Merge branch '3.4.x'

Closes gh-45553

Author: philwebb

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/features/external-config.adoc

@@ -821,7 +821,14 @@ include-code::MyProperties[]
 In this setup, the presence of a single parameterized constructor implies that constructor binding should be used.
 This means that the binder will find a constructor with the parameters that you wish to have bound.
 If your class has multiple constructors, the javadoc:org.springframework.boot.context.properties.bind.ConstructorBinding[format=annotation] annotation can be used to specify which constructor to use for constructor binding.
-To opt out of constructor binding for a class with a single parameterized constructor, the constructor must be annotated with javadoc:org.springframework.beans.factory.annotation.Autowired[format=annotation] or made `private`.
+
+To opt-out of constructor binding for a class, the parameterized constructor must be annotated with javadoc:org.springframework.beans.factory.annotation.Autowired[format=annotation] or made `private`.
+Kotlin developers can use an empty primary constructor to opt-out of constructor binding.
+
+For example:
+
+include-code::primaryconstructor/MyProperties[]
+
 Constructor binding can be used with records.
 Unless your record has multiple constructors, there is no need to use javadoc:org.springframework.boot.context.properties.bind.ConstructorBinding[format=annotation].
 

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/features/externalconfig/typesafeconfigurationproperties/constructorbinding/primaryconstructor/MyBean.java

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2012-2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.features.externalconfig.typesafeconfigurationproperties.constructorbinding.primaryconstructor;
+
+class MyBean {
+
+}

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/features/externalconfig/typesafeconfigurationproperties/constructorbinding/primaryconstructor/MyProperties.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2012-2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.features.externalconfig.typesafeconfigurationproperties.constructorbinding.primaryconstructor;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.context.properties.ConfigurationProperties;
+
+@ConfigurationProperties("my")
+public class MyProperties {
+
+	// @fold:on // fields...
+	final MyBean myBean;
+
+	private String name;
+
+	// @fold:off
+
+	@Autowired
+	public MyProperties(MyBean myBean) {
+		this.myBean = myBean;
+	}
+
+	// @fold:on // getters / setters...
+
+	public String getName() {
+		return this.name;
+	}
+
+	public void setName(String name) {
+		this.name = name;
+	}
+
+	// @fold:off
+
+}

spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/features/externalconfig/typesafeconfigurationproperties/constructorbinding/primaryconstructor/MyProperties.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2012-2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.features.externalconfig.typesafeconfigurationproperties.constructorbinding.primaryconstructor
+
+import org.springframework.boot.context.properties.ConfigurationProperties
+import org.springframework.boot.context.properties.bind.DefaultValue
+import java.net.InetAddress
+
+@ConfigurationProperties("my")
+class MyProperties() {
+
+	constructor(name: String) : this() {
+		this.name = name
+	}
+
+	// @fold:on // vars...
+	var name: String? = null
+	// @fold:off
+
+}

spring-boot-project/spring-boot/src/test/kotlin/org/springframework/boot/context/properties/bind/KotlinDefaultBindConstructorProviderTests.kt

@@ -61,6 +61,18 @@ class KotlinDefaultBindConstructorProviderTests {
 		assertThat(bindConstructor).isNull()
 	}
 
+	@Test
+	fun `type with no param primary constructor and secondary params constructor should not use constructor binding`() {
+		val bindConstructor = this.constructorProvider.getBindConstructor(NoParamPrimaryWithParamsSecondaryProperties::class.java, false)
+		assertThat(bindConstructor).isNull()
+	}
+
+	@Test
+	fun `type with params primary constructor and no param secondary constructor should use constructor binding`() {
+		val bindConstructor = this.constructorProvider.getBindConstructor(ParamsPrimaryWithNoParamSecondaryProperties::class.java, false)
+		assertThat(bindConstructor).isNotNull()
+	}
+
 	@Test
 	fun `type with autowired secondary constructor should not use constructor binding`() {
 		val bindConstructor = this.constructorProvider.getBindConstructor(AutowiredSecondaryProperties::class.java, false)
@@ -127,6 +139,18 @@ class KotlinDefaultBindConstructorProviderTests {
 		constructor(@Suppress("UNUSED_PARAMETER") foo: String) : this(foo, 21)
 	}
 
+	class NoParamPrimaryWithParamsSecondaryProperties() {
+
+		constructor(@Suppress("UNUSED_PARAMETER") name: String) : this()
+
+		var name: String? = null
+	}
+
+	class ParamsPrimaryWithNoParamSecondaryProperties(var name: String?) {
+
+		constructor() : this(null)
+	}
+
 	class AutowiredSecondaryProperties {
 
 		@Autowired