Any way to Invoke a private method?

java reflection private
Sheldon Ross picture Sheldon Ross · May 19, 2009 · Viewed 137.9k times · Source

I have a class that uses XML and reflection to return Objects to another class.

Normally these objects are sub fields of an external object, but occasionally it's something I want to generate on the fly. I've tried something like this but to no avail. I believe that's because Java won't allow you to access private methods for reflection.

Element node = outerNode.item(0);
String methodName = node.getAttribute("method");
String objectName = node.getAttribute("object");

if ("SomeObject".equals(objectName))
    object = someObject;
else
    object = this;

method = object.getClass().getMethod(methodName, (Class[]) null);

If the method provided is private, it fails with a NoSuchMethodException. I could solve it by making the method public, or making another class to derive it from.

Long story short, I was just wondering if there was a way to access a private method via reflection.

Answer

erickson picture erickson · May 19, 2009

You can invoke private method with reflection. Modifying the last bit of the posted code:

Method method = object.getClass().getDeclaredMethod(methodName);
method.setAccessible(true);
Object r = method.invoke(object);

There are a couple of caveats. First, getDeclaredMethod will only find method declared in the current Class, not inherited from supertypes. So, traverse up the concrete class hierarchy if necessary. Second, a SecurityManager can prevent use of the setAccessible method. So, it may need to run as a PrivilegedAction (using AccessController or Subject).

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