Introduction

Background

Querydsl was born out of the need to maintain HQL queries in a type-safe way. Building HQL queries incrementally requires string concatenation, which produces hard-to-read code. Referencing domain types and properties through plain strings is fragile and error-prone.

With a changing domain model, type safety brings huge benefits. Domain changes are reflected directly in queries, and IDE auto-complete makes query construction faster and safer.

HQL for Hibernate was the first target language for Querydsl. Today the framework supports JPA, SQL (JDBC), R2DBC, MongoDB, Collections, Spatial, Kotlin, and Scala as backends.

If you are new to database access in Java, this guide provides a good overview of the various parts and options and shows where Querydsl fits in.

Principles

Type safety is the core principle of Querydsl. Queries are constructed based on generated query types that reflect the properties of your domain types. Function and method invocations are constructed in a fully type-safe manner.

Consistency is another important principle. The query paths and operations are the same across all implementations, and the query interfaces share a common base interface.

To get an impression of the expressivity of the Querydsl query and expression types, explore the Javadocs for com.querydsl.core.Query, com.querydsl.core.Fetchable, and com.querydsl.core.types.Expression.