With bean validation in Jersey, you get JSON errors like this:

[
  {
    "message":"may not be null",
    "messageTemplate":"{javax.validation.constraints.NotNull.message}",
    "path":"Hello.register.arg0.email",
    "invalidValue":null
  }
]

That's great, but sometimes you have errors that appear after bean validation, such as when someone tries to register as an existing user.

You will only know about this when you talk to your data stores of existing users.

It would be nice to keep the above error format, but talk about more general errors. You do this by creating a custom ConstraintViolation and throwing a ConstraintViolationException.

The ConstraintViolationException takes a HashSet of ConstraintViolation. Let's create a dummy ConstraintViolation in Scala (the Java version is obviously just about the same):

new ConstraintViolation[Object] {
  def getConstraintDescriptor(): ConstraintDescriptor[_] = null
  def getExecutableParameters(): Array[Object] = null
  def getExecutableReturnValue(): Object = null
  def getInvalidValue(): Object = "The invalid valid"
  def getLeafBean(): Object = null
  def getMessage(): String = "Something went wrong"
  def getMessageTemplate(): String = ""
  def getPropertyPath(): javax.validation.Path = new MyPropertyPath(argPath)
  def getRootBean(): Object = obj
  def getRootBeanClass(): Class[Object] = classOf[Object]
  def unwrap[U](t: Class[U]): U = t.newInstance
}

Most of the values above are empty, except for getInvalidValue, getMessage, getPropertyPath, getRootBean and getRootBeanClass

getInvalidValue displays the invalid value in the JSON above. getMessage displays the messages. getPropertyPath helps display path above: it's just a dumy object with an dumy iterator and we use its toString method to fill out the rest of the "path"

The getRootBean gives us the name at the start of the "path" json reply. getRootBeanClass and unwrap need to be non-null but don't seem to affect the JSON output.

Let's put this altogether:

// Dumpy Path object: we only want its "toString"
class MyPropertyPath(var pathName: String) extends javax.validation.Path {
  def iterator():java.util.Iterator[javax.validation.Path.Node] = {
    return new java.util.Iterator[javax.validation.Path.Node] {
      def hasNext(): Boolean = false
      def next(): javax.validation.Path.Node = null
    }
  }
  override def toString:String = pathName
}

def customValidationErrorMessage(obj: Object, argPath: String, message: String, invalid: Object) = {
  var hs = new HashSet[ConstraintViolation[_]]
  hs.add(new ConstraintViolation[Object] {
    def getConstraintDescriptor(): ConstraintDescriptor[_] = null
    def getExecutableParameters(): Array[Object] = null
    def getExecutableReturnValue(): Object = null
    def getInvalidValue(): Object = invalid
    def getLeafBean(): Object = null
    def getMessage(): String = message
    def getMessageTemplate(): String = ""
    def getPropertyPath(): javax.validation.Path = new P(argPath)
    def getRootBean(): Object = obj
    def getRootBeanClass(): Class[Object] = classOf[Object]
    def unwrap[U](t: Class[U]): U = t.newInstance
  })
  throw new ConstraintViolationException("", hs)
}

If we call customValidationErrorMessage(register, "username", "Duplicate user", "dave") in a Jersey method, we'll get this returned:

[
  {
    "message":"Duplicate user",
    "messageTemplate":"",
    "path":"RegisterUser.username",
    "invalidValue":"dave"
  }
]

Voila!

java-jersey java-bean-validation scala java