21.2 Validating Null and Empty Strings

The Java programming language distinguishes between null and empty strings. An empty string is a string instance of zero length, whereas a null string has no value at all.

An empty string is represented as "". It is a character sequence of zero characters. A null string is represented by null. It can be described as the absence of a string instance.

Managed bean elements represented as a JavaServer Faces text component such as inputText are initialized with the value of the empty string by the JavaServer Faces implementation. Validating these strings can be an issue when user input for such fields is not required. Consider the following example, in which the string testString is a bean variable that will be set using input entered by the user. In this case, the user input for the field is not required.

if (testString==null) {
    doSomething();
} else {
    doAnotherThing();
}

By default, the doAnotherThing method is called even when the user enters no data, because the testString element has been initialized with the value of an empty string.

In order for the Bean Validation model to work as intended, you must set the context parameter javax.faces.INTERPRET_EMPTY_STRING_SUBMITTED_VALUES_AS_NULL to true in the web deployment descriptor file, web.xml:

<context-param>
    <param-name>
        javax.faces.INTERPRET_EMPTY_STRING_SUBMITTED_VALUES_AS_NULL
    </param-name>
    <param-value>true</param-value>
</context-param>

This parameter enables the JavaServer Faces implementation to treat empty strings as null.

Suppose, on the other hand, that you have a @NotNull constraint on an element, meaning that input is required. In this case, an empty string will pass this validation constraint. However, if you set the context parameter javax.faces.INTERPRET_EMPTY_STRING_SUBMITTED_VALUES_AS_NULL to true, the value of the managed bean attribute is passed to the Bean Validation runtime as a null value, causing the @NotNull constraint to fail.