CLASSPATH, Java Buld Path (eclipse), and WEB-INF\LIB : what to use, when, and why?

Question

I recently switched to J2EE from .NET, and am confused about where to put JAR files. I know that the CLASSPATH, WEB-INF, and Eclipse\'s Java Web Path are all places where J

Answer 1

Eclipse requires you to specify the path to your libraries, jar files (on Properties -> Java Build Path -> Libraries tab). This can be found on the .classpath project file.

Usually you have the JRE libs on its path (which would be on your classpath too), so adding the libs to the classpath and updating eclipse build path would work.

The WEB-INF directory should be the place that contains necessary information for your web application.

Answer 2

I'm not an Eclipse expert, but I think that your problem can be answered like this:

1) CLASSPATH is an environment variable that is read in when you launch java programs and is used by classloader to figure out where the classes are placed.

I would modify the CLASSPATH variable only in the case when you are launching an java program from a script, as this allows you to conveniently launch a program and make sure that the classes are found. This would not be the case for you, as you are developing the web application.

2) WEB-INF/lib is the directory under which the web application container's classloader (like tomcat or glassfish) looks into if your web application needs to resolve a class. So you put there the classes that are used in your web application.

Some IDE's do include the libraries/.jar files that you are using in a project automatically to the package.

3) Eclipse library/classpath resolving during the development time. I would assume, but apologies for assuming, as one really shouldn't do this ;), that you can define a library (add external .jar files to projects) and autocomplete/all the other interesting features should start working with this, as you basically make those classes visible for the IDE with that activity. I also would assume that you can then mark those libraries to be automatically added to the web projects etc., by the IDE.

In general a good reading about how classes are found during execution is here (it's a sun's official documentation). Also a good place to read about this is the ClassLoader class documentation.

Answer 3

Taken together the comments helped me as well. I had added all the jena .jars to the build path from eclipse but that wasn't sufficient. Following suggestion to "add to WEB-INF/lib" it seemed intuitive to drag from libraries folder to WEB-INF (from within eclipse), but that didn't work. Nor did copying the .jars to WEB-INF. I eventually drag-and-dropped from the windows desktop to the WEB-INF lib folder in Eclipse, and that fixed the problem. It would be nice if any .jars added to the build path were automatically copied to WEB-INF lib by Eclipse. In case it matters, this was eclipse EE IDE, Indigo release, on windows 7.

Answer 4

Also, I know that WEB-INF\LIB is a place where you can put JAR files that your web app is going to use. However, I've put JARs in WEB-INF\LIB only to have them be ignored. In what situations should I put JARs into WEB-INF\LIB folder? How do I get Eclipse or the web server to notice them?

The real problem you have here is likely that you didn't got Eclipse for Java EE developers and/or that you just created a generic Java Project instead of a Dynamic Web Project and built up the necessary folder structure yourself.

If you create a Dynamic Web Project in Eclipse for Java EE developers, then Eclipse will automagically add any libraries in WEB-INF/lib to the build path. The build path is roughly said just the classpath which is been used in both compiletime and runtime. With other words: just drop the 3rd party JAR's in there, really nothing more needs to be done.

Note that Java is case sensitive, thus it should be really called WEB-INF/lib, not WEB-INF/LIB. But anyway, if you create a Dynamic Web Project, then Eclipse will just automagically generate the correct folder/file structure for you.

As said by others, ignore the %CLASSPATH% environment variable. It is only used by javac.exe/java.exe and even then only when you do not specify any of the -cp, -classpath or -jar arguments. In real world this environment variable is seldom used, it is just some convenience for starters (and unfortunately also the most confusing one, they should never have invented it).

Answer 5

The CLASSPATH you set in your environment affects only standalone Java applications, i.e. ones you run from a command prompt or an icon. As you've noticed, Eclipse ignores this. It sets up its own per-project classpaths.

javac and java, if called from the command prompt, should/may honor this path, but it's no longer considered great practice to do this. It's turned out that every app needs its own set of stuff, so a global CLASSPATH isn't really doing any of them any good. Modern practice is to simply specify the classpath with the -cp option on the command line for javac or java.

A standalone Web Application server will also set up its own classpath. From the command line or GUI, WebAppServers are usually started by a script (.BAT or .sh) that sets up a classpath using -cp. Tomcat has a directory called common or common/lib where it expects to see libraries that should be available the the server and all programs running under it. But you will generally not need/want to mess with this, as it's customaries for applications to provide their own library collectons in WEB-INF/lib.

So for a Web app, you'd put your varous jars into the lib directory, under WEB-INF, assuming Eclipse pre-builds such a directory structure for you.

All the libs you need also need to be made known to Eclipse. In the Project Explorer, I select the whole slew of them at once, right-click and select Build Path | add to build path. That's easier than messing with Eclipse's project build path manually.

Answer 6

If you're dealing with web applications, /WEB-INF/lib is the portable place to put JARs. This is where web servers servlet containers expect to find an application's jar files.