{"id":533,"date":"2017-10-20T14:07:09","date_gmt":"2017-10-20T12:07:09","guid":{"rendered":"http:\/\/itblog.inginea.eu\/?p=533"},"modified":"2021-05-13T16:33:10","modified_gmt":"2021-05-13T14:33:10","slug":"using-hotswapagent-to-speed-up-development","status":"publish","type":"post","link":"https:\/\/ondro.inginea.eu\/index.php\/using-hotswapagent-to-speed-up-development\/","title":{"rendered":"Using HotswapAgent to speed up development"},"content":{"rendered":"

As a Java EE developer, I sometimes envy how fast it’s possible to see the result of a code change in a running application with interpreted languages like PHP or JavaScript. With Java, it’s always necessary to rebuild the source code in a bytecode, which can be then safely updated only by restarting the whole application. And all developers know that restoring the desired state of the application after a fresh restart takes time and is tedious. A while ago I’ve come across an opensource tool called HotswapAgent<\/a> that speeds up code reloading.<\/p>\n

Many developers know that JRebel<\/a> can help a lot with updating the code on the fly. There’s been a lot of effort put into it to support all sorts of code and resource changes and refresh them with virtually any Java framework used by the application. But the downside is that it’s pretty expensive for a casual developer, doing just some hacking on his\/her own or working on a non-commercial project. I have some experience with JRebel and I liked it a lot, but I was using it on a commercial project where I didn’t pay for the license. I tried HotswapAgent and it has worked very well as a free alternative of JRebel for me for my personal Java EE projects. I’m going to write up how I got it running in my IDE and my Java EE server of choice – Payara Server<\/a>.<\/p>\n

Getting started with HotswapAgent<\/h2>\n

Hotswap\u00a0Agent itself is just a java agent, which has to be attached to the application. It essentially scans the classpath to detect the presence of known frameworks and tries to refresh the frameworks after a code or resource change is detected. It’s also possible to write custom plugins for any project or even specifically for your application. However, getting the most of Hotswap\u00a0Agent requires installing an alternative DCEVM engine into your JRE installation, which does much better job in reloading code changes than the standard VM engine in the HotSpot VM. The alternative VM engine is basically a patched version of the standard engine, which improves code hot swap via the debugging interface, without impacting other functions of the JVM.<\/p>\n

So to summarize, we are going to do this to work with HotSwap from our IDE of choice:<\/p>\n

    \n
  1. Install the DCEVM<\/a> engine into a Java installation (JRE or JDK)<\/li>\n
  2. Configure the IDE to start the application with the HotswapAgent JAR<\/a> as java agent and using the DCEVM engine instead of the standard JVM engine<\/li>\n
  3. Configure the IDE to refresh plain text resources like XML, property files, in the running application (this has to be done by the IDE because it’s not supported by Java debugger)<\/li>\n
  4. Start the application in debug mode and connect to it via the debugger interface from withing IDE<\/li>\n
  5. Continue coding and see the changes in your app immediately \ud83d\ude09<\/li>\n<\/ol>\n

    Installing\u00a0the DCEVM engine<\/h2>\n

    First, download the DCEVM installer<\/a> for your Java installation.\u00a0The project is pretty quick to release new versions\u00a0for new Java updates. At the time of writing this blog post, there was a build for Java 8 update 144 available, released 12 days ago, just a month after the actual Java 8 update 144 had been released. There aren’t releases of DCEVM for every Java update, but I always just used the latest version with my Java installation and never had any issues.<\/p>\n

    The installer is a plain executable JAR file, which you can start with java -jar DCEVM-installer.jar<\/span>\u00a0. In Linux, I usually start this with admin privileges (using sudo), because it will add\/modify files in the Java installation, which is usually read-only for standard users if installed system-wide:\u00a0\u00a0sudo java -jar\u00a0DCEVM-installer.jar<\/span><\/p>\n

    The installer will open a window, which lists all the detected Java installations on the system.<\/p>\n

    \"Screenshot<\/a>

    The DCEVM installer windows showing all detected Java installations<\/p><\/div>\n

    set configs.config.server-config.java-config.java-home=”$DCEVM_JAVA_HOME”<\/p>\n

    It’s possible to install the DCEVM engine in 2 ways:<\/p>\n