Support new default values in WebDataBinder

Prior to this commit, binding a `@ModelAttribute` object as a Controller
handler paramater would instantiate the object and set all its
properties, fetching data from the request. When no data is available,
the WebDataBinder tries to bind default "empty" values:
* Boolean.FALSE for boolean types
* empty arrays for array types
* null by default

This commit adds the new default empty values:
* empty Collections for Collection types
* empty Maps for Map types

Rather than using empty implementations provided by `Collections.empty`
(which are not mutable), we're using the closest possible target type
and real implementations, provided by the `CollectionFactory`.

Issue: SPR-13502

spring-web/src/main/java/org/springframework/web/bind/WebDataBinder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -17,11 +17,13 @@
 package org.springframework.web.bind;
 
 import java.lang.reflect.Array;
+import java.util.Collection;
 import java.util.List;
 import java.util.Map;
 
 import org.springframework.beans.MutablePropertyValues;
 import org.springframework.beans.PropertyValue;
+import org.springframework.core.CollectionFactory;
 import org.springframework.validation.DataBinder;
 import org.springframework.web.multipart.MultipartFile;
 
@@ -40,6 +42,7 @@ import org.springframework.web.multipart.MultipartFile;
  *
  * @author Juergen Hoeller
  * @author Scott Andrews
+ * @author Brian Clozel
  * @since 1.2
  * @see #registerCustomEditor
  * @see #setAllowedFields
@@ -243,26 +246,41 @@ public class WebDataBinder extends DataBinder {
 
 	/**
 	 * Determine an empty value for the specified field.
-	 * <p>Default implementation returns {@code Boolean.FALSE}
-	 * for boolean fields and an empty array of array types.
-	 * Else, {@code null} is used as default.
+	 * <p>Default implementation returns:
+	 * <ul>
+	 *     <li>{@code Boolean.FALSE} for boolean fields
+	 *     <li>an empty array for array types
+	 *     <li>Collection implementations for Collection types
+	 *     <li>Map implementations for Map types
+	 *     <li>else, {@code null} is used as default
+	 * </ul>
 	 * @param field the name of the field
 	 * @param fieldType the type of the field
 	 * @return the empty value (for most fields: null)
 	 */
 	protected Object getEmptyValue(String field, Class<?> fieldType) {
-		if (fieldType != null && boolean.class == fieldType || Boolean.class == fieldType) {
-			// Special handling of boolean property.
-			return Boolean.FALSE;
-		}
-		else if (fieldType != null && fieldType.isArray()) {
-			// Special handling of array property.
-			return Array.newInstance(fieldType.getComponentType(), 0);
-		}
-		else {
-			// Default value: try null.
-			return null;
+		if (fieldType != null) {
+			try {
+				if (boolean.class == fieldType || Boolean.class == fieldType) {
+					// Special handling of boolean property.
+					return Boolean.FALSE;
+				}
+				else if (fieldType.isArray()) {
+					// Special handling of array property.
+					return Array.newInstance(fieldType.getComponentType(), 0);
+				}
+				else if (Collection.class.isAssignableFrom(fieldType)) {
+					return CollectionFactory.createCollection(fieldType, 0);
+				}
+				else if (Map.class.isAssignableFrom(fieldType)) {
+					return CollectionFactory.createMap(fieldType, 0);
+				}
+			} catch (IllegalArgumentException exc) {
+				return null;
+			}
 		}
+		// Default value: try null.
+		return null;
 	}
 
 	/**

spring-web/src/test/java/org/springframework/web/bind/support/WebRequestDataBinderTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -16,10 +16,15 @@
 
 package org.springframework.web.bind.support;
 
+import static org.hamcrest.Matchers.*;
+import static org.junit.Assert.*;
+
 import java.beans.PropertyEditorSupport;
 import java.util.Arrays;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
+import java.util.Set;
 
 import org.junit.Test;
 
@@ -34,8 +39,6 @@ import org.springframework.web.bind.ServletRequestParameterPropertyValues;
 import org.springframework.web.context.request.ServletWebRequest;
 import org.springframework.web.multipart.support.StringMultipartFileEditor;
 
-import static org.junit.Assert.*;
-
 /**
  * @author Juergen Hoeller
  */
@@ -126,6 +129,31 @@ public class WebRequestDataBinderTests {
 		assertFalse(target.isPostProcessed());
 	}
 
+	// SPR-13502
+	@Test
+	public void testCollectionFieldsDefault() throws Exception {
+		TestBean target = new TestBean();
+		target.setSomeSet(null);
+		target.setSomeList(null);
+		target.setSomeMap(null);
+		WebRequestDataBinder binder = new WebRequestDataBinder(target);
+
+		MockHttpServletRequest request = new MockHttpServletRequest();
+		request.addParameter("_someSet", "visible");
+		request.addParameter("_someList", "visible");
+		request.addParameter("_someMap", "visible");
+
+		binder.bind(new ServletWebRequest(request));
+		assertThat(target.getSomeSet(), notNullValue());
+		assertThat(target.getSomeSet(), isA(Set.class));
+
+		assertThat(target.getSomeList(), notNullValue());
+		assertThat(target.getSomeList(), isA(List.class));
+
+		assertThat(target.getSomeMap(), notNullValue());
+		assertThat(target.getSomeMap(), isA(Map.class));
+	}
+
 	@Test
 	public void testFieldDefaultPreemptsFieldMarker() throws Exception {
 		TestBean target = new TestBean();