multiple JDKs : Java Glossary

If you want to install multiple JDKs, first uninstall all your JDKs and JREs, in order, uninstalling most recent to oldest. Manually delete all the Java directories, and delete all copies of java.exe, javac.exe, javaws.exe etc. you find anywhere on your machine including in C:\Windows\System32. For extra safety delete all the Java registry entries too. Then install the JREs and JDKs the oldest version first and install them in order. If you want the JDK, there in no need to install the corresponding JRE as well. You will get two JREs bundled with the JDK automatically.

Why go to all this work? Sun tests the installs and uninstalls in the order they release the JDKs/JREs. It would be a huge amount of work to test every permutation of orders in install/uninstall of the JDK versions. Further, if a problem were detected, they could not very well modify an old version to correct the problem because many people already have the defective version installed. Old versions have no knowledge of newer versions. To prevent them from inadvertently meddling with newer versions, they should never be installed/uninstalled in the presence of newer version. On the other hand newer versions are aware of the quirks of older versions, so can be safely installed in their presence, especially the version immediately one prior, the most common case.

Then instead of saying java.exe, use the fully qualified name to select which version of the java run time you want e.g. J:\Program Files\java\jdk1.6.0_17\bin\javac.exe. You an use the Java control panel to select which JRE to use in Java Web Start and in Applets, but not from the command line. Also change set JAVA_HOME J:\Program Files\java\jdk1.6.0_17 to point to the JDK you want to use. Also put J:\Program Files\java\jdk1.6.0_17 on the path.

JAVA_HOME helps Java auxiliary programs like ANT find the JDK. The path lets Windows find the JDK executables you want to use such as javac.exe.

You can also use -source, -target and -bootclasspath to compile with a recent compiler version to approximate what you would have produced with an earlier version.

How Does Java figure out which JDK/JRE to use?

• When you use a browser, the Java plugin DLL looks in the registry to select the JRE to use. You have to the one to use with the Java control panel. It will be the most recent of the JVMs you have enabled. If you launch the Applet via Java Web Start and JNLP, then you have control over which JVM is preferentially used.
• When you use Java Web Start (either inside or outside a browser), the *.jnlp file describes its preferences for JRE. You can further control which JREs are available by configuring in the Java control panel.
• When you type java.exe on the command line, Windows will search the path for it. Usually it finds the dummy version C:\windows\system32\java.exe. It then looks in the registry to find the most recently installed JRE and uses that java.exe.
• When you type a fully qualified java.exe, e.g. C:\Program Files\java\jre6\bin\java.exe, it will use that version.
• When you type javac.exe, Windows will search the path for it. Usually you have put J:\Program Files\java\jdk1.6.0_17\bin on the path, so it will use J:\Program Files\java\jdk1.6.0_17\bin\java.exe. The path overrides anything you set up in the Java control panel or JAVA_HOME.
• When ANT wants to find the current java compiler, it looks in JAVA_HOME.
• In Linux, the correct Java is selected via the JAVA_HOME and PATH.