Opinionated JPA with Querydsl
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Opinionated JPA with Querydsl

About the Book

ORM is a neat idea, typically used together with rich domain model. JPA, currently in version 2.2, is a Java specification that is a common ground for most Java ORMs. But ORM is also a love-hate topic, with stickers like "Vietnam of computer science", often called an antipattern.

In this book we'll talk about many interesting JPA aspects and put them into the context of other persistence options. I'll also try to explore how to use JPA for queries that are more SQL friendly, but still benefiting from convenient features not available with plain old JDBC. We will discuss non-JPA feature called entity (or ad-hoc) joins - supported by both EclipseLink and Hibernate. We'll also cover some typical problems (lazy loading and related N+1 select).

Querydsl will be with us along the way, because compared to Criteria API it allows us to write much cleaner queries in a fluent API fashion.

About the Author

Richard "Virgo" Richter
Richard "Virgo" Richter

Software developer since forever, professionally since 2000. Very familiar with Java ecosystem, but open to learning new languages and technologies as I don’t believe in narrow specialization. I like the idea of an agile and learning environment (like Joy, Inc.). I also believe in testing automation, continuous delivery, clean code and also other, often rather peopleware-ish topics. I'm exploring functional programming and domain-driven design, among others.

Table of Contents

  • Preface
      • Is this book for you?
      • Not in the book
      • Other JPA books
      • Books on performance of the database and persistence layer
      • Looking back to Patterns of Enterprise Application Architecture
      • Structure of the book
      • Using code examples
      • My writing background
      • Feedback
  • I JPA Good Times, Bad Times
    • 1. Good Parts
      • JPA is standard
      • Database independence
      • Natural SQL-Java type mapping
      • Flexible mapping
      • Unit of work
      • Declarative transactions
      • Other JPA 2.1 goodies
    • 2. The Missing Parts
      • Compared to ORM providers
      • Comparing JPQL to SQL
      • Other missing parts
    • 3. Questionable parts
      • Lazy on basic and to-one fields
      • Generated SQL updates
      • Unit of work vs queries
      • We can’t escape SQL and relational model
      • Additional layer
      • Big unit of work
      • Other entity manager gotchas
      • JPA alternatives?
    • 4. Caching considerations
      • Caching control
      • Second-level cache vs queries
      • Explicit application-level cache
      • Conclusion
    • 5. Love and hate for ORM
      • Vietnam of Computer Science
      • Not much love for ORM
      • But a lot of hate
      • Is tuning-down a way out?
      • And there is more
  • II Opinionated JPA
    • 6. Tuning ORM down
      • Price for relations
      • How does this affect my domain model?
      • When to tune down and when not?
    • 7. No further step without Querydsl
      • Simple example with Querydsl
      • Comparison with Criteria API
      • Comparison with JPQL
      • What about the cons?
      • Be explicit with aliases
      • Functions and operations
      • Subqueries
      • Pagination
      • Tuple results
      • Fetching to-many eagerly
      • Querydsl and code style
      • More
    • 8. Troubles with to-one relationships
      • Simple example of @ManyToOne
      • How about going lazy?
      • Dual relationship mapping
      • Using projections for fetching
      • Entity update scenario
      • Alternative entity “views”
      • Summary
    • 9. Removing to-one altogether
      • Why do we need ON anyway?
      • Dropping to-one annotations
      • Ad hoc joins across tables
      • Loading the relationship from the entity
      • Loosing information without mapping annotations
      • Wrapping it up
    • 10. Modularity, if you must
      • Everybody knows layers
      • Components based on features
      • Multiple JARs with JPA entities
      • Distributing persistence.xml – do we need it?
      • It doesn’t start anyway
      • Shared cache mode
      • Putting the Spring configuration together
      • Common base entity classes in separate JAR
      • Cutting cyclic dependencies
      • Conclusion
  • III Common problems
    • 11. Avoid N+1 select
      • Anatomy of N+1 select
      • Eager to-one without joins
      • Eager to-many relationships
      • Lazy relationships triggered later
      • Paginating with to-many
      • Wrapping up N+1 problem
    • 12. Mapping Java enum
      • Naive approach
      • Encapsulated conversion
      • Conversion micro-framework
      • Refinement?
      • Simplified static resolving
      • Alternative conversions within entities
      • Java 8 and reverse enum resolution
      • Relational concerns
    • 13. Advanced Querydsl
      • Introducing handy utility classes
      • Tricky predicate parts
      • Groups of AND/OR predicates
      • Operator precedence and expression serialization
      • Querydsl Expression hierarchy
      • Constant expressions
      • Using FUNCTION template
      • Update/delete clauses and exception handling
      • JPAQueryFactory
      • Detached queries
      • Working with dates and java.time API
      • Result transformation
      • Note about Querydsl 3 versus 4
  • IV Final thoughts
      • JPA/ORM – yes or no?
      • JPA or concrete ORM provider?
      • JPA, Querydsl and beyond
      • This book
  • Appendices
    • H2 Database
      • Starting it
      • Using it as a devel/test DB
    • Project example
      • Querydsl and Maven
      • Querydsl and Gradle
    • Bugs discovered while writing this book
    • List of acronyms
  • Resources
      • Books
      • Online articles, blogs
      • Other online resources
  • Notes

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