Develop Reference

Is it possible to load an eclipse Java project using JDT in the headless mode?

I have an eclipse Java project and want to get information like project source dir, classpaths, etc. My current implementation parses the .project file. But as I didn't find any official documentation describing the structure of the .project file, I have some concerns for the robustness of this approach.
A more convenient and robust way would be to use JDT (headlessly) to load the project and get the relevant information from the IJavaProject object.
Although the developer guide of JDT says
JDT Core packages give you access to the Java model objects and headless Java IDE infrastructure.
all the examples I can find opening an existing Java project get the IJavaProject object from projects within a workspace or use an IProject object. But I couldn't find the way to add a project to the workspace or to construct a IProject/IJavaProject from a path to .project file.
Could anyone please help?

Yes, and can all be done through clear and stable API. org.eclipse.jdt.core is a plug-in, just like org.eclipse.core.resources (which is where you would get an IWorkspace instance), and they both expect to be running within an Eclipse runtime, which can be headless if that's how you write your Eclipse Application. JDT uses the .classpath file to record where sources, libraries, and build output are, and what abstracted references to libraries to use, while the .project file is what records what kind of project it is in general--Java, PHP, Web, some combination of those or others--and a little more information about what builders to execute.
So make yourself a headless Eclipse Application, or package your end-goal functionality inside of one.
https://wiki.eclipse.org/FAQ_What_is_an_Eclipse_application%3F
https://wiki.eclipse.org/FAQ_What_are_extensions_and_extension_points%3F
http://help.eclipse.org/mars/topic/org.eclipse.platform.doc.isv/reference/extension-points/org_eclipse_core_runtime_applications.html?cp=2_1_1_27
http://help.eclipse.org/mars/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/core/resources/ResourcesPlugin.html#getWorkspace--
http://help.eclipse.org/mars/topic/org.eclipse.jdt.doc.isv/reference/api/org/eclipse/jdt/core/JavaCore.html#getJavaCore--
http://help.eclipse.org/mars/topic/org.eclipse.jdt.doc.isv/reference/api/org/eclipse/jdt/core/IJavaProject.html#getResolvedClasspath-boolean-

See http://www.ant4eclipse.org/ for a project which allows you to work inside Eclipse projects.
I used it for a bit some years back, and found that this is too brittle for long term usage and build scripts. I would recommend against relying on internal Eclipse things. They break! Typically when mixing releases.
Instead I would suggest you move to Maven. This is a bit more work, but allows you to use the whole maven infrastructure and API's to do things. What you want to achieve here, may be possible already using a Maven plugin.

Info about version dependancies of enterprise java libraries

I am new to enterprise application developement and trying to create a REST server with Spring REST, JPA taking to mySQL database and Javascript on the client side. As I see loads of opensource libraries doing specific task, I started off with using 'maven project' with different 'arch type' (which is nothing but predefined POM with relevent libraries as I understand) provided by Eclipse. But I often run into version mismatch issues, in many cases found specific solution to that perticular libraries in StackOverflow or other sites.
Hence I started looking for a information these version dependancies such as, this version of JPA works with that version of Hibernate library and so on. I checked maven repository of major libraries, I did not find such information so far.
My queries are:
Where can I find these information about the versions dependancies?
Are these pre defined POM in eclipse reliable? Who owns them, where can I get last modified dates on these maven 'arch types'? (I find the ones I choose having fairly old version of libraries).
If I have to start off on my own creating a Maven dependacies, where will I get information about what are the dependent libraries, for example, if I need spring MVC, for sure it needs java servlet library. I am worried becuase the maven 'spring-mvc-jap' arch type whooping 50 libaries as dependancies.(Coming from embeded domain, I find to too hard to digest :D). So not sure if it is the right way.
Please correct if I am missing anything in my understanding.

You can find this information, if you are using maven and some IDE you can go to the dependancy and make click un > and this will show the librarys used for this dependancy or if you want to use the console you have http://maven.apache.org/plugins/maven-dependency-plugin/examples/resolving-conflicts-using-the-dependency-tree.html this will show the same that the IDE.
After normally in the documentation of the library used, you can find some doc about the dependancy about this.
Other solution is get the .jar and use 7zip to see the POM and know the dependancy used.
And for finish my answer if the IDE tell you that you dont have x dependancy normally you have to add this because any of the other dependancy used have it (they used only for the compiling task), but sometime somes projet change a lots of code between the version .a to .b so you will have some problem using the version .b, at this moment i didnt find one good and easy way to fix this, only using the way that i told you (only if i found some problem in the compilation)

3> I am not sure how it works in eclipse but in IntelliJ IDEA when you start using some class without proper dependency IDEA suggest you to add proper dependency automatically. The same approach should be in eclipse.

