intermediate~2h
Collections with Spring Boot
How collections show up across a real Spring Boot application — REST controller serialization, JPA entity collection mappings, and the specific pitfalls (mutable getters, uninitialized fields, lazy-loaded collections) that look fine in a code review and break in production.
Learning objectives
- Explain how List, Set, and Map fields serialize to JSON through Jackson in a Spring Boot controller
- Choose between List and Set for @OneToMany/@ManyToMany JPA mappings and explain the Cartesian product problem
- Identify the mutable-getter pitfall where a collection field leaks a live reference out of an entity or DTO
- Explain why an uninitialized collection field throws NullPointerException instead of behaving like an empty collection
- Describe how lazy-loaded JPA collections cause the N+1 query problem and how to recognize it in logs
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Collections with Spring Boot