Improve how the "java" executable is found

[cescoffier]

Right now it expects java to be in the system path.
there are two issues:

1. windows users (java.exe)
2. it may not be the case on all systems

Something like:

private File findJava() {
        String javaHome = System.getProperty("java.home");
        File found;
        if (javaHome == null) {
            found = findExecutableInSystemPath("java");
        } else {
            File bin = new File(javaHome, "bin");
            found = find("java", bin);
        }

        if (found == null || ! found.isFile()) {
            throw new IllegalStateException("Unable to find the java executable in JAVA_HOME or in the system path");
        }
        return found;
    }

public static File findExecutableInSystemPath(String executable) {
        String systemPath = System.getenv("PATH");

        // Fast failure if we don't have the PATH defined.
        if (systemPath == null) {
            return null;
        }

        String[] pathDirs = systemPath.split(File.pathSeparator);

        for (String pathDir : pathDirs) {
            File dir = new File(pathDir);
            if (dir.isDirectory()) {
                File file = findExecutableInDirectory(executable, dir);
                if (file != null) {
                    return file;
                }
            }
        }
        // :-(
        return null;
    }

public static File find(String executable, File... dirs) {
        // First try using the hints.
        if (dirs != null) {
            for (File hint : dirs) {
                File file = findExecutableInDirectory(executable, hint);
                if (file != null) {
                    return file;
                }
            }
        }

        // Not found, try to use the system path.
        return findExecutableInSystemPath(executable);
    }

private static File findExecutableInDirectory(String executable, File directory) {
        if (directory == null || !directory.isDirectory()) {
            // if the directory is null or not a directory => returning null.
            return null;
        }
        for (String extension : EXECUTABLE_EXTENSIONS) {
            File file = new File(directory, executable + extension);
            if (file.isFile() && file.canExecute()) {
                return file;
            }
        }
        // Not found.
        return null;
    }

would be more robust. I will work on a PR.

[dmlloyd]

On Linux we can be very accurate using Files.readSymbolicLink(Paths.get("/proc/self/exe")) (but only if the sun.java.launcher property is equal to SUN_STANDARD, otherwise we might have been launched via JNI).

But don't do stuff like looking for java.exe on Linux or java on Windows; just get the exact expected executable.

[cescoffier]

@dmlloyd the idea is to traverse the system path to find the right executable (and before that check $JAVA_HOME/bin/ if exist).

[lburgazzoli]

maybe you can use maven toolchains too ?

[cescoffier]

you mean the ability to extract java home from maven?

[lburgazzoli]

AFAIR the maven compiler plugin uses toolchains to determine the java executable but I do not know if it is the same problem here :)

[cescoffier]

It's a good idea, I didn't look at it for years! A great opportunity to refresh my memory!

[dmlloyd]

@dmlloyd the idea is to traverse the system path to find the right executable (and before that check $JAVA_HOME/bin/ if exist).

Well that is an idea. :) My point was just that there is only an executable extension on Windows, so we should look specifically for java.exe there and java elsewhere. And, that if you're running Java from Java, at least on Linux you can definitively determine your JVM location.

[cescoffier]

I will first check with toolchain, it would be more reliable.

[starksm64]

I have added toolchain support with fallback to the java.home used to launch maven and I'm testing it on windows.

[gsmet]

This one has been fixed.