利用可重用代码,第 2 部分:捕捉惯用模式
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清单 5. MaxLength 属性声明
@Retention(RetentionPolicy.RUNTIME)
public @interface MaxLength {
int length() default 0;
}
MaxLength 验证程序的实际功能存在于两个类中:名为 Validator 的一个抽象类及其具体实现 MaxLengthValidator。Validator 类出现在清单 6 中:
清单 6. 提取基于属性的 Validator 类
public abstract class Validator {
public void validate(Object obj) throws ValidationException {
Class clss = obj.getClass();
for(Method method : clss.getMethods())
if (method.isAnnotationPresent(getAnnotationType()))
validateMethod(obj, method, method.getAnnotation(getAnnotationType()));
}
protected abstract Class getAnnotationType();
protected abstract void validateMethod(
Object obj, Method method, Annotation annotation);
}
该类通过查看 getAnnotationType() 来迭代类中的方法,以确定这些方法是否修饰有特定属性;当它找到一个方法时,就执行 validateMethod() 方法。MaxLengthValidator 类的实现见清单 7:
清单 7. MaxLengthValidator 类
public class MaxLengthValidator extends Validator {
protected void validateMethod(Object obj, Method method, Annotation annotation) {
try {
if (method.getName().startsWith("get")) {
MaxLength length = (MaxLength)annotation;
String value = (String)method.invoke(obj, new Object[0]);
if ((value != null) && (length.length() < value.length())) {
String string = method.getName() + " is too long." +
"Its length is " + value.length() +
" but should be no longer than " + length.length();
throw new ValidationException(string);
}
}
} catch (Exception e) {
throw new ValidationException(e.getMessage());
}
}
@Override
protected Class getAnnotationType() {
return MaxLength.class;
}
}
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