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What's in an Eclipse .classpath/.project file?

We recently had an issue with an Eclipse project for one of our team members. Tomcat was not deploying JARs of the application.
We eventually noticed the .classpath Eclipse file was not the same as for the team members where the project was OK. We replaced the .classpath file with one from a project that was OK and the Tomcat deploy was complete.
Just out of curiosity and to know at what to look in the future if something is wrong, what is inside the .classpath and .project files. What can I add in there, what does it all mean?

Eclipse is a runtime environment for plugins. Virtually everything you see in Eclipse is the result of plugins installed on Eclipse, rather than Eclipse itself.
The .project file is maintained by the core Eclipse platform, and its goal is to describe the project from a generic, plugin-independent Eclipse view. What's the project's name? what other projects in the workspace does it refer to? What are the builders that are used in order to build the project? (remember, the concept of "build" doesn't pertain specifically to Java projects, but also to other types of projects)
The .classpath file is maintained by Eclipse's JDT feature (feature = set of plugins). JDT holds multiple such "meta" files in the project (see the .settings directory inside the project); the .classpath file is just one of them. Specifically, the .classpath file contains information that the JDT feature needs in order to properly compile the project: the project's source folders (that is, what to compile); the output folders (where to compile to); and classpath entries (such as other projects in the workspace, arbitrary JAR files on the file system, and so forth).
Blindly copying such files from one machine to another may be risky. For example, if arbitrary JAR files are placed on the classpath (that is, JAR files that are located outside the workspace and are referred-to by absolute path naming), the .classpath file is rendered non-portable and must be modified in order to be portable. There are certain best practices that can be followed to guarantee .classpath file portability.

.project
When a project is created in the workspace, a project description file is automatically generated that describes the project. The sole purpose of this file is to make the project self-describing, so that a project that is zipped up or released to a server can be correctly recreated in another workspace.
.classpath
Classpath specifies which Java source files and resource files in a project are considered by the Java builder and specifies how to find types outside of the project. The Java builder compiles the Java source files into the output folder and also copies the resources into it.

Complete reference is not available for the mentioned files, as they are extensible by various plug-ins.
Basically, .project files store project-settings, such as builder and project nature settings, while .classpath files define the classpath to use during running. The classpath files contains src and target entries that correspond with folders in the project; the con entries are used to describe some kind of "virtual" entries, such as the JVM libs or in case of eclipse plug-ins dependencies (normal Java project dependencies are displayed differently, using a special src entry).

This eclipse documentation has details on the markups in .project file: The project description file
It describes the .project file as:
When a project is created in the workspace, a project description file is automatically generated that describes the project. The purpose of this file is to make the project self-describing, so that a project that is zipped up or released to a server can be correctly recreated in another workspace. This file is always called ".project"

How to correctly version control (svn:ignore) a Java project (Maven, Spring)?

i was on a 2-day training introducing as to Java EE. We used there Java EE, Spring Framework, Maven, Springsource Tool Suite (Eclipse), Tomcat.
I took the Eclipse workspace we created there and run it on my work PC. I had, if i remember correctly, only to configure Tomcat properly, and it worked on my PC.
Now i want to save the created Eclipse workspace containing 5 "sub"-projects in subversion so that my work colleagues can checkout this to them and run it on their computers.
How to do this correctly? I found somewhere a svn:ignore rule:
.classpath
.project
.settings
target
Using tortoiseSVN i added to the folder with the workspace this ignore rule, but the found out that the underlying folders target were'nt deleted so i deleted them manually and "added to ignore list". But after that the project in spring source tool suite does not see the mevan dependencies (i think so) because the imports are broken. STS underlines org. in the imports and says it can not resolve this.
How do i correctly version control such a project?