How to get Google GIN working with Google-Eclipse plugin in DevMode?

I am experimenting with my first GWT app that uses GIN. I am essentially mimicking the exact same source code example found in their Getting Started tutorial. At the end of that tutorial, the author mentions that you need to supply the the GWT compiler with a classpath pointing to the compiled Java binaries. The example they use for doing this is done using an Ant target.
Although I will have an automated (Ant-driven) build that also emulates this build task, I want to be able to run/debug my GWT apps from inside of Eclipse without having to kick off an Ant build.
So I ask: when running my GWT app inside Eclipse (Dev Mode), how do I configure the GIN library to point to the compiled classes? Thanks in advance!

All you should need to do is add the gin.jar (and dependencies, probably just guice.jar) to your project's classpath.
It should not be required to copy these to WEB-INF/lib/, though depending on your server setup, you may require guice.jar there, but never gin.jar.

How is a Java Project Structured (Compared to a Visual Studio C# Project)?

I'm trying to learn project automation and I'm using the book Pragmatic Project Automation as a guide. It's examples are in Java so I find it easier to follow it along in Java. I don't have any experience using Java or any of its IDEs. However, I've learned some C# using Visual Studio (although I'm still a beginner).
I'm having trouble understanding some parts of setting up a Java project. I'm using Netbeans IDE 7.0 on Windows 7. With Visual Studio, I have a solution with projects underneath. With Netbeans, I appear to just have a project with directories determining the rest of the structure (and the IDE to some degree)? The equivalent to Add Reference appears to be adding a source to the Classpath. There also seems to be a degree of separation between the compiler and the IDE. I'm currently in a situation where I can compile my project just fine while the IDE tells me I still have errors (and I assume this is because I have the project set up incorrectly).
I'm basically looking for analogies that will help me better understand Java project structure.

Lots of similarities between the two languages and IDEs. I spent many years in both. For starters the equivalent to 'add reference' in VS is adding a library or jar in netbeans. In respect to a reference - a jar is pretty much the same thing as a module or .dll in VS. A jar is a compiled reference. To add a reference just go to the project menu and then properties then to the libraries menu from there you can add either pre-assembled netbeans libraries, which are collections of .jar's, or a single .jar, or even a project. Once you add a reference you can import them into your class just like you would in C#.
Netbeans doesn't really have a 'solution' as VS does. You deal with individual projects. It does however have the capability to add a project as a reference so you don't have to continually re-build the references when you change something between multiple projects. It also has project groups to group similar projects.
Lastly Apache ANT is responsible for tying everything together in the background. Netbeans creates a build.xml and build-impl.xml file in the background to tell ANT how to assemble the project into a .jar.
There are my other things to cover but I thing this answers most of your questions. Does this help?

I can't speak for NetBeans, as I use Eclipse, but you are on the right track with classpath being roughly equivalent to references in the Visual Studio world. Libraries (usually .jar files) are placed on the classpath and must be there both at compile time and runtime (you specify the classpath to the compiler at compile time, and to the JVM at runtime). The classpath can contain many different entries, and they can be anywhere in the project structure (or outside of it entirely).
Java itself doesn't impose many restrictions on your project structure, although various IDEs and build tools do. The one thing that is a universal restriction in all Java environments is that source files (and class files) are placed in a directory named after the package name. So if your package name is com.test.something, then your source files will be in SRC_DIR/com/test/something, and your class files in OUT_DIR/com/test/something (note: SRC_DIR and OUT_DIR are not special variables; each IDE will have a different way to specify those directories).
Java libraires tend to heavily build-on one-another, so at some point, you'll find that the classpath entries are too many to manage manually. Before you get there, you'll want to take a look at Apache Maven or Apache Ivy which are dependency management tools. You'll need to understand how they work (either one, not both) and how to integrate them with your IDE. If you use Eclipse and Maven, m2eclipse offers fairly complete integration between the IDE and the dependency management tool.

