Automatically detect the caller class!

Guys,
I have a simple but "sounds-like-general-and-interesting" doubt..
I have a class 'A' and another class 'B'. I have a method in 'A' called 'a' calling a method in class 'B' (which is 'b').
suppose the control is inside the method 'b' of the class 'B'.
Now, the doubt is
how can I find out that it was class 'A' who called the method 'b' of class 'B' from within the method 'b'?
the scenario is a simpe loging scenario where the class 'B' is a Logger class and I want to automatically detect the class calling this component from within this component so that it can be logged too.
Hope my problem is clear. Thankx in advance for the replies.. am stuck and confused too.. and hop there's a solution for this
regards
Manesh

mgumbs is right - typically a Logger class will accept a context (Object) to indicate where the message has come from. Sometimes people pass in "this", sometimes they pass in "this.class" or sometimes they'll log on behalf of another class. It's pretty much the same solution - if it's needed you should supply it.
One method made possible in J2SDK-1.4 is using a stack trace to find out what the calling class/method is. Perhaps the only way to do that would be to throw a new Exception and get hold of its stack trace elements. I suspect that this would add significant overhead, though, and I certainly wouldn't recommend it!

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To access a package, the calling class must reside in the root/parent direc

My goal was to be able to access my package from anywhere within the
high level root folder (d:\zJava). But it seems the invoking class MUST
RESIDE IN THE TOP LEVEL PARENT DIRECTORY of the package. Was Java
designed to behave this way?
D:\zJava\zsamples\com\zswingpackage1> <-- packages successfully compiled here
 swing_optgrp_eg1.{class/java}
 package com.zswingpackage1;
 public class swing_optgrp_eg1 extends JFrame implements ItemListener {
D:\zJava\zsamples\com> <-- classes residing here could NOT successfully access the package 'com.zswingpackage1'. The third level subfolder is 'com'.
 package_test4.java
 /* Following import stmt. causing RUN-TIME error "Exception in thread "main" java.lang.NoClassDefFoundError: com/zswingpackage1/swing_optgrp_eg1". */
 import com.zswingpackage1;
 Package_Test4.class
 /* Unsuccessful remedies:
 i) D:\zJava\zsamples\com>java Package_Test4
 -classpath .;d:\zJava\zsamples\com\zswingpackage1
 -classpath .;d:\zJava\zsamples\com
 -classpath .;d:\zJava\zsamples (also used in compile)
 -classpath .;d:\zJava
 ii) ran from the root/parent directory of the package d:\zJava\zsamples),with and without classpath i.e.
 D:\zJava\zsamples>java com\Package_Test4 [-classpath .;d:\zJava\zsamples]
 iii) ran from the top level directory (d:\zJava), with and without classpath i.e.
 D:\zJava>java zsamples\com\Package_Test4 [-classpath .;d:\zJava\zsamples]
 Possible causes:
 - maybe the above behaviours are by design i.e. the calling class/program CANNOT be stored within the package hierarchy?
D:\zJava\zsamples\net> <-- classes residing here could NOT successfully access the package 'com.zswingpackage1'. The third level subfolder is 'net'.
 package-test6.java
 /* Following import stmt. causing RUN-TIME error "Exception in thread "main" java.lang.NoClassDefFoundError: com/zswingpackage1/swing_optgrp_eg1". */
 import com.zswingpackage1;
 Package_Test6.class
 /* Unsuccessful remedies:
 i) D:\zJava\zsamples\net>java Package_Test6
 -classpath .;d:\zJava\zsamples\com\zswingpackage1
 -classpath .;d:\zJava\zsamples\com
 -classpath .;d:\zJava\zsamples (also used in compile)
 -classpath .;d:\zJava
 ii) ran from the root/parent directory of the package (d:\zJava\zsamples), with and without classpath i.e.
 D:\zJava\zsamples>java net\Package_Test6 [-classpath .;d:\zJava\zsamples]
 iii) ran from the top level directory (d:\zJava), with and without classpath i.e.
 D:\zJava>java zsamples\net\Package_Test6 [-classpath .;d:\zJava\zsamples]
 Possible causes:
 - maybe the above behaviours are by design i.e. the calling class/program MUST be stored in the parent folder of the package hierarchy?
**D:\zJava\zsamples> <-- classes residing here SUCCESSFULLY accessed package 'com.zswingpackage1'. NO -classpath parms. were necessary!
D:\zJava\ <-- classes residing here could NOT successfully access the package 'com.zswingpackage1'. Similar scenarios as above.
D:\zJavaB\ <-- classes residing here could NOT successfully access the package 'com.zswingpackage1'. I purposely created another folder with the 'B' suffix. Similar scenarios as above.
My PC configuration:
 java version "1.4.0_01"
 Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.0_01-b03)
 Java HotSpot(TM) Client VM (build 1.4.0_01-b03, mixed mode)
 JRun 4 (Build 47304)
 WinNT 4.0 SP5
 IE 5.50
 CLASSPATH=.;D:\jakarta-tomcat-3.3.1\lib\common\servlet.jar;
 C:\JRun4\servers\default\default-ear\default-war\WEB-INF\classes
 JAVA_HOME=C:\j2sdk1.4.0_01
 Path=C:\j2sdk1.4.0_01\bin;C:\WINNT\system32;C:\WINNT;
 C:\Program Files\Network Associates\PGPNT;C:\Program Files\MTS;d:\MSSQL7\BINN
Thank you so much for your expertise.

