Remove use of @PostConstruct from main code

When running on Java 11 (where `@PostConstruct` is no longer part of
the JRE) and without a dependency on jakarta-annotation-api,
`@PostContruct` annotions are silently dropped. This leads to obscure and
hard-to-track down changes in the behaviour of our auto-configuration
as the `@PostConstruct`-annotated methods are not invoked.

To allow users to run on Java 11 without having jakarta-annotation-api
on the classpath, this commit removes use of `@PostConstruct` from main
code. A Checkstyle rule has also been added to prevent its usage in
main code from being reintroduced.

Closes gh-23723

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/jackson/JsonComponentModule.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -20,8 +20,6 @@ import java.lang.reflect.Modifier;
 import java.util.Map;
 import java.util.function.BiConsumer;
 
-import javax.annotation.PostConstruct;
-
 import com.fasterxml.jackson.databind.JsonDeserializer;
 import com.fasterxml.jackson.databind.JsonSerializer;
 import com.fasterxml.jackson.databind.KeyDeserializer;
@@ -33,6 +31,7 @@ import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
 import org.springframework.beans.factory.HierarchicalBeanFactory;
+import org.springframework.beans.factory.InitializingBean;
 import org.springframework.beans.factory.ListableBeanFactory;
 import org.springframework.boot.jackson.JsonComponent.Scope;
 import org.springframework.core.ResolvableType;
@@ -51,7 +50,7 @@ import org.springframework.util.ObjectUtils;
  * @since 1.4.0
  * @see JsonComponent
  */
-public class JsonComponentModule extends SimpleModule implements BeanFactoryAware {
+public class JsonComponentModule extends SimpleModule implements BeanFactoryAware, InitializingBean {
 
 	private BeanFactory beanFactory;
 
@@ -60,7 +59,11 @@ public class JsonComponentModule extends SimpleModule implements BeanFactoryAwar
 		this.beanFactory = beanFactory;
 	}
 
-	@PostConstruct
+	@Override
+	public void afterPropertiesSet() {
+		registerJsonComponents();
+	}
+
 	public void registerJsonComponents() {
 		BeanFactory beanFactory = this.beanFactory;
 		while (beanFactory != null) {

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/jdbc/AbstractDataSourceInitializer.java

@@ -18,9 +18,9 @@ package org.springframework.boot.jdbc;
 
 import java.sql.DatabaseMetaData;
 
-import javax.annotation.PostConstruct;
 import javax.sql.DataSource;
 
+import org.springframework.beans.factory.InitializingBean;
 import org.springframework.core.io.ResourceLoader;
 import org.springframework.jdbc.datasource.init.DatabasePopulatorUtils;
 import org.springframework.jdbc.datasource.init.ResourceDatabasePopulator;
@@ -35,7 +35,7 @@ import org.springframework.util.Assert;
  * @author Stephane Nicoll
  * @since 1.5.0
  */
-public abstract class AbstractDataSourceInitializer {
+public abstract class AbstractDataSourceInitializer implements InitializingBean {
 
 	private static final String PLATFORM_PLACEHOLDER = "@@platform@@";
 
@@ -50,7 +50,11 @@ public abstract class AbstractDataSourceInitializer {
 		this.resourceLoader = resourceLoader;
 	}
 
-	@PostConstruct
+	@Override
+	public void afterPropertiesSet() {
+		initialize();
+	}
+
 	protected void initialize() {
 		if (!isEnabled()) {
 			return;