Where does Ant set its 'java.home' (and is

Question

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 1

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).

Answer 2

I had a JDK installed on a Windows box in d:\jdk but running its d:\jdk\bin\java.exe with -XshowSettings showed its built-in java.home pointing to the system-wide JRE install in c:\Program Files\Java\jre1.8.0_91. I suspect that I corrupted my install by deleting d:\jdk\jre without understanding the role of a private JRE vs public JRE.