In my project we are using Maven and Eclipse (Helios, currently) and the Maven plugins for Eclipse:
Maven Integration for Eclipse
Maven Integration for WTP
We only have the pom.xml file and the src/ directory tree in our version control system. We make sure not to add the eclipse files there. Then when a new developer starts in the project they do Import -> Maven -> Existing Maven Projects. The Maven plugins for Eclipse then set up perfect build paths, settings and so on.
This way it is also very easy to re-import your projects into Eclipse as needed.
So, my tip is to leave the Eclipse files out of SVN and make sure you can setup the project correctly automatically simply by importing a Maven project.

If I understand your problem the right way, you need to configure Eclipse in order to be able to launch tomcat from it. The key, here, is not maven anymore, but Eclipse, I think. As you've made modifications in your workspace that can't be put in your maven configuration file (the pom.xml), you become "Eclipse dependant".
The key here is that, as you're Eclipse dependant, you need the Eclipse configuration files to work. Consequently, I'm afraid you need to add back .classpath, .project, .settings to your versioning tool... It's not generic, because you force people who work on your project to use Eclipse. But if everybody in your team do so, it shouldn't be a problem.
As I don't use Eclipse anymore, I don't know if versioning theses files can lead to problems. However, I hope this answer will help you to configure your project back...
EDIT : to be more accurate... and maybe give a better answer.
When using a version control system, the main goal is often (always ?) to give all the keys to use the sources, and develop from them. Consequently, you need to put in your VCS your sources, and all the configurations needed to use them efficiently.
In your specific case, the key is that you've become Eclipse dependant through its Springsource Tool Suite plugin. Consequently, it becomes essential to add the configuration files for this tool, because they can't work without them, and if they can't work, you can't work.

I can tell you my way of subversioning maven eclipse projects. First, when you create the project structure you have to commit the .setting, .classpath, .project files into the subversion repository. If you can't do this the other colleagues will can not use the project structure after checkout. After you commit the project structure, the best way is not to commit these files except only when you change something important eclipse or build path settings, because the others will have conflicts due to the system dependent informations. Never commit the maven target directory. Sorry for my english. Hope it helps.

Checking in an Eclipse project into SVN

I want to checkin a Dynamic Web Project I created in eclipse into svn. Can someone tell me which files I have to check in and which one I should not? The idea is to be able to check out the project using the New Project Wizard so that I can create the Dynamic Web Project again. More specifically here are the files/directories I have in the project --
src
WebContent
build
dist
build.xml
.project
.classpath
.settings/
The build directory is not supposed to checked in obviously. What about the other ones?
I am guessing all the . files should not be checked in either. Can some one verify this?
What is this dist directory and the .settings directory?
Also where does eclipse store the Server information (tomcat)? I don't want to check it in either.
EDIT:
I initially checked in all of the above except the build directory of course. When I checked out the project from inside Eclipse it did not prompt me to create a new project since the .project is there but Eclipse was creating a JavaEE project or something instead of the Dynamic Web Project. Did anyone else run into this behavior?
** EDIT 2 **
Found it! Turns out I should not check in the following --
.project
.settings/
.classpath
Once these 3 are removed the New project Wizard works as expected and everything is fine.

If you check in .classpath/.project/.settings you make your project Eclipse-specific. What about developers who work with Netbeans or IntelliJ? IMO it is cleaner to keep your project IDE-independent and easy to set up.
I usually go for a Maven build. The pom.xml specifies all the required dependencies and mvn eclipse:eclipse generates the .classpath/.project files for you.
The .settings directory contains local settings (like which Java version you want to use). IMO it is not useful to check this in. You can enforce Java version compliance via the Maven2 pom.
Finally, for your next project, my protip is to svn-ignore the files or directories you don't want in SVN before your first commit. In a Maven2 setup that would be .settings .classpath .project target (the default output directory of Maven2) and any other generated stuff (log files, gfembed directories, etc). In your case you would ignore build and dist instead of target.
You can svn-ignore files or directories with RIGHT_MOUSE->Team->'Add to svn:ignore' (I use the Subclipse plugin). Ignore instructions are stored as svn-properties on the parent directory. The properties on a directory can be viewed by RIGHT_MOUSE->Team->Show properties. You can also edit the properties directly there by clicking on the value field. Make sure there is an end of line after each property.
Now that you have already committed and then removed these files, ignoring is not going to work anymore in my experience. Somehow I have never managed to successfully ignore generated files which have ever been checked into the SVN repository; they are like zombies, always coming back from the dead. Maybe by deleting their entries physically in the SVN repo this can be achieved, but I've never done it.

