Servlet & JSP - Decorating Requests and Responses

Servlet API 提供了四个包装类：ServletRequestWrapper、ServletResponseWrapper、HttpServletRequestWrapper、HttpServletReponseWrapper。这些包装类使用的频率很小，但却很有用。当 servlet 接收到请求，service 方法被调用时，我们可以使用这些包装类来包装 servlet 容器传递过来的 request 和 response 对象以改变 request 和 response 的行为。例如，当 HTTP 请求的参数包含中文字符时，不想在每个 servlet 里面都对其转换编码，可以使用过滤器去拦截请求，然后使用 HttpServletRequestWrapper 去装饰该请求，覆盖 getParameter、getParameterValues 等方法，使这些方法返回的参数是转换过编码的。

应用示例

自定义的 HttpServletRequestWrapper：

package com.huey.hello.wrapper;

import java.io.UnsupportedEncodingException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;

public class CharsetServletRequestWrapper extends HttpServletRequestWrapper {

private HttpServletRequest request;
private String oldCharset;
private String newCharset;

public CharsetServletRequestWrapper(HttpServletRequest request, String oldCharset, String newCharset) {
super(request);
this.request = request;
this.oldCharset = oldCharset;
this.newCharset = newCharset;
}

@Override
public String getParameter(String name) {
return convertCharset(request.getParameter(name));
}

@Override
public String[] getParameterValues(String name) {
return convertCharset(request.getParameterValues(name));
}

@Override
public Map<String, String[]> getParameterMap() {
final Map<String, String[]> parameterMap = request.getParameterMap();
Map<String, String[]> newMap = new Map<String, String[]>() {
@Override
public int size() {
return parameterMap.size();
}

@Override
public boolean isEmpty() {
return parameterMap.isEmpty();
}

@Override
public boolean containsKey(Object key) {
return parameterMap.containsKey(key);
}

@Override
public boolean containsValue(Object value) {
return parameterMap.containsValue(value);
}

@Override
public String[] get(Object key) {
return convertCharset(parameterMap.get(key));
}

@Override
public String[] put(String key, String[] value) {
return parameterMap.put(key, value);
}

@Override
public String[] remove(Object key) {
return parameterMap.remove(key);
}

@Override
public void putAll(Map<? extends String, ? extends String[]> m) {
parameterMap.putAll(m);
}

@Override
public void clear() {
parameterMap.clear();
}

@Override
public Set<String> keySet() {
return parameterMap.keySet();
}

@Override
public Collection<String[]> values() {
return convertCharset(parameterMap.values());
}

@Override
public Set<Map.Entry<String, String[]>> entrySet() {
return convertCharset(parameterMap.entrySet());
}
};

return newMap;
}

private String convertCharset(String str) {
String newStr;
try {
newStr = new String(str.getBytes(oldCharset), newCharset);
} catch (UnsupportedEncodingException e) {
newStr = str;
}
return newStr;
}

private String[] convertCharset(String[] strs) {
String[] newStrs = null;
if (strs != null) {
newStrs = new String[strs.length];
for (int i = 0; i < newStrs.length; i++) {
newStrs[i] = convertCharset(strs[i]);
}
}
return newStrs;
}

private Collection<String[]> convertCharset(Collection<String[]> valueCollection) {
Collection<String[]> newCollection = new ArrayList<String[]>();
for (String[] values : valueCollection) {
newCollection.add(convertCharset(values));
}
return newCollection;
}

private Set<Entry<String, String[]>> convertCharset(Set<Map.Entry<String, String[]>> entrySet) {
Set<Map.Entry<String, String[]>> newSet = new HashSet<Map.Entry<String, String[]>>();
for (final Map.Entry<String, String[]> entry : entrySet) {
Map.Entry<String, String[]> newEntry = new Map.Entry<String, String[]> () {
@Override
public String getKey() {
return entry.getKey();
}
@Override
public String[] getValue() {
return convertCharset(entry.getValue());
}
@Override
public String[] setValue(String[] value) {
return entry.setValue(value);
}
};
newSet.add(newEntry);
}
return newSet;
}

}

过滤器：

package com.huey.hello.filters;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.http.HttpServletRequest;

import com.huey.hello.wrapper.CharsetServletRequestWrapper;

@WebFilter(filterName="charsetFilter", urlPatterns = {"/*"},
initParams = {
@WebInitParam(name = "oldCharset", value = "ISO-8859-1"),
@WebInitParam(name = "newCharset", value = "UTF-8"),
})
public class CharsetFilter implements Filter {

private String oldCharset;
private String newCharset;

@Override
public void init(FilterConfig filterConfig) throws ServletException {
oldCharset = filterConfig.getInitParameter("oldCharset");
newCharset = filterConfig.getInitParameter("newCharset");
}

@Override
public void destroy() {
}

@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
request.setCharacterEncoding(newCharset);
response.setContentType("text/plain;charset=" + newCharset);

HttpServletRequest httpRequest = (HttpServletRequest) request;
CharsetServletRequestWrapper requestWrapper = new CharsetServletRequestWrapper(httpRequest, oldCharset, newCharset);
chain.doFilter(requestWrapper, response);
}

}