Spring5 @Valid

验证功能在Spring中是很常用的。你可以使用注解或自己的验证器并将其绑定到请求中。本文将重点介绍第一种解决方案。

第一部分将介绍注解验证流程。在第二部分中，将介绍基本实现的组件。最后一部分将包含Spring初学者开发人员常见错误的解释:是否有必要直接在验证对象之后放置BindingResult。

使用@Valid注解在Spring中进行验证流程

要了解使用标准Java @Valid或特定Spring @Validated注解的验证过程，我们首先需要了解Spring如何解析使用了@ModelAttribute注解的对象。它们在controller的方法签名进行注解。@ModelAttribute注解用于将动态请求参数转换为Java注解中指定的对象。例如，观察代码@ModelAttribute(“article”)Article article ,Spring会尝试将所有请求参数匹配到Article类的字段中。现在，假设这个类有两个字段:title和content。如果请求包含title和content参数，它们将被用作Article的title和content的值(后面会对@ModelAttribute方面的源码做进一步的分析)。

当我们有对象需要进行验证时，@ModelAttribute注解的处理器(org.springframework.web.method.annotation.ModelAttributeMethodProcessor)会检查是否必须应用验证注解。注解验证必须以“Valid”这个字眼开头。接下来，对象通过org.springframework.validation.DataBinder类中的public void validate(Object … validationHints)进行验证。该方法遍历所有可用的验证器，并调用每个验证器的validate方法。验证器取自带有validatorID的bean。这样，它可以与annotation-driven的xml配置相关联:

<mvc:annotation-driven validator="validator" >

如果未指定验证器bean，则将使用默认验证器:org.springframework.validation.beanvalidation.LocalValidatorFactoryBean。

如何在Spring中处理验证？

我们已经了解了验证流程。现在，我们可以专注于验证过程本身，即验证器是如何知道一个字段不正确的。LocalValidatorFactoryBean继承自同一个包下的SpringValidatorAdapter，但不会覆盖其的validate()方法。这些方法用于检查验证字段是否正确。更准确地说，SpringValidatorAdapter包含一个目标验证器字段(Validator类型的targetValidator)。它将在validate()方法中使用来验证已验证对象的所有字段。

public class SpringValidatorAdapter implements SmartValidator, javax.validation.Validator {
 private static final Set<String> internalAnnotationAttributes = new HashSet<>(3);
 static {
  internalAnnotationAttributes.add("message");
  internalAnnotationAttributes.add("groups");
  internalAnnotationAttributes.add("payload");
 }
 @Nullable
 private javax.validation.Validator targetValidator;
 /**
  * Create a new SpringValidatorAdapter for the given JSR-303 Validator.
  * @param targetValidator the JSR-303 Validator to wrap
  */
 public SpringValidatorAdapter(javax.validation.Validator targetValidator) {
  Assert.notNull(targetValidator, "Target Validator must not be null");
  this.targetValidator = targetValidator;
 }
 SpringValidatorAdapter() {
 }
 void setTargetValidator(javax.validation.Validator targetValidator) {
  this.targetValidator = targetValidator;
 }
...
 @Override
 public void validate(@Nullable Object target, Errors errors) {
  if (this.targetValidator != null) {
   processConstraintViolations(this.targetValidator.validate(target), errors);
  }
 }
 @Override
 public void validate(@Nullable Object target, Errors errors, @Nullable Object... validationHints) {
  if (this.targetValidator != null) {
   Set<Class<?>> groups = new LinkedHashSet<>();
   if (validationHints != null) {
    for (Object hint : validationHints) {
     if (hint instanceof Class) {
      groups.add((Class<?>) hint);
     }
    }
   }
   processConstraintViolations(
     this.targetValidator.validate(target, groups.toArray(new Class<?>[groups.size()])), errors);
  }
 }

此验证的结果是由在SpringValidatorAdapter内的protected void processConstraintViolations(Set<ConstraintViolation<Object>> violations, Errors errors)方法处理得到。它将错误从JSR-303验证器附加到给定的Spring的错误对象(觉得别扭请看下面方法上的英文注释)。

