Java reflection (interesting question)

hi folks,
class A {
void foo() {
Class B overrides method foo() in A
class B extends A {
void foo() {
Now i create a object of class B and assign to A
A aref = new B();
aref.foo() will call method foo()of class B. //polymorphism
Using reflection, is it possible to call method foo() of class A using the handle aref.
thanks
venkat

hi bondvenky,
What abt the answer for my original question. How to
access the base class methods using the handle for
child class object using reflection ?as far as i know, this isn't possible - your next question is probably going to be "why". It certainly seems slightly surprising that you can't do this, but you can access private methods. Unless you consider the latter a weaker way of breaking encapsulation (!?).
what was the sun's purpose behind allowing access to
the private methods of an object using Java
Reflection? good question.. its very useful but on the other hand i can't think of a time i've used it that couldn't be classed as a hack.
Is it not a security threat to java security model?it doesn't break anything - ie its not a security loophole. It links in with your question above though - would it have been possible/useful to not allow it period?
sorry for the vague answers :(
asjf

Similar Messages

Can Java reflect not only .Class file

Hi' i'm newbie in this topic, i'm really appreciate if somebody can help me..cos i'm really stuck in here...
My Problems are :
1. i want to ask about this, can Java reflect from .java file?
2. i'm using Eclipse IDE, i'm really interesting about how JTree or Package Explorer in Eclipse can always displaying update information about class structure? but .java files not compiled, how can? if Eclipse using reflection, .java files must be compiled first, correct me if i'm wrong?
The fact is Eclipse don't have to compiled .java files to get the update information about class structure and displaying in JTree or package Explorer...how implement like this?
what i mean like this :
ex : if i type int x = 100; (not only int, it could be anything else..) at the working files, JTree or Package Explorer in Eclipse can always update the informasion about class structure, but .java files not compiled..
i hope my question are easy to understand, i really need some help..
Thanks a lot..

hey, thanks for the answers, but i would like to ask :
1) Eclipse performs background compilation of the Java sources, then performs reflection on those temporary classes++ if i'm using this way, how about the performance? seems that it will be compiled all the time right?
2) Eclipse has access to the results of the Java source code parser, and can extract the information from the java syntax parser, before it gets to the compilation stage.++ how to implement this? what do you mean about java syntax parser?
do you know where i can find any article about this?
thanks a lot again...

Java Reflection and dynamic class loading

I am trying to load my classes 'dynamically' using java reflection, which is a feature absolutely necessary for my webapp. I could not get this to work as of yet. Could someone please give me a piece of sample code that would do the following :
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cheers,
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PS: Please do not give me any links to tutorials/articles that do not do the EXACT thing that I asked for. Thank you.

tried it, but it always gives me a MethodNotFoundException, because its trying to find my class in java.lang.String for some reason...
heres part of the code (its an altered version of the code given in the invoke tutorial):
public String getMethodReturnValue(String methodName, String className) {
String result = null;
Class theModuleClass = String.class;
Class[] parameterTypes = new Class[] {};
Method concatMethod;
//Object[] arguments = new Object[] {parameters};
try {
concatMethod = theModuleClass.getMethod(methodName, null);
result = (String) concatMethod.invoke(createObject(className), parameterTypes);
} catch (NoSuchMethodException e) {
result = e.toString();
} catch (IllegalAccessException e) {
result = e.toString();
} catch (InvocationTargetException e) {
result = e.toString();
return result;
private Object createObject (String className) {
Object object = null;
try {
Class classDefinition = Class.forName(className);
object = classDefinition.newInstance();
} catch (Exception e) {}
return object;
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Java Reflection

Hi Every1....i have a problem with java reflection n hope some1 will be able to resolve this me.
I am getting the name of the child class as a string argument.
eg String classname = this.getattributes().get("classname");
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plz help

but the problem is getting the method nameIf you don't have the method name what exactly is the plan? You've gotta have something ...
.....i have the object of base class with meActually the object is an instance of the derived class ...
....and the method i
need is in the child class.....so how do i invoke the
method???Sigh. You can lead a horse to water but you can't make him drink. See
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/reflect/Method.html#invoke(java.lang.Object,%20java.lang.Object...)

