How to reliably locate Java's rt.jar or equivalent?

Question

How can one reliably locate the jar containing Java's bootstrap classes (rt.jar or equivalent)? I was using the below code, but I've discovered that JAVA_HOME is used to refer to the JDK, not the JRE, and will fail entirely if there is no JDK installed.

def findJRE():
    try:
        home = os.environ['JAVA_HOME']
        path = os.path.join(home, 'jre', 'lib', 'rt.jar')
        if os.path.isfile(path):
            return path

        #For macs
        path = os.path.join(home, 'bundle', 'Classes', 'classes.jar')
        if os.path.isfile(path):
            return path
    except Exception as e:
        pass

Answer 1

As the location and name of the file differs between platforms and it seems there is no environment variable pointing to it I suppose that your best bet is to look for the file in the filesystem:

#!/usr/bin/env python
import subprocess
import re

rtjarPaths = subprocess.check_output(["locate", "rt.jar"])
paths = re.findall('^.*/jre/.*$', rtjarPaths, re.M)
print paths

vicent@deckard:~$ python findrt.py 
['/usr/lib/jvm/java-6-openjdk/jre/lib/rt.jar']

The above code works fine for me on my Ubuntu box. It can be easily extended for working on MacOS X too. Other linux distros may require to adapt the code too.

Update: After some googling I've found in the oracle documentation that the rt.jar file is located in the path stored in the sun.boot.class.path system property. Unfortunately I don't know how to get this property directly from Python or from the command line so I can only provide the following dirty alternative to the previous code.

Create and compile PropertiesTest.java:

public class PropertiesTest {
    public static void main(String[] args)
        throws Exception {
        String value = System.getProperty("sun.boot.class.path");
        System.out.println(value);
    }
}

Then execute the following Python script:

#!/usr/bin/env python
import subprocess
import re

jrePaths = subprocess.check_output(["java", "PropertiesTest"])
rt = re.findall('(?:.*:)?(.*/(?:rt|classes)\.jar):?', jrePaths)
print rt

which should work on both Linux and MacOS X platforms. On my Ubuntu system it gives the output

['/usr/lib/jvm/java-6-openjdk/jre/lib/rt.jar']

Answer 2

I like Vincent's more principled answer, but here is an even simpler hack:

java -verbose 2>/dev/null | sed -ne '1 s/\[Opened \(.*\)\]/\1/p'

prints the absolute path to rt.jar.

Tested with Sun Java 6 and 8 on Linux and Windows (Cygwin), and Sun Java 6 on OS X (where rt.jar is instead called classes.jar).