首先我们来描述一下在开发中遇到的问题，场景如下：
比如我们要拦截所有请求，获取请求中的某个参数，进行相应的逻辑处理：比如我要获取所有请求中的公共参数 token,clientVersion等等；这个时候我们通常有两种做法
前提条件是我们实现Filter类，重写doFilter方法
1、通过getParameter方法获得
HttpServletRequest hreq = (HttpServletRequest) req;
String param = hreq.getParameter("param");
这种方法有缺陷：它只能获取 POST 提交方式中的
Content-Type: application/x-www-form-urlencoded;
这种提交方式中，key为param ,若提交类型不是这种方式就获取不到param的值
2、获取请求对象中的数据流，通过解析流信息来获取提交的内容；代码示例如下：
/**
* 获取请求Body
*
* @param request
* @return
*/
public static String getBodyString(ServletRequest request) {
StringBuilder sb = new StringBuilder();
InputStream inputStream = null;
BufferedReader reader = null;
try {
inputStream = request.getInputStream();
reader = new BufferedReader(new InputStreamReader(inputStream, Charset.forName("UTF-8")));
String line = "";
while ((line = reader.readLine()) != null) {
sb.append(line);
}
} catch (IOException e) {
e.printStackTrace();
} finally {
if (inputStream != null) {
try {
inputStream.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (reader != null) {
try {
reader.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
return sb.toString();
}
通过这种简单的解析方式，将http请求中的body解析成 字符串的形式；然后再对字符串的格式进行业务解析；
这种处理方式的优点是：能解析 出content-Type 为 application/x-www-form-urlencoded ，application/json , text/xml 这三种提交方式的 数据 （至于另外一种带文件的请求：multipart/form-data ，本人没有亲自测试过，等后期遇到此类为题再来解决）。 上述方法返回的可能是key,value(对应第一种content-type),可能是json字符串（对应第二种），可能是xml的字符串（对应第三种）
但是这种方式有一个很大的缺点：那就是当从请求中获取流以后，流被filter中的这个 inputStreamToString(InputStream in) 这个方法处理后就被“消耗”了，这会导致，chain.doFilter(request, res)这个链在传递 request对象的时候，里面的请求流为空，导致责任链模式下，其他下游的链无法获取请求的body,从而导致程序无法正常运行，这也使得我们的这个filter虽然可以获取请求信息，但是它会导致整个应用程序不可用，那么它也就失去了意义；
针对第二种方式的缺陷：流一旦被读取，就无法向下传递整个问题，有如下解决方案；
解决思路如下：将取出来的字符串，再次转换成流，然后把它放入到新request 对象中，在chain.doFiler方法中 传递新的request对象；要实现这种思路，需要自定义一个类
继承HttpServletRequestWrapper，然后重写public BufferedReader getReader()方法，public ServletInputStream getInputStream()方法；（这两个方法的重写实现逻辑如下：getInputStream（）方法中将body体中的字符串转换为字节流（它实质上返回的是一个ServletInputStream 对象）；然后通过getReader()调用---->getInputStream（）方法；），继承实现重写逻辑以后，在自定义分filter（VersionCheckFilter）中，使用自定义的HttpServletRequestWrapper（BodyReaderHttpServletRequestWrapper）将原始的HttpServletRequest对象进行再次封装；再通过BodyReaderHttpServletRequestWrapper对象去做dofilter(req,res)的req对象；
涉及的三个类的代码如下：
自定义的HttpServletRequestWrapper
import java.io.BufferedReader; import java.io.ByteArrayInputStream; import java.io.IOException; import java.io.InputStreamReader; import java.nio.charset.Charset; import java.util.Enumeration; import javax.servlet.ServletInputStream; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletRequestWrapper; import com.yt.util.HttpHelper; public class BodyReaderHttpServletRequestWrapper extends HttpServletRequestWrapper { private final byte[] body; public BodyReaderHttpServletRequestWrapper(HttpServletRequest request) throws IOException { super(request); System.out.println("-------------------------------------------------"); Enumeration e = request.getHeaderNames() ; while(e.hasMoreElements()){ String name = (String) e.nextElement(); String value = request.getHeader(name); System.out.println(name+" = "+value); } body = HttpHelper.getBodyString(request).getBytes(Charset.forName("UTF-8")); } @Override public BufferedReader getReader() throws IOException { return new BufferedReader(new InputStreamReader(getInputStream())); } @Override public ServletInputStream getInputStream() throws IOException { final ByteArrayInputStream bais = new ByteArrayInputStream(body); return new ServletInputStream() { @Override public int read() throws IOException { return bais.read(); } }; } @Override public String getHeader(String name) { return super.getHeader(name); } @Override public Enumeration<String> getHeaderNames() { return super.getHeaderNames(); } @Override public Enumeration<String> getHeaders(String name) { return super.getHeaders(name); } }
