• A Java development-time tool that decreases turnaround by instantly reloading changes to your code, without having to restart the container or redeploy the application.
• A JVM -javaagent plugin. The -javaagent is a command line option that has been available since Java 5 to allow custom instrumentation plugins to be registered. JRebel uses this option for installation. 
• A JAR file about 1 MB big. That’s pretty much says it all – there is no long installation process: just unzip, copy, add options to the command line and enjoy!

• An IDE plugin. JRebel comes pre-bundled with the plugins for Eclipse, IntelliJ IDEA, NetBeans and JDeveloper, however, JRebel agent is designed to be IDE-agnostic and can be used outside the IDE.
• A framework. JRebel does not introduce any dependencies in your application. You can take it away at any moment and just continue developing the way you do normally.
• An application server. JRebel works with all prominent application servers, but it definitely isn’t one.
• A custom JVM. JRebel doesn’t require you to make any changes to the JVM and works on all prominent JVM implementations.

JRebel integrates with the JVM and application servers mainly on the class loader level. It does not create any new class loaders, instead, it extends the existing ones with the ability to manage reloaded classes.

When a class is loaded JRebel will try to find a corresponding .class file for it. It will search from the classpath (including an application classpath, like WEB-INF/classes) and from the places specified in the rebel.xml configuration file. If it find a .class file JRebel instruments the loaded class and associates it with the found .class file. The .class file timestamp is then monitored for changes in the loaded class and updates are propagated through the extended class loader, to your application.
JRebel can also monitor .class files in JARs if they are specified in rebel.xml.

Importantly, when loading updates to a class, JRebel preserves all of the existing instances of that class. This allows the application to just continue working, but also means that when adding a new instance field it will not be initialized in the existing instances, since the constructor will not be rerun.

Some common misconceptions:

• JRebel just wraps class loaders around classes. In fact JRebel does not add a single new class loader to your application. The solution of reloading Java classes by wrapping them in throwaway classloaders is a well-known one, but unfortunately also very limited. The problem is that unless you also throw away all of the instances of the classes loaded by said class loaders, the code will not reload. However throwing away those instances is only possible if they are somehow managed by the framework, e.g. as it’s done in Tapestry 5.
• JRebel just uses Instrumentation API. The Instrumentation API was introduced in Java 5 and included a limited ability to redefine Java classes on-the-fly. Unfortunately it is limited to only changing method bodies (as is HotSwap) and also suffers from several additional quirks, which makes it not too useful in a real environment. JRebel agent does use it to instrument the application server class loaders and other basic classes, but the API does not play part in the actual reloading process.

In one word: no. However there are several partial solutions:

• Hot deploy. This is basically when your application is redeployed on any change to the code. Most application servers will allow you to do that. The problem is that it only works for small or lightweight applications. Your typical enterprise application will redeploy for at least 30 seconds and this time is just wasted. With JRebel the reloading time is measured in milliseconds.

• HotSwap. HotSwap is a technology available in Java since 1.4 that allows you to instantly redefine Java classes inside a debugger session. This is supported by all prominent IDEs (e.g. IntelliJ IDEA, Eclipse and NetBeans), so if you attach a debugger to your application and make some changes, they will be reflected immediately. Unfortunately this technology is very limited, as it only allows changes to method bodies and doesn’t allow new classes to be added. JRebel can be thought of as improved HotSwap, as it allows changes to class structure, including adding methods, fields, constructors and even annotations as well adding classes and changing configurations.

• OSGi. OSGi is a runtime module container for Java. It allows you to split an application into modules with well-defined dependencies and manage them separately. One of the side-effects is that it allows you to update the modules (or, in OSGi-speak, bundles) one at a time. It is basically the same solution as Hot Deploy, but since the module size is smaller than the application size, the time wasted can be smaller. It will still likely be measured in tens of seconds, since all of the dependent modules should also be updated, but it can be significantly faster than vanilla WAR/EAR applications. JRebel reloading time is still orders of magnitude faster.

