How can I dynamically set a System property on the JVM, after it has started, without modifying my program's code?

I would like to be able to change a system property while a java application is running inside an application server. I know that this can be done with java.lang.System.setProperty or by adding the -DpropertyName=value as a JVM flag.

But I want to do this at runtime and without modifying the application code.

Is there a tool that can change system properties in the JVM at runtime without having to update the application-code ( e.g. just by attaching a tool the running process or by using JMX ) ?

The idea is to attach an agent that calls System.setProperty to a running application.
Note: this does not require modifying application code.

Here is the agent code:

import com.sun.tools.attach.VirtualMachine;

public class SetProperty {

    public static void main(String[] args) throws Exception {
        String url = SetProperty.class.getProtectionDomain().getCodeSource().getLocation().toString();
        if (args.length < 2 || !url.startsWith("file:")) {
            System.out.println("Usage: java -jar setproperty.jar pid property=value");
            System.exit(1);
        }

        VirtualMachine vm = VirtualMachine.attach(args[0]);
        try {
            int startIndex = "\\".equals(System.getProperty("file.separator")) ? 6 : 5;
            vm.loadAgent(url.substring(startIndex), args[1]);
        } finally {
            vm.detach();
        }
    }

    public static void agentmain(String args) {
        int eq = args.indexOf('=');
        System.setProperty(args.substring(0, eq), args.substring(eq + 1));
    }
}

1. Compile the source code:

javac -source 8 -target 8 SetProperty.java

2. Create MANIFEST.MF file with the following contents:

Main-Class: SetProperty
Agent-Class: SetProperty

3. Build .jar file from the compiled sources and the manifest:

jar cfm setproperty.jar MANIFEST.MF SetProperty.class 

4. Launch the agent passing the target process ID and the property to set (JDK 9+):

java -jar setproperty.jar 1234 MyProperty=SomeValue

If you are using JDK 8, you'll need to add tools.jar to the class path:

java -cp setproperty.jar:$JAVA_HOME/lib/tools.jar SetProperty 1234 MyProperty=SomeValue

I prepared setproperty.jar so you can skip steps 1-3 and proceed directly to the final command.