Java Glossary : flipping JVMs

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flipping JVMs

If you value your sanity, never install more than one JVM on a machine at a time. However, you may be forced into doing it. How do you flip back and forth between the different versions of the JDK/JREs?

Changing Command Line java.exe, javaw.exe javaws.exe and javac.exe

There are five ways to control which java.exe, javaw.exe, javaws.exe and javac.exe you get.

Put J:\j2sdk1.4.2_04\bin as the very first thing on the path even before C:\winnt\system32. To change the path, click control panel | system | advanced | environment | path | edit. Your changes won't take effect until you start a new DOS box. This will also switch which javac.exe you get.
Use explicit directory names in all your references to java.exe e.g. J:\j2sdk1.4.2_04\bin\java.exe and J:\j2sdk1.4.2_04\bin\javac.exe
Use the JAVA_HOME environment variable, or a temporary override:
```
SET JAVAHOME=J:\j2sdk1.4.2_04
```
Refer to %JAVA_HOME%\bin\java.exe and %JAVA_HOME%\bin\javac.exe in all your *.bat files.

Capture and restore the state of the registry and the C:\WINNT\system32 files. The idea is you capture the state of a JVM just after it is installed. You can the get back there, no matter what strange stuff has happened to the files or the registry. Once you have captured various configurations, you can restore whichever one you want. This is good protection even if you don't plan to switch JVMs on the fly.

The key is, to run a version of the JVM, the registry must be set up to point to it, and its java.exe, javaw.exe and javaws.exe must be in the C:\winnt\system32 directory as the first version encountered on the path. They can then use the registry to find the other related files.

@echo off
echo capture.bat saving Java System state in C:\env\%1
C:
md! \env\%1
cd \env\%1

REM export Java region of registry
regedit /E java.reg "HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft"

REM collect this version's java*.exe files

copy C:\winnt\system32\java.exe
copy C:\winnt\system32\javaw.exe
copy C:\winnt\system32\javaws.exe

REM collect the deployment.properties file for Java WebStart
copy C:\Documents and Settings\Administrator\Application Data\Java\Deployment\deployment.properties

dir

REM -30-

@echo off
echo restore.bat restoring Java System state from C:\env\%1
C:
cd \env\%1

REM import Java region of registry
regedit java.reg

REM restore this version's java*.exe files

copy java.exe C:\winnt\system32
copy javaw.exe C:\winnt\system32
copy javaws.exe C:\winnt\system32

Rem restore deployment.properties file for Java WebStart
copy deployment.properties C:\Documents and Settings\Administrator\Application Data\Java\Deployment

REM -30-

Write a program to fiddle the registry entries and copy the appropriate java.exe, javaw.exe and javaws.exe to C:\winnt\system32 where they will be the first copies visible on the path.
JVM Manager student project

Verifying

Verify you are getting the effect you want by typing:

java.exe -version

Unfortunately javac.exe does not have a -version option, but you can get to recognize the different versions by what they display when you type javac -X. javaw.exe and javaws.exe don't have a -version option either, but once you get java.exe working the others are usually correct to.

Controlling the JRE for your Browser

In theory, you should be able to use click Control Panel | Java and select which JVM of the ones installed you want to use in your browsers. Unfortunately, I have found once you install 1.5 beta, suddenly the other JVMs become invisible. You can also turn Java support on and off for each browser. If the browser can't see Java, click Control Panel | Java, Click browser support off, click apply, click browser support on again, then click apply.

Controlling the JRE for Java Web Start

You control which version of javaws.exe as for java.exe, however, the Java Web Start app itself uses the JNLP file to request which version of the JVM it prefers. It automatically selects the best of what you have available where the JNLP file defines best.

Java Web Start ¤ java.exe ¤ javac.exe ¤ JDK ¤ JRE

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