Snippet on Micha Kops' Tech Notes

Object Audit with Java and Javers

Mon, 18 Sep 2023 00:00:00 +0200

Figure 1. Object Audit with Java and Javers

Just a quick snippet

Maven Integration

pom.xml

<dependency>
  <groupId>org.javers</groupId>
  <artifactId>javers-core</artifactId>
  <version>${javers.version}</version>
</dependency>

Calculate Changes in Object Graph

package io.hascode;

import org.javers.core.Changes;
import org.javers.core.Javers;
import org.javers.core.JaversBuilder;
import org.javers.core.diff.Diff;

public <T> Changes diff(T snapshot, T latest) {
  Javers javers = JaversBuilder.javers().build();
  Diff diff = javers.compare(snapshot, latest);
  return diff.getChanges();
}

Resources

Javers Website

Forwardings Requests to static content in Spring Boot Webflux

Tue, 16 Aug 2022 00:00:00 +0200

The following WebFilter redirects incoming requests for / to a static HTML file, index.html.

ToIndexPageRedirection.java

package com.hascode.tutorial;

import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;
import org.springframework.web.server.WebFilter;
import org.springframework.web.server.WebFilterChain;
import reactor.core.publisher.Mono;

@Component
public class ToIndexPageRedirection implements WebFilter {
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
        if (exchange.getRequest().getURI().getPath().equals("/")) {
            return chain.filter(
                exchange.mutate().request(
                    exchange.getRequest().mutate().path("/index.html").build()
                ).build()
            );
        }

        return chain.filter(exchange);
    }
}

Ressources:

JavaDocs WebFilter

Fast Link Checks using Filiph Linkchecker and Docker

Mon, 02 May 2022 00:00:00 +0200

This Linkchecker claims to be way faster than blc and wummel/linkchecker.

Using Docker, we may validate our site’s links in no time and without complex setup.

docker run --rm tennox/linkcheck hascode.io

Resources

https://github.com/filiph/linkcheck

Setting up multiple Postgresql Instances with docker-compose

Tue, 08 Feb 2022 00:00:00 +0100

A simple setup when two Postgres databases prefilled with schema/data needed.

docker-compose.yml

version: '3.6'
services:
    postgres1:
        image: postgres
        restart: always
        environment:
            - DATABASE_HOST=127.0.0.1
            - POSTGRES_USER=root
            - POSTGRES_PASSWORD=root
            - POSTGRES_DB=root
        ports:
            - "15432:15432"
        volumes:
            - ./postgres1-init.sql:/docker-entrypoint-initdb.d/docker_postgres_init.sql

    postgres2:
        image: postgres
        restart: always
        environment:
            - DATABASE_HOST=127.0.0.1
            - POSTGRES_USER=root
            - POSTGRES_PASSWORD=root
            - POSTGRES_DB=root

        ports:
            - "25432:25432"
        volumes:
            - ./postgres2-init.sql:/docker-entrypoint-initdb.d/docker_postgres_init.sql

And our sample init scripts:

CREATE USER tester WITH PASSWORD 'tester' CREATEDB;
CREATE DATABASE testdb
    WITH
    OWNER = tester
    ENCODING = 'UTF8'
    LC_COLLATE = 'en_US.utf8'
    LC_CTYPE = 'en_US.utf8'
    TABLESPACE = pg_default
    CONNECTION LIMIT = -1;

Factory Reset for Google Pixel-C Android Tablet

Sun, 13 Jun 2021 00:00:00 +0200

Steps

Reboot the system by pressing "Power" and "Volume down" simultaneously
Boot menu should appear, use "Volume up/down" to select the menu item "Android Recovery", "Power" to confirm
System reboots, a screen appears with a message "No command" - this is no error though it looks like one
Press "Power" and "Volume up" together and a menu "Android Recovery" appears
Use "Volume up/down" to select the menu item "Wipe data/factory reset", "Power" to confirm
User data on the device is deleted and the original meu is shown
Select the menu item "Reboot system now" and confirm by pressing the "Power" Button
The device reboots and you may configure the Android system …

Setting up an OAuth2 Authorization Server and Resource Provider with Spring Boot

Sun, 13 Mar 2016 00:00:00 +0100

OAuth2 is a frequently used standard for authorization and with Spring Boot it is easy to set up authorization and resource server in no time.

In the following short tutorial I’d like to demonstrate how to set up an OAuth2 authorization server as well as a connected and secured resource server within a few minutes using Java, Maven and Spring Boot.

