 然后使用方法就和okhttp差不多了，可以使用call.enqueue()或call.execute()方法

 call.enqueue(new Callback() {
           @Override
           public void onResponse(Call call, retrofit2.Response response) {

           }

           @Override
           public void onFailure(Call call, Throwable t) {

           }
       });

        //或者
        try {
            call.execute();
        } catch (IOException e) {
            e.printStackTrace();
        }

他们分别代表了同步，和异步发起请求。
具体使用方法还请参考官网文档，这里就不再说了，接下来我们进入正题。

二、源码分析

我们在使用Retrofit的使用使用了这么 一段代码来创建retrofit

new Retrofit.Builder()
                .baseUrl(Constants.BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .client(okHttpClient)
                .build()

那么我们就先分析retrofit怎么被创建出来
这里使用了一个建造者模式，Build是Retrofit中的一个嵌套类

  public static final class Builder {
    private Platform platform;
    private okhttp3.Call.Factory callFactory;
    private HttpUrl baseUrl;
    private List converterFactories = new ArrayList<>();
    private List adapterFactories = new ArrayList<>();
    private Executor callbackExecutor;
    private boolean validateEagerly;

    Builder(Platform platform) {
      this.platform = platform;
      // Add the built-in converter factory first. This prevents overriding its behavior but also
      // ensures correct behavior when using converters that consume all types.
      converterFactories.add(new BuiltInConverters());
    }

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

可以看到这个类中几个重要的域 Platform、okhttp3.Call.Factory、List< Converter.Factory >、List< CallAdapter.Factory > 、Executor
从类名上大致可以猜想到他们的作用，platform 代表目前使用的平台，可以是android、ios、java8
okhttp3.Call.Factory 的实现实际是我们传入的 okhttpClient,它的作用是发送网络请求
List

 public Retrofit build() {
      if (baseUrl == null) {
        throw new IllegalStateException("Base URL required.");
      }

      okhttp3.Call.Factory callFactory = this.callFactory;
      if (callFactory == null) {
        callFactory = new OkHttpClient();
      }

      Executor callbackExecutor = this.callbackExecutor;
      if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
      }

      // Make a defensive copy of the adapters and add the default Call adapter.
      List adapterFactories = new ArrayList<>(this.adapterFactories);
      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

      // Make a defensive copy of the converters.
      List converterFactories = new ArrayList<>(this.converterFactories);

      return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,
          callbackExecutor, validateEagerly);
    }

在Builder 的构造方法中可以看出它获取了当前的平台：

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

class Platform {
  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();
  }

我们只看Android平台：

static class Android extends Platform {
    @Override public Executor defaultCallbackExecutor() {
      return new MainThreadExecutor();
    }

    @Override CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {
      return new ExecutorCallAdapterFactory(callbackExecutor);
    }

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

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

他有2个方法，一个是获得一个 CallAdapter.Factory ，它是一个制造CallAdapter的工厂

一个是获得一个Executor ，用来执行运行调用完成的结果。

接下来我们看，当我们调用了 retrofit.create(Api.class); 之后发生了什么

public  T create(final Class 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);
          }
        });
  }

看以看到这个方法使用了java的动态代理，关于动态代理可以参考我的上一篇文章http://blog.csdn.net/a992036795/article/details/52101685
可以知道执行的将是invoke(Object proxy, Method method, Object… args) 这个方法，也就是执行：

 // 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);

如果我们传入参数正确的话，它将走最后这三行代码
我们就先看 ServiceMethod serviceMethod = loadServiceMethod(method); 这句代码的作用

 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;
  }

serviceMethodCache 是一个Map，这里估计是考虑到了反射的效率问题，所以加了一个缓存机制，我们考到它使用new ServiceMethod.Builder(this, method).build(); 来创建一个
ServiceMethod这里又是一个建造者模式。我们点进去看一下

public ServiceMethod build() {
      callAdapter = createCallAdapter();
      responseType = callAdapter.responseType();
      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();

      for (Annotation annotation : methodAnnotations) {
        parseMethodAnnotation(annotation);
      }

      if (httpMethod == null) {
        throw methodError("HTTP method annotation is required (e.g., @GET, @POST, etc.).");
      }

      if (!hasBody) {
        if (isMultipart) {
          throw methodError(
              "Multipart can only be specified on HTTP methods with request body (e.g., @POST).");
        }
        if (isFormEncoded) {
          throw methodError("FormUrlEncoded can only be specified on HTTP methods with "
              + "request body (e.g., @POST).");
        }
      }

