You are building a data access layer that must work with both MySQL and PostgreSQL. You have MySQLConnection, MySQLStatement, MySQLResultSet and matching PostgreSQL classes. The risk: a MySQLConnection used together with a PostgreSQLStatement is a runtime error waiting to happen. The objects in a family must be used together consistently. The Abstract Factory pattern enforces that consistency.
This guide builds the pattern from the ground up using a realistic UI theme scenario — dark and light themes — so the “family” concept is immediately tangible. Once the structure is clear, you will see exactly how JDBC in the Java standard library uses the same idea.
You write a notification service. First it sends email. A month later you add SMS. Then push notifications. Then Slack. Each time, you open the same class and add another else if branch. The class that was tested and working now needs to change again — and again — every time a new channel appears. This is the problem Factory Method solves.
This guide builds the pattern from the ground up. You will see exactly why each piece exists before you see the code for it. By the end, you will understand how the pattern works, how to spot it in the Java standard library, and when to use (and skip) it.
Of every breaking change in the Spring Boot 2 → 3 era, the removal of WebSecurityConfigurerAdapter generated the most confused stack traces I’ve debugged — because security configuration is the one place where “it compiles and runs” tells you almost nothing about whether it still protects anything. This guide migrates Spring Security 5 configurations to the 6.x component model (Spring Boot 3.x), covers what tightens further in Spring Security 7 (Spring Boot 4), and flags the places where a mechanical conversion quietly changes your authorization behaviour.
Spring Boot 4 finally forced the issue: RestTemplate, in maintenance mode since Spring 5, is out of the recommended path, and codebases full of getForObject() and exchange() calls need a plan. The good news is that RestClient (introduced in Spring 6.1 / Boot 3.2) was designed as the synchronous successor, so this migration is mostly mechanical — if you know the method-by-method mapping and the three places where behaviour quietly differs. This guide gives you both.
I have now taken three codebases through the Hibernate 5 → 6 → 7 path, and the same pattern repeated each time: the compile errors are the easy part, and the silent behaviour changes — query result types, ID generation, fetch semantics — are what reach production. This guide walks the migration in two deliberate hops (5→6, then 6→7), with the exact errors you will see at each stage and how to fix them. Do not attempt 5→7 in one jump; the diagnostics assume you pass through 6.
The first time I ported a Zuul gateway to Spring Cloud Gateway, the routes took an afternoon — and the filters took two weeks. That ratio surprises every team that attempts this migration, because the route configuration looks superficially similar while the filter model is a different universe: Zuul 1 is a blocking servlet filter chain, Spring Cloud Gateway runs on Netty with Project Reactor, and a single hidden blocking call in a ported filter can stall your entire gateway. This guide covers the route conversion, the filter-by-filter port, and the blocking-call traps, in that order.
In 2020 I published a dozen tutorials on this site covering Eureka, Hystrix, Zuul, Ribbon, and Feign — the Spring Cloud Netflix stack. They were accurate then. Today, most of that stack is dead: Netflix put Hystrix into maintenance mode back in 2018, Zuul 1 and Ribbon followed, and the Spring team removed them from the Spring Cloud release train entirely. If you are still running any of these libraries on Spring Boot 2.x, this guide maps every Netflix component to its modern replacement and shows you the actual migration steps — including the parts that break.