Hello,
I think that you use the import-declarations incorrectly.
Your code says:
import com.zswingpackage1;
... The meaning of the above declaration is the following: Import the class "zswingpackage1" from the package "com". The compiler cannot find that class, but it is probably never asked to find it in your code, so it won't report an error.
If you want to import the whole package use the following declaration instead:
import com.zswingpackage1.*;
... That means: import all classes from from the package "com.zswingpackage1".
You can also import just the classes needed, e.g.:
import com.zswingpackage1.swing_optgrp_eg1;
... I hope that does the job.
Søren

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Name of the "caller" class

Hello,
Is it possible in a class to know the name of the class that instanciated it ?
Thank you in advance,
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Getting the calling object reference / class?

Dear forum members,
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class A {
A() {
public void someMethod(String text) {
    System.out.println("I was called with: " + text);
    System.out.println("I was called by: " + thatMagicWayToGetCallerClassOrReference());
class B {
public static void main(String[] args) {
    A a = new A();
    a.someMethod("HOLA HOLA");
}

Another way of getting the class of your caller is to use a SecurityManager object. getClassContext() is protected but you can get at it by subclassing SecurityManager (it works if you just instanciate the manager without installing it). This gives you a reference to the Class object itself where the stack trace returns only the class name. Bear in mind that the class in question will be several items up the array, the first entry is the SecurityManager itself, the second the called class.
However, I agree with others it doesn't sound like the best design. If you're having trouble with not being able to change the interface specifications to pass the extra information then consider using a static reference, if necessary a ThreadLocal. What happens if you want to call the offending method indirectly through some central method at some future time? You've lost the call origin data.
Why can't you simply add a parameter to the offending call?

Which is the best way for a called function to identify caller class name.

Which is the best way for a called function to identify the caller class name .
1)Using sun.reflect.Reflection from called function
                Class caller = Reflection.getCallerClass(2);
                System.out.println("Caller Class Name ::"+caller.getName());2) Analyzing current threads stack trace from called function
                StackTraceElement[] stElements=Thread.currentThread().getStackTrace();
                System.out.println("Caller Class Name ::"+stElements[3].getClassName());Is there any alternate ways to achieve the same .Which is the best way ?
Called function doesn’t have any arguments, I don’t want t pass any arguments from caller function to called function.
Plz help.
With kind regards
Paul

798185 wrote:
Which is the best way for a called function to identify the caller class name .
Is there any alternate ways to achieve the same.SecurityManager
    // 0 is the anonymous SecurityManager class
    // 1 is this class (also works in static context)
    // 2 is calling class
    static Class getClass(int i) {
        return new SecurityManager() {
            protected Class[] getClassContext() {
                return super.getClassContext();
        }.getClassContext();

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Call classes from another project

Hi,
i have a (i hope) simple question.
Meanwhile i have a lot of custom coding which i uploaded to my J2EE as PAR files from NWDS.
For several projects i have similar requirements (e.g. read customers information etc.), so I generated a "base" project in nwds and deployed it.
In NWDS project properties i can set the "Java Build Path" to this project and i am able to call the methods that i wrote in this project. But only in NWDS.
When i deploy the project to J2EE i get runtime errors that classes are not found, so i think i have to customize sth. in portalapp.xml to set the reference
I tried several things with "SharingReference" etc. but nothing worked.
Could anyone tell me the steps i have to do?
I know i could create a jar file which i link in my project but i want a central project where i can change coding, deploy it again, and all projects automatically use the actual classes.
Many thanks
regards
Alex

