Drop "[]" from parameter names in data binding

Closes gh-25836

Author: rstoyanchev

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -194,6 +194,7 @@ public boolean isBindEmptyMultipartFiles() {
 	protected void doBind(MutablePropertyValues mpvs) {
 		checkFieldDefaults(mpvs);
 		checkFieldMarkers(mpvs);
+		adaptEmptyArrayIndices(mpvs);
 		super.doBind(mpvs);
 	}
 
@@ -249,6 +250,27 @@ protected void checkFieldMarkers(MutablePropertyValues mpvs) {
 		}
 	}
 
+	/**
+	 * Check for property values with names that end on {@code "[]"}. This is
+	 * used by some clients for array syntax without an explicit index value.
+	 * If such values are found, drop the brackets to adapt to the expected way
+	 * of expressing the same for data binding purposes.
+	 * @param mpvs the property values to be bound (can be modified)
+	 * @since 5.3
+	 */
+	protected void adaptEmptyArrayIndices(MutablePropertyValues mpvs) {
+		for (PropertyValue pv : mpvs.getPropertyValues()) {
+			String name = pv.getName();
+			if (name.endsWith("[]")) {
+				String field = name.substring(0, name.length() - 2);
+				if (getPropertyAccessor().isWritableProperty(field) && !mpvs.contains(field)) {
+					mpvs.add(field, pv.getValue());
+				}
+				mpvs.removePropertyValue(pv);
+			}
+		}
+	}
+
 	/**
 	 * Determine an empty value for the specified field.
 	 * <p>The default implementation delegates to {@link #getEmptyValue(Class)}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -104,6 +104,15 @@ public void testFieldPrefixCausesFieldResetWithIgnoreUnknownFields() throws Exce
 		assertThat(this.testBean.isPostProcessed()).isFalse();
 	}
 
+	@Test // gh-25836
+	public void testFieldWithEmptyArrayIndex() {
+		MultiValueMap<String, String> formData = new LinkedMultiValueMap<>();
+		formData.add("stringArray[]", "ONE");
+		formData.add("stringArray[]", "TWO");
+		this.binder.bind(exchange(formData)).block(Duration.ofMillis(5000));
+		assertThat(this.testBean.getStringArray()).containsExactly("ONE", "TWO");
+	}
+
 	@Test
 	public void testFieldDefault() throws Exception {
 		MultiValueMap<String, String> formData = new LinkedMultiValueMap<>();

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -112,6 +112,18 @@ public void testFieldPrefixCausesFieldResetWithIgnoreUnknownFields() throws Exce
 		assertThat(target.isPostProcessed()).isFalse();
 	}
 
+	@Test // gh-25836
+	public void testFieldWithEmptyArrayIndex() {
+		TestBean target = new TestBean();
+		WebRequestDataBinder binder = new WebRequestDataBinder(target);
+
+		MockHttpServletRequest request = new MockHttpServletRequest();
+		request.addParameter("stringArray[]", "ONE");
+		request.addParameter("stringArray[]", "TWO");
+		binder.bind(new ServletWebRequest(request));
+		assertThat(target.getStringArray()).containsExactly("ONE", "TWO");
+	}
+
 	@Test
 	public void testFieldDefault() throws Exception {
 		TestBean target = new TestBean();