Building Apache Tomcat from source is very easy, and is the first step to contributing to Tomcat. The following is a step by step guide.

Tomcat requires a JDK (version 1.6 or later) to be installed.
The Sun JDK can be downloaded here.

IMPORTANT: Set an environment variable JAVA_HOME to the pathname of the directory into which you installed the JDK release.

Download a binary distribution of Ant 1.8.1 or later from here.

Unpack the binary distribution into a convenient location so that the Ant release resides in its own directory (conventionally named apache-ant-1.8.x). For the remainder of this guide, the symbolic name ${ant.home} is used to refer to the full pathname of the Ant installation directory directory.

IMPORTANT: Create an ANT_HOME environment variable to point the directory ${ant.home}, and modify the PATH environment variable to include directory ${ant.home}/bin in its list. This makes the ant command line script available, which will be used to actually perform the build.

Tomcat SVN repository URL: http://svn.apache.org/repos/asf/tomcat/trunk/

Tomcat source packages: http://tomcat.apache.org/download-70.cgi.

Checkout the source using SVN, or download and unpack a source package. For the remainder of this guide, the symbolic name ${tomcat.source} is used to refer to the location where the source has been placed.

Use the following commands to build Tomcat:

cd ${tomcat.source}
ant


NOTE: Users accessing the Internet through a proxy must use a properties file to indicate to Ant the proxy configuration. Read below for details.

WARNING: Running this command will download binary binaries to the /usr/share/java directory by default. Make sure this is appropriate to do so on your computer. On Windows, this usually corresponds to the C:\usr\share\java directory, unless Cygwin is used. Read below to customize the directory used to download the binaries.

The build can be controlled by creating a ${tomcat.source}/build.properties file and adding the following content to it:

# ----- Proxy setup -----
# Uncomment if using a proxy server.
#proxy.host=proxy.domain
#proxy.port=8080
#proxy.use=on

# ----- Default Base Path for Dependent Packages -----
# Replace this path with the directory path where
# dependencies binaries should be downloaded.
base.path=/usr/share/java


Once the build has completed successfully, a usable Tomcat installation will have been produced in the ${tomcat.source}/output/build directory, and can be started and stopped with the usual scripts.

IMPORTANT: This is not a supported means of building Tomcat; this information is provided without warranty :-). The only supported means of building Tomcat is with the Ant build described above. However, some developers like to work on Java code with a Java IDE, and the following steps have been used by some developers.

NOTE: This will not let you build everything under Eclipse; the build process requires use of Ant for the many stages that aren't simple Java compilations. However, it will allow you to view and edit the Java code, get warnings, reformat code, perform refactorings, run Tomcat under the IDE, and so on.

Sample Eclipse project files and launch targets are provided in the res/ide-support/eclipse directory of the source tree. The instructions below will automatically copy these into the required locations.

An Ant target is provided as a convenience to download all binary dependencies, and to create the Eclipse project and classpath files in the root of the source tree.

cd ${tomcat.source}
ant ide-eclipse


Start Eclipse and create a new Workspace.

Open the Preferences dialog and then select Java->Build Path->Classpath Variables to add two new Classpath Variables:

TOMCAT_LIBS_BASE	The same location as the base.path setting in build.properties, where the binary dependencies have been downloaded
ANT_HOME	the base path of Ant 1.8.1 or later

Use File->Import and choose Existing Projects into Workspace. From there choose the root directory of the Tomcat source tree (${tomcat.source}) and import the Tomcat project located there.

start-tomcat and stop-tomcat launch configurations are provided in res/ide-support/eclipse and will be available in the Run->Run Configurations dialog. Use these to start and stop Tomcat from Eclipse.
If you want to configure these yourself (or are using a different IDE) then use org.apache.catalina.startup.Bootstrap as the main class, start/stop etc. as program arguments, and specify -Dcatalina.home=... (with the name of your build directory) as VM arguments.

Tweaking a few formatting preferences will make it much easier to keep consistent with Tomcat coding conventions (and have your contributions accepted):

Java -> Code Style -> Formatter -> Edit...	Tab policy: Spaces only Tab and Indentation size: 4
General -> Editors -> Text Editors	Displayed tab width: 2 Insert spaces for tabs Show whitespace characters (optional)
XML -> XML Files -> Editor	Indent using spaces Indentation size: 2
Ant -> Editor -> Formatter	Tab size: 2 Use tab character instead of spaces: unchecked

The same general approach should work for most IDEs; it has been reported to work in IntelliJ IDEA, for example.