Pages with tag Spring Boot

A fix for "PersistentObjectException - detached entity passed to persist" in Spring Boot Error messages coming out of Spring can be more than inscrutible. As discussed in my solution to transient unsaved properties, I'm defining a REST API using Swagger/OpenAPI targeting a Spring Boot implementation. The object models are complex, with lots of nested objects. Today's inscrutible message, "detached entity passed to persist", doesn't make much sense as a phrase does it? Detached? Detached Entity? Say what? Turns out it's not that hard a problem to fix.
A fix for org.hibernate.TransientPropertyValueException in Spring Boot

Error messages coming out of Spring can be more than inscrutible. In this case I'm defining a REST API using Swagger/OpenAPI targeting a Spring Boot implementation. The object models are complex, with lots of nested objects. It turns out you can POST a big blob of JSON with all kinds of structure, and Spring/JPA/Hibernate will swallow the whole thing and automatically parcel everything out to the corresponding database table. If you define the JPA annotations correctly, that is.

Get an annotation incorrect, and you'll be left scratching your head over what the heck you're being told by the error message. The Spring Team promises on a huge stack of bibles that Spring Boot automatically takes care of zillions of things for you, and it's so easy to write your code. In my experience it's instead a matter of papering complexity over complexity over a system containing about a thousand kitchen sinks worth of excess capabilities. There's no amount of papering-over that kind of complexity that will make the system simpler. But, the boss-man says to use Spring Boot, so here goes.

Create a Spring Boot REST API using Swagger/OpenAPI "Generate Swagger OpenAPI REST API documentation for Spring Boot application"

The Swagger tools, and the OpenAPI format, are an excellent way to document REST API's and even to generate client or server stub libraries to ease implementation. The technology serves two purposes -- a) standardized documentation for REST API's, b) generating code from API documentation in several programming languages. An OpenAPI file is fairly simple to write, you declare REST endpoints, describe the parameters and the request type, and then describe responses. It allows you to define complex object models that can be used either as input to a service, or its output.

Unfortunately the Swagger website doesn't have adequate documentation of using the tools. And it proved difficult to find clear straight-forward tutorials showing how to get started. Even the most powerful tool can be hampered if folks are unable to use it.

The following tutorial is a complete demonstration of, starting from scratch, developing a small Spring Boot service using OpenAPI and the Swagger tools. We show how to go from an OpenAPI spec to generated Spring Boot code, and also how to generate an OpenAPI spec from running Spring Boot code. There are several issues with the workflow of generating code from the OpenAPI spec. It's more effective to instead write the service code, and add in the annotations required for the Swagger tools to generate the OpenAPI spec for you.

With the OpenAPI spec it's easy to produce interactive API documentation that programmers can try out directly in their web browser.

Persisting complex Embeddable/Embedded objects in Spring/Hibernate Hibernate and Spring and the Java Persistence packages make for a powerful combination. You can define a complex tree of objects, and the system automagically creates the database schema, and automagically saves and retrieves that object tree from the database. It means you can innocently code away, until one day you try to include an Embedded object but it silently fails to be saved to the database, Hibernate silently fails to create database columns for that object, and when retrieving the object for which the Embedded object is a field, the object is null. No error messages are printed in the log. Instead you're left wondering, why didn't Hibernate persist this object? What's presented here is a solution using EntityListener methods to convert values in and out of shadow values stored in the Entity.
Solving HttpMessageNotWritableException, Could not write JSON: Could not set field value in Spring Boot My experience of programming on the Spring platform is that you'll be coding along, everything is going great, until an inscrutible complex error message emerges from the depths of who knows where. This message seemed to involve some module or another having created a round peg to fit into a square hole. That is, I'd sent a GET request to retrieve a large object tree, and it seemed that while filling data into a field of a subsidiary object Spring/Hibernate/etc chose the wrong data type, and it threw up an error. The specific type of error was receiving no discussion anywhere, so it was impossible to see how others had solved the problem. The documentation was of no use in explaining anything about this. But, after a couple days of trials and errors and more errors, a solution was found. And, indeed, Spring/Hibernate had chosen the wrong data type to fill into one of the fields, and the solution is fairly simple.
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