Name: boT120536 Date: 01/02/2001


java version "1.3.0"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0)
Java HotSpot(TM) Client VM (build 1.3.0, mixed mode)


The lookup table used by Class's getConstructor(Class[] paramClasses) doesn't
find the constructor if any of the paramClasses are concrete implementations of
interfaces in the constructor.

Here's two classes that demonstrate the problem. Get the output by running "java
Invoker"

--------------

import java.io.Serializable;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class Invoker
{
    public Invoker()
    {}

    private static void printArray(Object[] array)
    {
        System.out.print("[ ");
        for(int i=0;i<array.length;i++)
            {
                System.out.print(array[i].toString()+" ");
            }
        System.out.println(" ]");
    }

    public static Victem works()
        throws
NoSuchMethodException,InstantiationException,IllegalAccessException,InvocationTargetException
    {
        System.out.println("Inside works");

        Class concrete = Victem.class;
        Serializable id = new Long(0);
        Object[] params = new Object[1];
        
        params[params.length-1] = id;
        System.out.println("params are "+params);
        printArray(params);

        //get the classes of the parameters
        Class[] paramClasses = new Class[params.length];
        for(int i=0;i<params.length;i++)
            {
                paramClasses[i] = params[i].getClass();
            }

        //I added this line to make the method work
        paramClasses[paramClasses.length-1]=Serializable.class;

        System.out.println("paramClasses are "+paramClasses);
        printArray(paramClasses);

        //Get the right constructor
        Constructor constructor = concrete.getConstructor(paramClasses);
        
        //create a new instance of the concrete class
        Victem result = (Victem)constructor.newInstance(params);
        
        System.out.println("-------------");
        //return the concrete class
        return result;
    }

    public static Victem broken()
        throws
NoSuchMethodException,InstantiationException,IllegalAccessException,InvocationTargetException
    {
        System.out.println("Inside broken");

        Class concrete = Victem.class;
        Serializable id = new Long(0);
        Object[] params = new Object[1];
        
        params[params.length-1] = id;
        System.out.println("params are "+params);
        printArray(params);

        //get the classes of the parameters
        Class[] paramClasses = new Class[params.length];
        for(int i=0;i<params.length;i++)
            {
                paramClasses[i] = params[i].getClass();
            }

        System.out.println("paramClasses are "+paramClasses);
        printArray(paramClasses);

        //Get the right constructor
        Constructor constructor = concrete.getConstructor(paramClasses);
        
        //create a new instance of the concrete class
        Victem result = (Victem)constructor.newInstance(params);
        
        //return the concrete class
        return result;
    }

    public static void main(String args[])
    {
        try
            {
                works();
                broken();
            }
        catch(Exception ex)
            {
                ex.printStackTrace();
            }
    }
}
--------------------------

import java.io.Serializable;

public class Victem
{
    private Serializable id;

    public Victem(Serializable identifier)
    {
        id = identifier;
    }
}
------------------------
System.out gets
Inside works
params are [Ljava.lang.Object;@310d42
[ 0 ]
paramClasses are [Ljava.lang.Class;@5d87b2
[ interface java.io.Serializable ]
-------------
Inside broken
params are [Ljava.lang.Object;@20c10f
[ 0 ]
paramClasses are [Ljava.lang.Class;@62eec8
[ class java.lang.Long ]
java.lang.NoSuchMethodException
        at java.lang.Class.getConstructor0(Native Method)
        at java.lang.Class.getConstructor(Class.java:927)
        at Invoker.broken(Invoker.java:82)
        at Invoker.main(Invoker.java:96)

-----------------------------------------

I realize that the bug involves an ambiguous case: if a paramClass implements
two different interfaces and the class to be constructed has two constructors,
each of which takes one interface as a parameter, then the getConstructor()
method won't be able to figure out which constructor to choose. However, this
problem is not unique to the getConstructor() method. The javac compiler won't
compile a class if it detects this problem. You could change the
getConstructor() (and the getDeclaredMethod() method) to throw either a runtime
exception or a subclass of NoSuchMethodException. An
AmbiguousMethodMatchException could include a list of possible matching methods.

Try and compile this code to see what javac does:
------------------

import java.io.Serializable;

public class Ambiguous
{
    private Serializable id;

    public Ambiguous(Serializable identifier)
    {
        id = identifier;
    }

    public Ambiguous(Comparable compit)
    {

    }

    public static void main(String args[])
    {
        try
            {
                new Ambiguous(new Long(2));
            }
        catch(Exception ex)
            {
                ex.printStackTrace();
            }
    }
}
(Review ID: 114421)
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