Spring @RequestAttribute

@RequestAttribute注解用法

@RequestAttribute用在方法入参上，作用：从request中取对应的值，至于request中是怎么存在该属性的，方式多种多样，拦截器中预存、ModelAttribute注解预存、请求转发带过来的；

该注解出现自Spring4.3版本

@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface RequestAttribute {

	/**
	 * Alias for {@link #name}.
	 */
	@AliasFor("name")
	String value() default "";

	/**
	 * The name of the request attribute to bind to.
	 * <p>The default name is inferred from the method parameter name.
	 */
	@AliasFor("value")
	String name() default "";

	/**
	 * Whether the request attribute is required.
	 * <p>Defaults to {@code true}, leading to an exception being thrown if
	 * the attribute is missing. Switch this to {@code false} if you prefer
	 * a {@code null} or Java 8 {@code java.util.Optional} if the attribute
	 * doesn't exist.
	 */
	boolean required() default true;

}

尝试访问一个request中不存在的值时，@RequestAttribute抛出异常：

@RequestMapping("/demo1")
    public String demo1(@RequestAttribute String name){
        System.out.println(name);
        return "test";
    }

请求request中预存属性的方式：

方式一.ModelAttribute注解

    @ModelAttribute
    public void storeEarly(HttpServletRequest request){
        request.setAttribute("name","lvbinbin");
    }

方式二.拦截器中request.setAttribute()

public class SimpleInterceptor implements HandlerInterceptor {

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        System.out.println(" Simple Interceptor preHandle");
        request.setAttribute("name",24);
        return true;
    }

    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
        System.out.println(" Simple Interceptor postHandle");
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        System.out.println(" Simple Interceptor afterCompletion");
    }
}

方式三.请求转发，转发过来的请求属性中存在；

@RequestAttribute解析流程

SpringMvc中对于方法入参解析采用HandlerMethodArgumentResolver接口，而@RequestAttribute采用RequestAttributeMethodArgumentResolver解析：

其中name属性默认为 @RequestAttribute中name/value值，如果不存在就去方法入参的名字！

@RequestAttribute属性required默认为true, request.getAttribute获取不到参数就会抛出异常 ServletRequestBindingException ；

                                 required设置为false，即使没有从request中获取到就忽略跳过，赋值为null;