Jackson Demo for Exception Reproduction

This is a simple reproduction of a sample Spring Boot/JPA application to show that @JsonIgnore does not work with java.util.Set, but does with java.util.List, when using the current latest version of Jackson and Spring Boot.

This project implements the @ManyToMany example on https://vladmihalcea.com/the-best-way-to-use-the-manytomany-annotation-with-jpa-and-hibernate/.

Github Issue

This reproduction is for the following issue: FasterXML/jackson-databind#2782.

Problem to Reproduce

• When using a @ManyToMany or @OneToMany annotation on a List, when using Jackson to serialize the owning entity, Jackson will recursively serialize the back-referenced entity and throw an exception.
• To fix this, you can add @JsonIgnore on the back part of the reference, as described in https://www.baeldung.com/jackson-bidirectional-relationships-and-infinite-recursion#json-ignore
• This works when the back-reference is a List, but not a Set.
• You may want to use a Set to optimize Hibernate queries, as explained in this vladmihalcea article.

Technologies Used

• Maven 3.6.3 (bundled as wrapper)
• Java 13.0.2
• Spring Boot Starter Data JPA 2.3.1
• Spring Boot Starter Web 2.3.1
• H2 Database 1.4.200
• Lombok 1.18.12
• Jackson dependencies (transitive) 2.11.0
• IntelliJ 2020.1.2 Ultimate (if it matters)

Installing and Running

Pre-requisites

You must have Java 13 installed.

If using IntelliJ, you must have the Lombok Plugin installed.

• In File > Settings > Other Settings > Lombok Plugin, ensure that Enable Lombok plugin is checked.
• In File > Settings > Build, Execution, Deployment > Compiler > Annotation Processors, ensure that Enable annotation processing is checked.

With IntelliJ

1. Import the root pom.xml as a Maven project and use default options.
2. Right-click DemoApplication in src/main/java/com/example/demo and click Run DemoApplication.
3. The application will build and run on http://localhost:8080.

With Command Line

# Navigate to demo directory
cd C:\<path-to-demo-directory>\jackson-demo

# Build project
mvnw clean install -DskipTests

# Run application
java -jar target\demo-0.0.1-SNAPSHOT.jar

Testing APIs

API with List Bidirectional Mappings

1. Make a GET request to http://localhost:8080/api/posts-with-list.

2. This should return:

   [
     {
       "id": 1,
       "title": "Post 1",
       "tagsWithList": [
         {
           "id": 1,
           "name": "foo"
         },
         {
           "id": 2,
           "name": "bar"
         }
       ]
     }
   ]

API with Set Bidirectional Mappings

1. Make a GET request to http://localhost:8080/api/posts-with-set.

2. It should return an HTTP 500 error page with the following error logged to the console:

   Resolved [org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON:
   Infinite recursion (StackOverflowError);
     nested exception is com.fasterxml.jackson.databind.JsonMappingException: Infinite recursion (StackOverflowError)
       (through reference chain: java.util.ArrayList[0]->com.example.demo.models.PostWithSet["tagsWithSet"])]
   

Running Unit Tests

With IntelliJ

1. Right-click PostControllerTest in src/test/java/com/example/demo/controllers and click Run 'PostControllerTest'.

With Command Line

mvnw test