Spring MVC过滤器-字符集过滤器（CharacterEncodingFilter）

Spring的字符集过滤通过用于处理项目中的乱码问题，该过滤器位于org.springframework.web.filter包中，指向类CharacterEncodingFilter，CharacterEncodingFilter源代码如下：

/* 
 * Copyright 2002-2007 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. 
 * You may obtain a copy of the License at 
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0 
 * 
 * Unless required by applicable law or agreed to in writing, software 
 * distributed under the License is distributed on an "AS IS" BASIS, 
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 
 * See the License for the specific language governing permissions and 
 * limitations under the License. 
 */  
  
package org.springframework.web.filter;  
  
import java.io.IOException;  
import javax.servlet.FilterChain;  
import javax.servlet.ServletException;  
import javax.servlet.http.HttpServletRequest;  
import javax.servlet.http.HttpServletResponse;  
  
/** 
 * Servlet 2.3/2.4 Filter that allows one to specify a character encoding for 
 * requests. This is useful because current browsers typically do not set a 
 * character encoding even if specified in the HTML page or form. 
 * 
 * <p>This filter can either apply its encoding if the request does not 
 * already specify an encoding, or enforce this filter's encoding in any case 
 * ("forceEncoding"="true"). In the latter case, the encoding will also be 
 * applied as default response encoding on Servlet 2.4+ containers (although 
 * this will usually be overridden by a full content type set in the view). 
 * 
 * @author Juergen Hoeller 
 * @since 15.03.2004 
 * @see #setEncoding 
 * @see #setForceEncoding 
 * @see javax.servlet.http.HttpServletRequest#setCharacterEncoding 
 * @see javax.servlet.http.HttpServletResponse#setCharacterEncoding 
 */  
public class CharacterEncodingFilter extends OncePerRequestFilter {  
  
    private String encoding;  
  
    private boolean forceEncoding = false;  
  
  
    /** 
     * Set the encoding to use for requests. This encoding will be passed into a 
     * {@link javax.servlet.http.HttpServletRequest#setCharacterEncoding} call. 
     * <p>Whether this encoding will override existing request encodings 
     * (and whether it will be applied as default response encoding as well) 
     * depends on the {@link #setForceEncoding "forceEncoding"} flag. 
     */  
    public void setEncoding(String encoding) {  
        this.encoding = encoding;  
    }  
  
    /** 
     * Set whether the configured {@link #setEncoding encoding} of this filter 
     * is supposed to override existing request and response 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, applying it as default response encoding as well. 
     * <p>Note that the response encoding will only be set on Servlet 2.4+ 
     * containers, since Servlet 2.3 did not provide a facility for setting 
     * a default response encoding. 
     */  
    public void setForceEncoding(boolean forceEncoding) {  
        this.forceEncoding = forceEncoding;  
    }  
  
  
    @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) {  
                response.setCharacterEncoding(this.encoding);  
            }  
        }  
        filterChain.doFilter(request, response);  
    }  
  
}  

        上述代码显示，在配置字符集过滤器时可设定两个参数的值，如下：
        l  encoding：字符集，即将过滤到的request的字符集设置为encoding指定的值，如UTF-8等，相当于：

request.setCharacterEncoding  

        l  forceEncoding：字面意思是强制字符集，但你大可不必按字面意思理解，因为这个参数的值只不过是指定response的字符集是否也设置成encoding所指定的字符集，所以你可以选择设置为true或false，当值为true时，相当于

request.setCharacterEncoding(“”);  
response.setCharacterEncoding(“”); 

        当值为false时，相当于：

request.setCharacterEncoding(“”); 

        默认值为false。
        示例：

       <filter>  
    <filter-name>characterEncodingFilter</filter-name>  
    <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>  
    <init-param>  
        <param-name>encoding</param-name>  
        <param-value>UTF-8</param-value>  
    </init-param>  
    <init-param>  
        <param-name>forceEncoding</param-name>  
        <param-value>true</param-value>  
    </init-param>  
</filter>  
<filter-mapping>  
    <filter-name>characterEncodingFilter</filter-name>  
    <url-pattern>/*</url-pattern>  
</filter-mapping>

 
        以上代码放置在web.xml中，相当于servlet中的：

request.setCharacterEncoding("UTF-8");  
response.setCharacterEncoding("UTF-8");