2、辅助类
HttpHelper
import java.io.BufferedReader; import java.io.IOException; import java.io.InputStream; import java.io.InputStreamReader; import java.nio.charset.Charset; import javax.servlet.ServletRequest; public class HttpHelper { /** * 获取请求Body * * @param request * @return */ public static String getBodyString(ServletRequest request) { StringBuilder sb = new StringBuilder(); InputStream inputStream = null; BufferedReader reader = null; try { inputStream = request.getInputStream(); reader = new BufferedReader(new InputStreamReader(inputStream, Charset.forName("UTF-8"))); String line = ""; while ((line = reader.readLine()) != null) { sb.append(line); } } catch (IOException e) { e.printStackTrace(); } finally { if (inputStream != null) { try { inputStream.close(); } catch (IOException e) { e.printStackTrace(); } } if (reader != null) { try { reader.close(); } catch (IOException e) { e.printStackTrace(); } } } return sb.toString(); } }
3.自定义的filter
import java.io.IOException; import java.io.InputStream; import java.io.PrintWriter; import java.net.URL; import java.net.URLDecoder; import java.util.Map; 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.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.xml.crypto.URIDereferencer; import com.yt.util.HttpHelper; import com.yt.util.JsonHelper; public class VersionCheckFilter implements Filter { @Override public void destroy() { } @Override public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException { HttpServletRequest hreq = (HttpServletRequest) req; String uri = hreq.getRequestURI(); if(uri.contains("uploadImageServlet")){ //图像上传的请求，不做处理 chain.doFilter(req, res); }else{ String reqMethod = hreq.getMethod(); if("POST".equals(reqMethod)){ PrintWriter out = null; HttpServletResponse response = (HttpServletResponse) res; response.setCharacterEncoding("UTF-8"); response.setContentType("application/json; charset=utf-8"); /* String requestStr = this.inputStreamToString(hreq.getInputStream()); String[] arrs = requestStr.split("&"); for (String strs : arrs) { String[] strs2 = strs.split("="); for (int i = 0; i < strs2.length; i++) { if (strs2[0].equals("param")) { if (strs2[1] != null &&!strs2[1].equals("")) { System.out.println("test=" + strs2[1]); } } } }*/ ServletRequest requestWrapper = new BodyReaderHttpServletRequestWrapper(hreq); String body = HttpHelper.getBodyString(requestWrapper); //如果是POST请求则需要获取 param 参数 String param = URLDecoder.decode(body,"utf-8"); //String param = hreq.getParameter("param"); //json串 转换为Map if(param!=null&¶m.contains("=")){ param = param.split("=")[1]; } Map paramMap = JsonHelper.getGson().fromJson(param, Map.class); Object obj_clientversion = paramMap.get("clientVersion"); String clientVersion = null; if(obj_clientversion != null){ clientVersion = obj_clientversion.toString(); System.out.println(clientVersion); /*try { out = response.getWriter(); Map remap = new HashMap<String, Object>(); remap.put("code", 9); remap.put("message", Constant.SYSTEM_ERR_DESC); out.append(JsonHelper.getGson().toJson(remap)); } catch (IOException e) { e.printStackTrace(); } finally { if (out != null) { out.close(); } }*/ chain.doFilter(requestWrapper, res); }else{ chain.doFilter(requestWrapper, res); } }else{ //get请求直接放行 chain.doFilter(req, res); } } } @Override public void init(FilterConfig arg0) throws ServletException { } /*public String inputStreamToString(InputStream in) throws IOException { StringBuffer out = new StringBuffer(); byte[] b = new byte[4096]; for (int n; (n = in.read(b)) != -1;) { out.append(new String(b, 0, n)); } return out.toString(); }*/ }