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");