Java Generics and reflection!

Question

I have a class that looks like this:

public class UploadBean {


    protected UploadBean(Map<String,?> map){ 
        //do nothing.
    }
}

To use reflection and create a object by invoking the corresponding constructor, I wrote code as follows:

Class<?> parTypes[] = new Class<?>[1];
parTypes[0] = Map.class;
Constructor ct = format.getMappingBean().getConstructor(parTypes);
Object[] argList  = new Object[1];
argList[0] = map;
Object retObj = ct.newInstance(argList);

This code fails at runtime with "No Such Method Exception". Now, how do I set the param type correctly?! such that the generic map argument in the constructor is identified?

Format is an implementation of this: public interface FormatFactory { public Map<String, Class> getFormat(); public Class<? extends UploadBean> getMappingBean(); } — Jay
– Jay, Commented Jun 2, 2009 at 14:44

Jon Skeet · Accepted Answer · 2009-06-02 14:48:00Z

7

The constructor is protected - if you make it public or use getDeclaredConstructor instead of getConstructor it should work.

(You'll need to use setAccessible if you're trying to call this from somewhere you wouldn't normally have access.)

EDIT: Here's a test to show it working okay:

import java.lang.reflect.*;
import java.util.*;

public class UploadBean {

    // "throws Exception" just for simplicity. Not nice normally!
    public static void main(String[] args) throws Exception {
        Class<?> parTypes[] = new Class<?>[1];
        parTypes[0] = Map.class;
        Constructor ct = UploadBean.class.getDeclaredConstructor(parTypes);
        Object[] argList  = new Object[1];
        argList[0] = null;
        Object retObj = ct.newInstance(argList);
    }

    protected UploadBean(Map<String,?> map){ 
        //do nothing.
    }
}

edited Jun 2, 2009 at 14:48

answered Jun 2, 2009 at 14:40

Jon Skeet

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5 Comments

Michael Myers Over a year ago

Or use getDeclaredConstructor(), as I just found out (but too late!).

Jay Over a year ago

doesn't work throws this exception again: java.lang.NoSuchMethodException: test.fileupload.XYZUploadBean.<init>(java.util.Map)

Michael Myers Over a year ago

Would getDeclaredConstructor() then require setAccessible(true)? In my tests it doesn't, but I didn't try separating main() from the UploadBean.

Jon Skeet Over a year ago

@Jay: Please provide a short but complete program then. If you're using the right class and it's got the right constructor, it should be okay. It works in my tests.

Jon Skeet Over a year ago

In particular, does XYZUploadBean have the right constructor? You've shown UploadBean, but not XYZUploadBean...

fortran · Accepted Answer · 2009-06-02 14:50:47Z

1

The generic information is not available at runtime, it's just for static analysis, so do it as if the generics didn't exist.

answered Jun 2, 2009 at 14:50

fortran

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1 Comment

Michael Myers Over a year ago

Yeah, the error really has nothing to do with generics; this is really just a reflection question. (But it's nice to clarify this point.)

Rob Di Marco · Accepted Answer · 2009-06-02 14:41:31Z

0

I believe you need to call

ct.setAccessible(true)

The setAccessible method allows you to override access methods.

answered Jun 2, 2009 at 14:41

Rob Di Marco

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