This page documents an idea on how to improve the java alternatives system. A proof of concept has been implemented.
Problem
All java related tools such as java, javac, javadoc are symlinks are handled by the alternatives system. Neither the non-privileged user nor the Debian packager can easily override which JVM should be used. There is some support for overriding in the java-wrappers package but the implementation is not very Debianish. That is why we will introduce sensible tools to the java-common package.
Use Cases
Basic Usage
sensible-java, sensible-javac, sensible-javadoc, ... will just run java, javac, and javadoc by default but you can change it by specifying JAVA_HOME:
JAVA_HOME=/usr/lib/jvm/default-java export JAVA_HOME sensible-java ...
You get more flexibility with JAVA_FLAVOR:
JAVA_FLAVOR="default /usr/lib/jvm/java-6-sun" export JAVA_FLAVOR sensible-javac ...
which is a space separated list of codenames or absolute directory names of candidates for JAVA_HOME. JAVA_FLAVOR will be ignored if JAVA_HOME is specified. Some examples for the codenames are:
default which points to default-java
sun which will try openjdk-6, sun-java6, and sun-java5
gcj which points to gcj
The complete list can be found in /etc/java/jvm.list and will be documented later.
The sensible commands will fall back to the alternatives system in case no valid JAVA_HOME can be found. -- IRC discussion: failing is probably a better idea
Setting up CLASSPATH
The contruction of the CLASSPATH can be simplified by specifying
JAVA_LIBS="item1 item2 item3"
where each item can be
a relative filename of a JAR file in /usr/share/java/,
- an absolute filename, or
- an absolute directory name that contains jar files.
The .jar extension is optional for filenames and will be added automatically. All JAR files will be added to the CLASSPATH in lexical order for the directory case. An explicitely specified CLASSPATH has priority over JAVA_LIBS.
Advanced Usage
The program sensible-java-home can be used to improve existing shell wrappers. Just add a line like
JAVA_FLAVOR=default JAVA_LIBS="hello-impl hello-api" export JAVA_FLAVOR JAVA_LIBS eval $(sensible-java-home java javac javadoc)
to the top of the wrapper. That will insert the following code into the wrapper:
JAVA_HOME=/usr/lib/jvm/default-java JAVA_CMD=/usr/lib/jvm/default-java/bin/java JAVAC_CMD=/usr/lib/jvm/default-java/bin/javac JAVADOC_CMD=/usr/lib/jvm/default-java/bin/javadoc CLASSPATH=/usr/share/java/hello-impl:/usr/share/java/hello-api export JAVA_HOME JAVA_CMD JAVAC_CMD JAVADOC_CMD CLASSPATH
The arguments to sensible-java-home are optional and default to java; sensible-java-home finds a JAVA_HOME that is valid for all specified commands.
Ant Support
The line
eval $(sensible-java-home java javac)
will be added to the ant wrapper and allows you to set JAVA_FLAVOR and JAVA_LIBS before calling ant.
Makefile Usage
You can beautify debian/rules without including helper fragments:
export JAVA_FLAVOR = default
export JAVA_LIBS = hello-impl hello-api
build-indep:
sensible-javac ...
ant
Implementation Details
The individual sensible commands are simple symlinks to sensible-java-home which are implemented in dash compatible shell scripts. Unit tests might use something more advanced like python doctests. An environment variable JAVA_DEBUG will enable debugging output.