不用重启使用新Class - JavaRebel Brings Class Reloading

作者:anotherbug 日期:2007-10-13 14:33:47

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JavaRebel Brings Class Reloading to Java

JavaRebel reloads changes to Java classes on-the-fly without redeploy or restart including new methods and fields. It is a generic solution that works for standalone Java applications as well as application servers. For the impatient: watch the demonstration screencast (~5 mins) and download.

Ever since HotSwap support has been introduced in Java 1.4 people have been craving for more. As witnessed by this bug in the Sun database and numerous other discussions changing method bodies was just not enough. Dynamic languages like Ruby were quick enough to capitalize on this lack, touting Java EE development as slow and cumbersome.

As time passed by, people have tried to work around the limitation by reloading classes along with their classloader. However since this requires reconstruction of the object state, it will only work in a controlled and limited environment ?e.g. that of a web framework like RIFE, Tapestry 5 and so on.

Just as many others, we were working on a workaround. However, instead of limiting ourselves to the facilities provided by the Java platform, we devised a whole new approach. Using bytecode modification and a little bit of magic, JavaRebel extends the JVM with the ability to reload almost arbitrary changes to classes on-the-fly. This includes adding and removing both methods and fields and the only forbidden changes are to 'extends' and 'implements' clauses. Since all the existing object instances are preserved the application and server can continue running and code changes are immediately visible in the application.

Watch the demonstration screencast (~5 mins), read the feature list or just download JavaRebel from ZeroTurnaround.com and give it a try. Disclaimer: JavaRebel is commercial software with a free trial for 14 days and developer seat cost at 99$.

Features

JavaRebel reloads only individual class files (e.g. the ones in WEB-INF/classes). This allows to use IDE to compile the classes to the output folder and see the changes immediately. The following changes are supported:

Changing, adding and removing class/interface methods

Changing, adding and removing class fields

Changing, adding and removing class constructors

Overriding existing methods

Creating new classes or renaming old ones (as long as some other class superclass doesn’t change)

Changing method parameter or return types or field type without changing its name

Changing, adding and removing class/interface constant fields is supported, but will not get reinitialized and thus is only useful for primitive types and Strings

Changing superclass and implemented interfaces is not supported and in such cases JavaRebel will issue a warning.

Note that JavaRebel adds a performance penalty to the application and should be used only in a development environment. Be sure to to switch it off for performance tests.

Comparison

This compares the changes that JavaRebel can reload to those that Sun JVM implementation reloads when debugger is attached (HotSwap).

	Sun JVM HotSwap	JavaRebel
Creating new classes	+	+
Changing method bodies	+	+
Adding/removing methods	-	+
Adding/removing constructors	-	+
Adding/removing fields	-	+
Overriding methods	-	+
Changing method or field types	-	+
Changing interfaces	-	+
Changing superclass	-	-
Changing implemented interfaces	-	-