Find absolute java.exe path programmatically from java code

Question

If I have a java jar or class file which is launched by the user (assuming java path is set in environment variables), so how can i from within the code, figure out absolute path of java.exe/javaw.exe from which this file is being launched.

Like on ubuntu we can run: % which java and it shows the path.

However on windows, if i check System.getenv() it may happen that there are multiple path's found e.g for old or new version. If through cmd line, I run java -version it does not show the path.

Can you tell me either through pure java or command line on windows how is it possible to find out the location of javaw.exe?

Solution 1

String javaHome = System.getProperty("java.home");

---

Can you tell me either through pure Java ... on windows how is it possible to find out the location of javaw.exe?

E.G.

import java.io.File;

class JavawLocation {

    public static void main(String[] args) {
        String javaHome = System.getProperty("java.home");
        File f = new File(javaHome);
        f = new File(f, "bin");
        f = new File(f, "javaw.exe");
        System.out.println(f + "    exists: " + f.exists());
    }
}

Output

C:\Program Files (x86)\Java\jdk1.6.0_29\jre\bin\javaw.exe    exists: true
Press any key to continue . . .

And yes, I am confident that will work in a JRE.

Solution 2

On Windows, the java.library.path System Property begins with the path to the bin directory containing whichever java.exe was used to run your jar file.

This makes sense, because on Windows the first place any executable looks for DLL files is the directory containing the executable itself. So naturally, when the JVM runs, the first place it looks for DLLs is the directory containing java.exe.

You can acquire the path to java.exe as follows:

final String javaLibraryPath = System.getProperty("java.library.path");
final File javaExeFile = new File(
  javaLibraryPath.substring(0, javaLibraryPath.indexOf(';')) + "\\java.exe"
);
final String javaExePath =
  javaExeFile.exists() ? javaExeFile.getAbsolutePath() : "java";

This code is Windows-specific - I hard-coded the path separator (;) and the file separator (). I also put in a fallback to just "java" in case the library path trick somehow doesn't work.

I have tested this with Java 6 and 7 on Windows 7. I tried a 32-bit and 64-bit version of Java.

Solution 3

Here's a slightly more generalised solution that I came up with. Maybe useful:

private static String javaExe()
{
    final String JAVA_HOME = System.getProperty("java.home");
    final File BIN = new File(JAVA_HOME, "bin");
    File exe = new File(BIN, "java");

    if (!exe.exists())
    {
        // We might be on Windows, which needs an exe extension
        exe = new File(BIN, "java.exe");
    }

    if (exe.exists())
    {
        return exe.getAbsolutePath();
    }

    try
    {
        // Just try invoking java from the system path; this of course
        // assumes "java[.exe]" is /actually/ Java
        final String NAKED_JAVA = "java";
        new ProcessBuilder(NAKED_JAVA).start();

        return NAKED_JAVA;
    }
    catch (IOException e)
    {
        return null;
    }
}

Comments

[Thomas]

Why do you need the location of the exe? Might you want to get the location of the jar instead?

[Johnydep]

@Thomas, i need both. Location of jar can be found easily. I need to install my application as a windows service, and for that i need the abs path of default javaw.exe Actually it is there in Path but i am confused as it can show multiple entries of old and new versions, wondering how would i now which one is the default one?

[Johnydep]

Thank you, actually on my machine if i check Path=C:/Program Files/Java/jre7/bin; while JAVA_HOME=C:\Program Files\Java\jdk1.6.0_26 So i am just wondering for system without jdk, would JAVA_HOME still be set with JRE Path?

[Johnydep]

Actually to test it i would have to uninstall jdk, but if you could confirm then i will have my answer, thank you

[a_horse_with_no_name]

But the JAVA_HOME is not necessarily the java.exe that is used to launch the currently running application. I have three different JDKs installed and only a single JAVA_HOME.

[Johnydep]

@a_horse_with_no_name, yes you are right but Andrew's answer is what i was looking for. I was trying System.getenv("JAVA_HOME") while what i needed was: System.getProperty("java.home");

[rogerdpack]

appears you can also programmatically tell whether you're running java vs. javaw or not, see stackoverflow.com/questions/14632272/…

[kevinarpe]

For future readers, the direction layout of JDK8 and JDK11 are different. However, this answer works reliable on both directory layouts. It is also useful to find keytool(.exe)