Code generation through Java Reflection

Hi
I am after some clarification about the possibility of mapping of method outputs to other method inputs, using java reflection.
The java objects are described in an XML based language (called DAML) as follows
<java:Method rdf:ID="meth1" java:priority="1">
<java:methodName>buildQuery</java:methodName>
<java:parentClass>afsw.query.QueryBuilder</java:parentClass>
<java:methodParameters rdf:parseType="daml:collection">
<java:Parameter>
<java:inORout>input</java:inORout>
<java:type>java.util.Hashtable</java:type>
</java:Parameter>
</java:methodParameters>
</java:Method>
and I want to map them back to method calls and instantiations, so to be able to generate code on the fly. I need to know if its is possible to pass the output of a method such as getQuery() which in this case is a Document to the input of constructor MsgModule as in the following example:
Document queryDoc =qc.getQuery();
MsgModule mg= new MsgModule(queryDoc);
regards
Charlie

This is possible. What you need to do in the 'new' instance case is to find the best constructor. So let's say you have Class clazz, the class you want to create a new instance of, and Class[] params, an array of objects to pass in the constructor. Using the Class api you can do:
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Java reflection and parameter's names

hi, I have to extract from a method of a given class all types and names of the parameters, i've tryed to use java reflection function:
Method[] metodo = temp.getDeclaredMethods();
for (int i = 0; i < metodo.length; i++) System.out.println(metodo.toString());
and the output is:
public void DinamicLoad.function1.method1()
public void DinamicLoad.function1.method2(java.lang.String)the function extract only the parameter's type, not his name

Probabily all the "dozens, hundreds, thousands of
other people who use reflection" knows very well
classes that they use.
Imagine to produce a class from a wsdl file, how can
you know the content?
How can you pass parameters if you know only the type
and not the MEANING?When building classes from XML it's rare to use the constructor to convey the attributes, far more often each attribute is set independantly using a single argument setter with a signature like:
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If constructors are to be used then parameters will be supplied in order.

Java reflection - any disadvantages

1. Are there any disadvantages in using java reflection api (too much)?
2. How do obfuscators deal with reflection based applications?
Thanks
Santosh

Sorry, i wanted to know how the code
Object obj = new MyClass();
is different from
Class clazz = Class.forName("MyClass");
Object obj = clazz.newInstance();
at the byte-code level.
That is, in both the cases the classloader has to
search for MyClass and invoke the constructor. Hope I
am clear :)I don't think there is any difference as far as performance is concerned between the above mentioned methods for instantiating the object of MyClass.
But here is what is generally said about using reflection(u can find it on google):
===============================================================
Don?t overuse the Reflection API. Programs that use it are more difficult to understand and maintain. In addition, accessing fields and calling methods through the Reflection API is slower than using them in the ?normal? way, so use reflection only when it is really necessary.
===============================================================
We too had concerns about reflection's performance, but as long as you cache the lookups, reflection is very fast. What is slow is doing things like looking up a method on a class or looking up an instance member. We do all these lookups once and then cache the results.
===============================================================
Why is it slow as stated above? - Maybe because flexibility always comes at a price !!! Maybe the internal implementation of the call sequence by the JVMs is such that it cannot be implemented any faster...

Java reflection and singletons

Using java reflection and singletons .. anyone seen this being used?

I've solved it
I had a singleton but then it turned out i had to use reflection instead, i tried to used reflection on 3 methods but the session value within was null upon the second method call. Relfection was re-creating the Object.
Sorry for wasting your time.

Help need regarding Java Reflection

I need help in programming the following:
I have to generate a test Harness for OOP.This program that automatically tests
classes by instantiating them, selecting methods of these instantiated objects randomly or
using some criteria, invoking them, and using the returned values as input parameters to
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//Test Harness class
public class ClassUnderTestTests {
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     Method[] methods;
     Method minVal;
     Method maxVal;
     Method setVal1;
     Method setVal2;
     Method getVal1;
     Method getVal2;
     Class<?> cut;
     ClassUnderTest<Integer> obj1;
     ClassUnderTest<String> obj2;
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          try {
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          print("List of Methods >>"+ getMethodNames(methods));
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          minVal.setAccessible(true);
          maxVal=cut.getDeclaredMethod("maxValue");
          maxVal.setAccessible(true);
          setVal1=cut.getDeclaredMethod("setVal1", Comparable.class);
          setVal1.setAccessible(true);
          setVal2 = cut.getDeclaredMethod("setVal2", Comparable.class);
          setVal2.setAccessible(true);
          getVal1=cut.getDeclaredMethod("getVal1");
          getVal1.setAccessible(true);
          getVal2 = cut.getDeclaredMethod("getVal2");
          getVal2.setAccessible(true);
          } catch (ClassNotFoundException e) {
               // TODO Auto-generated catch block
               e.printStackTrace();
          } catch (Exception e) {
               // TODO Auto-generated catch block
               e.printStackTrace();
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               print("MaxValue for Object2 when Integer treated as String>>> "+maxVal.invoke(obj2));
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               // TODO Auto-generated catch block
               e.printStackTrace();
          //obj2.setVal1(obj1.maxValue()
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          System.out.println(str);
     private String getMethodNames(Method[] methods) {
          StringBuffer sb= new StringBuffer();
          for(Method method: methods){
               sb.append(method.getName()+ " ");
          return sb.toString();
     private String getFieldNames(Field[] fields) {
          StringBuffer sb= new StringBuffer();
          for(Field field: fields){
               sb.append(field.getName()+ " ");
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     //Fields
     private T val1;
     private T val2;
     ClassUnderTest(){}
     public ClassUnderTest(T val1, T val2){
          this.val1=val1;
          this.val2=val2;
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     public T getVal1() {
          return val1;
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     public void setVal1(T val1) {
          this.val1 = val1;
     * @return the val2
     public T getVal2() {
          return val2;
     * @param val2 the val2 to set
     public void setVal2(T val2) {
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          T max=null;
          if(val1.compareTo(val2) >= 0){
               max= val1;
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               max= val2;
          return max;
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          T min=null;
          if(val1.compareTo(val2) < 0){
               min= val1;
          }else {
               min= val2;
          return min;
This code throws Null Pointer Exception if CutStringtest function(I think on calling obj1 in tht function) Please suggest!

