Java reflection error: NoSuchMethodException

Question

I just read about reflection and decided to give it a try, but I seem to run into an error I can not find the cause for.

I got the following code in a class:

String hashType = "md5";
Method method = DigestUtils.class.getDeclaredMethod(hashType + "Hex");
String hash = (String) method.invoke("hello");

This piece of code that should store the hashed string into the variable hash, throws the following error at runtime:

java.lang.NoSuchMethodException: org.apache.commons.codec.digest.DigestUtils.md5Hex()
        at java.lang.Class.getDeclaredMethod(Unknown Source)
        at stringHasher.stringHasher.hashString(stringHasher.java:37)

According to the API documentation the method does exist: https://commons.apache.org/proper/commons-codec/apidocs/org/apache/commons/codec/digest/DigestUtils.html

Beside not understanding what is the cause of this error I also don't understand why I need to cast the returned value by the method to a String as the API states it returns a string (should type safety not be in the hands of the programmer in this case instead of enforced by eclipse?).

There are 3 different methods called md5Hex but all of them have a parameter. You are not declaring any parameter in the getDeclaredMethod call so it looks for a method without parameters md5Hex() that doesn't exist. But may I ask why you are using reflection in this case at all? If you want type safety directly calling the method and not using reflection seems a better solution. — user6073886
– user6073886, Commented Aug 13, 2018 at 14:51
As @OHGODSPIDERS said, there are 3 methods called md5Hex. So try this one: DigestUtils.class.getDeclaredMethod(hashType + "Hex", String.class); — Bilal EL CHAMI
– Bilal EL CHAMI, Commented Aug 13, 2018 at 14:53
I think this is a duplicate of stackoverflow.com/questions/15465407/… — foundationer
– foundationer, Commented Aug 13, 2018 at 14:53
I am using it because I would like to try it out and see if I can get it to work. I might use it to reduce code at a later time. How would declare parameters in the getDeclaredMethod call? — Piet Hein
– Piet Hein, Commented Aug 13, 2018 at 14:55

Daria Pydorenko · Accepted Answer · 2018-08-13 15:26:53Z

3

You should add an argument type as a second argument in getDeclaredMethod. And you should pass something (better null) as a first argument to invoke a static method.

String hashType = "md5";
Method method = DigestUtils.class.getDeclaredMethod(hashType + "Hex", String.class);
String hash = (String) method.invoke(null, "hello");

And for a non-static methods you can do that:

DigestUtils instance = new DigestUtils();
String hash = (String) method.invoke(instance, "hello");

edited Aug 13, 2018 at 15:26

answered Aug 13, 2018 at 14:53

Daria Pydorenko

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1 Comment

GotoFinal Over a year ago

for static methods you should use null - but argument is just ignored so it is not a big issue and you can use anything. But using any arguments looks like maybe it is target of this method.

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