With Netbeans, I appear to just have a
project with directories determining
the rest of the structure (and the IDE
to some degree)?
Visual Studio dictates a particular project layout and since the compiler is so tightly integrated into the IDE there's no real concept of a build script. In contrast, Java has no such structure (although certain 'best practices' have emerged such as having a 'src' directory for source files, 'lib' for libraries, 'test' for test source, etc.) and a build script is usually required to tell the compiler were to find source files and libraries, what artefacts to produce and a miscellany of other chores (running tests, deployment, creating code metrics and so forth).
In simple cases, the IDE will take care of this for you (if you follow the convention for that particular IDE) but ultimately you will probably want to take a look at a build tool to understand what's going on behind the scenes. Apache Ant and Apache Maven are both prominent offerings. Ant is very flexible whereas Maven attempts to dictate a common layout. I suggest you investigate both and see which suits.
There also seems to be a degree of
separation between the compiler and
the IDE. I'm currently in a situation
where I can compile my project just
fine while the IDE tells me I still
have errors
If your code compiles, it is correct. The IDE is simply acting in an advisory capacity (and will highlight issues beyond compiler errors, such as warning you of potential code mistakes or bad practice).
and I assume this is because I have
the project set up incorrectly
This is a possibility although, as stated above, there are many other explanations.

how to repackage eclipse for my team

I'd like to set up eclipse with a bunch of plugins and DB connection configurations, etc and re-zip it up so my team-mates and new starters can all be working on the same platform easily.
It seems that installing plugins is fine, but when I add in custom jars (e.g. ivy2, ojdbc, etc) they all save with full, absolute paths which probably dont exist on others machines (particularly if they unzip in a different location to me).
Anyway, I'm hoping that this idea is not silly and am working if this sort of process is documented somewhere or if anyone has any tips in general.
Thanks,

I would recommend against requiring all developers to place eclipse in the same location. There are times when some developers may want to try an alternate version of eclipse to explore a technology that requires a different set of plugins or a different eclipse base version.
Let developers install eclipse where they would like.
However, for jars that you need to run plugins (external dependencies that you need to configure for proper plugin usage):
Hardwire a directory for those jars (as opposed to the entire eclipse dir), like c:\eclipse-helpers or something.
To deal with third-party library dependencies (in the code you're developing), you have a few good choices:
Create project(s) to hold the third-party libs and check them into your source version control system (which you are using, right?). You can then add the libs to the build path(s) of the project(s) - make sure you mark them for export in the "order and export" tab of the build path page. You can then simply add these third-party projects as project dependencies.
Reference the third-party jars as CLASSPATH variables when adding them to the build path of your projects. This allows other developers to store the dependencies in different locations. Perhaps define a CLASSPATH variable (in eclipse's Window->Preferences->Java->Build Path->Classpath Variables) called THIRD_PARTY_JARS; each developer can map it to a different path where they want to hold their deps.
Reference the third-party jars as a "user library" (Window->Preferences->Java->Build Path->User library). This is similar to classpath variables, but acts as an explicit set of jars.
Include the third-party jars directly in your projects. Use this option only if you need the deps in a single location.

Although not exactly in line with the direction of the question, you could use Yoxos OnDemand. It allows you to "roll-your-own" Eclipse distro and download it as a zip. They add in their own perspective where you can add more plugins (direct from their repo), or update the plugins that you have.
Although I've never used the feature, you can make make your own stacks and name them, allowing anyone to go to the site later and download it (with the most up-to-date versions of the plugins). Also, dependencies for plugins are resolved automatically if need be.

In eclipse - in many places it's possible to use workspace relative paths or system environment infos to reference external files, too.
Another option could be to place your jars into a workspace project so that every team member can check it out from cvs/subversion/whatever and start working. Working like this ensures a reproducible environment for server builds or for desktops even after years.
Talking about Yoxos...
it provides "Workspace Provisioning" as well. This means you can attach Eclipse Preferences, checkstyle configurations and Mylyn setups additionally to your list of needed tools/plugins for your IDE to your yoxos profile.
This means your team could share a profile and would be able to start working with the same setup regardless of their OS or whatever. (Its possible to use multiple profiles at once, too.)

We did a similar thing with our development environment (it needed both Eclipse and our own plug-in which, in the early stages, had to run in a known location).
We just put it in c:\eclipse_<projName> and made that a requirement for the team. That's probably the easiest solution for you.
It's your team, you can dictate this as a requirement. Unless your team members are absolute idiots, they'll work with you.

I found Yoxos really good and it does very good work in determining dependencies.
Its really a good tool and worth giving a look.

I just started using git to manage my eclipse install. I did a write-up. The approach might work for you, and it's probably worth looking at.

If developers all don't have the same paths on their machine, instead of adding independent JAR files you could create what Eclipse calls a "library" and include a bunch of jars in that. Then another developer just has to change the location of the library and it'll pick up all the jars in there.