I am writing a java program using JAVA 6. Our company server is using JAVA 5. They refuse to upgrade it to 6, so the workaround would be install another JRE 6 inside the same machine. They wonder, will installing different version of JRE causes instability?
What the installation process do? Simply copy over the files and setting up environment variable? Will it change any registry or other setting?
Multiple JREs can reside on the same machine. However, if you install JRE6, that's the same as upgrading to Java 6. Java 6 is (as far as I can tell) able to run all older Java code. However, Java 6 binary (.class and .jar files) can not be executed using Java 5, unless they were compiled to target the previous version.
If you need to, you can target Java 5 using a Java 6 JDK. There are command line arguments for javac that you can use (or incorporate into Ant, and probably other build tools) to specify a target JRE. For example, if you used the -target 1.5 command-line option using your JDK, the .class or .jar files that are produced will be executable using the Java 5 JRE.
It's been a while since I have ran two JREs side-by-side, but unless things have changed, there will be two separate java.exe (on Windows, anyway) files - one for the previous Java 5 JRE and a new one for the Java 6 JRE. Due to naming, only one can be in the path at a time - all of the files have the same names, so you can't include the Java 5 and Java 6 java.exe at the same time and expect the right one to magically run. However, you can leave the Java 5 JRE in your path and manually invoke the Java 6 java.exe when you execute your application.
If you use JAVA_HOME set to the Java 5 JRE and set a new environment variable to the Java 6 Java Home, let's say JAVA_1.6, as long as you properly reference the right environment variable, you should be fine.
You can definitely have both. If you are "allowed" maybe you could bundle the jre you want with your app? This URL talks about how.. http://forums.sun.com/thread.jspa?threadID=708451