The Runtime. This implementation misses a lot of detail and optimizations, but you get the idea. Great article on this topic by Dave Booth. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. How does JRebel work? Ask Question. Asked 10 years, 9 months ago. Active 1 year, 5 months ago. Viewed 7k times. Improve this question. I found a nice article on dynamic class loading here: toptal.
Add a comment. Active Oldest Votes. Improve this answer. Olufsen Jevgeni Kabanov Jevgeni Kabanov 2, 23 23 silver badges 23 23 bronze badges. This is why you cannot just redeploy your application in the application server again and again all day.
It will crash with the out of memory errors. Since the problem is quite apparent, especially when you work with larger applications, it received some attention a long time ago. In , Sun introduced a technology into the Java 1. It was incorporated within the Debugger API, and allowed debuggers to update class bytecode in place, using the same class identity.
This meant that all objects could refer to an updated class and execute new code when their methods were called, preventing the need to reload a container whenever class bytecode was changed.
Unfortunately, this redefinition is limited only to changing method bodies. It cannot add methods or fields or otherwise change anything else, except for the method bodies. This limits the usefulness of HotSwap severely. And enhancing HotSwap to work on the other changes beyond the method bodies only is a hard challenge. It is because the JVM is a heavily optimized piece of software, running on many platforms.
Performance and stability are the highest priorities. To support them in different environments the JVM features:.
These features make evolving the class schema adding or changing fields or methods, or changing the class hierarchy a considerable challenge. When loaded into the JVM, an object is represented by a structure in memory, occupying a continuous region of memory with a specific size - its fields and the metadata. In order to add a field, we would need to resize that structure. But since nearby regions may already be occupied, we would need to relocate the whole structure to a different region where there is enough free space to fit it in.
JRebel is much more powerful than HotSwap. It can reload the code that contains all kinds of changes to the Java code. It supports adding and removing static or instance fields in the objects, adding, removing, or changing method bodies and signatures, constructors, annotations, enum values, and even changing the implemented interfaces or the class hierarchy.
It is possible because JRebel works on a different level of abstraction than HotSwap. Whereas HotSwap works at the virtual machine level and is dependent on the inner workings of the JVM, JRebel makes use of two remarkable features of the JVM — abstract bytecode and classloaders. Classloaders allow JRebel to recognize the moment when a class is loaded, then translate the bytecode on-the-fly to create another layer of abstraction between the virtual machine and the executed code.
When a classloader tries to load a class, JRebel intercepts that process and modifies the class into a combination of a proxy class and a number of support classes that represent the versions of the class as it goes through the changes due to the app development.
What makes the real difference is that the system is heavily optimized for the JIT transformation runtime and allows modifications to take place without any visible degradation in performance or compatibility. In a nutshell, JRebel:. Nowadays, applications are not just classes and resources; they are wired together by extensive configuration and metadata of the frameworks in use.
When that configuration changes it should be reflected in the running application. Keep in mind that running out of memory or permgen space can cause the JVM to crash altogether. With some application servers e.
WebSphere and GlassFish , it is possible that the banner is shown, but hidden in one of the console output logs. Check those logs to find it. Or check jrebel. Seeing this message means that the day JRebel evaluation is over. However, sometimes this is shown even when you have a valid license. In that case:. Check that jrebel. To verify jrebel.
The limitedUntil field defines when the license is going to expire. These are the possible problems and solutions when JRebel is set up and started correctly JRebel startup banner visible in the console , but the classes do not get reloaded:. Keep this in mind: when you are testing JRebel and think that classes are not being reloaded, be absolutely sure. Note that JRebel only issues a console statement when a previous version of the class has already been loaded.
Just changing some class does not cause it to be reloaded — unless you accessed it before. Are the classes monitored? The first thing to check is that JRebel actually monitors the directories with compiled classes in your workspace. To do this, search jrebel. If you do not find any, then probably rebel. If the monitored folders are present, you know exactly where JRebel is expecting to find updated classes and resources. Check these accordingly. Refer to rebel.
If you cannot find any monitored folders, it could mean that your rebel. It will show you both where the rebel. You can search jrebel. Once you are convinced that JRebel is monitoring these folders, it should pick up changes to the resources.
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