Code Generation

About

Code Generation plugins in Maven are used to automate the creation of source code during the build process. These plugins reduce manual coding effort by generating boilerplate code, stubs, client/server APIs, proxies, or model classes from various specifications (e.g., OpenAPI, WSDL, XSD, Protocol Buffers, etc.).

Automated code generation ensures:

• Consistency between contract (specs) and implementation.

• Productivity by eliminating repetitive tasks.

• Synchronization of APIs across services and clients.

Common Scenarios for Code Generation

• Generating Java classes from XML schemas (XSD).

• Generating SOAP client stubs from WSDL files.

• Generating REST clients or servers from OpenAPI/Swagger specs.

• Generating gRPC or Protocol Buffers classes.

• Creating JPA entities, DTOs, mappers, or boilerplate code.

JAXB2 Maven Plugin

Generates Java classes from XSD schemas using JAXB 2. It's useful in projects where XML configuration or communication is required.

Common Goals

Goal	Description
xjc	Compile .xsd into Java classes

Basic Configuration

<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>jaxb2-maven-plugin</artifactId>
  <version>2.5.0</version>
  <executions>
    <execution>
      <goals>
        <goal>xjc</goal>
      </goals>
    </execution>
  </executions>
  <configuration>
    <schemaDirectory>src/main/resources/schema</schemaDirectory>
    <outputDirectory>${project.build.directory}/generated-sources/jaxb</outputDirectory>
  </configuration>
</plugin>

Useful Config Options

Option	Description
schemaDirectory	Path to XSD files
outputDirectory	Directory to generate Java classes
clearOutputDir	Clean output before generation
packageName	Override package name

OpenAPI Generator Maven Plugin

Generates client/server code from OpenAPI (Swagger) definitions. Supports many languages and frameworks.

Common Goals

Goal	Description
generate	Generates code from openapi.yaml

Basic Configuration

<plugin>
  <groupId>org.openapitools</groupId>
  <artifactId>openapi-generator-maven-plugin</artifactId>
  <version>6.6.0</version>
  <executions>
    <execution>
      <goals>
        <goal>generate</goal>
      </goals>
    </execution>
  </executions>
  <configuration>
    <inputSpec>${project.basedir}/src/main/resources/openapi.yaml</inputSpec>
    <generatorName>java</generatorName>
    <output>${project.build.directory}/generated-sources/openapi</output>
    <apiPackage>com.example.api</apiPackage>
    <modelPackage>com.example.model</modelPackage>
  </configuration>
</plugin>

Popular Generator Types:

• java (Spring, JAX-RS)

• typescript, python, go, kotlin, etc.

wsimport (JAX-WS Maven Plugin)

Generates Java classes from WSDL for SOAP-based web service clients.

Basic Configuration

<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>jaxws-maven-plugin</artifactId>
  <version>2.3</version>
  <executions>
    <execution>
      <goals>
        <goal>wsimport</goal>
      </goals>
      <configuration>
        <wsdlUrls>
          <wsdlUrl>https://example.com/service?wsdl</wsdlUrl>
        </wsdlUrls>
        <packageName>com.example.ws</packageName>
      </configuration>
    </execution>
  </executions>
</plugin>

protobuf-maven-plugin

Used to compile .proto files into Java or other languages using the Protocol Buffers compiler.

Basic Configuration

<plugin>
  <groupId>org.xolstice.maven.plugins</groupId>
  <artifactId>protobuf-maven-plugin</artifactId>
  <version>0.6.1</version>
  <executions>
    <execution>
      <goals>
        <goal>compile</goal>
        <goal>compile-custom</goal>
      </goals>
    </execution>
  </executions>
  <configuration>
    <protoSourceRoot>${project.basedir}/src/main/proto</protoSourceRoot>
  </configuration>
</plugin>

Generated Output

By default, .java classes will be generated from .proto files under:

target/generated-sources/protobuf

mapstruct-processor (for Annotation Processing)

Used for annotation-based generation of Java bean mappers.

Basic Configuration

<dependency>
  <groupId>org.mapstruct</groupId>
  <artifactId>mapstruct</artifactId>
  <version>1.5.5.Final</version>
</dependency>
<dependency>
  <groupId>org.mapstruct</groupId>
  <artifactId>mapstruct-processor</artifactId>
  <version>1.5.5.Final</version>
  <scope>provided</scope>
</dependency>

Additionally, enable annotation processing in your build tool (IDE or compiler).