如何无需重启Hazelcast集群，从客户端代码添加新的Identified Serializable类？

Great question! Restarting your entire Hazelcast cluster every time you add a new IdentifiedSerializable class is a major productivity hit—luckily, there are several solid alternatives to eliminate that overhead. Here are the most practical solutions:

1. Use Hazelcast's User Code Deployment (Recommended) #

Hazelcast's built-in User Code Deployment feature is designed specifically for this scenario. It lets cluster nodes dynamically fetch required classes from connected clients or designated file locations, removing the need to pre-deploy JARs to nodes or restart the cluster entirely.

How to set it up: #

• Enable the feature in your Hazelcast configuration (XML or programmatic):

  <hazelcast>
      <user-code-deployment enabled="true">
          <!-- Let nodes load classes directly from connected clients -->
          <client-provider enabled="true"/>
          <!-- Allow nodes to load classes from their local user-lib/classpath -->
          <classpath-provider enabled="true"/>
          <!-- Optional: Restrict loaded classes to trusted packages for security -->
          <class-filter>
              <prefix>com.yourcompany.yourpackage.</prefix>
          </class-filter>
      </user-code-deployment>
  </hazelcast>
  

• Update your client application: When you add class2 to your client's JAR, the client will automatically send the class definition to cluster nodes on demand. Nodes will dynamically load the class without needing a restart.
• Optional shared directory deployment: If you prefer not to rely on client-side provisioning, configure nodes to pull updated JARs from a shared network directory using the <directory-provider> setting in the configuration.

2. Dynamic Class Loading via Hot Deployment #

If you want to keep deploying JARs to the Hazelcast/bin/user-lib directory but skip restarts, you can set up Hazelcast to monitor this directory for changes and load new classes dynamically.

How to implement this: #

• Use Java's URLClassLoader to programmatically load new JARs from the user-lib directory at runtime. You can hook into Hazelcast's lifecycle events or set up a file watcher to detect new JARs as they're added.
• Alternatively, use Hazelcast's UserCodeDeploymentService API to register updated JARs dynamically:

  UserCodeDeploymentService service = hazelcastInstance.getConfig().getUserCodeDeploymentConfig().getService();
  service.addJar(new File("/path/to/updated-classes.jar"));
  

  This requires a small utility or integration into your deployment pipeline to trigger the class load.

3. Fall Back to Standard Serialization (If Performance Permits) #

While IdentifiedSerializable offers better performance and control, if you don't need absolute maximum efficiency, switching to Java's built-in Serializable interface can simplify things. With standard serialization, Hazelcast can often resolve classes dynamically without pre-deployment to nodes (assuming the client provides the class definition). Note that this comes with a minor performance tradeoff compared to IdentifiedSerializable.

Key Notes for All Solutions: #

• Keep Serialization IDs Consistent: Never modify getFactoryId() or getId() values for existing IdentifiedSerializable classes—this will cause deserialization failures even with dynamic loading.
• Security First: When using User Code Deployment, use the <class-filter> to restrict loaded classes to trusted packages, preventing arbitrary code execution risks.
• Version Check: Ensure you're running Hazelcast 3.8 or newer, as User Code Deployment was introduced in this version.

内容的提问来源于stack exchange，提问作者Vijay Patel