reloading classes : Java Glossary

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reloading classes
If the code for a class changes, Java will not automatically notice and load the new version, until you start up the JVM (Java Virtual Machine) again. What you can do is create a new ClassLoader, and use it to load the new code for the class. You still have a very difficult problem of tracking down all instances of the old class and getting rid of them, which will eventually cause the old class to unload. There is no direct way to unload the old class, or upgrade the objects. The JVM does not mind having objects of both the old and new version of the class lying about, however, the ClassLoader may get into its head it has to reload many of the other classes, since it has no record of loading them itself. You will end up with duplicates of many classes.

You are probably better off not to use a new ClassLoader, but instead to rename the class, and then you can track down the old objects and pass them to the new constructor to upgrade them. You then have to update all the references to make them point to the new object. By using one layer of indirection, you can make that task easier since there is only one link to update. You have to ensure old and new share a common base class or interface that you can use for references.

Another possible object-upgrading tool is serialization where you serialise version-marked objects and reconstitute them with new code. It is slow but convenient. I have not experimented with any of these techniques personally.

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