Fix compatibility with JDK9

Cast runtime object retrieved reflectively to their public interface
counterpart.

See gh-7226

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/ExpressionTree.java

 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 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.
@@ -47,7 +47,7 @@ class ExpressionTree extends ReflectionWrapper {
 			"getInitializers");
 
 	ExpressionTree(Object instance) {
-		super(instance);
+		super("com.sun.source.tree.ExpressionTree", instance);
 	}
 
 	public String getKind() throws Exception {

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/ReflectionWrapper.java

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2016 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.
@@ -31,14 +31,9 @@ class ReflectionWrapper {
 
 	private final Object instance;
 
-	ReflectionWrapper(Object instance) {
-		this.type = instance.getClass();
-		this.instance = instance;
-	}
-
 	ReflectionWrapper(String type, Object instance) {
 		this.type = findClass(instance.getClass().getClassLoader(), type);
-		this.instance = instance;
+		this.instance = this.type.cast(instance);
 	}
 
 	protected final Object getInstance() {

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/Trees.java

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2016 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.
@@ -30,7 +30,7 @@ import javax.lang.model.element.Element;
 final class Trees extends ReflectionWrapper {
 
 	private Trees(Object instance) {
-		super(instance);
+		super("com.sun.source.util.Trees", instance);
 	}
 
 	public Tree getTree(Element element) throws Exception {

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/VariableTree.java

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2016 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.
@@ -30,7 +30,7 @@ import javax.lang.model.element.Modifier;
 class VariableTree extends ReflectionWrapper {
 
 	VariableTree(Object instance) {
-		super(instance);
+		super("com.sun.source.tree.VariableTree", instance);
 	}
 
 	public String getName() throws Exception {