Attempting to find java.exe, on a Windows machine, in a way that's semi-reliable across Java versions, operating system versions, 32 vs 64-bit and whatever else can be thrown at us.

Find-JavaExe.ps1

function Find-JavaExe {
    <#

    .SYNOPSIS

    Locates java.exe on the current system via common conventions.


    .DESCRIPTION

    Searches $env:JAVAHOME, path variables, and the registry (http://stackoverflow.com/a/5415532/211747)


    .EXAMPLE

    Find java.exe, or throw an error if it can't be found.

    $JavaExePath = Find-JavaExe


    .EXAMPLE

    Find java.exe, or return null if it can't be found.

    $JavaExePathOrNull = Find-JavaExe -ErrorAction SilentlyContinue

    #>
    [CmdletBinding()]
    param ()

    $JavaExeSuffix = 'bin\java.exe'

    if ($env:JAVAHOME -eq $null) {
        Write-Debug "`$env:JAVAHOME doesn't exist, going to look elsewhere"
    }
    else {
        $JavaHomeBasedPath = Join-Path $env:JAVAHOME $JavaExeSuffix
        Write-Debug "Testing for $JavaHomeBasedPath, based on `$env:JAVAHOME"
        if (Test-Path $JavaHomeBasedPath) {
            Write-Debug "Found $JavaExePath"
            return $JavaExePath
        }
    }

    $FileSearchPaths = $env:Path.Split(';')
    $JavaExePath = $FileSearchPaths |
        where {
            $PotentialPath = Join-Path $_ $JavaExeSuffix
            Write-Debug "Testing for $PotentialPath, based on path variable $_"
            if (Test-Path $PotentialPath) { $PotentialPath }
        } |
        select -First 1

    if ($JavaExePath -ne $null) {
        Write-Debug "Found $JavaExePath"
        return $JavaExePath
    }

    $RegistrySearchPaths = @('HKLM:\SOFTWARE\JavaSoft\Java Runtime Environment\', 'HKLM:\SOFTWARE\Wow6432Node\JavaSoft\Java Runtime Environment\')
    $JavaExePath = $RegistrySearchPaths |
        where { Test-Path $_ } |
        % {
            $CurrentVersion = (Get-ItemProperty $_).CurrentVersion
            Write-Debug "Current Java version is $CurrentVersion, based on $($_)"
            $VersionKey = Join-Path $_ $CurrentVersion
            $JavaHomeBasedPath = Join-Path (Get-ItemProperty $VersionKey).JavaHome $JavaExeSuffix
            Write-Debug "Testing for $JavaHomeBasedPath, based on $VersionKey\JavaHome"
            if (Test-Path $JavaHomeBasedPath) { $JavaHomeBasedPath }
        } |
        select -First 1

    if ($JavaExePath -ne $null) {
        Write-Debug "Found $JavaExePath"
        return $JavaExePath
    }

    Write-Error "We couldn't resolve java.exe. We looked for `$env:JAVAHOME, then checked environment paths and the registry. The full set of search paths were written to the debug output stream."
}

I seem to recall, but don't have a machine I can test it on to hand, that some JDK/JRE releases put java.exe in %WINSYSDIR%. I don't recall if that directory is explicitly or implicitly in the Windows PATH env variable.

If it isn't, you should also check there directly and early, as it's the version that will be found by most programs.

%WINSYSDIR% only contains a pseudo java executable, which reads the registry to check what the default Java version is, then passes on the execution including arguments to the real Java binary.

Also, here's my implementation. It avoids the registry and any other config, which are sometimes unreliable and just scans the Program directory for any Java directory that still has a \bin directory (Java updates sometimes remove old versions by deleting the \bin dir)

https://github.com/Orbixx/findjava/blob/master/findJava.ps1