Well, I can't help you specifically since I'm not into reflection too much, but here are some ideas:
* Either learn how to set breakpoints and single step through your code, or pepper your code with a lot more meaningful System.out.println() statements.
* Come up with simple test cases, pass them first, then progressively more complex test cases, until you test all possible situations. Try to get each test case to test only a single concept and not multiple concepts at the same time. If your code is broken on more than one concept, you will not be able to isolate each concept by itself to fix it without separating it.
* Consider refactoring your code so each sub-module accomplishes one task in isolation and verify that module can handle each possible situation that its asked to perform.
* The exception usually tells you what line in your code threw the exception. Look for a line in the exception printout that has the name of your package or class in it.

Performance difference between java.beans.Expression and Java Reflection

What is the Performance difference between using java.beans.Expression class and Java Reflection classes like Class,Method ??

negligible

Questions on Java Reflection in EJB

Hi,
Recently, I use reflection technology on EJB to get/set properties of a bean.
We need it because we need to encapsulate data in a map to transfer data between
presentation layer and business back end(i.e. the so-called value data object).
A bean is packed into a map as following:
The property name of a bean becomes the key in the map, and its value becomes the
corresponding value in the map.
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1)Given a bean, convert it to a map;
2)Given a data map, assign the value to a bean
It would be nice if we can implement the two requirements in a base class. So I use
reflection. And succeed to achieve the goal.
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schema) I got a ClassNotFoundException. A workaround is to jar the BMP or CMP bean class
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So, I want to know why there is such restriction.
2)For the classes java.lang.reflect.Method, java.lang.reflect.InvocationTargetException
I reference to in bean class, the IDE(I use IntelliJ Idea) give me a
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dangerous in EJB environment?
Can anyone explain me these pluzzles?
Thank you in advance!
BTW, I develop under weblogic 7.0. Now, my program functions well, I just can't understand
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Regards,
Justine

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As for using a map for transferring data, I would strongly recommend against it, especially in this case, because you've not only lost the strong typing you get with ValueObjects, but you have to do a lot of extra work on both "sides" (client and EJB) to make sure all your data is present and/or correct.

Interesting question in Java!!!!!!!!

Hi,
I am in need of getting some help on the following Java problem,
I have a String variable with a value of,
"if (flag) i=i+1 else i=i-1;" or "...some other arthmetic operation.."
and at one poing of time i need to execute the code inside this String. Is there anyway? Pls do help me. Thanks in advance.
Crao

I don't know of anything that will handle both expression evaluation and program logic...
http://www.beanshell.org/intro.html
http://jep.sourceforge.net/

Question about java reflection and "int" type

hi, using reflections I should invoke a Method (meth3) that have as parameter an INT type (not integer). I've tryed with this code:
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Works fine when I try it.
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Java/OO Design question. Please help!

Brian,
I'd say your better off trying the
weblogic.developer.interest.ejb newsgroup
or alternatively try the discussion forums in
www.theserverside.com
or
http://forum.java.sun.com/
For what its worth I'd return an empty array rather than an exception as I associate
exceptions with real errors whereas a search that happens to return nothing is a
valid result.
Cheers,
mairt
"Brian Snyder" <[email protected]> wrote:
>
I'm certain this is not the best forum for my question, but I didn't any
other
that seemed obvious.
I have a design issue/ OO question. Our system has the very common requirement
of searching. We have build a session EJB which actually performs the search
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or
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Simple Java Network Programming Question

import java.io.*;
import java.net.*;
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georgemc wrote:
BlueNo yel-- Auuuuuuuugh!
But the real question is: What is the air-speed velocity of an unladen swallow?

Java reflection (interesting question)

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