Creating a dynamic function in Java

Question

So, I'm trying to figure out if there is any method to dynamically create / assign a method to a class in Java. If it were C, I would just do it this way with pointers:

public class Foo { void bar(void *ptr) {....} }; int main() { Foo f = new Foo(); f.bar({"my function" ...}) } 

However, Java, of course, has no pointers, so is there a way to get similar functionality from a Java application?

Answer 1

In Java, you usually declare an interface with a called method. For example, if your function just wants to execute some code, you should declare Runnable and implement your execution method.

 public class Foo { void bar(Runnable function) { for(int i = 0; i < 5; i++) { function.run(); } } static void myFunction() { System.out.println("my Function!"); } public static void main(String[] ignored) { Foo f = new Foo(); f.bar( new Runnable() { public void run() { myFunction(); }}); } } 

Answer 2

To generate truly dynamic methods, you will need a bytecode manipulation library such as Javassist or cglib .

Answer 3

In java this is achieved by what are called anonymous classes, here is an example -

 abstract class Bar { public void myfunc(); } public class Client { public void execute() { doSomething(new Bar() { // define your dynamic function here ie provide its implementation public void myfunc() { //do whatever } }); } public void doSomething(Bar b) { b.myfunc(); } } 

Answer 4

You can use the Java Scripting API, create a function as a Script, and call it. But do this only if your functions are really fully defined at runtime, because interpreting scripts is always slower than implementing in native Java.

Answer 5

If you really want to change the classes at runtime, the only way is to actually change the bytecode, assuming your setup allows it (Java security usually works). However, in Java 6 there is a java.lang.instrument that can help:

http://download.oracle.com/javase/6/docs/api/java/lang/instrument/package-summary.html

You can also use the cglib project:

http://sourceforge.net/projects/cglib/

Answer 6

See http://functionaljava.org/ for the entire functional library for Java.

Answer 7

Here's a link to how you can use the built-in version of javac to compile classes that you define on the fly.