Question

On my current project, I’ve felt the need to create a sort of simulated callback system in Java using reflection. However, I’m having issues getting my reflection to actually function. The code at fault follows:

public Callback(Object parentObj, String methodName, Class<?>...parameters)
{
    if(parentObj == null)
        throw new IllegalArgumentException("parentObj cannot be null", new NullPointerException());

    Class<?> clazz = parentObj.getClass();

    // Trace debugging, see output
    for(Method m : clazz.getDeclaredMethods())
        if(m.getName().equals("myMethod")) System.out.println (m);

    try { this.method = clazz.getMethod(methodName, parameters); }
    catch(NoSuchMethodException nsme) { nsme.printStackTrace(); } // Exception caught
    catch(SecurityException se) { se.printStackTrace(); }

    this.parentObj = parentObj;
    this.parameters = parameters;
}

When I construct the Callback object, I’m using syntax like this:

new Callback(this, "myMethod", boolean.class)

When I try to create my pseudo-callback, it hits the NoSuchMethodException catch block. I’ve included some trace debugging above to show the output of one of my methods failing. The output:

private void my.package.MyClass.myMethod(boolean)
java.lang.NoSuchMethodException: my.package.MyClass.myMethod(boolean)
    at java.lang.Class.getMethod(Class.java:1605)
    at my.package.other.Callback.<init>(Callback.java:63)

I couldn’t figure the problem out, so I started hunting, to little avail. The best I could find was mention of versioning conflict between the compiled JAR and the runtime. However, MyJar.jar/META-INF/MANIFEST.MF contains Created-By: 1.6.0_02 (Sun Microsystems Inc.). My IDE is running C:\Program Files\Java\jdk1.6.0_02\bin\javac.exe to compile my project. I’m using C:\Program Files\Java\jdk1.6.0_02\bin\java.exe to run my JAR.

I’m at a loss why Class.getMethod is claiming the method doesn’t exist, but Class.getMethods seems to have no problem finding it. Help? 🙁

Answer 1

Your method is private but getMethod() only returns public method.

You need to use getDeclaredMethod().