javax.validation.constraints如何校验参数合法性

import javax.validation.constraints.*;

public class User {

    @NotNull(message = "ID 不能为空")
    private Long id;

    @NotEmpty(message = "用户名不能为空")
    @Size(min = 2, max = 20, message = "用户名长度必须在 2 到 20 个字符之间")
    private String username;

    @Min(value = 18, message = "年龄必须大于等于 18 岁")
    @Max(value = 100, message = "年龄必须小于等于 100 岁")
    private int age;

    @Email(message = "电子邮件格式不正确")
    private String email;

    // Getter and Setter ...
}

import javax.validation.Constraint;
import javax.validation.Payload;
import java.lang.annotation.*;

@Documented
@Constraint(validatedBy = PasswordValidator.class)
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface Password {

    String message() default "密码必须包含至少一个大写字母、一个小写字母和一个数字";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};
}

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

public class PasswordValidator implements ConstraintValidator<Password, String> {

    private static final String PASSWORD_PATTERN = "^(?=.*[a-z])(?=.*[A-Z])(?=.*\\d)[a-zA-Z\\d]{8,}$";

    @Override
    public void initialize(Password constraintAnnotation) {
    }

    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        if (value == null) {
            return false;
        }
        return value.matches(PASSWORD_PATTERN);
    }
}

public class User {

    // ... 其他字段

    @Password
    private String password;

    // Getter and Setter ...
}

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.MethodArgumentNotValidException;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.RestControllerAdvice;

@RestControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<String> handleValidationException(MethodArgumentNotValidException exception) {
        StringBuilder message = new StringBuilder();
        exception.getBindingResult().getFieldErrors().forEach(fieldError ->
                message.append(fieldError.getField()).append(": ").append(fieldError.getDefaultMessage()).append("; ")
        );
        return ResponseEntity.status(HttpStatus.BAD_REQUEST).body(message.toString());
    }
}