Develop Reference

Replace Eclipse library class with my own

I'm developing an Eclipse plugin in which I want to overwrite the functionality of a method contained in a class that is in the Eclipse library.
What I've tried so far is creating an identical (same package and file names) file in my plugin source, and making the changes I want there. Then, I set the build path order such that my source is above the Plug-in Dependencies. Based on my limited understanding, this should mean that when Java looks for that class, it should use mine over the one in the library.
However, this is not working. The behavior that I want to override is not changing, and I don't see the print statements I put in my code either.
How can I "replace" a class in the Eclipse library with one of my own?

I did it once (not proud of it :-)) in the following way:
Import the plugin you wish to hack by Import->Plugin Development->Plug-ins and Fragments (Import as Projects with source folders).
Set the project to build automatically, edit the file and find its resulting class file.
Open the jar of the plugin (the one containing its class files), inject your class file instead of the original one.
If jar file is signed remove all signature information from MANIFEST.MF (and maybe other files).
I admit it's ugly but it's the best way I've found.

I ended up using the JVM JavaAgent to achieve this, by overriding the class loader and loading in my own class to replace the one in the library.
This was a useful tutorial for me: https://stackoverflow.com/a/11898653/634324

AEM 6.1 Sightly Java class does not update on build

I have recently started using Sightly in AEM 6.1, which allows the Java files to be located in the same folder as the component and use the WCMUse class. However, I am noticing that whenever I do a Maven build that involves a change to one of those Java files that the page functionality operates as if the Java class had not been changed, though the Java file in the crx does include the changes. As a workaround I have been been able to modify the Java file in the crx, save it, then modify it back and save again in order to update the functionality, but I do not have that capability on all of my instances.
Anybody have an idea how to force the recompile of the Sightly Java within the components either during or following the build?

A few things to validate:
1- Do you change the version of the software when doing a maven build/deploy? Sometime if your zip or jar does not have -SNAPSHOT in it AEM won't update the code when maven deploys.
2- there a /var/classes/sightly in CRX/DE that you can delete compiled classes, I think even in the system console there is an entry
Hope this help.

Bambara actually helped me get to the answer that I needed. It turns out the /var/classes folder holds the compiled sightly files, but it doesn't naturally recompile on build. Deleting that folder on build, then running a page that uses the sightly code forces a recompile and shows the new functionality.

Hopefully others can answer why this happens and how to avoid it. Having faced this a number of times, I'm beginning to think placing Java code into the component folders is not a very good idea. Using a maven multi-module project with a Services/Core bundle, then all Java code could go there. Calling it from the view just requires using the fully qualified classname (including the package). Placing the Java into the services bundle has the following benefits
Allows the classes to be extended. For some reason compilation was unpredictable when extending classes from component folders.
Easier IDE setup. Java classes in component folders in the view module have a Sling specific folder structure, so getting IDE's to provide code assist requires extra effort.
Sling folder conventions follow URI practices and might have dashes, Java packages cannot have dashes... import apps.my-cool-project.components.pages.base.Header; // won't compile

classpath issue with jar files

I'm having a classpath issue with commons-httpclient.jar.
The application uses a newer version of the commons-httpclient.jar, which is commons-httpclient-3.0.1.jar.
Once JBOSS is installed it installs the original version named commons-httpclient.jar in two different directories.
/opt/appname/lib and /opt/appname/server/default/lib
It would be easy to simply remove them, but it turns out that they are being used by another application on the system. I would like to know if there is any way to designate an order to which classes are used, for example.
Use commons-httpclient-3.0.1.jar first then use commons-httpclient.jar
Fairly new to Java, so any help would be appreciated.

I believe the only way to do this under the constraints you mention is to move the right JAR into the server/default/lib folders of each of the apps on your JBoss, and remove it from /opt/appname/lib.
That said, this may still be problematic, as JBoss itself may also need the JAR. In that case, you may be out of luck.
As a side note, you can create a folder /jboss/lib/endorsed. Libs placed there take precedence over those is jboss/lib.

How to load a resource from compound JAR?

I have the following problem:
I am writing an application that uses some of the JARs from the Netbeans Platform. To be exact, I am only using the Netbeans Visual Library for creating some graphs. This can be done without using the Netbeans Platform by extracting 3 JARs from the platform. This is all working well, except for 1 problem.
Some Background
I am using the Java Simple Plugin Framework (JSPF) to handle my plugin management. So I have an application that basically consists of a skeleton framework, and then depending on which plugin JARs it finds, it can do various things, one of which is drawing graphs. The JAR plugin for this functionality has all it's dependant libraries inside. This is done by exporting the JAR as an artifact in IntelliJ, which will unJAR all the dependant libraries and reJAR them inside yours (so everything you need is there).
The Problem
What seems to be happening though, is that when it tries to start use the classes from the embedded libraries, it works fine, but when it needs resources (.png specifically in my case), it complains that it cannot find it.
My Thoughts
The only thing I can think of why it is not working, is that it could be since the plugin JAR is not in a classpath. Could this be it?
Is there anyway to specify a classpath directory in the MANIFEST maybe? Otherwise must I create my own ClassLoader and manually load all the JARs in the plugins directory?
Thank you!
UPDATE:
I have subsequently pinpointed that it is indeed a problem with the classpath. If I place my compound library on the classpath, everything works perfectly. The problem I experience now though is:
If I copy the library to /Library/Java/Home/lib/ext/ it works fine. If I execute the application with java -cp "/path/to/plugins/myLib.jar" -jar Application.jar it does not work.
How can I load all the jars in the plugins directory into my application so the resources inside them can be used?
Thanks again!

