I’m start series of articles devoted to download, build and install latest version of OpenJDK 7 under Debian Lenny. And in first article i want to describe how we can get latest source code from official mercurial repository.
At first stage we must read Repository Guide and install mercurial >= 0.9.4. Lenny already include mercurial 0.9.5 (1.0 version can be included in release). Also i create separate dir for openjdk repository in /opt/repos/mercurial/openjdk/openjdk7
Please be attentive, to proper clone openjdk repository you must use fclone commant (this command provided by extension called forest and not included in official mercurial package and Debian repository). Therefore you must manual install this extension:

     mkdir -p /opt/repos/mercurial/mercurial/forest/
     cd /opt/repos/mercurial/mercurial/forest/
     hg clone http://hg.akoha.org/hgforest/
     cp hgforest/forest.py /usr/share/python-support/mercurial/hgext/
   

And the configure your .hgrc to enable the extension by adding following lines:

     [extensions]
     hgext.forest=
     # or, if forest.py is not in the hgext dir:
     # forest=/usr/share/python-support/mercurial/hgext/forest.py
   

After this we are ready to clone openjdk repository:

     mkdir -p /opt/repos/mercurial/openjdk/openjdk7
     cd /opt/repos/mercurial/openjdk/openjdk7
     hg fclone http://hg.openjdk.java.net/jdk7/jdk7
   

This action require around 70 Mb and around 10-20 minutes of time. But after this you would have own copy of main openjdk development tree.
ps: You need fclone because openjdk repository use nested mercurial trees inside main tree. If you try to use clone instead of fclone you dowload only some documentation files without source code.

Added on 23 October 2008: Now OpenJDK 6 is official part of Debian repository: openjdk-6