tostring()省略String属性中的双引号

Question

我怎么能打印一个注释的双引号字段的值，当它是一个字符串?例如:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@interface MyAnnotation {
    String myField();
}
@MyAnnotation(myField = "bla")
public class Test {
    public static void main(String[] args) {
        for (Annotation annotation : Test.class.getAnnotations()) {
            System.out.println(annotation);
        }
    }
}

上面的代码产生:

@MyAnnotation(myField=bla)

如何将其更改为生成@MyAnnotation(myField="bla")

Answer 1

反射是唯一的方法:

for (Annotation annotation : Test.class.getAnnotations()) {
    System.out.println(toString(annotation));
}
private String toString(Annotation annotation) throws InvocationTargetException, IllegalAccessException {
    Map<String, Object> paramValues = getParamValues(annotation);
    StringBuilder result = new StringBuilder("@")
            .append(annotation.annotationType().getSimpleName())
            .append("(");
    for (Map.Entry<String, Object> param : paramValues.entrySet()) {
        result.append(param.getKey()).append("="").append(param.getValue()).append("", ");
    }
    result.delete(result.length() - 2, result.length());
    return result.append(")").toString();
}
private Map<String, Object> getParamValues(Annotation annotation) throws InvocationTargetException, IllegalAccessException {
        Map<String, Object> params = new HashMap<>();
        for (Method param : annotation.annotationType().getMethods()) {
            if (param.getDeclaringClass() == annotation.annotationType()) { //this filters out built-in methods, like hashCode etc
                params.put(param.getName(), param.invoke(annotation));
            }
        }
        return params;
}

这将准确地打印你所要求的，对于任何类的任何注释(你只需要看看你想如何打印非字符串属性)。

但是…你为什么需要这个?

Answer 2

Annotation接口不提供对特定注释的任何见解。但是我认为你可以这样做:

public static void main(String[] args) {
  for (Annotation annotation : Test.class.getAnnotations()) {
    if (annotation instanceof MyAnnotation) {
       ... then you can cast to that specific type ...
       ... and then access/print 
    }
}

通常情况下，这种带有向下转换的instanceof检查应该避免，但是如果不这样做，我看不出有什么办法可以解决你的问题。