Java 'StackOverflowError' from recursive toString/equals/hashCode or JSON serialization (Jackson) of bidirectional JPA relationships (parent <-> child): how do I fix it?

Cause: StackOverflowError is thrown when a thread recurses too deeply for its stack. With bidirectional object graphs (an Order has items, each item points back to its Order), generated or hand-written toString, equals, hashCode, or a JSON serializer such as Jackson walk from parent to child and back to the parent forever. IDE-generated or Lombok @Data / @ToString / @EqualsAndHashCode on both entities is a common source; Jackson reports it as a 'Document nesting depth' or an infinite recursion error wrapped in a JsonMappingException, depending on version. Fix (best first): 1. Break the cycle in toString, equals and hashCode: exclude the back-reference field (for Lombok, @ToString.Exclude and @EqualsAndHashCode.Exclude), and base equals/hashCode on an id or business key rather than on the whole relationship. 2. For JSON, avoid serializing entities directly: return DTOs or projections that contain only what the client needs. This is the cleanest fix. 3. If you must serialize the entities, mark the pair with Jackson's @JsonManagedReference on the parent side and @JsonBackReference on the child's back-pointer (the back side is omitted when serializing), or use @JsonIdentityInfo so repeated objects are written as references to an earlier id, or ignore the specific back-reference property with @JsonIgnoreProperties. 4. If the recursion is legitimate but deep (a very long chain), convert it to iteration or add a base case rather than only enlarging the thread stack size. Verify: Call toString, equals and the JSON endpoint on a parent with children and confirm they return finite output; add a unit test with a two-way link. Sources: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/StackOverflowError.html https://javadoc.io/static/com.fasterxml.jackson.core/jackson-annotations/2.18.9/com/fasterxml/jackson/annotation/JsonManagedReference.html https://javadoc.io/static/com.fasterxml.jackson.core/jackson-annotations/2.18.9/com/fasterxml/jackson/annotation/JsonIdentityInfo.html