Ensure ParamTag release resources

Update ParamTag to reset values when resources are released. This
prevents problems in containers that pool tags.

Issue: SPR-10769

spring-webmvc/src/main/java/org/springframework/web/servlet/tags/ParamTag.java

 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2013 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.
@@ -26,6 +26,7 @@ import javax.servlet.jsp.tagext.BodyTagSupport;
  * <p>This tag must be nested under a param aware tag.
  *
  * @author Scott Andrews
+ * @author Nicholas Williams
  * @since 3.0
  * @see Param
  * @see UrlTag
@@ -37,16 +38,16 @@ public class ParamTag extends BodyTagSupport {
 
 	private String value;
 
-	private Param param;
+	private boolean valueSet;
 
 	// tag lifecycle
 
 	@Override
 	public int doEndTag() throws JspException {
-		param = new Param();
-		param.setName(name);
-		if (value != null) {
-			param.setValue(value);
+		Param param = new Param();
+		param.setName(this.name);
+		if (this.valueSet) {
+			param.setValue(this.value);
 		}
 		else if (getBodyContent() != null) {
 			// get the value from the tag body
@@ -90,6 +91,15 @@ public class ParamTag extends BodyTagSupport {
 	 */
 	public void setValue(String value) {
 		this.value = value;
+		this.valueSet = true;
+	}
+
+	@Override
+	public void release() {
+		super.release();
+		this.name = null;
+		this.value = null;
+		this.valueSet = false;
 	}
 
 }

spring-webmvc/src/test/java/org/springframework/web/servlet/tags/ParamTagTests.java

 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2013 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.
@@ -25,9 +25,10 @@ import org.springframework.mock.web.test.MockBodyContent;
 import org.springframework.mock.web.test.MockHttpServletResponse;
 
 /**
- * Unit tests for ParamTag
+ * Unit tests for {@link ParamTag}
  *
  * @author Scott Andrews
+ * @author Nicholas Williams
  */
 public class ParamTagTests extends AbstractTagTests {
 
@@ -67,7 +68,7 @@ public class ParamTagTests extends AbstractTagTests {
 		assertEquals("value", parent.getParam().getValue());
 	}
 
-	public void testParamWithNullValue() throws JspException {
+	public void testParamWithImplicitNullValue() throws JspException {
 		tag.setName("name");
 
 		int action = tag.doEndTag();
@@ -77,6 +78,43 @@ public class ParamTagTests extends AbstractTagTests {
 		assertNull(parent.getParam().getValue());
 	}
 
+	public void testParamWithExplicitNullValue() throws JspException {
+		tag.setName("name");
+		tag.setValue(null);
+
+		int action = tag.doEndTag();
+
+		assertEquals(Tag.EVAL_PAGE, action);
+		assertEquals("name", parent.getParam().getName());
+		assertNull(parent.getParam().getValue());
+	}
+
+	public void testParamWithValueThenReleaseThenBodyValue() throws JspException {
+		tag.setName("name1");
+		tag.setValue("value1");
+
+		int action = tag.doEndTag();
+
+		assertEquals(Tag.EVAL_PAGE, action);
+		assertEquals("name1", parent.getParam().getName());
+		assertEquals("value1", parent.getParam().getValue());
+
+		tag.release();
+
+		parent = new MockParamSupportTag();
+		tag.setPageContext(createPageContext());
+		tag.setParent(parent);
+		tag.setName("name2");
+		tag.setBodyContent(new MockBodyContent("value2",
+				new MockHttpServletResponse()));
+
+		action = tag.doEndTag();
+
+		assertEquals(Tag.EVAL_PAGE, action);
+		assertEquals("name2", parent.getParam().getName());
+		assertEquals("value2", parent.getParam().getValue());
+	}
+
 	public void testParamWithNoParent() {
 		tag.setName("name");
 		tag.setValue("value");