      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];
        if (parameterAnnotations == null) {
          throw parameterError(p, "No Retrofit annotation found.");
        }

        parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
      }

      if (relativeUrl == null && !gotUrl) {
        throw methodError("Missing either @%s URL or @Url parameter.", httpMethod);
      }
      if (!isFormEncoded && !isMultipart && !hasBody && gotBody) {
        throw methodError("Non-body HTTP method cannot contain @Body.");
      }
      if (isFormEncoded && !gotField) {
        throw methodError("Form-encoded method must contain at least one @Field.");
      }
      if (isMultipart && !gotPart) {
        throw methodError("Multipart method must contain at least one @Part.");
      }

      return new ServiceMethod<>(this);
    }

这里执行了一些方法，我们看些主要的：
先看第一个createCallAdapter

private CallAdapter createCallAdapter() {
      Type returnType = method.getGenericReturnType();
      if (Utils.hasUnresolvableType(returnType)) {
        throw methodError(
            "Method return type must not include a type variable or wildcard: %s", returnType);
      }
      if (returnType == void.class) {
        throw methodError("Service methods cannot return void.");
      }
      Annotation[] annotations = method.getAnnotations();
      try {
        return retrofit.callAdapter(returnType, annotations);
      } catch (RuntimeException e) { // Wide exception range because factories are user code.
        throw methodError(e, "Unable to create call adapter for %s", returnType);
      }
    }

它调用了retrofit.callAdapter(returnType, annotations);

public CallAdapter callAdapter(Type returnType, Annotation[] annotations) {
    return nextCallAdapter(null, returnType, annotations);
  }


  public CallAdapter nextCallAdapter(CallAdapter.Factory skipPast, Type returnType,
      Annotation[] annotations) {
    checkNotNull(returnType, "returnType == null");
    checkNotNull(annotations, "annotations == null");

    int start = adapterFactories.indexOf(skipPast) + 1;
    for (int i = start, count = adapterFactories.size(); i < count; i++) {
    // 这里就调用了我们前文讲的那个 工厂ExecutorCallAdapterFactory，然后掉了它的get方法
      CallAdapter adapter = adapterFactories.get(i).get(returnType, annotations, this);
      if (adapter != null) {
        return adapter;
      }
    }

    StringBuilder builder = new StringBuilder("Could not locate call adapter for ")
        .append(returnType)
        .append(".\n");
    if (skipPast != null) {
      builder.append("  Skipped:");
      for (int i = 0; i < start; i++) {
        builder.append("\n   * ").append(adapterFactories.get(i).getClass().getName());
      }
      builder.append('\n');
    }
    builder.append("  Tried:");
    for (int i = start, count = adapterFactories.size(); i < count; i++) {
      builder.append("\n   * ").append(adapterFactories.get(i).getClass().getName());
    }
    throw new IllegalArgumentException(builder.toString());
  }

这里调用了 adapterFactories.get(i).get(returnType, annotations, this);
因为在Retrofit.Builder.build的时候 向adapterFactories 加入了我们ExecutorCallAdapterFactory，所以它最终调用的是这个类的get方法

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

      @Override public  Call adapt(Call call) {
        return new ExecutorCallbackCall<>(callbackExecutor, call);
      }
    };
  }

可以看它的get方法，它返回了一个匿名类 CallAdapter< Call< ? > >

也就是说serviceMethod中的这个callAdapter 实际指向的就是这个匿名类
build中还有一个方法就是 parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
它最终解析了方法的注解，来获得请求的类型。

然后我们看retrofit.create的最后 2句

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

我们看到它创建出了一个OkHttpCall 然后调用serviceMethod.callAdapter.adapt 的adapter方法，并将okhttpCall传了进去，上文我们说到serviceMethod.callAdapter就是我们看的那个匿名类

 return new CallAdapter>() {
      @Override public Type responseType() {
        return responseType;
      }

      @Override public  Call adapt(Call call) {
        return new ExecutorCallbackCall<>(callbackExecutor, call);
      }
    };
  }

  static final class ExecutorCallbackCall implements Call {
    final Executor callbackExecutor;
    final Call delegate;

    ExecutorCallbackCall(Executor callbackExecutor, Call delegate) {
      this.callbackExecutor = callbackExecutor;
      this.delegate = delegate;
    }

    @Override public void enqueue(final Callback callback) {
      if (callback == null) throw new NullPointerException("callback == null");

      delegate.enqueue(new Callback() {
        @Override public void onResponse(Call call, final Response response) {
          callbackExecutor.execute(new Runnable() {
            @Override public void run() {
              if (delegate.isCanceled()) {
                // Emulate OkHttp's behavior of throwing/delivering an IOException on cancellation.
                callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
              } else {
                callback.onResponse(ExecutorCallbackCall.this, response);
              }
            }
          });
        }

        @Override public void onFailure(Call call, final Throwable t) {
          callbackExecutor.execute(new Runnable() {
            @Override public void run() {
              callback.onFailure(ExecutorCallbackCall.this, t);
            }
          });
        }
      });
    }

