方法入口常出现重复的校验逻辑：

result.setSuccess(false);
if (StringUtils.isEmpty(order.getExternalId())) {
    result.setError("外部订单号不能为空");
    return result;
}

if(order.getExternalId().length() > 100){
    result.setError("外部订单号长度需在100以内");
    return result;
}

if (StringUtils.isEmpty(order.getProductCode())) {
    result.setError("商品编码不能为空");
    return result;
}

现有方案如JSR 303或Spring MVC Validator虽成熟但较重。本文实现一个轻量级校验框架，核心设计如下：

设计目标

1. 轻量化：无第三方依赖，代码精简
2. 链式调用：支持 new Checker().require(name).maxLength(address) 语法
3. 注解驱动：通过注解声明校验规则
4. 可扩展：支持自定义校验器

注解校验模块

定义校验注解：

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Mandatory {
    String fieldLabel();
}

实现注解处理器：

public class MandatoryHandler implements AnnotationHandler {
    @Override
    public CheckResult process(Field field, Object value) {
        CheckResult result = new CheckResult();
        if(field.isAnnotationPresent(Mandatory.class)){
            Mandatory rule = field.getAnnotation(Mandatory.class);
            if(value == null || value.toString().isEmpty()){
                result.setError(rule.fieldLabel() + "为必填项");
            }
        }
        return result;
    }
}

注解校验执行器：

public class AnnotationChecker {
    private static final List<AnnotationHandler> handlers = new ArrayList<>();
    static {
        handlers.add(new MandatoryHandler());
        handlers.add(new LengthLimitHandler());
    }

    public static <T> CheckResult verify(T obj) {
        for (Field field : obj.getClass().getDeclaredFields()) {
            field.setAccessible(true);
            Object value = field.get(obj);
            for (AnnotationHandler handler : handlers) {
                CheckResult result = handler.process(field, value);
                if(!result.valid()) return result;
            }
        }
        return new CheckResult(true);
    }
}

使用示例：

class User {
    @Mandatory(fieldLabel="用户名")
    private String username;
}

public class Demo {
    public static void main(String[] args) {
        User user = new User();
        CheckResult result = AnnotationChecker.verify(user);
        if(!result.valid()) {
            System.out.println(result.getError());
        }
    }
}

链式校验模块

校验规则接口：

public interface ValidationRule {
    String getError();
    boolean isValid();
}

链式校验器：

public class Checker {
    private boolean valid = true;
    private String error;

    public Checker apply(ValidationRule rule) {
        if(valid) {
            valid = rule.isValid();
            error = rule.getError();
        }
        return this;
    }

    public Checker require(Object value, String field) {
        if(valid && (value == null || value.toString().isEmpty())) {
            valid = false;
            error = field + "不能为空";
        }
        return this;
    }
}

使用示例：

Checker checker = new Checker()
    .require(order.getId(), "订单ID")
    .require(order.getAddress(), "收货地址");
if(!checker.isValid()) {
    System.out.println(checker.getError());
}

扩展机制

自定义日期校验器：

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface DatePattern {
    String format();
    String fieldName();
}

public class DateValidator implements AnnotationHandler {
    @Override
    public CheckResult process(Field field, Object value) {
        if(field.isAnnotationPresent(DatePattern.class)){
            DatePattern rule = field.getAnnotation(DatePattern.class);
            try {
                new SimpleDateFormat(rule.format()).parse(value.toString());
            } catch (Exception e) {
                return new CheckResult(rule.fieldName() + "格式错误");
            }
        }
        return new CheckResult(true);
    }
}

// 注册使用
AnnotationChecker.register(new DateValidator());