ValidationLib

A lightweight, fluent, and extensible Java validation library. Validate any value with built-in or custom rules, collect all error messages at once, and localize messages out of the box, with zero external dependencies.

ValidationResult result = Validator.of("user@example.com")
        .rule(new NotBlankRule())
        .rule(new EmailRule())
        .validate();

if (!result.isValid()) {
    result.getErrors().forEach(System.out::println);
}

Features

• Fluent API — chain any number of rules on a single value
• Built-in rules — EmailRule, NotNullRule, NotBlankRule, MinLengthRule, MaxLengthRule, RangeRule, PatternRule
• Lambda rules — define ad-hoc rules inline with a predicate and a message
• All errors at once — validation collects every failed rule, not just the first
• i18n support — localized default messages (English and Persian included, easily extendable)
• Immutable results — ValidationResult is defensive and thread-safe
• Pure Java 21 — no external runtime dependencies
• Fully documented — JavaDoc on every public class and method, covered by a JUnit 5 test suite

Installation

The library is published via JitPack.

Gradle

repositories {
    maven { url 'https://jitpack.io' }
}

dependencies {
    implementation 'com.github.30nap:ValidationLib:v1.0.0'
}

Maven

<repositories>
    <repository>
        <id>jitpack.io</id>
        <url>https://jitpack.io</url>
    </repository>
</repositories>

<dependency>
    <groupId>com.github.30nap</groupId>
    <artifactId>ValidationLib</artifactId>
    <version>v1.0.0</version>
</dependency>

Usage

Chaining rules

ValidationResult result = Validator.of(password)
        .rule(new NotBlankRule())
        .rule(new MinLengthRule(8))
        .rule(new MaxLengthRule(64))
        .validate();

Inline rules with lambdas

ValidationResult result = Validator.of(password)
        .rule(new MinLengthRule(8))
        .rule(p -> p.chars().anyMatch(Character::isDigit), "Password must contain a digit")
        .validate();

Numeric ranges

RangeRule works with any Comparable type:

Validator.of(age)
        .rule(new RangeRule<>(18, 120))
        .validate();

Reading the result

ValidationResult result = Validator.of(email).rule(new EmailRule()).validate();

result.isValid();     // true / false
result.getErrors();   // unmodifiable list of all error messages
result.firstError();  // Optional<String> with the first error

Built-in rules

Rule	Applies to	Checks
EmailRule	String	valid email address syntax
NotNullRule<T>	any	value is not null
NotBlankRule	String	not null, empty, or whitespace-only
MinLengthRule	String	at least n characters
MaxLengthRule	String	at most n characters (null passes)
RangeRule<T>	Comparable<T>	value within [min, max] inclusive
PatternRule	String	full match against a regular expression

Every rule has two constructors: one that takes a custom error message, and a default one that uses a localized message from the resource bundle.

Custom rules

Implement the Rule<T> interface — records work great:

public record PositiveRule(String message) implements Rule<Integer> {
    @Override
    public boolean isValid(Integer value) {
        return value != null && value > 0;
    }
}

Or create one from a predicate without a new class:

Rule<String> noSpaces = Rule.of(s -> s != null && !s.contains(" "), "Must not contain spaces");

Localization (i18n)

Default messages are resolved from the i18n/messages resource bundle. English and Persian are included:

Resources.get("invalid.email");            // JVM default locale
Resources.get("invalid.email", "fa");      // ایمیل معتبر نیست
Resources.format("invalid.minLength", 8);  // "Value must be at least 8 characters long"

To add a language, place a messages_<lang>.properties file under i18n/ on the classpath.

Building from source

git clone https://github.com/30nap/ValidationLib.git
cd ValidationLib
./gradlew build   # compiles, runs tests, and builds the jar

Requires JDK 21 (a Gradle toolchain will download one automatically if missing).

License

MIT License © 2026 Sina Pezeshki