Retrofit2.0通过注解来配置请求的方式，比如请求的类型，Header，参数等等。通过动态代理的方式来解释注解。每一个参数注解都对应着RequestBuilder的类型，如何将注解标注的参数和RequestBuilder的参数准确的对应起来就是由ParameterHandler来做的。
1.首先拿到所有方法参数的注解
方法参数的注解是按照你声明的顺序返回的

  this.parameterAnnotationsArray = method.getParameterAnnotations();

2.每一个参数注解都对应一种ParameterHandler
比如url和path 分别对应 ParameterHandler.RelativeUrl和ParameterHandler.Path类，这两个类负责给RequestBuilder的参数和注解标识的方法的参数关联起来

  if (annotation instanceof Url) {
           ......
                if (type == HttpUrl.class
                        || type == String.class
                        || type == URI.class
                        || (type instanceof Class && "android.net.Uri".equals(((Class<?>) type).getName()))) {
                    return new ParameterHandler.RelativeUrl();
                } else {
                    throw parameterError(p,
                            "@Url must be okhttp3.HttpUrl, String, java.net.URI, or android.net.Uri type.");
                }

            } else if (annotation instanceof Path) {
        ......
                Path path = (Path) annotation;
                String name = path.value();
                validatePathName(p, name);

                Converter<?, String> converter = retrofit.stringConverter(type, annotations);
                return new ParameterHandler.Path<>(name, converter, path.encoded());

并且将每一种参数注解对应的ParameterHandler都保存在一个ParameterHandler数组中，这个数组中存储数据的顺序同样也是参数声明的顺序。因为注解声明的顺序和参数声明的顺序一致，每次解析一个参数注解就创建一个ParameterHandler保存在数组中。

   for (int p = 0; p < parameterCount; p++) {
                Type parameterType = parameterTypes[p];
               .....
               .....
              parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
            }

3.将参数和RequestBuilder关联起来
ParameterHandler接口的定义以及一个子类。apply的目的就是将参数和RequestBuilder对应起来

abstract class ParameterHandler<T> {
    abstract void apply(RequestBuilder builder, T value) throws IOException;


  static final class Header<T> extends ParameterHandler<T> {
        private final String name;
        private final Converter<T, String> valueConverter;

        Header(String name, Converter<T, String> valueConverter) {
            this.name = checkNotNull(name, "name == null");
            this.valueConverter = valueConverter;
        }

        @Override
        void apply(RequestBuilder builder, T value) throws IOException {
            if (value == null) return; // Skip null values.
            builder.addHeader(name, valueConverter.convert(value));
        }
    }
.....
}

ServiceMethod的toRequest方法将参数和RequestBuilder关联起来

   /**
     * Builds an HTTP request from method arguments.
     * 将请求的参数和保存的Handler一一对应起来，参数顺序和handler的保存顺序是一致的
     */
    Request toRequest(Object... args) throws IOException {
        RequestBuilder requestBuilder = new RequestBuilder(httpMethod, baseUrl, relativeUrl, headers,
                contentType, hasBody, isFormEncoded, isMultipart);
       ......
        for (int p = 0; p < argumentCount; p++) {
//这里requestbuilder和参数就依依对应起来了
            handlers[p].apply(requestBuilder, args[p]);
        }

        return requestBuilder.build();
    }