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jackson

Use when working with Jackson JSON serialization - migrating from Jackson 2.x to 3.x, configuring JsonMapper, handling date/time types, or troubleshooting serialization issues.

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SKILL.md

Jackson 3 Skill

Guidance for working with Jackson 3.x JSON serialization in the lib-electronic-components library.

Overview

This project uses Jackson 3.0.3 for JSON serialization. Jackson 3 is a major version upgrade with significant API changes from Jackson 2.x.

Quick Reference

Import Changes

java
// Jackson 2.x
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;

// Jackson 3.x
import tools.jackson.databind.json.JsonMapper;
import tools.jackson.databind.SerializationFeature;
// No JavaTimeModule needed - built-in

Creating a Mapper

java
// Simple mapper
JsonMapper mapper = JsonMapper.builder().build();

// With configuration
JsonMapper mapper = JsonMapper.builder()
    .enable(SerializationFeature.INDENT_OUTPUT)
    .build();

Serialization

java
JsonMapper mapper = JsonMapper.builder().build();

// To JSON string
String json = mapper.writeValueAsString(object);

// Pretty print
String prettyJson = mapper.writerWithDefaultPrettyPrinter()
    .writeValueAsString(object);

Deserialization

java
JsonMapper mapper = JsonMapper.builder().build();

// From JSON string
MyClass obj = mapper.readValue(json, MyClass.class);

// From file
MyClass obj = mapper.readValue(new File("data.json"), MyClass.class);

Maven Dependencies

xml
<properties>
    <jackson.version>3.0.3</jackson.version>
</properties>

<dependencies>
    <!-- Jackson 3.x core -->
    <dependency>
        <groupId>tools.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>${jackson.version}</version>
    </dependency>

    <!-- Annotations stay on 2.x -->
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-annotations</artifactId>
        <version>2.20</version>
    </dependency>

    <!-- XML format (optional) -->
    <dependency>
        <groupId>tools.jackson.dataformat</groupId>
        <artifactId>jackson-dataformat-xml</artifactId>
        <version>${jackson.version}</version>
    </dependency>
</dependencies>

Module System (JPMS)

java
module my.module {
    // Jackson 3.x modules
    requires transitive tools.jackson.core;
    requires transitive tools.jackson.databind;

    // Annotations still use 2.x module name
    requires transitive com.fasterxml.jackson.annotation;

    // Open packages for Jackson reflection
    opens my.package.model to tools.jackson.databind;
}

Annotations

Annotations stay on Jackson 2.x (com.fasterxml.jackson.annotation):

java
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.annotation.JsonSubTypes;

public class MyClass {
    @JsonProperty("custom_name")
    private String name;

    @JsonIgnore
    private String internal;

    @JsonCreator
    public MyClass(@JsonProperty("name") String name) {
        this.name = name;
    }
}

Date/Time Handling

Jackson 3.x has built-in support for Java 8 date/time types:

java
// No module registration needed!
JsonMapper mapper = JsonMapper.builder().build();

public class Event {
    private LocalDate date;           // "2024-03-15"
    private LocalDateTime timestamp;  // "2024-03-15T10:30:00"
    private Instant instant;          // "2024-03-15T10:30:00Z"
    private Duration duration;        // "PT1H30M"
}

Common Configuration

java
JsonMapper mapper = JsonMapper.builder()
    // Pretty print output
    .enable(SerializationFeature.INDENT_OUTPUT)

    // Don't fail on unknown properties
    .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)

    // Include null values
    .serializationInclusion(JsonInclude.Include.ALWAYS)

    .build();

Migration from Jackson 2.x

Step-by-Step

  1. Update pom.xml

    • Change groupId: com.fasterxml.jackson.core → tools.jackson.core
    • Remove jackson-datatype-jdk8 and jackson-datatype-jsr310 (built-in)
    • Keep annotations at com.fasterxml.jackson.core:jackson-annotations:2.20

  2. Update module-info.java

    • Change: com.fasterxml.jackson.databind → tools.jackson.databind
    • Change opens: to com.fasterxml.jackson.databind → to tools.jackson.databind
    • Remove jsr310/jdk8 module requires

  3. Update code

    • ObjectMapper → JsonMapper
    • new ObjectMapper() → JsonMapper.builder().build()
    • Remove mapper.registerModule(new JavaTimeModule())
    • Update imports: com.fasterxml.jackson.databind → tools.jackson.databind.json

  4. Keep annotations unchanged

    • @JsonProperty, @JsonCreator, etc. stay at com.fasterxml.jackson.annotation

Before/After Examples

java
// BEFORE (Jackson 2.x)
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(new JavaTimeModule());
mapper.enable(SerializationFeature.INDENT_OUTPUT);
mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
String json = mapper.writeValueAsString(obj);

// AFTER (Jackson 3.x)
import tools.jackson.databind.json.JsonMapper;
import tools.jackson.databind.SerializationFeature;

JsonMapper mapper = JsonMapper.builder()
    .enable(SerializationFeature.INDENT_OUTPUT)
    .build();
String json = mapper.writeValueAsString(obj);

Learnings & Quirks

ObjectMapper vs JsonMapper

  • ObjectMapper still exists in Jackson 3 but is deprecated
  • Use JsonMapper which provides the builder pattern
  • JsonMapper.builder().build() is the canonical way

Annotations Compatibility

  • Jackson 3 databind works with Jackson 2 annotations
  • This is intentional to ease migration
  • Don't try to find Jackson 3 annotations - they don't exist

Maven Central Search Lag

  • Maven Central's search API may not show the latest Jackson 3 versions
  • The artifacts download fine with Maven
  • Trust mvn dependency:tree over search.maven.org

SerializationFeature Location

  • SerializationFeature moved to tools.jackson.databind.SerializationFeature
  • Not in tools.jackson.databind.json

No More Fluent Configuration on Instance

  • Jackson 2: mapper.enable(feature) returns this
  • Jackson 3: Use builder pattern instead

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