The regex that broke a server
Introduction I’ve never thought I would see an unresponsive server due to a bad regex matcher but that’s just happened to one of our services, yielding it unresponsive. Let’s assume we parse some external dealer car info. We are trying to find all those cars with “no air conditioning” among various available input patterns (but without matching patterns such as “mono air conditioning”).
How to detect the Hibernate N+1 query problem during testing
Introduction In this article, you are going to learn how to automatically detect the N+1 query problem when using JPA and Hibernate using the Hypersistence Utils open-source project. With Hibernate, you manage entity state transitions, which are then translated to SQL statements. The number of generated SQL statements is affected by the current fetching strategy, Criteria queries, or Collection mappings, and you might not always get what you expected. Ignoring SQL statements is risky, and it may eventually put a heavy toll on the overall application performance. I’m a strong advocate of… Read More
JPA Criteria API can generate unexpected SQL statements
Introduction In this article, we are going to learn why you should always check the SQL statements generated by JPA Criteria API queries when using Hibernate. Criteria API is very useful for dynamically building queries, but that’s the only use case where I’d use it. Whenever you have a UI with N filters that may arrive in any M combinations, it makes sense to have an API to construct queries dynamically since concatenating strings is always a path I’m running away from. The question is, are you aware of the SQL queries… Read More
JOOQ Facts: From JPA Annotations to JOOQ Table Mappings
JOOQ is a neat framework, and it addresses a long-time issue I’ve had with advanced dynamic filtered queries. While Hibernate and JPA come with a useful Criteria API, which I’ve been using for quite some time, there are understandable limits to what you can do with those. For instance, you cannot go beyond simple SQL operations (e.g JOINS, NESTED SLECTS, AGGREGATION) and do something like: window functions, user-defined functions or easy sequencing to name a few. JOOQ doesn’t feel like competing with Hibernate, but instead, I feel like it completes it. I’ve… Read More
Hibernate integration testing strategies
Introduction I like integration testing. As I explained in this article, it’s a good way to check what SQL queries are generated by Hibernate behind the scenes. But integration tests require a running database server, and this is the first choice you have to make. Using a production-like local database server for Integration Testing For a production environment, I always prefer using incremental DDL scripts, since I can always know what version is deployed on a given server, and which scripts required to be deployed. I’ve been relying on Flyway to manage… Read More
How to retry JPA transactions after an OptimisticLockException
Introduction In this article, I will discuss how we can implement an automatic Retry mechanism when dealing with the JPA OptimisticLockException. You can find the introductory part here and the MongoDB implementation here.
21st century logging
I think logging should get more attention than we are currently giving it. When designing an application a great deal of effort goes into modeling the customer business logic, making sure all use cases are covered and handled properly. The business model is mapped to a persistence storage (be at a RDBMS or a NoSQL solution), frameworks are being chosen: web, middle-ware, batch jobs, and probably SLF4J with log4j or logback. This has been the case of almost all applications I’ve been involved with, and logging was always a second-class citizen, relying… Read More
How to implement Equals and HashCode for JPA entities
Introduction Every Java object inherits the equals and hashCode methods, yet they are useful only for Value objects, being of no use for stateless behavior-oriented objects. While comparing references using the “==” operator is straightforward, for object equality things are a little bit more complicated.
How to fetch entities multiple levels deep with Hibernate
Introduction It’s quite common to retrieve a root entity along with its child associations on multiple levels. In our example, we need to load a Forest with its Trees and Branches and Leaves, and we will try to see have Hibernate behaves for three collection types: Sets, Indexed Lists, and Bags.
A beginner’s guide to Hibernate fetching strategies
Introduction When it comes to working with an ORM tool, everybody acknowledges the importance of database design and Entity-to-Table mapping. These aspects get a lot of attention, while things like fetching strategy might be simply put-off. In my opinion, the entity fetching strategy shouldn’t ever be separated from the entity mapping design, since it might affect the overall application performance unless properly designed. Before Hibernate and JPA got so popular, there was a great deal of effort put into designing each query, because you had to explicitly select all the joins you… Read More
