Add whenInstanceOf to PropertyMapper

Add an operation on PropertyMapper that takes care of casting. Returns
a source for the requested type if the current value is of the right
type.

Closes gh-11788

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/PropertyMapper.java

@@ -257,6 +257,17 @@ public final class PropertyMapper {
 			return when(object::equals);
 		}
 
+		/**
+		 * Return a filtered version of the source that will only map values that are an
+		 * instance of the given type.
+		 * @param <R> the target type
+		 * @param target the target type to match
+		 * @return a new filtered source instance
+		 */
+		public <R> Source<R> whenInstanceOf(Class<R> target) {
+			return when(target::isInstance).as(target::cast);
+		}
+
 		/**
 		 * Return a filtered version of the source that won't map values that match the
 		 * given predicate.

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/PropertyMapperTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -142,6 +142,18 @@ public class PropertyMapperTests {
 		this.map.from(() -> "123").whenEqualTo("321").toCall(Assert::fail);
 	}
 
+	@Test
+	public void whenInstanceOfToWhenValueIsTargetTypeShouldMatch() {
+		Long result = this.map.from(() -> 123L).whenInstanceOf(Long.class)
+				.toInstance((value) -> value + 1);
+		assertThat(result).isEqualTo(124L);
+	}
+
+	@Test
+	public void whenInstanceOfWhenValueIsNotTargetTypeShouldNotMatch() {
+		this.map.from(() -> 123).whenInstanceOf(Double.class).toCall(Assert::fail);
+	}
+
 	@Test
 	public void whenWhenValueMatchesShouldMap() {
 		String result = this.map.from(() -> "123").when("123"::equals)