HandlerMethodArgumentResolver(二)：Map参数类型和固定参数类型【享学Spring MVC】（上）

public class RequestHeaderMethodArgumentResolver extends AbstractNamedValueMethodArgumentResolver {
  // 必须标注@RequestHeader注解，并且不能，不能，不能是Map类型
  // 有的小伙伴会说：`@RequestHeader Map headers`这样可以接收到所有的请求头啊
  // 其实不是本类的功劳，是`RequestHeaderMapMethodArgumentResolver`的作用
  @Override
  public boolean supportsParameter(MethodParameter parameter) {
    return (parameter.hasParameterAnnotation(RequestHeader.class) &&
        !Map.class.isAssignableFrom(parameter.nestedIfOptional().getNestedParameterType()));
  }
  // 理解起来很简单：可以单值，也可以List/数组
  @Override
  @Nullable
  protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest request) throws Exception {
    String[] headerValues = request.getHeaderValues(name);
    if (headerValues != null) {
      return (headerValues.length == 1 ? headerValues[0] : headerValues);
    } else {
      return null;
    }
  }
}

    @ResponseBody
    @GetMapping("/test")
    public Object test(@RequestHeader("Accept-Encoding") String encoding,
                       @RequestHeader("Accept-Encoding") List<String> encodingList) {
        System.out.println(encoding);
        System.out.println(encodingList);
        return encoding;
    }

return request.getAttribute(name, RequestAttributes.SCOPE_REQUEST);

public abstract class AbstractCookieValueMethodArgumentResolver extends AbstractNamedValueMethodArgumentResolver {
  ...
  @Override
  public boolean supportsParameter(MethodParameter parameter) {
    return parameter.hasParameterAnnotation(CookieValue.class);
  } 
  @Override
  protected void handleMissingValue(String name, MethodParameter parameter) throws ServletRequestBindingException {
    throw new MissingRequestCookieException(name, parameter);
  }
  ... // 并木有实现核心resolveName方法
}

public class ServletCookieValueMethodArgumentResolver extends AbstractCookieValueMethodArgumentResolver {
  private UrlPathHelper urlPathHelper = new UrlPathHelper();
  ...
  public void setUrlPathHelper(UrlPathHelper urlPathHelper) {
    this.urlPathHelper = urlPathHelper;
  }
  @Override
  @Nullable
  protected Object resolveName(String cookieName, MethodParameter parameter, NativeWebRequest webRequest) throws Exception {
    HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
    Assert.state(servletRequest != null, "No HttpServletRequest");
    // 工具方法，底层是：request.getCookies()
    Cookie cookieValue = WebUtils.getCookie(servletRequest, cookieName);
    // 如果用javax.servlet.http.Cookie接受值，就直接返回了
    if (Cookie.class.isAssignableFrom(parameter.getNestedParameterType())) {
      return cookieValue;
    } else if (cookieValue != null) { // 否则返回cookieValue
      return this.urlPathHelper.decodeRequestString(servletRequest, cookieValue.getValue());
    } else {
      return null;
    }
  }
}

    @ResponseBody
    @GetMapping("/test")
    public Object test(@CookieValue("JSESSIONID") Cookie cookie,
                       @CookieValue("JSESSIONID") String cookieValue) {
        System.out.println(cookie);
        System.out.println(cookieValue);
        return cookieValue;
    }

// @since 3.2
public class MatrixVariableMethodArgumentResolver extends AbstractNamedValueMethodArgumentResolver {
  // @MatrixVariable注解是必须的。然后技能处理普通类型，也能处理Map
  @Override
  public boolean supportsParameter(MethodParameter parameter) {
    if (!parameter.hasParameterAnnotation(MatrixVariable.class)) {
      return false;
    }
    if (Map.class.isAssignableFrom(parameter.nestedIfOptional().getNestedParameterType())) {
      MatrixVariable matrixVariable = parameter.getParameterAnnotation(MatrixVariable.class);
      return (matrixVariable != null && StringUtils.hasText(matrixVariable.name()));
    }
    return true;
  }
  ...
}

// @since 3.1
public class ExpressionValueMethodArgumentResolver extends AbstractNamedValueMethodArgumentResolver {
  // 唯一构造函数  支持占位符、SpEL
  public ExpressionValueMethodArgumentResolver(@Nullable ConfigurableBeanFactory beanFactory) {
    super(beanFactory);
  }
  //必须标注有@Value注解
  @Override
  public boolean supportsParameter(MethodParameter parameter) {
    return parameter.hasParameterAnnotation(Value.class);
  }
  @Override
  protected NamedValueInfo createNamedValueInfo(MethodParameter parameter) {
    Value ann = parameter.getParameterAnnotation(Value.class);
    return new ExpressionValueNamedValueInfo(ann);
  }
  private static final class ExpressionValueNamedValueInfo extends NamedValueInfo {
    // 这里name传值为固定值  因为只要你的key不是这个就木有问题
    // required传固定值false
    // defaultValue：取值为annotation.value() --> 它天然支持占位符和SpEL嘛
    private ExpressionValueNamedValueInfo(Value annotation) {
      super("@Value", false, annotation.value());
    }
  }
  // 这里恒返回null，因此即使你的key是@Value，也是不会采纳你的传值的哟~
  @Override
  @Nullable
  protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest webRequest) throws Exception {
    // No name to resolve
    return null;
  }
}

// 在MVC子容器中导入外部化配置
@Configuration
@PropertySource("classpath:my.properties") // 此处有键值对：test.myage = 18
@EnableWebMvc
public class WebMvcConfig extends WebMvcConfigurerAdapter { ... }
    @ResponseBody
    @GetMapping("/test")
    public Object test(@Value("#{T(Integer).parseInt('${test.myage:10}') + 10}") Integer myAge) {
        System.out.println(myAge);
        return myAge;
    }