spring-boot/config/checkstyle/import-control.xml

<?xml version="1.0"?>
<!DOCTYPE import-control PUBLIC "-//Checkstyle//DTD ImportControl Configuration 1.4//EN" "https://checkstyle.org/dtds/import_control_1_4.dtd">
<import-control pkg="org.springframework.boot">

	<!-- Only use Micrometer Observation -->
	<allow pkg="io.micrometer.observation" />
	<disallow pkg="io.micrometer" />

	<!-- Improve DevEx with fluent APIs -->
	<allow class="org.springframework.lang.CheckReturnValue" />

	<!-- Use JSpecify for nullability (not Spring)  -->
	<allow class="org.springframework.lang.Contract" />
	<disallow pkg="org.springframework.lang" />

	<!-- Allow other imports -->
	<allow pkg=".*" regex="true" />

	<!-- Keep foundation packages away from 'org.springframework.boot' package -->
	<subpackage name="ansi">
		<disallow pkg="org.springframework.boot" exact-match="true"/>
	</subpackage>
	<subpackage name="availability">
		<disallow pkg="org.springframework.boot" exact-match="true"/>
	</subpackage>
	<subpackage name="bootstrap">
		<disallow pkg="org.springframework.boot" exact-match="true"/>
	</subpackage>
	<subpackage name="context">
		<subpackage name="properties">
			<subpackage name="bind">
				<disallow pkg="org.springframework.boot" exact-match="true"/>
			</subpackage>
			<subpackage name="source">
				<disallow pkg="org.springframework.boot" exact-match="true"/>
			</subpackage>
		</subpackage>
	</subpackage>
	<subpackage name="env">
		<disallow pkg="org.springframework.boot" exact-match="true"/>
	</subpackage>
	<subpackage name="info">
		<disallow pkg="org.springframework.boot" exact-match="true"/>
	</subpackage>
	<subpackage name="io">
		<disallow pkg="org.springframework.boot" exact-match="true"/>
	</subpackage>
	<subpackage name="ssl">
		<disallow pkg="org.springframework.boot" exact-match="true"/>
	</subpackage>
	<subpackage name="origin">
		<disallow pkg="org.springframework.boot" exact-match="true"/>
	</subpackage>
	<subpackage name="util">
		<disallow pkg="org.springframework.boot" exact-match="true"/>
	</subpackage>

	<!-- Open direct use of micrometer to specific subpackages  -->
	<subpackage name="metrics">
		<allow pkg="io.micrometer" />
	</subpackage>
	<subpackage name=".*\.metrics" regex="true">
		<allow pkg="io.micrometer" />
	</subpackage>
	<subpackage name="(.*\.autoconfigure|autoconfigure)(\..*)?" regex="true">
		<allow pkg="io.micrometer" />
	</subpackage>
	<subpackage name="docs">
		<allow pkg="io.micrometer" />
	</subpackage>
	<subpackage name="test">
		<subpackage name="autoconfigure">
			<subpackage name="actuate">
				<allow pkg="io.micrometer" />
			</subpackage>
		</subpackage>
	</subpackage>

	<!-- Keep logging focused on lower level packages (binding and AOT) -->
	<subpackage name="logging">
		<allow pkg="org.springframework.boot.context.properties" />
		<allow pkg="org.springframework.boot.context.properties.bind" />
		<allow pkg="org.springframework.context.aot" />
		<disallow pkg="org.springframework.context" />
		<disallow pkg="org.springframework.boot.context" />
	</subpackage>

	<!-- Web related concerns -->
	<subpackage name="web">

		<!-- Lock things down -->
		<disallow pkg="org.springframework.boot.web" />
		<disallow pkg="org.springframework.web.servlet" />
		<disallow pkg="jakarta.servlet" />

