如何检查当前方法的参数是否有注释并在 Java 中检索该参数值? [英] How to check if a parameter of the current method has an annotation and retrieve that parameter value in Java?

问题描述

考虑这个代码:

public example(String s, int i, @Foo Bar bar) {
  /* ... */
}

我想检查该方法是否有注释 @Foo 并获取参数或在找不到 @Foo 注释时抛出异常.

I want to check if the method has an annotation @Foo and get the argument or throw an exception if no @Foo annotation is found.

我目前的做法是先获取当前方法，然后遍历参数注解:

My current approach is to first get the current method and then iterate through the parameter annotations:

import java.lang.annotation.Annotation;
import java.lang.reflect.Method;

class Util {

    private Method getCurrentMethod() {
        try {
            final StackTraceElement[] stes = Thread.currentThread().getStackTrace();
            final StackTraceElement ste = stes[stes.length - 1];
            final String methodName = ste.getMethodName();
            final String className = ste.getClassName();   
            final Class<?> currentClass = Class.forName(className);
            return currentClass.getDeclaredMethod(methodName);
        } catch (Exception cause) {
            throw new UnsupportedOperationException(cause);
        }  
    }

    private Object getArgumentFromMethodWithAnnotation(Method method, Class<?> annotation) {
        final Annotation[][] paramAnnotations = method.getParameterAnnotations();    
            for (Annotation[] annotations : paramAnnotations) {
                for (Annotation an : annotations) {
                    /* ... */
                }
            }
    }

}

这是正确的方法还是有更好的方法?forach 循环中的代码是什么样的?我不确定我是否理解 getParameterAnnotations 实际返回的内容...

Is this the right approach or is there a better one? How would the code inside the forach loop look like? I'm not sure if I have understood the what getParameterAnnotations actually returns...

推荐答案

外部 for 循环

for (Annotation[] annotations : paramAnnotations) {
   ...
}

应该使用显式计数器，否则你不知道你现在正在处理什么参数

should use an explicit counter, otherwise you don't know what parameter you are processing right now

final Annotation[][] paramAnnotations = method.getParameterAnnotations();
final Class[] paramTypes = method.getParameterTypes();
for (int i = 0; i < paramAnnotations.length; i++) {
    for (Annotation a: paramAnnotations[i]) {
        if (a instanceof Foo) {
            System.out.println(String.format("parameter %d with type %s is annotated with @Foo", i, paramTypes[i]);
        }
    }
}

还要确保您的注释类型使用 @Retention(RetentionPolicy.RUNTIME)

Also make sure your annotation type is annotated with @Retention(RetentionPolicy.RUNTIME)

从您的问题来看，并不完全清楚您要做什么.我们同意形式参数与实际参数的区别:

From your question it is not entirely clear what you are trying to do. We agree on the difference of formal parameters vs. actual arguments:

void foo(int x) { }

{ foo(3); }

其中 x 是参数，3 是参数?

where x is a parameter and 3 is an argument?

无法通过反射获取方法的参数.如果可能的话，您将不得不使用 sun.unsafe 包.不过我不能告诉你太多.

It is not possible to get the arguments of methods via reflection. If it is possible at all, you would have to use the sun.unsafe package. I can't tell you much about that though.

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