Emacs. You can take a look at JDEE to get started.

IDEs aren't meant to be lightweight. Use a text editor like Sublime and just compile it in the terminal yourself.

But I wouldn't suggest not using an IDE with Java. Stick with IntelliJ, Eclipse, or Netbeans. Cut the plugins down to a minimum to cut down resource usage.

If you are running into memory issues, that is certainly a RAM issue. This article might be helpful.

Most IDEs are going to consume a lot of memory, because they're complex applications that load many different tools into memory. Many low-power systems can hardly handle the lexing that is so useful in IDEs.

I suggest you give a text editor a try, such as Atom or Sublime. You can then use plugins and the command line tools for Hibernate/Spring and Maven to manually perform the tasks the IDEs were doing for you. This will have a learning curve, but the command line often provides greater control and power over a GUI. A text editor will certainly use less memory.

How low end is it? I used to use IntelliJ Idea and loved, it also ran faster than eclipse for me. DrJava is also very small and light weight. But personally I prefer vim + javac the best. :)