ObjectAid UML Explorer

Is what I used. It is easily installed from the repository:

Name:     ObjectAid UML Explorer
Location: http://www.objectaid.com/update

And produces quite nice UML diagrams:

Description from the website:

The ObjectAid UML Explorer is different from other UML tools. It uses the UML notation to show a graphical representation of existing code that is as accurate and up-to-date as your text editor, while being very easy to use. Several unique features make this possible:

• Your source code and libraries are the model that is displayed, they are not reverse engineered into a different format.
• If you update your code in Eclipse, your diagram is updated as well; there is no need to reverse engineer source code.
• Refactoring updates your diagram as well as your source code. When you rename a field or move a class, your diagram simply reflects the changes without going out of sync.
• All diagrams in your Eclipse workspace are updated with refactoring changes as appropriate. If necessary, they are checked out of your version control system.
• Diagrams are fully integrated into the Eclipse IDE. You can drag Java classes from any other view onto the diagram, and diagram-related information is shown in other views wherever applicable.

What is your codebase? Java or C++?

eUML2 for Java is a powerful UML modeler designed for Java developper in Eclipse. The free edition can be used for commercial use. It supports the following features:

• CVS and Team Support
• Designed for large project with multiple and customizable model views
• Helios Compliant
• Real-time code/model synchronization
• UML2.1 compliant and support of OMG XMI
• JDK 1.4 and 1.5 support

• The commercial edition provides:

• Advanced reversed engineering

• Powerful true dependency analyze tools
• UML Profile and MDD
• Database tools
• Customizable template support

EDIT: If you're a designer then Papyrus is your best choice it's very advanced and full of features, but if you just want to sketch out some uml diagrams and easy installation then ObjectAid is pretty cool and it doesn't require any plugins I just installed it over Eclipse-JEE and works great !.

Trust Me in June 2012 it's Papyrus, I wish I could write about my experience with the rest of tools like eUML2, UML Lab, ObjectAid, I've tried around 11 UML plugin some I couldn't pass installation process bugs and others turned out to be commercial eUML2 the free edition is uninstallable and the UML Lab (Fancy and great but) is commercial !

Notes:

1. Be aware that most of the plugins hangs when processing so many classes at once but papyrus seems the strongest to hold.
2. The old papyrus project had Synchronization option but it wasn't shipped when they merged with eclipse until this happen let's follow another way

So let's talk about Papyrus Installation I will guide you to how I did what you wanted to do which is Java To UML

1. To save lots of plugins incompatibility and trust me it happens you need to have the latest eclipse modeling tool IDE version.

2. Help-> install modeling components and install the following before you install papyrus {xtext SDK ,Acceleo SDK, Modisco make sure modisco discoverer) then install papyrus

Generating UML

1. Project -> right click-> Discovery -> Discoverer -> Discover KDM code from java project model --> You'll get a property dialog box at the serialization line make set it to TRUE that would generate a file project_KDM.xmi

2. Right click that file -> Discovery -> Discoverer -> Discover UML model from KDM code again you'll get a property dialog set the serialization prop to TRUE to generate a file named project.UML

3. create a folder in your project and copy that UML file to it, this is because sometimes when you delete the papyrus files along with UML files it messes up with the .project file and makes your project loses the Library runtime settings so just put them in that folder instead of main project folder "/" .

4. now go to that folder you created, and pasted the UML file there, then right click on the project.uml file --> new --> papyrus model -> and finish

5. This will create empty project.di file but it in someway references the uml (I wish I knew at the moment how to import the UML file and create so many *.di files but for now I don't so just do it the way i did)

6. ALT+W -> show view -> papyrus model explorer

7. In that view you'll see that your classes are in there and imported but they aren't yet imported to any diagrams so in the next picture you can create a class diagram NOTE: the classes you see in the diagram are because I've already another class diagram done.

8. 

9. Then start dragging and dropping any class

10. To show the class elements (vars, functions etc) right click on any class -> Filters -> show/hide contents Voila !!

11. You can go to pereferences -> papyrus -> class diagram -> change class colors for more friendly color than the empty white boxes

12. one very important setting is Arrange when you drop the classes they get cramped right click on any empty space at class diagram and click Arrange All

13. You'll also find in the Root model (Model explorer view) some elements with Arrow icon beside called unnamed , actually these are the relations between the classes if you grab and drop them you'll see the relation each arrow represents

That's all, Don't buy commercial stuff, papyrus is actually great and sophisticated instead buy them a beer (donate). Disclaimer: I've no relation to the papyrus people infact i didn't like papyrus at first until I did lots of research and experienced it with some patience.

I don't have enough privileges to comment on answers,but just to add to first answer, I used eUML2 and it was really easy to download,install and use.At least for Galileo and Ganymede it works fine.

How about PlantUML? It's not for reverse engineering!!! It's for engineering before you code.

I would recommend EclipseUML from Omondo for general usage, although I did experience some problems a few months back, with my web projects. They had a free edition at one point in time, but that is supposedly no longer promoted.

If you are really keen on reverse engineering sequence diagrams from source code, I would recommend jTracert.

As far as Eclipse projects themselves are concerned, the Eclipse UML2 Tools project might support reverse engineering, although I've have never seen its use in practice.

The MoDisco (Model Discovery) project Eclipse GMT project seems to be clearer in achieving your objective. The list of technology specific tools would be a good place to start with.

You could also give the netbeans UML modeller a try. I have used it to generate javacode that I used in my eclipse projects. You can even import eclipse projects in netbeans and keep the eclipse settings synced with the netbeans project settings.

I tried several UML modellers for eclipse and wasn't satisfied with them. They were either unstable, complicated or just plain ugly. ;-)

You can use the 30 days evaluation build of EclipseUML for Eclipse 3.5 : http://www.uml2.org/eclipse-java-galileo-SR2-win32_eclipseUML2.2_package_may2010.zip This is not the latest 3.6 build but pretty good and don't need to buy it for testing reverse engineering.

Reverse engineering : http://www.forum-omondo.com/documentation_eclipseuml_2008/reverse/reverse/reverse_engineering_example.html

Live flash demo: http://www.ejb3.org/reverse.swf

EclipseUML Omondo is the best tool in the world for Java. Only eUML seems to compete with him on this live java synchronization market but eUML add model tags in the code which is really very very bad and a definitive no go for me.

I found Green plugin very simple to use and to generate class diagram from source code. Give it a try :). Just copy the plugin to your plugin dir.

Something very simillar has been asked before.

By far the best tool I have used for reverse engineering, and round tripping java -> UML is Borland's Together. It is based on Eclipse (not just a single plugin) and really works well.

IBM Rational Systems Developer has this feature built-in. You can do C++ to UML or java to UML transformation..