    @Override public boolean isExecuted() {
      return delegate.isExecuted();
    }

    @Override public Response execute() throws IOException {
      return delegate.execute();
    }

    @Override public void cancel() {
      delegate.cancel();
    }

    @Override public boolean isCanceled() {
      return delegate.isCanceled();
    }

    @SuppressWarnings("CloneDoesntCallSuperClone") // Performing deep clone.
    @Override public Call clone() {
      return new ExecutorCallbackCall<>(callbackExecutor, delegate.clone());
    }

    @Override public Request request() {
      return delegate.request();
    }
  }

这里看到 adapter 返回的是一个 ExecutorCallbackCall，我们看这里是一个装饰者模式
它将 OkHttpCall 装饰成了一个ExecutorCallbackCall 然后返回给了调用程序
实际他是将请求委托给了 OkHttpCall。然后我们来继续看OkHttpCall

final class OkHttpCall implements Call {
  private final ServiceMethod serviceMethod;
  private final Object[] args;

  private volatile boolean canceled;

  // All guarded by this.
  private okhttp3.Call rawCall;
  private Throwable creationFailure; // Either a RuntimeException or IOException.
  private boolean executed;

  OkHttpCall(ServiceMethod serviceMethod, Object[] args) {
    this.serviceMethod = serviceMethod;
    this.args = args;
  }

  @SuppressWarnings("CloneDoesntCallSuperClone") // We are a final type & this saves clearing state.
  @Override public OkHttpCall clone() {
    return new OkHttpCall<>(serviceMethod, args);
  }

  @Override public synchronized Request request() {
    okhttp3.Call call = rawCall;
    if (call != null) {
      return call.request();
    }
    if (creationFailure != null) {
      if (creationFailure instanceof IOException) {
        throw new RuntimeException("Unable to create request.", creationFailure);
      } else {
        throw (RuntimeException) creationFailure;
      }
    }
    try {
      return (rawCall = createRawCall()).request();
    } catch (RuntimeException e) {
      creationFailure = e;
      throw e;
    } catch (IOException e) {
      creationFailure = e;
      throw new RuntimeException("Unable to create request.", e);
    }
  }

  @Override public void enqueue(final Callback callback) {
    if (callback == null) throw new NullPointerException("callback == null");

    okhttp3.Call call;
    Throwable failure;

    synchronized (this) {
      if (executed) throw new IllegalStateException("Already executed.");
      executed = true;

      call = rawCall;
      failure = creationFailure;
      if (call == null && failure == null) {
        try {
          call = rawCall = createRawCall();
        } catch (Throwable t) {
          failure = creationFailure = t;
        }
      }
    }

    if (failure != null) {
      callback.onFailure(this, failure);
      return;
    }

    if (canceled) {
      call.cancel();
    }

    call.enqueue(new okhttp3.Callback() {
      @Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse)
          throws IOException {
        Response response;
        try {
          response = parseResponse(rawResponse);
        } catch (Throwable e) {
          callFailure(e);
          return;
        }
        callSuccess(response);
      }

      @Override public void onFailure(okhttp3.Call call, IOException e) {
        try {
          callback.onFailure(OkHttpCall.this, e);
        } catch (Throwable t) {
          t.printStackTrace();
        }
      }

      private void callFailure(Throwable e) {
        try {
          callback.onFailure(OkHttpCall.this, e);
        } catch (Throwable t) {
          t.printStackTrace();
        }
      }

      private void callSuccess(Response response) {
        try {
          callback.onResponse(OkHttpCall.this, response);
        } catch (Throwable t) {
          t.printStackTrace();
        }
      }
    });
  }

  @Override public synchronized boolean isExecuted() {
    return executed;
  }

  @Override public Response execute() throws IOException {
    okhttp3.Call call;

    synchronized (this) {
      if (executed) throw new IllegalStateException("Already executed.");
      executed = true;

      if (creationFailure != null) {
        if (creationFailure instanceof IOException) {
          throw (IOException) creationFailure;
        } else {
          throw (RuntimeException) creationFailure;
        }
      }

      call = rawCall;
      if (call == null) {
        try {
          call = rawCall = createRawCall();
        } catch (IOException | RuntimeException e) {
          creationFailure = e;
          throw e;
        }
      }
    }

    if (canceled) {
      call.cancel();
    }

    return parseResponse(call.execute());
  }

它封装了okhttp 的call，最后请求实际是通过okhttp来进行了请求。

也即是说调用程序执行请求的过程是，拿到的实际是一个实现了retrofit2.Call< T > 接口的
ExecutorCallbackCall< T > 然后ExecutorCallbackCall 将请求转给 OkHttpCall
然后 OkHttpCall 调用 okhttp 发起请求