Very Computer

public class JavaTest
{
        public static void main(String[] argv)
        {
                System.out.println(System.getProperty("java.home"));
                System.out.println(System.getProperty("java.version");
        }

Quote:}

Make the executable jar:
$ echo 'Main-Class: JavaTest' > manifest
$ jar cmf manifest JavaTest JavaTest.class
$ chmod +x JavaTest

When I execute it using the java command it uses the JVM of the first
java on the PATH - exactly as expected:
$ java JavaTest
/opt/jdk1.5.0/jre
1.5.0

The JVM is also confirmed by:
$ java -version
java version "1.5.0"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0-b64) Java
HotSpot(TM) Client VM (build 1.5.0-b64, mixed mode, sharing)

When I execute the jar I get what I expect:
$ ./JavaTest
/opt/jdk1.5.0/jre
1.5.0

If I change the PATH so another JVM is first I get unexpected results:

$ PATH=/usr/java1.2/bin:$PATH java -version
java version "1.2.2"
Solaris VM (build Solaris_JDK_1.2.2_10, native threads, sunwjit)

$ PATH=/usr/java1.2/bin:$PATH java JavaTest
/usr/java1.2/jre
1.2.2

$ PATH=/usr/java1.1/bin:$PATH java -version
java version "1.1.8"

$ PATH=/usr/java1.2/bin:$PATH ./JavaTest
/opt/jdk1.5.0/jre
1.5.0

$ PATH=/usr/java1.1/bin:$PATH ./JavaTest
/opt/jdk1.5.0/jre
1.5.0

With another machine:
$ java -version
java version "1.4.1_01"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.1_01-b01) Java
HotSpot(TM) Client VM (build 1.4.1_01-b01, mixed mode)

$ ./JavaTest
/usr/java1.2/jre
1.2.2

If I compile with java v1.5 it still execs an old version which duly fails.

"truss -ef ./JavaTest" shows that it is execve(2) executing the jar file,
it also shows the PATH active for execve, but in choosing a JVM execve is
not using the PATH.

Finally the question:  How does execve decide which JVM to use when
executing a jar file?

For test I have made a trivial Java program that prints the java version
and directory from the System properties:

public class JavaTest
{
        public static void main(String[] argv) {
                System.out.println(System.getProperty("java.home"));
                System.out.println(System.getProperty("java.version");
        }

Quote:}

Make the executable jar:
$ echo 'Main-Class: JavaTest' manifest
$ jar cmf manifest JavaTest JavaTest.class
$ chmod +x JavaTest

When I execute it using the java command it uses the JVM of the first java
on the PATH - exactly as expected:
$ java JavaTest
/opt/jdk1.5.0/jre
1.5.0

The JVM is also confirmed by:
$ java -version
java version "1.5.0"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0-b64) Java
HotSpot(TM) Client VM (build 1.5.0-b64, mixed mode, sharing)

When I execute the jar I get what I expect:
$ ./JavaTest
/opt/jdk1.5.0/jre
1.5.0

If I change the PATH so another JVM is first I get unexpected results:

$ PATH=/usr/java1.2/bin:$PATH java -version java
version "1.2.2"
Solaris VM (build Solaris_JDK_1.2.2_10, native threads, sunwjit)

$ PATH=/usr/java1.2/bin:$PATH java JavaTest
/usr/java1.2/jre
1.2.2

$ PATH=/usr/java1.1/bin:$PATH java -version
java version "1.1.8"

$ PATH=/usr/java1.2/bin:$PATH ./JavaTest
/opt/jdk1.5.0/jre
1.5.0

$ PATH=/usr/java1.1/bin:$PATH ./JavaTest
/opt/jdk1.5.0/jre
1.5.0

With another machine:
$ java -version
java version "1.4.1_01"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.1_01-b01) Java
HotSpot(TM) Client VM (build 1.4.1_01-b01, mixed mode)

$ ./JavaTest
/usr/java1.2/jre
1.2.2

If I compile with java v1.5 it still execs an old version which duly
fails.

"truss -ef ./JavaTest" shows that it is execve(2) executing the jar file,
it also shows the PATH active for execve, but in choosing a JVM execve is
not using the PATH.

Finally the question:  How does execve decide which JVM to use when
executing a jar file?

--
 .-.   .-.    Yes, I am an agent of Satan, but my duties are largely
(_  \ /  _)   ceremonial.
     |

Thank you.

So it seems to do that; that's weak!  I would have hoped it invokes
something like isexec which found a suitable java from the PATH and the
version info.  I can set the link for myself but means I can't use
executable JARs for other people in places unknown.  I must add use
wrapper script and it all gets messy as it's delivered in more parts.

set javaexec:jexec = "/usr/java/bin/java"

/usr/java/bin/java would be replaced with your java interpreter,
e.g. /usr/j2se/jre/bin/java or /usr/j2se/bin/java, etc., which
is then called with -jar and your JAR file as arguments (you can
change -jar by setting javaexec:jexec_arg).

Markus

Quote:> Finally the question:  How does execve decide which JVM to use when
> executing a jar file?

The only way to change this is to put the following in /etc/system and
reboot :
set javaexec:jexec = "/path/to/my/jexec"
set javaexec:jexec_arg = "-other -options"

There should be little from stopping you writing your own jexec which is
PATH-aware, just be aware that it might break other things on the system
if you're not careful.

  Scott.

As of Solaris 8 FCS, setuid java executables weren't supported, so one
wouldn't have to reset PATH on there to a safe default value (like
/usr/bin for non-root or /usr/sbin:/usr/bin for root) if effective uid !=
real uid (or effective gid != real gid or one of the gid list members),
although it would probably be very wise to do that extra work anyway, in
case javaexec at some later version does support setuid java executables.

So, think that would work???

--

Lasik/PRK theme music:
    "In the Hall of the Mountain King", from "Peer Gynt"

Quote:Richard L. Hamilton writes:
> To get more flexibility, could one create an executable wrapper that
> attempted to locate "java" via PATH, and if it couldn't, fell back to the
> original hardcoded value (typically /usr/java/jre/lib/{i386|sparc}/jexec),
> and passed all its args to it; then set javaexec:jexec in /etc/system to
> point to that? Seems that would let people have it locate the java
> interpreter via PATH if possible, else fall back to the default one.

Markus

----

Bye,
Roland

--
  __ .  . __

  \__\/\/__/  MPEG specialist, C&&JAVA&&Sun&&Unix programmer
  /O /==\ O\  TEL +49 641 7950090
 (;O/ \/ \O;)

My original question was how Solaris selected a JVM but of course I would
like to be sure this worked in a wider OS context.  Java is supposed to be
platform independent but its invocation isn't.  Improved executable JARs
could do it.