In our case, we have checked in all you mentioned in the list except, .settings/.
With .classpath and .project checked in, users can quickly check out the project and fire up Eclipse on a new computer and just start working on it; the alternative being to configure the project manually and adding in all the jar dependencies painstakingly (if you use ant). Many open source projects do this.
Read this, there are some really good points to ponder about.

Good Question... Many of us are in a dilemma on whether we want to check in IDE related files or not. I normally go for checking in .classpath for eclipse and I use eclipse variables to make sure that team needs to just change the variable value and it works. We also check in .project so that team need not to create new project in their workspace.

I would omit the .project, .settings/, dist, and build.
The .classpath can be left in if you use variables instead of hardcoded paths. This is useful so you don't have to rebuild your classpath every time you check out the project.

Which files in a Java Netbeans project should be placed under version control?

I'm working on a Java Netbeans 6.7 roject with a few developers and we're using Mercurial for version control. I was wondering what I should put in the .hgignore file (ie. what files should not be added to the repository). I know I will definitely want the src, test, and lib folders in the repository and the build folder to not be in the repository. But I'm not sure what to do about the nbproject folder.

You must ignore the following folders: build (or nbbuild), dist (or nbdist), and the nbproject/private.
And if you want others to open it with netbeans (as you do in netbeans) you must add nbproject folder.
Though nbproject/private should be ignored, nbproject should be
checked into the version control system.
If you want more information, see netbeans help on http://netbeans.org/kb/docs/java/import-eclipse.html#versioning

If you have created these projects under an existing clone/repository, the NetBeans/Mercurial integration will already have a good idea of what to include or ignore.
NetBeans will automatically include files in nbproject that are considered to be environment independent. Files that include environmental dependencies (like directory paths) are usually placed in nbproject/private directory. You probably do not need to save those to your repo.
You may want to exclude the nbproject/build-impl.xml... it is generated when the project is opened... But, if you do not have it in your repo you will run into problems if you attempt to build the project with ant, independent of the IDE.

Anything that can be generated from what you check in should not be in the repository: no .class, .war, generated .jar, etc.

Eclipse and SVN: Missing .project file

I'm working on a uni project with a few other people using SVN. Much to my annoyance the .project file was removed from the repository since "it contains platform specific information". However, this has obviously broken my setup in Eclipse, giving me the error:
Problems occurred opening the selected resources.
The project description file (.project) for '_________' is missing. This file contains important information about the project. The project will not function properly until this file is restored.
Any suggestions? Thank you.

Simply revert the project to a revision that featured the .project file, then put in .svnignores and to back to the current revision again. Or even simpler:
svn cp -r15 .project .project
Where the number after -r is a revision featuring the .project file.

.project files don't belong in an SCM, they contain developer-specific information. Common configuration belongs in some standardized file like (e.g.) a maven pom.xml, from which a .project file is automatically generated, but things like .project, .settings, .classpath should always be in svn:ignore, which means you can keep your own copy without overwriting others

That should be easy - checkout an older revision of the project where the .project file is still available, to a temporary location and copy this old version of .project to your actual project folder. You can use the Navigator view which is more convenient for this special copy'n'paste task. (Or do it on the file system outside eclipse)
Once the .project file is back in place you can continue to work as usual.
Alternative: create an empty java project, copy the autogenerated .project to your active project and reconfigure that project.