NSIS - How to Detect Whether Java is Installed in the System

While writing my installer I wanted to install JRE (embed JRE installer in my installer) only if Java is not installed already. So I had to detect whether Java is already installed.

My previous post on NSIS points to a quick start guide (in Harshani's blog) for NSIS, including how to install NSIS and how to write a simple installer. So at this point you should have installed NSIS and been somewhat familiar with NSIS scripting (sections, etc..). So here I am posting the piece of script I used to detect whether Java is installed in the system.

Section "find java" FINDJAVA

  StrCpy $1 "SOFTWARE\JavaSoft\Java Runtime Environment"
  StrCpy $2 0
  ReadRegStr $2 HKLM "$1" "CurrentVersion"
  StrCmp $2 "" DetectTry2
  ReadRegStr $5 HKLM "$1\$2" "JavaHome"
  StrCmp $5 "" DetectTry2
  goto done

  DetectTry2:
  ReadRegStr $2 HKLM "SOFTWARE\JavaSoft\Java Development Kit" "CurrentVersion"
  StrCmp $2 "" NoJava
  ReadRegStr $5 HKLM "SOFTWARE\JavaSoft\Java Development Kit\$2" "JavaHome"
  StrCmp $5 "" NoJava done

  done:
  ;All done. 

  NoJava:
  ;Write the script to install Java here

SectionEnd

In 3rd line of the script, it copies the path "Software\JavaSoft\Java Runtime Environment" to the variable 1. It is the Windows registry entry in which we should look for Java. In line 5 it reads the value of registry attribute "Current Version" which belongs to "SOFRWARE\JavaSoft\Java Runtime Environment", to the variable 2. ReadRegStr is the command used to read the registry. Here, "HKLM" means that it should look for this key under "HKEY_LOCAL_MACHINE" registry main key. 

In line 6 the command StrCmp compares the value of var 2 with "empty". If it is empty then that means there is no JRE installed in the system. So it passes the control to label "DetectTry2" which tries to detect if there is any Jdk installed. Line 7 is executed only if there is a value for JRE version. If so it tries to find the value for JavaHome attribute in key "Software\JavaSoft\Java Runtime Environment\1.x.x". In line 8 it checks whether JavaHome is null and if so, then again it jumps to label "DetectTry2". If it is not null that means there is a JRE installed and line 9 passes it to label "done".

Now let's see what "DetectTry2" label does. It is executed only if there is no JRE installed. In line 12 it reads the value of  "CurrentVersion" attribute of key "SOFTWARE\JavaSoft\Java Development Kit". In line 13 it checks whether the value it got is empty. If so it will hit the label "NoJava" means that neither JRE nor JDK is installed. If it is not empty then line 14 will check whether JavaHome is empty like before. Again, if JavaHome is empty it jumps to "NoJava". If not it jumps to "done".

If we ever hit "done" then that means either JRE or JDK is installed in the system. So there is nothing to do there. If we hit NoJava then we are going to install Java. I will post the script for that in a later post, NSIS - How to embed some other installer in your own installer.

This is valid for any other software like .NET framework, MS Office, etc as long as you know the correct registry entries to look in.