How do I set the default Java installation/runtime (Windows)?
I'm in the situation where I've installed the JDK, but I can't run applets in browsers (I may not have installed the JRE).
However, when I install the JRE, it clobbers my JDK as the default runtime. This breaks pretty much everything (Eclipse, Ant) - as they require a server JVM.
There's no JAVA_HOME environment variable these days - it just seems to use some registry magic (setting the system path is of no use either). Previously, I've just uninstalled the JRE after I've used it to restore the JDK. This time I want to fix it properly.
This also manifests itself with the jre autoupdater - once upon a time, I had a working setup with the JDK and JRE, but it updated and bust everything.
Answer
This is a bit of a pain on Windows. Here's what I do.
Install latest Sun JDK, e.g. 6u11, in path like c:\install\jdk\sun\6u11, then let the installer install public JRE in the default place (c:\program files\blah). This will setup your default JRE for the majority of things.
Install older JDKs as necessary, like 5u18 in c:\install\jdk\sun\5u18, but don't install the public JREs.
When in development, I have a little batch file that I use to setup a command prompt for each JDK version. Essentially just set JAVA_HOME=c:\jdk\sun\JDK_DESIRED and then set PATH=%JAVA_HOME%\bin;%PATH%. This will put the desired JDK first in the path and any secondary tools like Ant or Maven can use the JAVA_HOME variable.
The path is important because most public JRE installs put a linked executable at c:\WINDOWS\System32\java.exe, which usually overrides most other settings.