So I have finally figured out what was happening. When creating a executable jar, the MANIFEST.MF file overrides any classpath you specify in the command-line, which basically renders it useless if you want to specify external jars. This seems to be a general problem that has been logged since Java 1.3 already.
My simple solution is to simply not create a executable jar, and then launch the application with a script:
java -cp App.jar:plugins/* my.package.structure.App
which works perfectly.

The default classloader's do not load classes in nested jars. You'll need to write your own classloader to get the classes in the nested jars.
You can check out this jspf article...
"I forgot: Adding dependencies as JARs inside JARs is not possible, because it would not work in all scenarios (e.g., applets); IIRC also tools like Eclipse would have problems if you used classes with unresolved (read: runtime-resolved-dependencies). To my knowledge there is no established way yet to gracefully handle nested JARs in all circumstances."
http://code.google.com/p/jspf/wiki/UsageGuide

Easiest way to manage my CLASSPATH?

I'm beginning to play with Clojure a bit and my Java experience is pretty limited. I'm coming from the dynamic world of Ruby and OO, so the functional side of things is very interesting!
Anyway, as I discover libraries and various tools for use (and the tutorial files for the Pragmatic Clojure Book), everything typically calls for placing files in the CLASSPATH in order for Clojure to see the library for use.
Is there such thing as good CLASSPATH practice? Would I ever want to only have a CLASSPATH with just the external libraries of files I need or can I go ahead toss any library or file I would ever need in a directory and simply define it as my CLASSPATH and only require what's needed?
If it helps, I'm an OSX and Emacs user (Using slime and swank-clojure).

I recommend using leiningen and lein-swank to manage this. You can start a REPL in the directory and connect to it from Emacs.

Personally, I'm using a variant of a clojure-project elisp function by Phil Hagelberg, see source in this post to the Clojure group. It sets up the classpath appropriately for the project you'll be working on, then launches SLIME. (EDIT: You'll need to change the value which gets assigned to swank-clojure-jar-path to point to clojure.jar. I'm using (expand-file-name "~/.clojure/clojure.jar") as the default.)
To answer the question about having everything on the classpath all the time vs only throwing in what's needed: to the best of my knowledge, nothing will actually break if you take the first approach (I know I do that for experimental purposes), but apparently things might break with the first approach (see cjstehno's comment below) and in a proper project I find the second to be cleaner. At some point it'll be necessary to determine what libs are being used (and which versions of them), if only to tell leiningen (or maven) about it -- why not keep tabs on it as you go.

We are using Clojure and use a number of infrastructure tools, especially Eclipse (IDE) (http://en.wikipedia.org/wiki/Eclipse_%28software%29) and maven (http://en.wikipedia.org/wiki/Apache_Maven). maven manages libraries and jar dependencies so if you have a number of these and they are likely to grow start using maven.
In answer to your original question you can just put your jars in one directory and you can access them by name every time you run. But you will benefit from the tools...
If you are just exploring then Eclipse will probably manage your jar files fairly painlessly. You can add these to the project as required through the Build Path -> Configure Build Path option.
As your work progresses you will possibly wish to split it into Projects which Eclipse supports so you can add your (or other projects) to your Build Path.
If you use external Clojure libraries look to see if they have been packaged as maven projects (they will have a pom.xml file). The POM will give a list of dependencies.
#

The usual CLASSPATH practice for Java is to put only the jar files needed for a project into this projects class path, which means to have potentially different class paths for diffent projects. This is usually managed by the IDE as part of it's project properties.
Since you are using Emacs and thus probably don't have or use something like projects it might be more convinient for you to set up and use a single global class path for all your clojure related stuff or maybe even simply put all the needed jar files into the java2se/jre/lib/ext directory of your java installation.
The two main problems that could arise from having unneded jar files in your class path are: 1. it has a minor negative impact on the start up time of the JVM and 2. it becomes more difficult to make sure that you are not having classes with different versions in the same class path (i.e. different classes with the same package and name in different jar files).

Since Java SE 1.6 (or JDK 1.6) you can include class path entries by wildcard. If your class files live in .\bin, and your library jar files live in .\lib, then on Windows you could define your class path like this:
set CLASSPATH=bin;lib\*;
This will let you add jar files into the .\lib directory and they will automatically be added to the class path for new instances of the JRE.
See this link for details: Setting the class path
Prior to JDK 1.6 you had to add each jar file onto the ClassPath individually.

I just discovered this bit which I need to give a shot:
(setq swank-clojure-extra-classpaths (list "/class/path/1" "/class/path/2" "/class/path/3" "etc"))

clojure-contrib/launchers/bash/clj-env-dir has an interesting property that you can point it at a directory and it will basically include anything in there. In the past I've had a ~/classpath directory which I would dump any jars into and link any commonly used directories and it worked great. Very simple way to dump and use. Now I tend to use Maven clojure-maven-plugin and that works well also though can be a bit tedious when you just want to muck around with some ideas.