		<!-- Common -->
		<subpackage name="context">
			<allow pkg="org.springframework.context" />
			<subpackage name="servlet">
				<allow pkg="org.springframework.boot.web.servlet" />
				<allow pkg="jakarta.servlet" />
			</subpackage>
		</subpackage>
		<subpackage name="server">
			<allow pkg="org.springframework.boot.web.error" />
			<allow pkg="org.springframework.boot.web.server" />
			<disallow pkg="org.springframework.context" />
			<subpackage name="autoconfigure">
				<allow pkg="org.springframework.context" />
				<subpackage name="servlet">
					<allow pkg="jakarta.servlet" />
					<allow pkg="org.springframework.boot.web.servlet" />
				</subpackage>
			</subpackage>
			<subpackage name="client">
				<allow pkg="jakarta.servlet" />
				<allow pkg="org.springframework.context" />
			</subpackage>
			<subpackage name="context">
				<allow pkg="org.springframework.context" />
			</subpackage>
			<subpackage name="reactive">
				<subpackage name="context">
					<allow pkg="org.springframework.context" />
					<allow pkg="org.springframework.web.context" />
					<allow pkg="org.springframework.boot.web.context.reactive" />
				</subpackage>
			</subpackage>
			<subpackage name="servlet">
				<allow pkg="jakarta.servlet" />
				<allow pkg="org.springframework.boot.web.servlet" />
				<subpackage name="context">
					<allow pkg="org.springframework.context" />
					<allow pkg="org.springframework.web.context" />
					<allow pkg="org.springframework.boot.web.context.servlet" />
				</subpackage>
			</subpackage>
			<subpackage name="test">
				<allow pkg=".*" regex="true" />
			</subpackage>
		</subpackage>

		<!-- Servlet -->
		<subpackage name="servlet">
			<allow pkg="jakarta.servlet" />
			<subpackage name="filter">
				<allow pkg="jakarta.servlet" />
				<allow pkg="org.springframework.boot.web.servlet" />
			</subpackage>
			<subpackage name="support">
				<allow pkg="jakarta.servlet" />
				<allow pkg="org.springframework.boot.web.error" />
				<allow pkg="org.springframework.boot.web.context.servlet" />
				<allow pkg="org.springframework.boot.web.servlet" />
			</subpackage>
			<subpackage name="view">
				<allow pkg="org.springframework.web.servlet" />
			</subpackage>
			<subpackage name="error">
				<allow pkg="org.springframework.boot.web.error" />
				<allow pkg="org.springframework.web.servlet" />
			</subpackage>
		</subpackage>

		<!-- Reactive -->
		<subpackage name="reactive">
			<subpackage name="context">
				<allow pkg="org.springframework.boot.web.context" />
			</subpackage>
			<subpackage name="error">
				<allow pkg="org.springframework.boot.web.error" />
			</subpackage>
			<subpackage name="result">
				<subpackage name="view">
					<allow pkg="org.springframework.boot.web.reactive.result.view" />
				</subpackage>
			</subpackage>
		</subpackage>

	</subpackage>

	<!-- Actuator endpoint infrastructure -->
	<subpackage name="actuate">
		<subpackage name="endpoint">
			<disallow pkg="org.springframework.http" />
			<disallow pkg="org.springframework.web" />
			<subpackage name="web">
				<allow pkg="org.springframework.http" />
				<allow pkg="org.springframework.web" />
				<subpackage name="servlet">
					<disallow pkg="org.springframework.web.reactive" />
				</subpackage>
				<subpackage name="reactive">
					<disallow pkg="org.springframework.web.servlet" />
				</subpackage>
			</subpackage>
		</subpackage>
	</subpackage>

	<!-- Web Servers -->
	<subpackage name="tomcat|undertow|jetty|reactor" regex="true">
			<allow pkg="org.springframework.boot.web.server" />
			<disallow pkg="org.springframework.boot.web" />
			<disallow pkg="org.springframework.web.servlet" />
			<disallow pkg="org.springframework.web.reactive" />
		<subpackage name="servlet">
			<allow pkg="jakarta.servlet" />
			<allow pkg="org.springframework.boot.web.server" />
			<allow pkg="org.springframework.boot.web.servlet" />
			<allow pkg="org.springframework.boot.web.error" />
			<allow pkg="org.springframework.web.servlet" />
		</subpackage>
		<subpackage name="reactive">
			<allow pkg="org.springframework.boot.web.server" />
			<allow pkg="org.springframework.boot.web.codec" />
			<allow pkg="org.springframework.boot.web.reactive" />
			<allow pkg="org.springframework.web.reactive" />
		</subpackage>
		<subpackage name="autoconfigure">
			<subpackage name="servlet">
				<allow pkg="jakarta.servlet" />
				<allow pkg="org.springframework.boot.web.servlet" />
				<allow pkg="org.springframework.boot.web.error" />
				<allow pkg="org.springframework.web.servlet" />
			</subpackage>
			<subpackage name="reactive">
				<allow pkg="org.springframework.boot.web.codec" />
				<allow pkg="org.springframework.boot.web.reactive" />
				<allow pkg="org.springframework.web.reactive" />
			</subpackage>
		</subpackage>
	</subpackage>

</import-control>