Hi Min,
i tried this but get errors.
The project par with the baseclasses has the name "com.xxx.baseclasses.par", the package in the project where the classes are placed is "com.xxx.baseclasses.usermanagement;"
So i tried it in portalapp.xml of my other project with the following lines
<application-config>
<property name="SharingReference" value="usermanagement,com.xxx.baseclasses"/>
<property name="SharingAccess" value="shared"/>
</application-config>
I also tried com.xxx.baseclasses.usermanagement, but no luck. In default trace i get:
java.lang.NoClassDefFoundError: com.xxx.baseclasses.usermanagement.ZBVUser
Do i have to set the reference in a different way or do i have to change sth. in my baseclasses project?
Many thanks
Regards
Alex

Detecting the source of a method invocation.

Within a given method, is there a way to detect what class/method made the invocation call?
I need to find a way to modify instance variables within a method, based on the invoking class ... I thought of navigating back through the Stack, but I am working with a multi-threaded application so I fear that I cannot absolutely depend on the last Stack call being the actual invoking class/method. Any ideas? - please help.
Many thanks,
Thomas

How does one access the method invocation stack?I'm not sure if an easier way exists, but you can extend SecurityManager to fetch the Class context of the current Thread, which will provide you with an array of Class obejcts, ordered according to the method invocation stack. Thus, you can determine what Class invoked a particular method, but you can not determine what Object (not via the SecurityManager funcationality).
Here is a simple class I often use for such a need (excuse any formating issues- or misspellings in the documenation)... ... although, it should be stated, this class may not work in certain environments- specifically ones that deny permission "createSecurityManager".
* This class provides static utility methods for retrieving information
* on the class context. The methods in this class can provide a
* resource with a list of the classes that invoked it, thus allowing
* clases to determine if a particular method is being accessed by
* the proper caller or allow Class variables to be initialized statically,
* with out having to instantiate the type.
* This class is meant to provide simple functionality in familiar
* environments. Its funcationality may not work properly when executing
* in an enviroment with strigent security permissions. Specifically, if
* permission is denied to construct a new SecurityManager instance, the
* methods of this class will fail.
* @author: Sean Flanerry
public final class ClassLookup extends SecurityManager {
private static final ClassLookup instance = new ClassLookup();
private ClassLookup() { }
* This method returns the class context for the current thread in format
* <ul>
* <li>Class for currently executing method
* <li>Class that invoked the above method
* <li>Class that invoked the above method...
* </ul>
* The first two elements in the returned array will be of type ClassLookup.
* @see java.lang.SecurityManager#getClassContext()
public static Class [] fetchClassContext() {
 return instance.getClassContext();
* This method returns the Class that invoked this method. This method
* is used primarily by static initializers that have to retrieve a handle
* to the java.lang.Class instance that wraps the current type, but that
* can not instantiate the current type, eg 
* <code>public static final Class currentClass = ClassLookup.getCallingClass();
public static Class getCallingClass() {
 return instance.getClassContext()[3];
* This returns the Class that invoked the current method on the calling
* Class. If for example, Race.startRace() invokes Runner.sprint(), Runner.sprint()
* can determine what class invoked it by calling this method. In other
* words, this method lets other methods determine which class invoked
* it.
* Remember that this method returns a Class instance for which
* ever type contains the method which invoked the caller so the
* Object returned by this method might be the caller's type if the
* caller was spawned by a method with in the same class. Consider,
* <ul>
* <li>Race.start() invokes
* <li>Runner.sprint() which invokes
* <li>Runner.fatigue()
* </ul>
* If Runner.fatigue() invokes this method, an instance of Runner.class will
* be returned, not Race.class because fatigue was invoked by sprint.
* This method is typically used by resources that wish to determine
* if the current method is being invoked via the proper context, ie the
* invoking class is a trused resource.
public static Class getInvokingClass() {
 return instance.getClassContext()[4];
public static void main(String args[]) {
 Class [] c = fetchClassContext();
 for (int i = 0; i < c.length; i++) {
 System.out.println(c.getName());

Where does the java classes go?

