The default field error message in struts 2 is Invalid field value for field "[fieldName]". This can be change by creating a property file with a file name pattern [ClassName].properties under the package of the Action class. Then add the example line below (Example for a birthDate field):

    invalid.fieldvalue.birthDate = Birthdate is invalid

You can add another field error message in the file. Just code it line by line. Below is the project file heirarchy.