Starting a Java agent after program start

Question

Is it possible to insert a javaagent after virtual machine start from within the same VM?

Lets say for example we have an agent in a jar myagent.jar with the appropr

Answer 1

You should be able to do it in Java 6, see the package documentation chapter "Starting Agents After VM Startup"

edit: Maybe it was possible in Java 5 already and just the javadocs didn't mention it that explicitly

Answer 2

https://web.archive.org/web/20141014195801/http://dhruba.name/2010/02/07/creation-dynamic-loading-and-instrumentation-with-javaagents/ has a great example of how to write an agent as well as how to start one on the fly.

Answer 3

Yes, you just have to pass the JVM process ID to the VirtualMachine.attach(String pid) method, and load the agent jar. The VirtualMachine class is available in the JDK_HOME/lib/tools.jar file. Here's an example of how I activate an agent at runtime:

public static void attachGivenAgentToThisVM(String pathToAgentJar) {
  try {                                                                               
    String nameOfRunningVM = ManagementFactory.getRuntimeMXBean().getName();                                                   
    String pid = nameOfRunningVM.substring(0, nameOfRunningVM.indexOf('@'));                                                   
    VirtualMachine vm = VirtualMachine.attach(pid);                                                                            
    vm.loadAgent(pathToAgentJar, "");
    vm.detach();   
  } catch (Exception e) {
    e.printStackTrace();
  }
}                                                                                                            

Answer 4

Having encountered the same issue, I have found a far more comprehensive solution, from the ByteBuddy library.

ByteBuddy thoroughly attempts to load its java agent dynamically:

Installs an agent on the currently running Java virtual machine. Unfortunately, this does not always work. The runtime installation of a Java agent is supported for:

JVM version 9+: For Java VM of at least version 9, the attachment API was moved into a module and the runtime installation is possible if the {@code jdk.attach} module is available to Byte Buddy which is typically only available for VMs shipped with a JDK.

OpenJDK / Oracle JDK / IBM J9 versions 8-: The installation for HotSpot is only possible when bundled with a JDK and requires a {@code tools.jar} bundled with the VM which is typically only available for JDK-versions of the JVM.

When running Linux and including the optional junixsocket-native-common dependency, Byte Buddy emulates a Unix socket connection to attach to the target VM.