I normally use IntelliJ which automatically puts the java class and the jsp where they are meant to go.
Can some help me.
Can someone please tell me where to put the java class, i have put the jsp pages in the webapps\ROOT but where does the java classes go?
Edited by: Tinkerbelle on Jul 24, 2008 1:46 AM

Tinkerbelle wrote:
sorry being stupid i do that sorry,
If i change the class to type (as someone said in a previous post)That would only work if you already put the bean in the session scope.
Did you try what PaulOckford wrote?
>
i get this error:
HTTP Status 500 -
type Exception report
message
description The server encountered an internal error () that prevented it from fulfilling this request.
exception
org.apache.jasper.JasperException: Unable to compile class for JSP:
An error occurred at line: 6 in the generated java file
Only a type can be imported. com.database.contactDB resolves to a packageDo you have a package structure that looks like this:
com
com/database/ <-- This is where contactDB.class is located
com/database/contactDB/
Because that is what the compiler is saying. Though it may be thrown off by case (see below)
P.S. If you use the jsp:useBean you do not need the import statement which appears to be where the error occurs. (One of the things that makes JSPs so hard to debug is the fact that the error line in the generated java file as referenced above is never the same as the line in the JSP file).
>
An error occurred at line: 3 in the jsp file: /login.jsp
database.contactDB cannot be resolved to a type
1: <%@ page import="com.database.contactDB" %>
2:
3: <jsp:useBean id='db'
4: scope='session'
5: type='database.contactDB'/>
6: <html>
And to be ultra clear - you did compile contactDB from a .java file to a .class file, and the class is called contactDB and not ContactDB correct? The class name is case sensitive and should be the exact same case in the useBean and import statements as in the real class name.
>
An error occurred at line: 3 in the jsp file: /login.jsp
database.contactDB cannot be resolved to a type
1: <%@ page import="com.database.contactDB" %>
2:
3: <jsp:useBean id='db'
4: scope='session'
5: type='database.contactDB'/>
6: <html>
Stacktrace:
 org.apache.jasper.compiler.DefaultErrorHandler.javacError(DefaultErrorHandler.java:93)
 org.apache.jasper.compiler.ErrorDispatcher.javacError(ErrorDispatcher.java:330)
 org.apache.jasper.compiler.JDTCompiler.generateClass(JDTCompiler.java:435)
 org.apache.jasper.compiler.Compiler.compile(Compiler.java:298)
 org.apache.jasper.compiler.Compiler.compile(Compiler.java:277)
 org.apache.jasper.compiler.Compiler.compile(Compiler.java:265)
 org.apache.jasper.JspCompilationContext.compile(JspCompilationContext.java:564)
 org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:302)
 org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:329)
 org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
 javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
note The full stack trace of the root cause is available in the Apache Tomcat/5.5.26 logs.
Apache Tomcat/5.5.26Edited by: stevejluke on Jul 24, 2008 7:33 AM
Fixed compile from .class file to a .java file to compile from a .java file to .class file

Capturing the caller object

I used to be doing like this:
myTextObj.addCaretListener(new MyCustomCaretListener(myTextObj, ..., ...));because the constructor can't access the event and its source object. But current project has so many of these types of calls. Is there a way by which the constructor could automatically access the caller or registerer object? This is not a Swing question, but a more general one I suppose.

There are a couple of ways. I agree with the last poster, an explanation of why you need this might lead to more helpful suggestions.
1 - Use a Factory object. Assuming you are passing the same parent to multiple children, you could create a Factory object that takes the parent in its constructor. this object would then have methods to create the children and pass the parent as part of the parameters to child constructor.
2 - Use inner classes which have a hidden parent in them.
public class Outter {
     private String name;
     public Outter(String name) {
          this.name = name;
     public String getName() {
          return name;
     class Inner {
                 // The hidden parent
          Outter getParent() {
               return Outter.this;
     public static void main(String[] args) {
          Outter outer = new Outter("Fred");
          Inner inner = outer.new Inner();
          System.out.println("Outter:" + outer.getName() +
                                 " Inner:" + inner.getParent().getName());
} Both these suggestions, while workable, should be examined in the context of your project. It would be very easy to make the code less readable or just plain wrong, by using either of these techniques.
Edited by: m0smith on Oct 10, 2007 9:27 PM

Automatically detect the caller class!

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