Add support for binding to immutable collection

Fixes gh-9290

spring-boot/src/main/java/org/springframework/boot/context/properties/bind/CollectionBinder.java

@@ -51,9 +51,26 @@ class CollectionBinder extends IndexedElementsBinder<Collection<Object>> {
 	@Override
 	protected Collection<Object> merge(Collection<Object> existing,
 			Collection<Object> additional) {
-		existing.clear();
-		existing.addAll(additional);
-		return existing;
+		try {
+			existing.clear();
+			existing.addAll(additional);
+			return existing;
+		}
+		catch (UnsupportedOperationException ex) {
+			return createNewCollection(additional);
+		}
+	}
+
+	@SuppressWarnings("unchecked")
+	private Collection<Object> createNewCollection(Collection<Object> additional) {
+		try {
+			Collection<Object> merged = additional.getClass().newInstance();
+			merged.addAll(additional);
+			return merged;
+		}
+		catch (Exception e) {
+			throw new IllegalStateException("Adding bound values to collection failed.");
+		}
 	}
 
 }

spring-boot/src/test/java/org/springframework/boot/context/properties/bind/CollectionBinderTests.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.context.properties.bind;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Set;
@@ -289,4 +290,15 @@ public class CollectionBinderTests {
 		List<String> values = result.stream().map(JavaBean::getValue).collect(Collectors.toList());
 		assertThat(values).containsExactly("a", "b", "c");
 	}
+
+	@Test
+	public void bindToImmutableCollectionShouldReturnPopulatedCollection() throws Exception {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo.values", "a,b,c");
+		this.sources.add(source);
+		Set<String> result = this.binder.bind("foo.values",
+				STRING_SET.withExistingValue(Collections.emptySet())).get();
+		assertThat(result).hasSize(3);
+	}
+
 }