How can I perform Hot Code Replace in Tomcat web application running outside eclipse?

I am using Hot Code Replace feature when Tomcat is running from eclipse and it works great.

But, how can I do this manually when Tomcat is running outside eclipse?

After some searching, I have found that I need to use an agent like HotswapAgent. But, they are using this agent with modified JDK called DCEVM. I don't want to use modified JDK. I want to achieve the same thing with OpenJDK.

I know that modification will be limited to method body only but, that's not a problem for me. How can I achieve the exact same thing eclipse is doing for Hot Code Replace for an externally running Tomcat without using IDE?

Edit : Eclipse example is just to clarify what I want to achieve. I do not want to use eclipse at all. I just want to do Hot Code Replace in an application running in Tomcat.

Yes, it's possible to perform Hot Code Replace in a running JVM. This involves several steps.

1. Prepare (compile) the new version of classes you want to replace. Let's say, you want to replace org.pkg.MyClass, and the new version of this class is located at /new/path/org/pkg/MyClass.class

2. Create a Java Agent that uses Instrumentation API to redefine the given class. Here is how the simplest agent may look like:

    import java.lang.instrument.*;
    import java.nio.file.*;
   
    public class HotCodeReplace {
   
        public static void agentmain(String args, Instrumentation instr) throws Exception {
            Class oldClass = Class.forName("org.pkg.MyClass");
   
            Path newFile = Paths.get("/new/path/org/pkg/MyClass.class");
            byte[] newData = Files.readAllBytes(newFile);
   
            instr.redefineClasses(new ClassDefinition(oldClass, newData));
        }
    }
   

3. Compile the above agent and pack it into .jar with the following MANIFEST.MF

    Agent-Class: HotCodeReplace
    Can-Redefine-Classes: true
   

The command to create HotCodeReplace.jar:

    jar cvfm HotCodeReplace.jar MANIFEST.MF HotCodeReplace.class

4. Load the agent .jar into the target JVM. This can be done with Attach API or simply with jattach utility:

    jattach <pid> load instrument false /path/to/HotCodeReplace.jar
   

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