The Java & Design Patterns series explains design principles and patterns through runnable Java code: SOLID, the GoF patterns, dependency injection, Repository and Specification with JPA, DDD, Hexagonal Architecture, ...
For twenty years, a Java class that "only holds data" came with about thirty lines of ceremony: private final fields, a constructor, getters, and then equals, hashCode and toString, written by hand or...
Every team has one: a seventy-line method buried in nested if statements that nobody dares to touch because it calculates real money. When the boss asks for "a new coupon for new customers", a develop...
Open the test suite of an order system after two years of development and you usually see the same scene: every test is thirty lines long, the first twenty only build a Customer, an Address and some O...
Some designs look reasonable when you first write them. The whole team may even praise them as "clean". A year later they are the source of every slow bug fix. The term anti-patterns describes exactly...
When a request splits into several parallel tasks, such as fetching customer data and order status at the same time, we usually reach for ExecutorService and Future. It works, but it hides a flaw that...
A typical order-processing service is written in the "one thread per request" style: the handler receives a request, calls the database, calls the payment gateway, and returns a result. The code is si...
The Visitor pattern is the most awkward design in the GoF catalog. To total the price of two kinds of products, you write a Visitor interface, add an accept method to every class, and then turn each n...
Call three warehouses to compare prices, calculate shipping, then merge the results into one invoice. Written sequentially, each call waits for the previous one and the response time equals the sum of...
When an order system is split into microservices, a request that sounds simple, like "place an order", suddenly touches three databases: inventory reservation, payment, and shipping. There is no singl...
An order is paid through a third-party gateway. One evening the gateway does not go down completely, it just gets slow: every call takes 30 seconds instead of 300 milliseconds. Tomcat threads are held...
An online shop starts with an orders table and one OrderService class that does everything. A year later, the admin screen needs to list orders with totals, the query that joins five tables runs slowl...
Many order and payment systems still run on Spring Boot 2.7 with Java 8 or 11, because "if it works, don't touch it." Standing still has a price: security patches stop, new libraries drop support for...
A Spring Boot project starts out clean: the controller calls a service, and the service calls a repository. Two years later, an @Entity shows up in the JSON response, a service imports HttpServletRequ...
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