    /**
  * Process the given JSR-303 ConstraintViolations, adding corresponding errors to
  * the provided Spring {@link Errors} object.
  * @param violations the JSR-303 ConstraintViolation results
  * @param errors the Spring errors object to register to
  */
 protected void processConstraintViolations(Set<ConstraintViolation<Object>> violations, Errors errors) {
  for (ConstraintViolation<Object> violation : violations) {
   String field = determineField(violation);
   FieldError fieldError = errors.getFieldError(field);
   if (fieldError == null || !fieldError.isBindingFailure()) {
    try {
     ConstraintDescriptor<?> cd = violation.getConstraintDescriptor();
     String errorCode = determineErrorCode(cd);
     Object[] errorArgs = getArgumentsForConstraint(errors.getObjectName(), field, cd);
     if (errors instanceof BindingResult) {
      // Can do custom FieldError registration with invalid value from ConstraintViolation,
      // as necessary for Hibernate Validator compatibility (non-indexed set path in field)
      BindingResult bindingResult = (BindingResult) errors;
      String nestedField = bindingResult.getNestedPath() + field;
      if ("".equals(nestedField)) {
       String[] errorCodes = bindingResult.resolveMessageCodes(errorCode);
       bindingResult.addError(new ObjectError(
         errors.getObjectName(), errorCodes, errorArgs, violation.getMessage()));
      }
      else {
       Object rejectedValue = getRejectedValue(field, violation, bindingResult);
       String[] errorCodes = bindingResult.resolveMessageCodes(errorCode, field);
       bindingResult.addError(new FieldError(
         errors.getObjectName(), nestedField, rejectedValue, false,
         errorCodes, errorArgs, violation.getMessage()));
      }
     }
     else {
      // got no BindingResult - can only do standard rejectValue call
      // with automatic extraction of the current field value
      errors.rejectValue(field, errorCode, errorArgs, violation.getMessage());
     }
    }
    catch (NotReadablePropertyException ex) {
     throw new IllegalStateException("JSR-303 validated property '" + field +
       "' does not have a corresponding accessor for Spring data binding - " +
       "check your DataBinder's configuration (bean property versus direct field access)", ex);
    }
   }
  }
 }

验证错误直接附加到DataBinder的private AbstractPropertyBindingResult bindingResult字段。

public class DataBinder implements PropertyEditorRegistry, TypeConverter {
 /** Default object name used for binding: "target" */
 public static final String DEFAULT_OBJECT_NAME = "target";
 /** Default limit for array and collection growing: 256 */
 public static final int DEFAULT_AUTO_GROW_COLLECTION_LIMIT = 256;
 /**
  * We'll create a lot of DataBinder instances: Let's use a static logger.
  */
 protected static final Log logger = LogFactory.getLog(DataBinder.class);
 @Nullable
 private final Object target;
 private final String objectName;
 @Nullable
 private AbstractPropertyBindingResult bindingResult;
 @Nullable
 private SimpleTypeConverter typeConverter;

此时它的值会在ModelAttributeMethodProcessor中检索:

if (binder.getBindingResult().hasErrors()) {
    if (isBindExceptionRequired(binder, parameter)) {
        throw new BindException(binder.getBindingResult());
    }
}

controller方法内获取BindingResult

需要注意的是，要在控制器的方法中检索BindingResult，必须将BindingResult实例直接放在经过验证的对象之后。具体请看public String addArticle(@ModelAttribute(“article”) @Valid Article article, BindingResult result)，BindingResult的实例将包含所有的验证错误。这时，如果你在Article和BindingResult实例之间放置另一个对象(例如:HttpServletRequest request)，将抛出如下异常:

An Errors/BindingResult argument is expected to be declared immediately after the  model attribute, 
the @RequestBody or the @RequestPart arguments to which they apply

