Purpose of the "Build Automatically" option in eclipse

What is the purpose of Build Automatically option in Eclipse (Project-->Build Automatically)? Mine will always be checked. But when ever I have some java code changes I still have to do a full build. I was told that it should always be checked. I don't see any benefit in doing so. Please somebody explain.

Yes, you still have to do a build - but it's done automatically when you save (not an external build like an Ant build, but the "internal" build). If you didn't want to build on save, you'd uncheck the box. Having it build automatically makes some people nervous, I believe.

Eclipse has a good answer on their website:

> You have two modes of working: auto-build mode and manual build mode. By default, you are in auto-build mode and Eclipse takes care of compiling source files automatically. Builds occur automatically in the background every time you change files in the workspace (for example saving an editor). Auto-build is convenient because it means problems view, binaries, etc. are are up-to-date at all times. The downside is that in large workspaces auto-builds can be time-consuming if you are changing files in projects with lots of downstream dependent projects. > > If auto-build is taking too long and is interfering with ongoing development, it can be turned off. Once in manual build mode, the user is in complete control over when builds occur and what gets built. Project > Build All (Ctrl+B) can be invoked at any time to trigger what auto-build was doing automatically. This allows you to build up a larger set of changes before invoking a build (Eclipse remembers which files have changed so that it does not have to do more work than required when you do ask for a build.

Note that when they say "auto-build mode" they mean if you have checked "Build Automatically", when they say "manual build mode" they mean you do not have "Build Automatically" checked.

NOTE
For C/C++ users it's told (from help.eclipse.org):

>By default, the Eclipse workbench is configured to build projects automatically. However, for C/C++ development you should disable this option, otherwise your entire project will be rebuilt whenever, for example, you save a change to your makefile or source files. Click Project > Build Automatically and ensure there is no checkmark beside the Build Automatically menu item.

Project - Disable Autobuild option does not always mean autobuild is off. For example "Makegood" test automation plugin will trigger autobuild when Preferences - Run/Debug - launching - (General opt) Build before launch is ON. So turn it off if manual build needed.

One thing that people didn't mention here (and it took a while for me to figure this out too) is that eclipse build (either automatic or manual) would basically follow what you have outlined in the project's "Java Build Path" source tab. So when a file is covered in that build source tab, when you change the file, it gets copied/compiled to the output folder that was specified for that directory/file.

One more difference is:
Most of the time while working on JSPs it helps me when checking "Build Automatically". It automatically picks up the changes in JSPs.
But if you make changes in your java classes/XMLs you need a manual build. Since I'm using resin server which is run outside eclipse via command prompt, i prefer checking the option.
When i have my server setup in eclipse, i turn it off and use Publish on server.

This is the way i use and the answer is based on my experience.

If Build Automatically is checked, the IDE will automatically compile your Java code whenever you modify and save a file. This does not mean it will completely build and package your entire solution as if you were going to deploy it such as if you are working in a Web project on a WAR module. It basically really just compiles your code.