Make spring-boot-loader its Java 9 friendly

This commit updates the integration tests of "spring-boot-loader" to
not use `@PostConstruct` but rather `InitializingBean`

Closes gh-10274

spring-boot-tools/spring-boot-loader/src/it/executable-props-lib/src/main/java/org/springframework/boot/launcher/it/props/SpringConfiguration.java

@@ -19,8 +19,7 @@ package org.springframework.boot.launcher.it.props;
 import java.io.IOException;
 import java.util.Properties;
 
-import javax.annotation.PostConstruct;
-
+import org.springframework.beans.factory.InitializingBean;
 import org.springframework.context.annotation.ComponentScan;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.io.ClassPathResource;
@@ -32,12 +31,12 @@ import org.springframework.core.io.ClassPathResource;
  */
 @Configuration
 @ComponentScan
-public class SpringConfiguration {
+public class SpringConfiguration implements InitializingBean {
 
 	private String message = "Jar";
 
-	@PostConstruct
-	public void init() throws IOException {
+	@Override
+	public void afterPropertiesSet() throws IOException {
 		Properties props = new Properties();
 		props.load(new ClassPathResource("application.properties").getInputStream());
 		String value = props.getProperty("message");

spring-boot-tools/spring-boot-loader/src/it/executable-props/src/main/java/org/springframework/launcher/it/props/SpringConfiguration.java

@@ -19,8 +19,7 @@ package org.springframework.boot.load.it.props;
 import java.io.IOException;
 import java.util.Properties;
 
-import javax.annotation.PostConstruct;
-
+import org.springframework.beans.factory.InitializingBean;
 import org.springframework.context.annotation.ComponentScan;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.io.ClassPathResource;
@@ -32,12 +31,12 @@ import org.springframework.core.io.ClassPathResource;
  */
 @Configuration
 @ComponentScan
-public class SpringConfiguration {
+public class SpringConfiguration implements InitializingBean {
 
 	private String message = "Jar";
 
-	@PostConstruct
-	public void init() throws IOException {
+	@Override
+	public void afterPropertiesSet() throws IOException {
 		Properties props = new Properties();
 		props.load(new ClassPathResource("application.properties").getInputStream());
 		String value = props.getProperty("message");