arthas/tutorials/katacoda/command-mc-redefine-en/redefine.md

Load the external *.class files to re-define the loaded classes in JVM.

Reference: Instrumentation#redefineClasses

Frequently asked questions

• The class of redefine cannot modify, add or delete the field and method of the class, including method parameters, method names and return values.

• If mc fails, you can compile the class file in the local development environment, upload it to the target system, and use redefine to hot load the class.

• At present, redefine conflicts with watch / trace / jad / tt commands. Reimplementing redefine function in the future will solve this problem.

Notes: Re-defined classes cannot be restored. There are chances that redefining may fail due to some reasons, for example: there's new field introduced in the new version of the class, pls. refer to JDK's documentation for the limitations.

The reset command is not valid for classes that have been processed by redefine. If you want to reset, you need redefine the original bytecode.

The redefine command will conflict with the jad/watch/trace/monitor/tt commands. After executing redefine, if you execute the above mentioned command, the bytecode of the class will be reset. The reason is that in the JDK redefine and retransform are different mechanisms. When two mechanisms are both used to update the bytecode, only the last modified will take effect.

Options

Name	Specification
[c:]	hashcode of the class loader
[classLoaderClass:]	The class name of the ClassLoader that executes the expression.
[p:]	absolute path of the external *.class, multiple paths are separated with 'space'

Restrictions of the redefine command

• New field/method is not allowed
• The function that is running, no exit can not take effect.