Figure 1. OAuth2 Flow with Spring Boot in Action

JAX-RS Server API Snippets

Sun, 28 Sep 2014 00:00:00 +0200

Because a lot of my current projects are using JAX-RS in different versions I’d like to write down and share some frequently used snippets for implementing RESTful web-services with the JAX-RS specification here.

Using RegEx in Path Expressions

Sometimes we need to extract multiple parameters from a path expression e.g. in the following example where year, month and day are fragments if the path.

@GET
@Path("/orders/{year:\\d{4}}-{month:\\d{2}}-{day:\\d{2}}")
@Produces(MediaType.TEXT_PLAIN)
public Response getOrders(@PathParam("year") final int year, @PathParam("month") final int month, @PathParam("day") final int day) {
	return Response.ok("Year: " + year + ", month: " + month + ", day: " + day).build();
}

Snippet: Mixing Scala, Java in a Maven Project

Fri, 23 Mar 2012 00:00:00 +0100

Having just returned from the Atlassian Camp 2012 I just toyed around with Java and Scala and wanted to share the following snippet that demonstrates how to mix code from both languages in a Maven project using the maven-scala-plugin.

Setting up the Maven Project

First create a new Maven project in your IDE or by running mvn archetype:generate.

In the next step, add the dependency for scala-library and the scala maven repositories to your pom.xml and hook the maven-scala-plugin to Maven’s lifecycle. My pom.xml finally looks like this one:

How to create an Android App using Google’s App Inventor

Wed, 04 Aug 2010 00:00:00 +0200

Today we’re going to take a look at Google’s App Inventor feature that offers programming-novices a nice possibility to enter the fabulous world of Android App programming without deeper knowledge of the API or complex SDK installations.

So lets build some stuff ..

Prerequisites

Java 6 JDK
App Inventors Extras Software
A Google App Inventor Beta Account – request one here

What we are going to build

We are building a simple GUI with a Textbox and a button
A click on the button starts an event that queries the acceleration sensor for coordinates
If the sensor is active and enabled then the coordinates are displayed in the text box

Creating a simple Gesture App with Android

Fri, 14 May 2010 00:00:00 +0200

The integration of gestures into your android app adds some nice functionality and is made very easy using Google’s GestureBuilder application and the integrated GestureLibrary and Gesture Overlay API – so let’s build a sample app.

If you need some basic information regarding gestures on android first – take a look at this article.

Creating a gesture library

First you need to define the gestures that should be captured in the application later. For this reason there’s the GestureBuilder delivered with the Android SDK. You can find the app in the samples directory of your android sdk – e.g. <installation-directory>/android-sdk-linux_86/platforms/android-2.1/samples/GestureBuilder.

Playing around with QR Codes

Tue, 11 May 2010 00:00:00 +0200

Sometimes QR codes are a nice way to distribute information like calendar events, contact information, e-mail, geo-locations or internet addresses.

In the following article we’re going to encode information to QR code images using the ZXing library and afterwards decode information from a given QR code.

Finally we’re taking a look on online QR code generators and how to integrate the ZXing library in a Maven project.

The ZXing Library

Download the ZXing Libraries from http://code.google.com/p/zxing/downloads/list – the file name is ZXing-<version>.zip
Unpack the downloaded archive somewhere
Change to the extracted directory and run ant. If you don’t have JavaME installed – and you don’t have to for the samples below – run ant buildwithoutj2me – that will do the job
Having compiled the libraries you’re now free to include the core.jar from zxing-<version>/core/ and the javase.jar from zxing-<version>/javase as dependency in your project

Snippets: Getting License Information from the Confluence API

Thu, 06 May 2010 00:00:00 +0200

Sometimes one needs to look up license details of a running Confluence system .. perhaps for creating a commercial plugin or to display recommendations dependant from the license used. For this reason there are a few possibilities for receiving some license information from the Confluence API or the velocity context.

Note: This article is outdated since the Atlassian Marketplace was launched and a shiny new licensing API was added. Until this article is updated I strongly recommend to take a closer look at the detailed information that Atlassian is providing in the Developer Documentation.

Observability Snippets

Mon, 01 Mar 2010 00:00:00 +0100

The USE Methodology

USE stands for Utilization, Saturation, and Errors. Developed by Brendan Gregg, it’s a low-level diagnostic tool for infrastructure and system resources.

Utilization – How much of a resource is used? (e.g., 70% CPU)
Saturation – Is the system over capacity? (e.g., CPU run queue)
Errors – Are there hardware or system faults? (e.g., disk errors)

Best for: Bottleneck analysis on servers, network, disk, memory

Source: https://grafana.com/docs/grafana/latest/dashboards/build-dashboards/best-practices/

The RED Methodology

RED focuses on service-level performance and is tailored for microservices and HTTP-based systems.