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...

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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 :
- return the value (String) of known method y from class x
- class x is only known at runtime (from the query-string in this case)
- method y is known
Thanks in advance.
cheers,
Tom
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) {}
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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...

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
invoke other methods. I have to use Java Reflection for this and methods must be of generic type!
For this
I have to generate a tree of functions(say 2 or 3).The functions could be say:int f(int){} and int g(int){}. or Int f(String), String g(float),Float h(int)..I have to create a tree of these function set and traverse them in BFS or DFS and generate combinations of these functions keeping some upper bound for the number of functions genrated(10) and size of combinations of functions(say 5).The output of one function should be the input to other function(so recursive).If any combination failed,then record(or throw exception).This all should be done using java reflection.The input can be provided though annotations or input file.This file can result in recording even the output for each function. I have tried using Junit for testing methods.My code with is following:I have two classes ClassUnderTestTests(which tests the methods of ClassUnderTest) and ClassUnderTest shown below
//Test Harness class
public class ClassUnderTestTests {
     Field[] fields;
     Method[] methods;
     Method minVal;
     Method maxVal;
     Method setVal1;
     Method setVal2;
     Method getVal1;
     Method getVal2;
     Class<?> cut;
     ClassUnderTest<Integer> obj1;
     ClassUnderTest<String> obj2;
     @Before public void setUp(){
          try {
               cut = Class.forName("com.the.ClassUnderTest");
          fields=cut.getDeclaredFields();
          methods=cut.getDeclaredMethods();
          print("Name of the CUT class >>"+ cut.getName());
          print("List of fields >>"+ getFieldNames(fields));
          print("List of Methods >>"+ getMethodNames(methods));
          //creating method objects for the methods declared in the class
          minVal=cut.getDeclaredMethod("minValue");
          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();
     @Test public void cutIntegertest(){
          //instantiating the class using the argument constructor for Object 1
          Constructor<?> constr1;
          assertNotNull(cut);
          try {
               constr1 = cut.getDeclaredConstructor(Comparable.class,Comparable.class);
               obj1=(ClassUnderTest<Integer>)constr1.newInstance(Integer.valueOf(102),Integer.valueOf(20));
               assertNotNull(obj1);
               print("The object of CUT class instantiated with Integer Type");
               // invoking methods on Object1
          assertEquals(minVal.invoke(obj1),20);
          //Object x = minVal.invoke(obj1);
          print("tested successfully the min of two integers passed");
          assertEquals(maxVal.invoke(obj1),102);
          // maxVal.invoke(obj2);
          //print();
          print("tested successfully the max of two integer values" );
          } catch (Exception e) {
               // TODO Auto-generated catch block
               e.printStackTrace();
@Test public void cutStringTest(){
     assertNotNull(cut);
     Constructor<?> constr2;
          try {
               constr2 = cut.getDeclaredConstructor(Comparable.class,Comparable.class);
               obj2=(ClassUnderTest<String>)constr2.newInstance(String.valueOf(obj1.maxValue().toString()),obj2.minValue().toString());
               //obj2=(ClassUnderTest<String>) cut.newInstance();
               assertNotNull(obj2);
               //assertNotNull("obj1 is not null",obj1);
               //print(obj1.maxValue().toString());
               print("Object 2 instantiated as String type ");
               print("setting values on object2 of type string from values of obj1 of type Integer by using toSting()");
               setVal1.invoke(obj2, obj1.maxValue().toString());
               setVal2.invoke(obj2, obj1.minValue().toString());//obj2.setVal2(obj1.minValue()
               assertEquals(getVal1.invoke(obj2),maxVal.invoke(obj1).toString());
               assertEquals(getVal2.invoke(obj2),minVal.invoke(obj1).toString());
               print("validating the Assert Equals set from object 1 after conversion to String");
               print("MinValue for Object2 when Integer treated as String>>> "+minVal.invoke(obj2));
               print("MaxValue for Object2 when Integer treated as String>>> "+maxVal.invoke(obj2));
          } catch (Exception e) {
               // TODO Auto-generated catch block
               e.printStackTrace();
          //obj2.setVal1(obj1.maxValue()
     private static void print(String str){
          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()+ " ");
          return sb.toString();
//ClassUnderTest
public class ClassUnderTest <T extends Comparable<T>>{
     //Fields
     private T val1;
     private T val2;
     ClassUnderTest(){}
     public ClassUnderTest(T val1, T val2){
          this.val1=val1;
          this.val2=val2;
     * @return the val1
     public T getVal1() {
          return val1;
     * @param val1 the val1 to set
     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) {
          this.val2 = val2;
     public T maxValue(){
          T max=null;
          if(val1.compareTo(val2) >= 0){
               max= val1;
          }else {
               max= val2;
          return max;
     public T minValue(){
          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.
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* 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.

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I am getting the name of the child class as a string argument.
eg String classname = this.getattributes().get("classname");
I need to dynamically create the object of this instance using the classname and call a specific method...i know the name of the base class but the base class doesnt have the method i need....i m not allowed to redesign the base class...
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
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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">
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regards
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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
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thanks
venkat

hi bondvenky,
What abt the answer for my original question. How to
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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.
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Edited by: Nitin.V on Feb 12, 2008 11:08 PM
Edited by: Nitin.V on Feb 12, 2008 11:09 PM

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