Hot deployment and redeployment are powerful features that allow developers to update and modify Enterprise JavaBeans (EJB) applications on-the-fly without restarting the application server. This capability is particularly useful during development and testing phases, enabling quick iterations and continuous development. This tutorial will guide you through the steps of performing hot deployment and redeployment of EJB applications in an application server.
Prerequisites
Before you begin, make sure you have the following:
- Basic understanding of EJB and Java EE
- An application server (such as GlassFish, WildFly, or WebSphere) installed and configured
- A sample EJB application ready for deployment
Step 1: Enable Hot Deployment
The first step in performing hot deployment is to enable this feature in your application server. Most application servers have hot deployment enabled by default. However, if it is not enabled, you can configure it by editing the server configuration file or through the administration console of your application server. Here's an example of enabling hot deployment in the GlassFish server by modifying the server configuration file (domain.xml):
<system-applications>
...
<application name="deployment" location="${com.sun.aas.instanceRootURI}/applications/deployment" type="ear" enabled="true"/>
...
</system-applications>
Ensure that the enabled attribute is set to true for the application element representing the deployment module.
Step 2: Deploy the Initial Application
Before you can perform hot deployment or redeployment, you need to deploy the initial version of your EJB application. This involves packaging your EJB modules into an archive file (such as a JAR or EAR file) and deploying it to the application server. The exact steps for deployment vary depending on the application server you are using. Here's an example command using the asadmin tool to deploy an application to the GlassFish server:
./asadmin deploy myapp.ear
Replace myapp.ear with the appropriate file name for your packaged EJB application.
Step 3: Perform Hot Deployment or Redeployment
With hot deployment enabled and the initial version of your EJB application deployed, you can now perform hot deployment or redeployment. Here's the typical process:
- Make the necessary changes or updates to your EJB application's source code or configuration files.
- Build and package the updated version of your EJB application into a new archive file.
- Perform hot deployment or redeployment by replacing the existing application with the updated version. Here's an example command for redeploying an application in the GlassFish server:
./asadmin redeploy myapp.ear
Replace myapp.ear with the appropriate file name for your updated EJB application.
The application server will detect the changes and automatically deploy the updated version of your EJB application. The EJB container will then handle the necessary initialization and activation steps to make the updated application available for use.
Common Mistakes
- Forgetting to enable hot deployment in the application server configuration.
- Not properly packaging the updated version of the application before redeployment.
- Overlooking dependency changes or conflicts that may arise during hot deployment or redeployment.
Frequently Asked Questions
Q1: What is the advantage of hot deployment over server restart?
Hot deployment allows for quick updates and continuous development by avoiding the need to restart the entire application server. It saves time and enables developers to see the changes immediately without interrupting the server's operation.
Q2: Can I perform hot deployment or redeployment in a production environment?
Hot deployment or redeployment is primarily intended for development and testing environments. In production environments, it is recommended to follow a more controlled and structured deployment process to ensure stability and minimize downtime.
Q3: What happens to existing client connections during hot deployment?
During hot deployment, the application server tries to maintain existing client connections as much as possible. However, depending on the changes made, certain connections may be terminated or interrupted. It is important to handle these scenarios gracefully in your EJB application.
Q4: Can I roll back a failed hot deployment or redeployment?
Some application servers provide the option to roll back a failed deployment or redeployment to the previous version. This feature allows you to revert to a stable state in case of any issues or failures during the update process. Check the documentation of your specific application server for details on rollback capabilities.
Q5: Are there any limitations to hot deployment or redeployment?
While hot deployment and redeployment offer convenience, there are some limitations to consider. Certain changes, such as modifications to EJB interfaces or database schemas, may require a server restart. Additionally, not all application servers support hot deployment or redeployment for all types of applications or configurations. Consult the documentation of your specific application server for any limitations or restrictions.
Summary
Hot deployment and redeployment are valuable features that enable developers to update EJB applications quickly and efficiently during development and testing phases. By enabling hot deployment, deploying the initial application, and performing hot deployment or redeployment, you can iteratively improve and modify your EJB applications without the need for a server restart. Remember to follow the proper steps, avoid common mistakes, and consult the documentation of your specific application server for detailed instructions. Now you have the knowledge to perform hot deployment and redeployment with confidence!