Observability of the Java Virtual Machine

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The JVM is one of the most observable runtimes. It provides us lots of tools for troubleshooting a JVM application in production. 1. Thread observability Threads are how the JVM actually does work. When something is wrong in production, the symptom is almost always a thread: stopped, blocked, leaking etc. Thread dumps work on any JVM with no  instrumentation, no agents, no restarts. <Example project link with /threaddump endpoint>         // (1) Deadlock — two threads grab the same pair of locks in opposite order.         new Thread(() -> grab(LOCK_A, LOCK_B), "deadlock-A-then-B").start();         new Thread(() -> grab(LOCK_B, LOCK_A), "deadlock-B-then-A").start(); http://localhost:8080/actuator/threaddump To list the JVMS, we can use the command below. PS C:\observe-jvm> jps -lv 25296 jdk.jcmd/sun.tools.jps.Jps -Dapplication.home=C:\Program Files\Microsoft\jdk-21.0.3.9-hotspot -Xms8m -Djdk.module.main=...
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Revisiting the Axon Framework

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Axon is a CQRS Framework for scalable and high-performance applications. Lets look at the evolution of the API from Axon 1 to Axon 3:


With Axon 3, Aggregate  interface is introduced and we pass the method to the execute() method:

Axon 1:

@CommandHandler
public void handle(final ChangeClientNameCommand command, UnitOfWork unitOfWork) {

    Client client = clientRepository.load(new StringAggregateIdentifier(command.getClientIdentifier()));

    client.changeNameAs(command.getNewName());
}


Axon 3:

@CommandHandler
public void handle(final ChangeClientNameCommand command) {

    Aggregate client = clientRepository.load(command.getClientIdentifier().toString());

    client.execute(aggregateRoot -> aggregateRoot.changeNameAs(command.getNewName()));
}





In the past Client entity was derived from a class called "AbstractAnnotatedAggregateRoot". Now in Axon 3, it is just a POJO and the Repository does not return a Client entity but an "aggregate". We pass the function as a parameter. At the time of Axon 1.0, Java language did not have support for lambda expressions.







Axon 1:









public class Client extends AbstractAnnotatedAggregateRoot {



    private String name;



    private Address address;



    private String phoneNumber;









Axon 3:








@AggregateRootpublic class Client {

    @AggregateIdentifier    private ClientId companyId;

    private String name;
    private Address address;
    private String phoneNumber;





Domain entities are not dependent to a base class anymore. We see this pattern in many frameworks like Unit Test frameworks. There is even a project called Lombok that uses annotations for simpler code.







To be continued..




















































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