Selectively force encoding in CharacterEncodingFilter

Prior to this commit, setting the `forceEncoding` option would force
encoding on both requests and responses.

This commit adds two new setters and a new constructor to differentiate
both options: forcing the encoding on the request and/or on the
response.

You can now define this filter programmatically using those options or
update your servlet XML configuration like:

```
<filter>
  <filter-name>characterEncodingFilter</filter-name>
  <filter-class>o.sf.web.filter.CharacterEncodingFilter</filter-class>
  <init-param>
     <param-name>encoding</param-name>
     <param-value>UTF-8</param-value>
  </init-param>
  <init-param>
    <param-name>forceRequestEncoding</param-name>
    <param-value>true</param-value>
  </init-param>
  <init-param>
    <param-name>forceResponseEncoding</param-name>
    <param-value>false</param-value>
  </init-param>
</filter>

```

Issue: SPR-14240

spring-web/src/main/java/org/springframework/web/filter/CharacterEncodingFilter.java

 /*
- * 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.
@@ -17,6 +17,7 @@
 package org.springframework.web.filter;
 
 import java.io.IOException;
+
 import javax.servlet.FilterChain;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServletRequest;
@@ -46,7 +47,9 @@ public class CharacterEncodingFilter extends OncePerRequestFilter {
 
 	private String encoding;
 
-	private boolean forceEncoding = false;
+	private boolean forceRequestEncoding = false;
+
+	private boolean forceResponseEncoding = false;
 
 
 	/**
@@ -77,12 +80,28 @@ public class CharacterEncodingFilter extends OncePerRequestFilter {
 	 * @see #setForceEncoding
 	 */
 	public CharacterEncodingFilter(String encoding, boolean forceEncoding) {
+		this(encoding, forceEncoding, forceEncoding);
+	}
+
+	/**
+	 * Create a {@code CharacterEncodingFilter} for the given encoding.
+	 * @param encoding the encoding to apply
+	 * @param forceRequestEncoding whether the specified encoding is supposed to
+	 * override existing request encodings
+	 * @param forceResponseEncoding whether the specified encoding is supposed to
+	 * override existing response encodings
+	 * @since 4.3.0
+	 * @see #setEncoding
+	 * @see #setForceRequestEncoding(boolean)
+	 * @see #setForceResponseEncoding(boolean)
+	 */
+	public CharacterEncodingFilter(String encoding, boolean forceRequestEncoding, boolean forceResponseEncoding) {
 		Assert.hasLength(encoding, "Encoding must not be empty");
 		this.encoding = encoding;
-		this.forceEncoding = forceEncoding;
+		this.forceRequestEncoding = forceRequestEncoding;
+		this.forceResponseEncoding = forceResponseEncoding;
 	}
 
-
 	/**
 	 * Set the encoding to use for requests. This encoding will be passed into a
 	 * {@link javax.servlet.http.HttpServletRequest#setCharacterEncoding} call.
@@ -101,20 +120,50 @@ public class CharacterEncodingFilter extends OncePerRequestFilter {
 	 * {@link javax.servlet.http.HttpServletRequest#getCharacterEncoding()}
 	 * returns a non-null value. Switch this to "true" to enforce the specified
 	 * encoding in any case, applying it as default response encoding as well.
+	 * <p>This is the equivalent to setting both {@link #setForceRequestEncoding(boolean)}
+	 * and {@link #setForceResponseEncoding(boolean)}.
+	 * @see #setForceRequestEncoding(boolean)
+	 * @see #setForceResponseEncoding(boolean)
 	 */
 	public void setForceEncoding(boolean forceEncoding) {
-		this.forceEncoding = forceEncoding;
+		this.forceRequestEncoding = forceEncoding;
+		this.forceResponseEncoding = forceEncoding;
+	}
+
+	/**
+	 * Set whether the configured {@link #setEncoding encoding} of this filter
+	 * is supposed to override existing request encodings.
+	 * <p>Default is "false", i.e. do not modify the encoding if
+	 * {@link javax.servlet.http.HttpServletRequest#getCharacterEncoding()}
+	 * returns a non-null value. Switch this to "true" to enforce the specified
+	 * encoding in any case.
+	 * @since 4.3.0
+	 */
+	public void setForceRequestEncoding(boolean forceRequestEncoding) {
+		this.forceRequestEncoding = forceRequestEncoding;
 	}
 
+	/**
+	 * Set whether the configured {@link #setEncoding encoding} of this filter
+	 * is supposed to override existing response encodings.
+	 * <p>Default is "false", i.e. do not modify the encoding.
+	 * Switch this to "true" to enforce the specified encoding
+	 * for responses in any case.
+	 * @since 4.3.0
+	 */
+	public void setForceResponseEncoding(boolean forceResponseEncoding) {
+		this.forceResponseEncoding = forceResponseEncoding;
+	}
 
 	@Override
 	protected void doFilterInternal(
 			HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
 			throws ServletException, IOException {
-
-		if (this.encoding != null && (this.forceEncoding || request.getCharacterEncoding() == null)) {
-			request.setCharacterEncoding(this.encoding);
-			if (this.forceEncoding) {
+		if (this.encoding != null) {
+			if (this.forceRequestEncoding || request.getCharacterEncoding() == null) {
+				request.setCharacterEncoding(this.encoding);
+			}
+			if (this.forceResponseEncoding) {
 				response.setCharacterEncoding(this.encoding);
 			}
 		}

spring-web/src/test/java/org/springframework/web/filter/CharacterEncodingFilterTests.java

 /*
- * 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.
@@ -145,4 +145,26 @@ public class CharacterEncodingFilterTests {
 		verify(filterChain).doFilter(request, response);
 	}
 
+	// SPR-14240
+	@Test
+	public void setForceEncodingOnRequestOnly() throws Exception {
+		HttpServletRequest request = mock(HttpServletRequest.class);
+		request.setCharacterEncoding(ENCODING);
+		given(request.getAttribute(WebUtils.ERROR_REQUEST_URI_ATTRIBUTE)).willReturn(null);
+		given(request.getAttribute(FILTER_NAME + OncePerRequestFilter.ALREADY_FILTERED_SUFFIX)).willReturn(null);
+
+		HttpServletResponse response = mock(HttpServletResponse.class);
+		FilterChain filterChain = mock(FilterChain.class);
+
+		CharacterEncodingFilter filter = new CharacterEncodingFilter(ENCODING, true, false);
+		filter.init(new MockFilterConfig(FILTER_NAME));
+		filter.doFilter(request, response, filterChain);
+
+		verify(request).setAttribute(FILTER_NAME + OncePerRequestFilter.ALREADY_FILTERED_SUFFIX, Boolean.TRUE);
+		verify(request).removeAttribute(FILTER_NAME + OncePerRequestFilter.ALREADY_FILTERED_SUFFIX);
+		verify(request, times(2)).setCharacterEncoding(ENCODING);
+		verify(response, never()).setCharacterEncoding(ENCODING);
+		verify(filterChain).doFilter(request, response);
+	}
+
 }