Java: NoSuchMethodException when method clearly exists

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().

Answer 2

You need the parameter list to be absolutely correct for the method you want for the call to succeed.

I've found that tiny steps are important when doing reflection because the compiler doesn't help. Write a small snippet which actually invokes exactly the method you want to in this particular case, and then when that works, generalize it into the framework here. I would focus on the parameters passed.