• Throwaway class loaders. Several frameworks (including Tapestry 5, RIFE, Seam, Grails and so on) will instantly reload the changes to the code of the managed components (e.g. beans, pages, controllers, etc). This is possible by wrapping a throwaway class loader around the managed component and recreating its instance after every change. Unfortunately this solution works only for managed components and the rest of the code cannot be reloaded. It can also cause weird ClassCastExceptions and other similar problems due to class loader migration problems. JRebel works for any class in your system, preserves all the current object instances and does not exhibit any class loader-related problems.

Reloading changes to Java classes is not always enough. Applications include configurations for Java code, annotations, XML files, property files and so on. JRebel uses configuration-specific plugins to reload those changes instantly, just as it does changes to classes. JRebel comes with plugins for Spring, Guice, Stripes, Tapestry 4 and others out of the box. To find out more check out check out our OSS projects’ overview.

JRebel is meant and licensed only as a development tool. If you’re interested in helping your production’s team check out LiveRebel which releases apps across QA, staging and production environments along with database and configuration changes, quickly, safely and seamlessly.

LiveRebel’s one-page overview lets you conveniently manage all your apps, servers and environments. You can deploy apps from the user console, command line or your continuous integration tool, with no downtime. Failed deployments are rolled back automatically before your users are impacted.

JRebel Base - This license type is meant for individual developers and smaller development teams (10 and fewer). Each base license is tied to an individual and cannot be transferred. Name and email address must be provided for the user of each base license.

JRebel Enterprise - This license type is meant for larger development teams, or anyone looking to take advantage of a floating license. The enterprise license is a floating license, meaning it can be transferred and used by different people (only 1 at a time per license). The enterprise license also provides management reporting and usage statistics.

JRebel is distributed as an IDE plugin for all major IDEs - Eclipse, IntelliJ IDEA, NetBeans, JDeveloper. You will find JRebel in the list of plugins available for your IDE. Please refer to reference manual for more details.

In case if you require a standalone archive of JRebel, you can download a ZIP archive from the web site.

If you are using an IDE, you can just enable JRebel per the instructions on the download page.

##################################################

  JRebel 5.2.0 (201302271426)
  (c) Copyright ZeroTurnaround OU, Estonia, Tartu.

  Over the last 30 days JRebel prevented 
  at least 56 redeploys/restarts saving you about 2.6 hours.

  You are running with an evaluation license.
  You have 12 days until the license expires. 

  You will see this notification until you obtain a 
  full license for your installation. 

  Visit www.jrebel.com for instructions on obtaining    
  a full license. If you wish to continue your evaluation  
  please e-mail to support@zeroturnaround.com.     
       
  If you think you should not see this message contact     
  support@zeroturnaround.com or check that you have your   
  license file in the same directory as the JAR file.      

  If you think you should not see this message contact     
  support@zeroturnaround.com or check that you have your   
  license file in the same directory as the JAR file. 

##################################################

• Does the application start without JRebel?
• Is there an exception in the output? If it is too cryptic just send it to support@zeroturnaround.com or post it on the forum.

if (myCondition) {
 System.out.println(); //Breakpoint
}

• Non-static fields will be initialized once a new class instance is created. This is due to the way Java handles field initialization, which gets run only on constructor calls. Since primitive types cannot be initialized to nulls such fields will cause a NullPointerException on access. Object types will cause a NullPointerException when dereferenced.
• Static fields will never get initialized. Again, Java appends the static fields initialization to the ends of the single static initializer method <clinit>. Rerunning this method is dangerous, since it can overwrite or alter meaningful state. This does not concern constants (final static primitive or String fields) as they are replaced by their proper values and therefore will get initialized.

• Start your application with -Drebel.log=true and a jrebel.log file is generated to the same directory as JRebel.jar
  • The log file contains all the necessary system information. JDK version, operating system, properties passed to the application, classloading logic etc.
  • If the file is too large, compress it with ZIP, GZip or your favorite compression algorithm.
• If for some reason the log is not generated or you don't have the time to generate it, please provide JDK, application server and any important library versions. Also knowing the operating system can help a lot!
• If you need private support just drop us an email at support@zeroturnaround.com with the information mentioned above.