Category: Java/.NET Runtime

Q

How is the "default" JVM picked at runtime?

A

As you probably know, the Codemesh runtime will normally attempt to pick a default JVM to load if you did not explicitly specify which JVM to use. We say "normally" because you can prohibit this behavior by using an JvmLoader singleton that prohibits default JVM use.

The selection algorithm that is used is relatively simple but you should use this documentation only as a guideline and not rely on the fact that it will remain unmodified. We will definitely tweak this algorithm over time, so the behavior that you observe when you run with a default JVM might change.

Here's the high-level overview:

  1. We look for an environment variable called JAVA_HOME. If set, we interpret it as refering to a directory that is the root directory of a JDK or JRE and try to identify a JVM relative to it.
  2. If JAVA_HOME is not set or if we can't find a valid JVM relative to it, we look in several wellknown directories for a Java distribution. The list of these directories is platform-specific and subject to change.
  3. Inside a potential Java distribution root directory, we use the following preference for selecting a JVM:

                 server

                 client

                 hotspot

                 classic

Finally, a word of caution: relying on a default JVM is not a good idea. We provide this feature because it can be a lifesaver in certain, relatively well-controlled intranet deployment scenarios, but in general you should always explicitly specify a JVM.

Remember: you can always implement your own custom algorithm for picking a "default" JVM to make sure that you only end up with a default JVM that satisfies your requirements. Specify your search criteria and use the configuration API to explicitly choose a found JVM.


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