ingyhere ingyhere - 5 months ago 16
Java Question

Where does Ant set its 'java.home' (and is it wrong) and is it supposed to append '/jre'?

Okay, I'm using Ant version 1.7.1 (default install) on CentOS 6.3:

[theuser@dev-ingyhere ~]$ ant -version
Apache Ant version 1.7.1 compiled on August 24 2010
[theuser@dev-ingyhere ~]$ cat /etc/*-release
CentOS release 6.3 (Final)


I have
JAVA_HOME
set and I run
ant
:

[theuser@dev-ingyhere ~]$ export JAVA_HOME=/usr/java/jdk1.7.0_17 ; echo $JAVA_HOME ;
/usr/java/jdk1.7.0_17
[theuser@dev-ingyhere ~]$ ant -diagnostics | grep java\\.home
java.home : /usr/java/jdk1.7.0_17/jre


This is even more fun:

[theuser@dev-ingyhere ~]$ export JAVA_HOME=/a/fools/folly ; echo $JAVA_HOME ; ant -diagnostics | grep java\\.home
/a/fools/folly
java.home : /usr/java/jdk1.7.0_17/jre
[theuser@dev-ingyhere ~]$ env | grep JAVA
JAVA_HOME=/a/fools/folly


So, I do get one thing -- apparently Oracle's Java 7 Javadoc for Class System is WRONG (aghast!) where it describes the
java.home
System Property as the "Java installation directory." I know that because the Java(TM) Tutorials for System Properties describes the
java.home
System Property as the "Installation directory for Java Runtime Environment (JRE)." In other words the
JAVA_HOME
in the environment does not necessarily equal
java.home
in the JVM System Properties. (What sets that?!)

QUESTION: Where and how does
Ant
get/set the system property
java.home
?

Answer

Really a JVM internals question

Since Ant is just echoing the java.lang.System properties (see comment above under original post), this is really a JVM question. The Java HotSpot Virtual Machine is the core interpreter. Code is available online at hg.openjdk.java.net.

On line 309 of the C++ code for HotSpot (os_linux.cpp) there is a an init_system_properties_values() method in the os class. It does some mild heuristics to kind of sniff out the location for a variable named home_path which ends up being set to what Java users see as "java.home". Comments in the code indicate that '<java_home>/jre' is being formally specified as the java.lang.System property value for "java.home" (in the case of a JDK install).