Where do you come from?

Sometimes it’s hard to figure out where a Java class has been loaded from. In a typical Enterprise System, there could be tens of web applications, maybe contained in an enterprise portal, running on several kinds of web and application servers, clustered over hundreds of servers. The common library used at run time can contain thousands of jars. A Java web application can load classes from several sources (his private WEB-INF/lib, the web server common library, some custom shared library, etc…), each of which can be associated with a different ClassLoader. Then it is no surprise that, if you work for years in an environment like this, you come to hate violently the class loading mechanism of Java, and every NoClassDefFoundError is seen as a voodoo curse.

Here’s a trick that I usually use when I work with Eclipse or an IDE that derives from it (the infamous RAD, for example). You have to be able to debug your program in order to perform the trick on the fly.

Place a breakpoint in an appropriate position (for example, just after some “new” instruction of an object whose class you want to know about). Then open the “Display” view. To do so, go to Windows ->Show view -> Other… . Then select the “Display” view (see figures below).



The “Display” view lets you input Java instructions that are interpreted interactively and affect the execution of the program. Now you can discover where a certain class definition is taken from. Just type something like


ctrl+a (select all) and ctrl+shift+d (Display), and you will get the output of the invokation:

(java.net.URL) file:/C:/Users/currentUser/workspace/myProject/bin/

This always gives you the exact location (i.e. the folder, as in this example, or the jar file) from which the class definition of the object has been loaded. You can also discover a bunch of fancy information about your object, such as the ClassLoader that has been used to load the class.

It might look like a very basic thing that every Java developer should know, but it is not. In fact, I cannot recall all the times that this activity saved me a lot of troubles and made me look like a smartass.

If you cannot debug the application that gives you problems then, well, i guess your only option is to insert some smart debug trace and wait for the next production release.


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