Hippo CMS treats internal links differently from external ones. Even when they are a part of an HTML field in a content document, they are stored as a reference to the referred content document and not as a regular hyperlink. When you're using the hst:html tag for rendering HTML fields in the template, it will take care of that automatically. But if you want to use the raw HTML string in some other way, the internal links inside it won't be valid.
Hippo CMS has built-in support for configuring components through auto-generated dialogs in the Channel Editor. These can easily be defined by using annotations in the component's ParametersInfo interface. Although there's an option to open a document picker, the documentation only hints at different configurations for it. After reading, it wasn't clear how to use it for picking a content folder instead of a document.
In a Hippo CMS site, all URLs are under its control. Using the sitemap configuration, you can to some extent influence how URLs will map to content but there's no obvious or well-documented way to make Hippo CMS ignore the URL and let it be processed by a different servlet.
In Java, .properties files are by default saved in ISO-8859-1 encoding. Since many languages use characters not included in this encoding, these characters will need to be expressed using Unicode escape sequences. Fortunately, IntelliJ IDEA can automatically handle conversion between the Unicode literals and their corresponding escape sequences, but the feature is not enabled by default.
One could argue that a JSON REST service should never return an empty body, but you might not always have a say in how a service you need to call will behave. Unfortunately, CRISP API in Hippo CMS throws a NullPointerException if the response body is empty.
The recommended way for integrating external services in Hippo CMS is the CRISP API. The official documentation provides detailed enough instructions for installing, configuring and using it. However, when I needed to customize the default Jackson ObjectMapper, it wasn't all that obvious how I could do it.
In Log4j, Mapped Diagnostic Context (MDC) can be used to provide additional context information for every log message. In server applications it will usually be initialized for every request in a filter. In Hippo CMS, a custom valve must be injected into the request pipeline for that purpose.
Generating the proxy client classes for a web service is only the first step. While the official documentation includes a basic sample for invoking a method on the web service, I had a hard time finding any guidance on how to pass in the credentials for basic authentication.
Apache CXF is a common choice for calling SOAP web services from Java. It's most convenient to use when you generate proxy client classes from the web service WSDL file. There's a command line tool available for that but if you don't want to put the generated sources in source control, you'll probably prefer the Maven plugin.
Most built-in components in Hippo CMS have a link to one or more documents as they are designed to render the CMS content documents. Sooner or later you'll want to achieve that with your own custom component as well.
As a part of getting acquainted with Hippo CMS I took on the task of creating a custom Hippo component for including a Stencil web component in a portal page using the CMS editor. I based my work on a similar component I found on GitHub for a Polymer web component.
Tree Fractal applet interactively demonstrates how easy it is to construct interesting images by adjusting the fractal parameters. The archive contains complete source code, a jar archive with compiled classes and a HTML page for viewing in a browser.
There probably isn't a computer user out there who hasn't heard of Java before. The language is already shipped with many ready to use classes and every release has more of them. I have written classes for some common tasks myself and you can download them from here for free.