Dynamic Function Creation in Java

问题

So I'm trying to figure out if there is some method to dynamically create/assign a method to a class in Java. If it were C, I would just do it as follows using 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 any way to get a similar functionality out of a Java application?

回答1:

In Java, you would normally declare an interface with a method to be called. For example, if your function simply wants to execute some code, you would declare a Runnable and implement its run 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();
         }});
    }

}

回答2:

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

回答3:

In java it is achieved by something 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();
    }
}

回答4:

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

回答5:

If you really want to change classes at runtime, the only way is to actually modify the bytecode, assuming your set-up allows it (Java security would normally kick in). That said, there's an java.lang.instrument package in Java 6 which may help:

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

You might find the cglib project of use also:

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