Using Java reflection to create eval() method

Question

I have a question about reflection I am trying to have some kind of eval() method. So i can call for example:

eval("test('woohoo')");

Now I understand the there is no eval method in java but there is reflection. I made the following code:

String s = "test";
Class cl = Class.forName("Main");
Method method = cl.getMethod(s, String.class);
method.invoke(null, "woohoo");

This works perfectly (of course there is a try, catch block around this code). It runs the test method. However I want to call multiple methods who all have different parameters.

I don't know what parameters these are (so not only String.class). But how is this possible? how can I get the parameter types of a method ? I know of the following method:

Class[] parameterTypes = method.getParameterTypes();

But that will return the parameterTypes of the method I just selected! with the following statement:

Method method = cl.getMethod(s, String.class);

Any help would be appreciated !

The eval method looks like Ruby's eval, so maybe you could look at how JRuby does it. — JRL
– JRL, Commented Apr 12, 2010 at 10:24

Koedlt · Accepted Answer · 2023-04-22 07:04:58Z

15

You will need to call Class.getMethods() and iterate through them looking for the correct function.

For (Method method : clazz.getMethods()) {
  if (method.getName().equals("...")) {
    ...
  }
}

The reason for this is that there can be multiple methods with the same name and different parameter types (ie the method name is overloaded).

getMethods() returns all the public methods in the class, including those from superclasses. An alternative is Class.getDeclaredMethods(), which returns all methods in that class only.

edited Apr 22, 2023 at 7:04

Koedlt

6,11110 gold badges25 silver badges47 bronze badges

answered Apr 12, 2010 at 10:25

cletus

627k169 gold badges922 silver badges945 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

heldopslippers Over a year ago

You guys are soo fast!..:P thanxs for the help!..the following code works perfectly!: Class cl = Class.forName("Main"); for (Method method : cl.getMethods()){ if(method.getName().equals("test")){ Class[] parameterTypes = method.getParameterTypes(); method.invoke(null, "this is", " so cool!"); } }

heldopslippers Over a year ago

(ehm could anybody help with the style ? ) (sorry)

heldopslippers Over a year ago

Well i posted some code but the style isn't really good.. But never mind. I will post better code when it is finished (for people who might have the same problem).

Danny Over a year ago

I dont think people can edit comments. But have a look at the following link for future reference (or just click the help link next to the comment). meta.stackexchange.com/a/24794/167981

Bozho · Accepted Answer · 2010-04-12 10:24:47Z

5

You can loop over all methods of a class using:

cls.getMethods(); // gets all public methods (from the whole class hierarchy)

or

cls.getDeclaredMethods(); // get all methods declared by this class

.

for (Method method : cls.getMethods()) {
    // make your checks and calls here
}

answered Apr 12, 2010 at 10:24

Bozho

599k147 gold badges1.1k silver badges1.2k bronze badges

Comments

Itay Maman · Accepted Answer · 2010-04-12 10:25:27Z

2

You can use getMethods() which returns an array of all methods of a class. Inside the loop you can inspect the parameters of each method.

for(Method m : cl.getMethods()) {
   Class<?>[] params = m.getParameterTypes();
   ...
}

Otherwise you can use getDelcaredMethods() which will allow you to "see" private methods (but not inherited methods). Note that if you want to invoke a private methods you must first apply setAccessible(boolean flag) on it:

for(Method m : cl.getDelcaredMethods()) {
   m.setAccessible(true);
   Class<?>[] params = m.getParameterTypes();
   ...
}

answered Apr 12, 2010 at 10:25

Itay Maman

30.8k12 gold badges93 silver badges124 bronze badges

Comments

heldopslippers · Accepted Answer · 2010-04-12 11:09:20Z

2

Ok thanxs to all the people who answered my question here the final solution:

import java.lang.reflect.Method;
public class Main {
   public static void main(String[] args){
      String func = "test";
      Object arguments[] = {"this is ", "really cool"};
      try{
         Class cl = Class.forName("Main");
         for (Method method : cl.getMethods()){
            if(method.getName().equals(func)){
               method.invoke(null, arguments);
            }
          }
       } catch (Exception ioe){
          System.out.println(ioe);
       }
    }
  public static void test(String s, String b){
     System.out.println(s+b);
  }
}

answered Apr 12, 2010 at 11:09

heldopslippers

8402 gold badges9 silver badges21 bronze badges

2 Comments

Stephen C Over a year ago

A problem with this code is that it is not robust. Various changes to the signature of test will result in exceptions; e.g. if the method is not static, if the number of args is different, it the types of the args are different, if the method is not visible.

heldopslippers Over a year ago

yes that is right.. but there is no other way to do this. I need to be able to call functions from a string (this is because I need to be able to setup a socket connection to a java program and then send json that will have the appropriate method name and arguments.) If i take care of my exception handling and have good unittests I think it will be robust enough (or i hope so :P ).

Collectives™ on Stack Overflow

Using Java reflection to create eval() method

4 Answers 4

4 Comments

Comments

Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

4 Comments

Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related