build.gradle

version '1.0-SNAPSHOT'

apply plugin: 'java'
apply plugin: 'application'

mainClassName = 'sandbox.SandboxMain'

sourceCompatibility = 1.8

repositories {
    mavenCentral()
}

dependencies {
    compile "javax.validation:validation-api:1.1.0.Final"
    compile "org.glassfish.web:el-impl:2.2"
    compile "org.hibernate:hibernate-validator:5.4.1.Final"

    compileOnly 'org.projectlombok:lombok:1.16.10'
    testCompile group: 'junit', name: 'junit', version: '4.11'
}

NotNonNull.java

package sandbox;

import java.util.Set;

import javax.validation.ConstraintViolation;
import javax.validation.Validation;
import javax.validation.Validator;
import javax.validation.ValidatorFactory;
import javax.validation.constraints.NotNull;

import lombok.NonNull;

public class SandboxMain {
    public static void main(String[] args) throws Exception {
        String foo = null;

        try {
            nonNull(foo);　 // => Null Pointer Exception
        } catch (NullPointerException e) {
            System.out.println("@NonNull: " + e.getMessage());
        }
        notNullWrong(foo);
        notNull();
    }

    private static void nonNull(@NonNull String foo) {
        System.out.println(foo);
    }

    private static void notNullWrong(@NotNull String foo) {
      // 何も起こらない
      System.out.println("Wrong @NotNull: " + foo);
    }

    private static void notNull() {
        Foo foo = new Foo(null);
        validate(foo);
    }

    private static <T> void validate(T obj) {
        ValidatorFactory validatorFactory = Validation.buildDefaultValidatorFactory();
        Validator validator = validatorFactory.getValidator();
        Set<ConstraintViolation<T>> violations = validator.validate(obj);
        for (ConstraintViolation<T> violation : violations) {
            System.out.println(violation.getMessage());
        }
    }

    static class Foo {
        @NotNull
        private String foo = null;

        Foo(String foo) {
            this.foo = foo;
        }

        private Foo() { }

        public void foo() {
            System.out.println(foo);
        }
    }
}
/* => outputs
@NonNull: foo
Wrong @NotNull: null
[DATE] org.hibernate.validator.internal.util.Version <clinit>
INFO: HV000001: Hibernate Validator 5.4.1.Final
may not be null
*/