How many JRE on my Windows machine

Today we are doing some discussion on JRE and one of my friends Lawrence asked me a question "How to find how many JRE are installed on system by Java Code ? " Now I don't think Java have any such API which will tell how many JRE are installed on System and what are they ? But my another friend Vikram has a saying that JRE installation write information in Registry. And here I tried to write this code. It will only run on Windows :) because again I have used Runtime class. I would love to know how the same could be achieved in Unix Systems. This code is not doing anything, just do query from registry and reflects the answer on the console.

import java.io.*;
class NoofJRE {
static String REG_PATH = "reg query " +
"\"HKLM\\Software\\JavaSoft\\Java Runtime Environment";

public static void getJREInfo() {
try {
Process process = Runtime.getRuntime().exec(REG_PATH);
InputStream inputstream = process.getInputStream();
InputStreamReader inputstreamreader = new InputStreamReader(inputstream);
BufferedReader bufferedreader = new BufferedReader(inputstreamreader);
String line;
while ((line = bufferedreader.readLine()) != null) {
System.out.println(line);
}
}
catch (Exception e) {
System.out.println("I am in Exception");
}
}
public static void main(String s[]) {
getJREInfo();
}
}

And here is my output:

E:\Program Files\Java\jdk1.6.0\bin>java NoofJRE
! REG.EXE VERSION 3.0
HKEY_LOCAL_MACHINE\Software\JavaSoft\Java Runtime Environment
CurrentVersion REG_SZ 1.6
BrowserJavaVersion REG_SZ 1.6.0_01
HKEY_LOCAL_MACHINE\Software\JavaSoft\Java Runtime Environment\1.4.2_17
HKEY_LOCAL_MACHINE\Software\JavaSoft\Java Runtime Environment\1.6
HKEY_LOCAL_MACHINE\Software\JavaSoft\Java Runtime Environment\1.6.0
HKEY_LOCAL_MACHINE\Software\JavaSoft\Java Runtime Environment\1.6.0_01

Which sounds true in my case. It is clearly telling me that I have JRE 1.4.2_17, 1.6.0 and 1.6.0_01. I have old bad habit on not uninstalling JRE's :). Please let me know if there is any other way to know how many and which JRE is/are installed in my system.

And to know where it is installed is also easy by querying JAVA_HOME in registry value.

Listing Java Process from Java

Month Ago, one of my colleagues was making an application, actually the UI flavor of JStack. Since JStack asks you the process ID for attaching the particular Java Application with JStack. In a UI based tool, telling user to do Alt-Ctrl-Del and see the process ID,not sounds good. So, my first impression was that you have to show the java process in the drop down and user will select in that. Some day back even, I want to find all process running on my machine from java code for some stupid purpose. I am trying to write some code for both of them. Java can't play with system process and hence invoking a runtime is only solution to get all process and here it is:


import java.io.*;
class ListProcess {
public static void main(String[] args)throws IOException
{
Runtime runtime = Runtime.getRuntime();
String cmds[] = {"cmd", "/c", "tasklist"};
Process proc = runtime.exec(cmds);
InputStream inputstream = proc.getInputStream();
InputStreamReader inputstreamreader = new InputStreamReader(inputstream);
BufferedReader bufferedreader = new BufferedReader(inputstreamreader);
String line;
while ((line = bufferedreader.readLine()) != null) {
System.out.println(line);
}
}
}

Code is written exclusively for Windows Machine :). And one line change in this code will list you only java running process.

String cmds[] = {"cmd", "/c", "jps"}; this is nothing but running jps.exe file in bin (jdk6 onwards). Its not all done. Writing Runtime code is not the real solution as there is little of platform dependencies. So, I have decide to write the code for getting List of Java Process. Again, I have checked by OpenJDK code for jps(search on jps.java file :) ) and I got some hint how to do it and here it goes:

import java.util.*;
import sun.jvmstat.monitor.*;

public class ListJavaProcess {
public static void main(String[] args) throws Exception {

/* Checking for local Host, one can do for remote machine as well */
MonitoredHost local = MonitoredHost.getMonitoredHost("localhost");
/* Take all active VM's on Host, LocalHost here */
Set ids = new HashSet(local.activeVms());
for (Object id : ids) {
/* 1234 - Specifies the Java Virtual Machine identified by lvmid 1234 on an unnamed host.
This string is transformed into the absolute form //1234, which must be resolved against
a HostIdentifier. */
MonitoredVm vm = local.getMonitoredVm(new VmIdentifier("//" + id));
/* take care of class file and jar file both */
String processname = MonitoredVmUtil.mainClass(vm, true);
System.out.println(id + " ------> " + processname);
}
}
}

I have written good amount of comment as it is all together a sun import rather than java or javax import(so no javadoc). This import resides in tools.jar, so even running simple javac and java will not work. So, running the program will go here:

E:\Program Files\Java\jdk1.6.0_10\bin>javac -classpath "E:\Program Files\Java\jd
k1.6.0_10\lib\tools.jar" ListJavaProcess.java


E:\Program Files\Java\jdk1.6.0_10\bin>java -classpath .;"E:\Program Files\Java\j
dk1.6.0_10\lib\tools.jar" Vista

3700 ------> ListJavaProcess

Right now only one java process is running. Now in the second code, you can play with some of the java process, but with native process in the above code you can't do anything except watching it :)