There are many rt.jar

Question

Question 1:
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- I started with JDK1.2.2 but lately installed JBUILDER development tools that has JDK1.3 I assume I could delete the 1.2.2 files
- I re-adjusted the PATH env variable to contain c:\jbuilder\jdk1.3\bin instead of the previous c:\jdk1.2.2\bin. It did made my life easier when I compile my codes using javac (since I have deleted the old 1.2.2 folder)
This is not a classpath problem/question. I am not having any problem to find my user class when I try to run javac or java.
I assume javac (invoke it under the command prompt in DOS) will find its needed Java platform classes from the new directory jdk1.3 and not from the previous jdk1.2.2 folder. Question is:how does it know? I was told I could safely delete all files under c:\jdk1.2.2 after installing Jbuilder and I did.
Question 2:
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This is similar situation when running an application. I have installed the Java plug-in because I think I need them if I need to test an Applet running under a web browser. For some reasons I installed different versions of Java run time environments onto my same machine. In my machine folder c:\program files\javasoft\ I've got three(3) different subfolders 1.2, 1.3.1, and 1.3.1_02 now.
When I run the JVM command java or the Appletviewer command, how do I know whether the system will find it Java platform classes from the new c:\program files\javasoft\jre\1.3.1.02\lib\rt.jar or from the former c:\program files\javasoft\jre\1.3.1\lib\rt.jar folder?
Question 3:
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How about the rt.jar classes in the development kit. For example, the one residing under c:\jbuilder\jdk1.3\jre\lib\ folder? Which one is my machine choosing, this one or the one under javasoft\jre\1.3.1_02. And how could I find out?
Hope these questions make sense.

Answer 1

Question 1:
I assume javac (invoke it under the command prompt in DOS) will find its needed Java platform classes from the new directory jdk1.3 and not from the previous jdk1.2.2 folder. Question is:how does it know?
The PATH (not CLASSPATH) variable is used by Windows/DOS to locate executable programs. The current (working) directory is automatically included in this list. When you type "javac foo.java", Windows/DOS starts looking for an executable named "javac" - the extension could be exe, com, or bat - first in the current directory, then in each of the directories listed in the PATH, from left to right. The first match is executed; if no match is found, the error message "Bad command or file name" is produced. For the rest of the answer, see Question 3
Question 2:
I assume javac (invoke it under the command prompt in DOS) will find its needed Java platform classes from the new directory jdk1.3 and not from the previous jdk1.2.2 folder. Question is:how does it know?
This is a messy one. First, the current Java Plugin Control Panel has a tab that allows you to specify the version of the plugin. This has evolved from version to version, and the greatest change was made between 1.2 and 1.3. Because of the evolving changes, I'm not sure what you see. (I've forgotten, I'm using 1.4.1_02) Anyway, you can specify a version to be used. If it's left at "Default", then normally the last installed version is used (but problems that Sun has had can change this - see the Buglist). If applets are involved, then it gets really messy - see the Plugin Developer's Guide http://java.sun.com/j2se/1.4.1/docs/guide/plugin/developer_guide/contents.html for the details.
Question 3:
How about the rt.jar classes in the development kit. For example, the one residing under c:\jbuilder\jdk1.3\jre\lib\ folder? Which one is my machine choosing, this one or the one under javasoft\jre\1.3.1_02. And how could I find out?
Depends on what executable is invoked - e.g., the rt.jar that was installed with the version of the executable will be used (in a perfect world). In reality, Sun has had a number of problems getting this matter sorted out (the evolution, ya know), so there could be a number of posssibilities. And you're right, there are a couple of Registry key/values that are involved. The easiest way to find out is to experiment - just rename one of the copies and try it out.
You can delete all of the older versions - unless you need to maintain multiple versions for compatibility purposes. Sun (and me!) recommends this - in fact, you'll get the cleanest update if you delete the old version first, then install the new version. This is about to change though - Sun is working on a real update-in-place - and it may make it into 1.4.2.