Java中@Constraint实现自定义校验注解

import javax.validation.Constraint;
import javax.validation.Payload;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Constraint(validatedBy = ValidLengthValidator.class)
@Target({ ElementType.METHOD, ElementType.FIELD, ElementType.ANNOTATION_TYPE, ElementType.PARAMETER })
@Retention(RetentionPolicy.RUNTIME)
public @interface ValidLength {
    String message() default "长度必须在1到20个字符之间";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
    int min() default 1;
    int max() default 20;
}

import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;

public class ValidLengthValidator implements ConstraintValidator<ValidLength, String> {

    private int min;
    private int max;

    @Override
    public void initialize(ValidLength constraintAnnotation) {
        this.min = constraintAnnotation.min();
        this.max = constraintAnnotation.max();
    }

    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        if (value == null) {
            return true; // 允许为空
        }
        return value.length() >= min && value.length() <= max;
    }
}

import javax.validation.constraints.NotNull;

public class User {

    @NotNull
    @ValidLength(min = 1, max = 20)
    private String username;

    // Getters and Setters
    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }
}

import javax.validation.Validation;
import javax.validation.Validator;
import javax.validation.ValidatorFactory;
import javax.validation.ConstraintViolation;
import java.util.Set;

public class Main {
    public static void main(String[] args) {
        User user = new User();
        user.setUsername("12345678901234567890"); // 20位，合法

        ValidatorFactory factory = Validation.buildDefaultValidatorFactory();
        Validator validator = factory.getValidator();

        Set<ConstraintViolation<User>> violations = validator.validate(user);
        for (ConstraintViolation<User> violation : violations) {
            System.out.println(violation.getMessage());
        }

        if (violations.isEmpty()) {
            System.out.println("校验通过");
        } else {
            System.out.println("校验失败");
        }
    }
}