Fix binding of overlapping nested maps

When binding a nested map structure, RelaxedDataBinder pre-populates
the target object with default empty maps. Previously, when these
structures overlapped, each step in pre-population process could
potentially overwrite what had come before it. This led to the output
of the pre-population process being incomplete which would lead to a
binding failure.

This commit updates the pre-population process so that it checks to see
if a property's value has already been set by an earlier step in the
process. If it has been set, the existing value is now reused rather
than being overwritten by a new empty map.

Fixes #328

spring-boot/src/main/java/org/springframework/boot/bind/RelaxedDataBinder.java

@@ -18,6 +18,7 @@ package org.springframework.boot.bind;
 
 import java.net.InetAddress;
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
@@ -232,11 +233,19 @@ public class RelaxedDataBinder extends DataBinder {
 				&& !descriptor.getType().equals(Object.class)) {
 			return;
 		}
+		String extensionName = path.prefix(index + 1);
+		if (wrapper.isReadableProperty(extensionName)) {
+			Object currentValue = wrapper.getPropertyValue(extensionName);
+			if ((descriptor.isCollection() && currentValue instanceof Collection)
+					|| (!descriptor.isCollection() && currentValue instanceof Map)) {
+				return;
+			}
+		}
 		Object extend = new LinkedHashMap<String, Object>();
 		if (descriptor.isCollection()) {
 			extend = new ArrayList<Object>();
 		}
-		wrapper.setPropertyValue(path.prefix(index + 1), extend);
+		wrapper.setPropertyValue(extensionName, extend);
 	}
 
 	private String getActualPropertyName(BeanWrapper target, String prefix, String name) {

spring-boot/src/test/java/org/springframework/boot/bind/RelaxedDataBinderTests.java

@@ -380,6 +380,22 @@ public class RelaxedDataBinderTests {
 		assertEquals("123", map.get("value"));
 	}
 
+	@SuppressWarnings("unchecked")
+	@Test
+	public void testBindOverlappingNestedMaps() throws Exception {
+		Map<String, Object> target = new LinkedHashMap<String, Object>();
+		BindingResult result = bind(target, "a.b.c.d: abc\na.b.c1.d1: efg");
+		assertEquals(0, result.getErrorCount());
+
+		Map<String, Object> a = (Map<String, Object>) target.get("a");
+		Map<String, Object> b = (Map<String, Object>) a.get("b");
+		Map<String, Object> c = (Map<String, Object>) b.get("c");
+		assertEquals("abc", c.get("d"));
+
+		Map<String, Object> c1 = (Map<String, Object>) b.get("c1");
+		assertEquals("efg", c1.get("d1"));
+	}
+
 	private BindingResult bind(Object target, String values) throws Exception {
 		return bind(target, values, null);
 	}