These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild     U ] dev-java/log4j-1.2.15-r1 [1.2.15] USE="jms jmx -doc -javamail -source" 0 kB 

Total: 1 package (1 upgrade), Size of downloads: 0 kB

Would you like to merge these packages? [Yes/No] 
>>> Verifying ebuild Manifests...

>>> Emerging (1 of 1) dev-java/log4j-1.2.15-r1 to /
 * apache-log4j-1.2.15.tar.gz RMD160 SHA1 SHA256 size ;-) ...             [ ok ]
 * checking ebuild checksums ;-) ...                                      [ ok ]
 * checking auxfile checksums ;-) ...                                     [ ok ]
 * checking miscfile checksums ;-) ...                                    [ ok ]
 * checking apache-log4j-1.2.15.tar.gz ;-) ...                            [ ok ]
 * Using: sun-jdk-1.6
>>> Unpacking source...
>>> Unpacking apache-log4j-1.2.15.tar.gz to /var/tmp/portage/dev-java/log4j-1.2.15-r1/work
tar: A lone zero block at 28567
>>> Source unpacked.
>>> Compiling source in /var/tmp/portage/dev-java/log4j-1.2.15-r1/work/apache-log4j-1.2.15 ...
Changing ignoresystemclasses to true for available tasks in /var/tmp/portage/dev-java/log4j-1.2.15-r1/work/apache-log4j-1.2.15/build.xml
Rewriting /var/tmp/portage/dev-java/log4j-1.2.15-r1/work/apache-log4j-1.2.15/build.xml
Rewriting attributes
Rewriting ./tests/build.xml
Rewriting ./src/ntdll/build.xml
Rewriting ./build.xml
 * Disabling all optional ANT_TASKS
!!! ERROR: Package  was not found!
 * 
 * ERROR: dev-java/log4j-1.2.15-r1 failed.
 * Call stack:
 *               ebuild.sh, line   49:  Called src_compile
 *             environment, line 3995:  Called java-pkg-2_src_compile
 *             environment, line 2162:  Called eant 'src_compile' 'src_compile' 'jar' '-f' 'build.xml' '-Djaxp-present=true' '-Djmx-present=true'
 *             environment, line  873:  Called java-pkg_getjars '-Djms-present=true'
 *             environment, line 2805:  Called die
 * The specific snippet of code:
 *       [[ $? != 0 ]] && die "java-config --classpath=${pkgs} failed";
 *  The die message:
 *   java-config --classpath=,sun-jmx failed
 * 
 * If you need support, post the topmost build error, and the call stack if relevant.
 * A complete build log is located at '/var/tmp/portage/dev-java/log4j-1.2.15-r1/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-java/log4j-1.2.15-r1/temp/environment'.
 * 
!!! When you file a bug report, please include the following information:
GENTOO_VM=sun-jdk-1.6  CLASSPATH="" JAVA_HOME="/opt/sun-jdk-1.6.0.07"
JAVACFLAGS="-source 1.4 -target 1.6" COMPILER="javac"
and of course, the output of emerge --info