Can a Java class add a method to itself at runtime?

user_187145 0
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Can a class add a method to itself at runtime (like from a static block), so that if someone is performing reflection on this class, they'll see the new method, even though it wasn't defined at compile time?

Background:

A framework I'm using expects Action classes to be defined that have a doAction(...) method, by convention. The framework inspects these classes at runtime to see what type of parameters are available in their doAction() method. For example: doAction(String a, Integer b)

I'd like each class to be able to programatically generate its doAction() method with various parameters, just-in-time when it is inspected. The body of the method can be empty.

javassist code-generation reflection java
11 Answers

What I suggest should work for your situation: 1. You have an existing class MyClass with n methods 2. You want to include (n+1) th method which is not in the class while compiling in another .java source file

My way to solve it is Inheritance. Create a new .java source file for a Class MyClassPlusOne extending the first class MyClass. Compile this class and use the object. How can I compile and deploy a java class at runtime?

class MyClassPlusOne extends MyClass
{
     void doAction(String a, Integer b)
     {
         int myNPlus1 = a+b;
         //add whatever you want before compiling this code
      }
}
user_97218020

This is a rather old question, but I still found myself looking at it today so, just in case, I'll add my two cents.

If you are using Java 8+, you can define "default" implementations of an interface method, so you can just define the interface with all the extra methods with empty default implementations, and add the implements clause in the desired classes. This approach, in some cases, may be the easiest one.

If you don't have control over the definition of the classes, or you need compatibility with older Java versions, you can still define an interface containing all the required extra methods; but in this case, implement a "Decorator" class with a method that receives the object to "decorate" as parameter, and returns a DynamicProxy instance, wrapping the passed object with this interface.

If you are using Spring, the decorator can be added to the context as a @Component, so you can inject it wherever you need to use it. If any of the objects you need to inject are Spring Beans, you could implement a FactoryBean that uses the decorator to return the instances, so you can just forget about calling the decorator explicitly for them.

user_38125080
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