Hey there! I’ve spent a fair amount of time working with HotSwapAgent for runtime class reloading, so let’s tackle your two questions with practical, actionable steps:

HotSwapAgent can handle changes in dependent JARs, but you need to make sure the agent is monitoring those JARs and you’re replacing the classes correctly. Here’s how:

• First, ensure your HotSwapAgent is configured to watch resources:
  When starting your container, add the Javaagent parameter with watchResources=true to let the agent track changes in JAR files:

  java -javaagent:/path/to/hotswap-agent.jar=autoHotswap=true,watchResources=true [your container startup command]
  

• Replace the class in the target JAR:
  1. Compile the modified class file from your tool JAR’s source code.
  2. Locate where the tool JAR is stored in your container: for most web apps, this is WEB-INF/lib inside your deployed WAR, or a shared library directory (like Tomcat’s lib folder or Weblogic’s domain lib).
  3. Use a file archiver (like 7-Zip or the jar command-line tool) to open the JAR and replace the old .class file with your new compiled one.
• Trigger a reload if needed:
  If the agent doesn’t pick up the change automatically, you can manually trigger a reload. If you have the HotSwapAgent console enabled, run:

  reload com.your.tool.package.YourModifiedClass
  

  Alternatively, use your IDE’s hot-deploy feature to sync the modified class directly into the JAR (many IDEs like IntelliJ/Eclipse support this when configured correctly).

Note: Remember HotSwap’s core limitation—you can only modify method bodies. If you’re adding/removing fields, methods, or changing inheritance, HotSwap won’t handle this, and you’ll need to restart the container.

Each container has its own settings to disable caching and enable runtime detection of JAR modifications. Let’s break them down:

Tomcat Configuration #

• Disable resource caching:
  Edit your app’s META-INF/context.xml (or the global conf/context.xml in Tomcat) to add this resource configuration, which prevents Tomcat from caching JAR contents:

  <Context>
    <Resources cachingAllowed="false" cacheMaxSize="0"/>
  </Context>
  

• Adjust default servlet settings:
  In conf/web.xml, find the default servlet entry and set readonly to false to allow modifications to WEB-INF files:

  <servlet>
    <servlet-name>default</servlet-name>
    <servlet-class>org.apache.catalina.servlets.DefaultServlet</servlet-class>
    <init-param>
      <param-name>readonly</param-name>
      <param-value>false</param-value>
    </init-param>
    <!-- Keep other params as is -->
  </servlet>
  

• Verify auto-deploy settings:
  Ensure autoDeploy="true" and deployOnStartup="true" are set in conf/server.xml under the Host node (this is the default, but double-check if you’ve modified it before).

Weblogic Configuration #

• Enable hot deploy mode:
  First, start Weblogic in Development Mode (Production Mode disables hot deploy by default). Add this JVM parameter to your startup script:

  -Dweblogic.ProductionModeEnabled=false
  

  Then, edit your domain’s config.xml to enable hot deploy for the server:

  <Server>
    <Name>your-server-name</Name>
    <HotDeployEnabled>true</HotDeployEnabled>
  </Server>
  

• Configure class loading and refresh intervals:
  In your app’s WEB-INF/weblogic.xml, add these settings to prioritize WEB-INF/lib classes and set a refresh interval for detecting changes:

  <weblogic-web-app>
    <container-descriptor>
      <prefer-web-inf-classes>true</prefer-web-inf-classes>
      <refresh-period-seconds>5</refresh-period-seconds> <!-- Check for changes every 5 seconds -->
    </container-descriptor>
  </weblogic-web-app>
  

• Add HotSwapAgent to Weblogic’s startup:
  Include the Javaagent parameter in Weblogic’s startup script:

  java -javaagent:/path/to/hotswap-agent.jar=autoHotswap=true -Dweblogic.debug.DebugHotSwap=true -jar startWebLogic.jar
  

Pro Tip: For shared libraries in Weblogic, you’ll need to enable hot deploy for the library itself via the Admin Console (under Deployments > Your Shared Library > Configuration > General > Enable Hot Deploy).

内容的提问来源于stack exchange，提问作者Prateek Chachra