PomTestWrapper pom = buildPom("build-extension-inheritance/sub");
        assertEquals(3, ((List<?>) pom.getValue("build/extensions")).size());
        assertEquals("b", pom.getValue("build/extensions[1]/artifactId"));
        assertEquals("a", pom.getValue("build/extensions[2]/artifactId"));
        assertEquals("0.2", pom.getValue("build/extensions[2]/version"));

        Objects.requireNonNull(projectRealm, "projectRealm cannot be null");

        if (cache.containsKey(key)) {
            throw new IllegalStateException("Duplicate project realm for extensions " + key);
        }

        CacheRecord record = new CacheRecord(projectRealm, extensionArtifactFilter);

        cache.put(key, record);

        return record;
    }

        if (build != null) {
            for (Extension extension : build.getExtensions()) {
                Plugin plugin = new Plugin();
                plugin.setGroupId(extension.getGroupId());
                plugin.setArtifactId(extension.getArtifactId());
                plugin.setVersion(extension.getVersion());
                XmlNode configuration = extension.getDelegate().getConfiguration();

          </description>
          <type>String</type>
        </field>
        <field>
          <name>extensions</name>
          <version>4.0.0+</version>
          <description>A set of build extensions to use from this project.</description>
          <association>
            <type>Extension</type>
            <multiplicity>*</multiplicity>
          </association>
        </field>
      </fields>

     *
     * @param classRealm The class realm hosting the build extensions of this project, may be {@code null}.
     */
    public void setClassRealm(ClassRealm classRealm) {
        this.classRealm = classRealm;
    }

    /**
     * Gets the project's class realm. This class realm hosts the build extensions of the project.

  </parent>

  <artifactId>maven-api</artifactId>
  <packaging>pom</packaging>
  <name>Maven 4 API</name>
  <description>A new immutable API for Maven 4 to better manage what plugins and extensions can influence.</description>

  <modules>
    <module>maven-api-meta</module>
    <module>maven-api-di</module>
    <module>maven-api-xml</module>
    <module>maven-api-model</module>

     * <p>
     * This type registry is NOT used by Maven Core: Maven replaces it during Session creation with a type registry
     * that supports extending it (i.e. via Maven Extensions).
     * <p>
     * Important: this "static" list of types should be in-sync with core provided types.
     */
    protected ArtifactTypeRegistry getArtifactTypeRegistry() {

Use this directory to add third party extensions to Maven Core. These extensions can either extend or override

package org.apache.maven.api.spi;

import org.apache.maven.api.annotations.Consumer;
import org.apache.maven.api.annotations.Experimental;

/**
 * Marker interface to indicate services that can be provided by plugins and extensions.
 */
@Experimental
@Consumer

    <module>plexus-tools</module>
    <module>plexus-utils</module>
  </modules>
  <build>
    <extensions>
      <extension>
        <groupId>org.apache.maven.wagon</groupId>
        <artifactId>wagon-webdav</artifactId>
        <version>1.0-beta-1</version>
      </extension>
    </extensions>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>