此错误消息的内容可以在org.springframework.web.method.annotation.ErrorsMethodArgumentResolver类中找到。此类用于从方法签名中解析错误实例。如果问为什么用ErrorsMethodArgumentResolver来解析BindingResults？简单来说，这是由于BindingResult接口扩展了Errors接口的缘故。所以，两者都可以用相同的参数解析器解决。

/**
 * Resolves {@link Errors} method arguments.
 *
 * <p>An {@code Errors} method argument is expected to appear immediately after
 * the model attribute in the method signature. It is resolved by expecting the
 * last two attributes added to the model to be the model attribute and its
 * {@link BindingResult}.
 *
 * @author Rossen Stoyanchev
 * @since 3.1
 */
public class ErrorsMethodArgumentResolver implements HandlerMethodArgumentResolver {
 @Override
 public boolean supportsParameter(MethodParameter parameter) {
  Class<?> paramType = parameter.getParameterType();
  return Errors.class.isAssignableFrom(paramType);
 }
 @Override
 public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
   NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
  Assert.state(mavContainer != null, "Errors/BindingResult argument only supported on regular handler methods");
  ModelMap model = mavContainer.getModel();
  if (model.size() > 0) {
   int lastIndex = model.size()-1;
   String lastKey = new ArrayList<>(model.keySet()).get(lastIndex);
   if (lastKey.startsWith(BindingResult.MODEL_KEY_PREFIX)) {
    return model.get(lastKey);
   }
  }
  throw new IllegalStateException(
    "An Errors/BindingResult argument is expected to be declared immediately after the model attribute, " +
    "the @RequestBody or the @RequestPart arguments to which they apply: " + parameter.getMethod());
 }
}

从上面代码可以看出，由于BindingResult的放置的位置不正确，而导致验证过程失败的方法其实很简单:

ModelMap model = mavContainer.getModel();
if (model.size() > 0) {
    int lastIndex = model.size()-1;
    String lastKey = new ArrayList<String>(model.keySet()).get(lastIndex);
    if (lastKey.startsWith(BindingResult.MODEL_KEY_PREFIX)) {
        return model.get(lastKey);
    }
}

可以看到，它获得用于构建视图部分的模型数据的ModelMap。所要验证对象和BindingResult如果放置正确，那么所要打印的日志应该如下:

model equals to {article=Article {text = }, org.springframework.validation.BindingResult.article=org.springframework.validation.BeanPropertyBindingResult: 1 errors
Field error in object 'article' on field 'text': rejected value []; codes [NotEmpty.article.text,NotEmpty.text,NotEmpty.java.lang.String,NotEmpty]; arguments [org.springframework.context.support.DefaultMessageSourceResolvable: codes [article.text,text]; arguments []; default message [text]]; default message [Text can't be empty]}

之后，将值放在ArrayList中，并获取最后一个 entry key。然后，检查此键是否以org.springframework.validation.BindingResult开头(BindingResult 接口的常量值)。如果是，该方法返回发现的Errors实例。否则，将抛出一个IllegalStateException异常。

public interface BindingResult extends Errors {
 /**
  * Prefix for the name of the BindingResult instance in a model,
  * followed by the object name.
  */
 String MODEL_KEY_PREFIX = BindingResult.class.getName() + ".";
 /**
  * Return the wrapped target object, which may be a bean, an object with
  * public fields, a Map - depending on the concrete binding strategy.
  */
 @Nullable
 Object getTarget();

这篇文章讲了Spring 验证的一些过程细节。它的第一部分介绍了验证流程，从@ModelAttribute开始，并以验证器集合结束。第二部分看了看基本的Spring验证器。在最后，我们看到一个非常常见的bug，基于直接在验证对象之后放置BindingResult实例，并解释了其中的原理所在。

转载于:https://my.oschina.net/oosc/blog/1647766

Spring5 @Valid

使用@Valid注解在Spring中进行验证流程

如何在Spring中处理验证？

controller方法内获取BindingResult

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