jsonschema2pojo is a Java library and Maven plugin that generates Java POJOs (Plain Old Java Objects) from JSON Schema definitions. It simplifies the process of creating Java classes that represent JSON data structures, making it easier to work with JSON in Java applications.

Pros

• Automates the creation of Java classes from JSON Schema, saving development time
• Supports various JSON Schema formats and can generate annotations for popular libraries like Jackson and Gson
• Offers customization options through configuration settings and custom rule classes
• Integrates well with Maven and Gradle build processes

Cons

• Generated code may not always be optimal or follow best practices for complex schemas
• Limited control over the generated class structure without custom rule implementations
• May require manual adjustments for edge cases or specific requirements
• Learning curve for advanced customization and rule creation

Code Examples

1. Basic usage with Maven plugin:

<plugin>
    <groupId>org.jsonschema2pojo</groupId>
    <artifactId>jsonschema2pojo-maven-plugin</artifactId>
    <version>1.1.1</version>
    <configuration>
        <sourceDirectory>${basedir}/src/main/resources/schema</sourceDirectory>
        <targetPackage>com.example.types</targetPackage>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>generate</goal>
            </goals>
        </execution>
    </executions>
</plugin>

This Maven configuration generates Java classes from JSON Schema files in the specified source directory.

2. Programmatic usage:

import org.jsonschema2pojo.*;
import org.jsonschema2pojo.rules.*;

GenerationConfig config = new DefaultGenerationConfig();
SchemaMapper mapper = new SchemaMapper(new RuleFactory(config, new Jackson2Annotator(config), new SchemaStore()), new SchemaGenerator());
mapper.generate(codeModel, "ClassName", "com.example", "schema.json");

This code snippet demonstrates how to use jsonschema2pojo programmatically to generate Java classes from a JSON Schema file.

3. Custom rule implementation:

public class CustomStringRule implements Rule<JClassContainer, JType> {
    @Override
    public JType apply(String nodeName, JsonNode node, JsonNode parent, JClassContainer container, Schema schema) {
        JType stringType = container.owner().ref(String.class);
        JFieldVar field = container.getOwner().field(JMod.PRIVATE, stringType, nodeName);
        field.annotate(Pattern.class).param("regexp", "^[A-Z]{3}$");
        return stringType;
    }
}

This example shows a custom rule implementation that adds a regex pattern annotation to string fields.

Getting Started

To use jsonschema2pojo in your Maven project:

1. Add the plugin to your pom.xml:

<plugin>
    <groupId>org.jsonschema2pojo</groupId>
    <artifactId>jsonschema2pojo-maven-plugin</artifactId>
    <version>1.1.1</version>
    <configuration>
        <sourceDirectory>${basedir}/src/main/resources/schema</sourceDirectory>
        <targetPackage>com.example.types</targetPackage>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>generate</goal>
            </goals>
        </execution>
    </executions>
</plugin>

2. Place your JSON Schema files in src/main/resources/schema.
3. Run mvn generate-sources to generate Java classes.
4. Use the generated classes in your project.