动态修改字段注释值 [英] Modify field annotation value dynamically

问题描述

是否可以在运行时更改字段注释值?

Is is possible to change field annotation values at runtime?

我可以访问这些值，但找不到更改它们的方法.

I can access the values, but can't find a way to change them.

可以通过以下方式访问:

Access is possible with:

Article.class.declaredFields.find {it.name="annotatedField"}.declaredAnnotations

推荐答案

我认为除了 Field(或 Object)之外，最好保留对 Annotation 对象的引用，并在更改 Annotation 引用时更新它值.这样，当 Class.java 中注解的实现发生变化时，您的代码不太可能被破坏.

I think it would be best to keep a reference to an Annotation object in addition to your Field (or Object), and update the Annotation reference as you change its values. This way, when the implementation of annotations in Class.java changes, your code is less likely to break.

问题评论中链接的答案对于处理包含单个元素的注释很有用，但如果您需要设置多个元素，这里有一个使用代理的更通用的解决方案:

The answer linked in the question comments is useful for dealing with annotations containing a single element, but if you have multiple elements that you need to set, here is a more general solution that makes use of a proxy:

import java.lang.annotation.Annotation;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class Test {
    public static void main(String[] args) throws Exception {
        Foo foo = new Foo();
        Field field = foo.getClass().getDeclaredFields()[0];

        Anno anno = field.getAnnotation(Anno.class);
        System.out.println(String.format("Old properties: %s, %s, %s", anno.value(), anno.bar(), anno.barr()));

        Anno anno2 = (Anno) setAttrValue(anno, Anno.class, "value", "new");
        System.out.println(String.format("New properties: %s, %s, %s", anno2.value(), anno2.bar(), anno2.barr()));

        Anno anno3 = (Anno) setAttrValue(anno2, Anno.class, "bar", "new bar");
        System.out.println(String.format("New properties: %s, %s, %s", anno3.value(), anno3.bar(), anno3.barr())); 
    }

    public static Annotation setAttrValue(Annotation anno, Class<? extends Annotation> type, String attrName, Object newValue) throws Exception {
        InvocationHandler handler = new AnnotationInvocationHandler(anno, attrName, newValue);
        Annotation proxy = (Annotation) Proxy.newProxyInstance(anno.getClass().getClassLoader(), new Class[]{type}, handler);
        return proxy;
    }
}

class AnnotationInvocationHandler implements InvocationHandler {
    private Annotation orig;
    private String attrName;
    private Object newValue;

    public AnnotationInvocationHandler(Annotation orig, String attrName, Object newValue) throws Exception {
        this.orig = orig;
        this.attrName = attrName;
        this.newValue = newValue;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        // "override" the return value for the property we want
        if (method.getName().equals(attrName) && args == null)
            return newValue;

        // keep other properties and methods we want like equals() and hashCode()
        else {
            Class<?>[] paramTypes = toClassArray(args);
            return orig.getClass().getMethod(method.getName(), paramTypes).invoke(orig, args);
        }
    }

    private static Class<?>[] toClassArray(Object[] arr) {
        if (arr == null)
            return null;
        Class<?>[] classArr = new Class[arr.length];
        for (int i=0; i<arr.length; i++)
            classArr[i] = arr[i].getClass();
        return classArr;
    }

}

class Foo {
    @Anno(value="old", bar="bar", barr="barr")
    public Object field1;
}

@Retention(RetentionPolicy.RUNTIME)
@interface Anno {
    String value();
    String bar();
    String barr();
}

程序输出:

Old properties: old, bar, barr
New properties: new, bar, barr
New properties: new, new bar, barr

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