Why @NotEmpty Misses Blank Strings and How @NotBlank Fixes It

Introduction

A bug was reported where a source field passed as an empty string could still be updated successfully, prompting a suggestion to add validation.

@NotEmpty(message = "source must not be empty")
private String source;

The @NotEmpty annotation appears to behave like StringUtils.isEmpty, but examining the source reveals it is used incorrectly.

public static boolean isEmpty(CharSequence cs) {
    return cs == null || cs.length() == 0;
}

When a blank string (length > 0) is passed, @NotEmpty cannot detect the problem.

1. Reproduce Demo

String source = "  ";
System.out.println(StringUtils.isEmpty(source));
System.out.println(source.length());

false
2

Because isEmpty checks that the value is not null and that its length is greater than 0; a blank string has length > 0, so the method returns false.

2. Correct Validation

To validate blank strings correctly, use StringUtils.isBlank, which checks for null, empty, or whitespace‑only strings.

String source = "  ";
System.out.println(StringUtils.isBlank(source));

true

The source code of isBlank is:

public static boolean isBlank(CharSequence cs) {
    int strLen = length(cs);
    if (strLen == 0) {
        return true;
    } else {
        for (int i = 0; i < strLen; ++i) {
            if (!Character.isWhitespace(cs.charAt(i))) {
                return false;
            }
        }
        return true;
    }
}

Note the use of Character.isWhitespace to detect whitespace characters.

3. Differences Among @NotNull, @NotEmpty, and @NotBlank

@NotNull : the value cannot be null, but empty strings or empty collections are allowed.

@NotEmpty : the value cannot be null; empty strings are allowed only if the length is greater than 0 (i.e., the string must contain at least one character).

@NotBlank : the value cannot be null and cannot be a blank string (null, empty, or whitespace‑only).