Spring MVC之HandlerMethodArgumentResolver参数解析器

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Spring MVC有几种常见的数据绑定的方法，如@PathVariable，@ModelAttribute，@RequestParam等这些数据绑定注解。有了这些注解，我们可以很方便的去获取参数，但是偶尔我们需要自定义的去进行数据绑定，可以通过HandlerMethodArgumentResolver实现。

下面简单介绍一下如何绑定 HttpServletRequest header中的信息到Service对象中：

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@AllArgsConstructor
public class Service {

    public final String code;
    public final String token;

}
实现HandlerMethodArgumentResolver类
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public class ServiceArgumentResolver implements HandlerMethodArgumentResolver {

	@Override
	public boolean supportsParameter(MethodParameter parameter) {
		return Service.class.isAssignableFrom(parameter.getParameterType());
	}

	@Override
	public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, 
	                             NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
		HttpServletRequest request = (HttpServletRequest)webRequest.getNativeRequest();
		
        return new Service(request.getHeader("x-code"), request.getHeader("x-token"));
	}

}
WebMvcConfig类中重写addArgumentResolvers方法
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@Configuration
public class WebMvcConfig extends WebMvcConfigurerAdapter {

    @Bean
    public ServiceArgumentResolver serviceArgumentResolver() {
        return new ServiceArgumentResolver();
    }
    
    @Override
    public void addArgumentResolvers(List<HandlerMethodArgumentResolver> argumentResolvers) {
        argumentResolvers.add(serviceArgumentResolver);
    }
    
    ......
}
controller中直接从Service对象获取参数
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@RestController
@RequestMapping("/service")
public class ServiceController {

    @RequestMapping(method = RequestMethod.GET)
    public void getServices(Service param) throws Exception{
        System.out.println(param.code);
    }
    
}

其实上面提到的 @PathVariable 注解也是通过上面这种方式实现的，代码如下：

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public class PathVariableMethodArgumentResolver extends AbstractNamedValueMethodArgumentResolver
		implements UriComponentsContributor {

	private static final TypeDescriptor STRING_TYPE_DESCRIPTOR = TypeDescriptor.valueOf(String.class);


	@Override
	public boolean supportsParameter(MethodParameter parameter) {
		if (!parameter.hasParameterAnnotation(PathVariable.class)) {
			return false;
		}
		if (Map.class.isAssignableFrom(parameter.nestedIfOptional().getNestedParameterType())) {
			String paramName = parameter.getParameterAnnotation(PathVariable.class).value();
			return StringUtils.hasText(paramName);
		}
		return true;
	}

	@Override
	protected NamedValueInfo createNamedValueInfo(MethodParameter parameter) {
		PathVariable annotation = parameter.getParameterAnnotation(PathVariable.class);
		return new PathVariableNamedValueInfo(annotation);
	}

	@Override
	@SuppressWarnings("unchecked")
	protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest request) throws Exception {
		Map<String, String> uriTemplateVars = (Map<String, String>) request.getAttribute(
				HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);
		return (uriTemplateVars != null ? uriTemplateVars.get(name) : null);
	}

	@Override
	protected void handleMissingValue(String name, MethodParameter parameter) throws ServletRequestBindingException {
		throw new MissingPathVariableException(name, parameter);
	}

	@Override
	@SuppressWarnings("unchecked")
	protected void handleResolvedValue(Object arg, String name, MethodParameter parameter,
			ModelAndViewContainer mavContainer, NativeWebRequest request) {

		String key = View.PATH_VARIABLES;
		int scope = RequestAttributes.SCOPE_REQUEST;
		Map<String, Object> pathVars = (Map<String, Object>) request.getAttribute(key, scope);
		if (pathVars == null) {
			pathVars = new HashMap<String, Object>();
			request.setAttribute(key, pathVars, scope);
		}
		pathVars.put(name, arg);
	}

	@Override
	public void contributeMethodArgument(MethodParameter parameter, Object value,
			UriComponentsBuilder builder, Map<String, Object> uriVariables, ConversionService conversionService) {

		if (Map.class.isAssignableFrom(parameter.nestedIfOptional().getNestedParameterType())) {
			return;
		}

		PathVariable ann = parameter.getParameterAnnotation(PathVariable.class);
		String name = (ann != null && !StringUtils.isEmpty(ann.value()) ? ann.value() : parameter.getParameterName());
		value = formatUriValue(conversionService, new TypeDescriptor(parameter.nestedIfOptional()), value);
		uriVariables.put(name, value);
	}

	protected String formatUriValue(ConversionService cs, TypeDescriptor sourceType, Object value) {
		if (value == null) {
			return null;
		}
		else if (value instanceof String) {
			return (String) value;
		}
		else if (cs != null) {
			return (String) cs.convert(value, sourceType, STRING_TYPE_DESCRIPTOR);
		}
		else {
			return value.toString();
		}
	}


	private static class PathVariableNamedValueInfo extends NamedValueInfo {

		public PathVariableNamedValueInfo(PathVariable annotation) {
			super(annotation.name(), annotation.required(), ValueConstants.DEFAULT_NONE);
		}
	}

}