拦截请求返回值的方法

在web应用开发时，我们经常会需要拦截请求，在请求前后做一些事，比如打印日志等，这种方式就相当于spring中的面向切面编程，以下总结了下实现这种方式的一些方法。

###1. 使用拦截器+过滤器

这种方法是目前比较通用的方法，不受限于框架限制，不管是用SSH还是springmvc都可以采用，实现方式大致如下：
（1）定义一个拦截器，比如WebRequestInterceptor，在preHandle里面拦截request，从中取出参数等信息，然后打印出来即可。
（2）定义一个过滤器，比如WebResponseFilter，在doFilter里面使用HttpServletResponseWrapper包装传进来的response，然后重写一些方法即可实现。
（注意：这里有个坑，就是直接使用拦截器无法从response里面取出返回值！）
（3）相关代码大致如下：

WebInterceptor.java

package com.zuolin.interceptor;
import java.util.Enumeration;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;
import com.zuolin.filter.WrapperResponse;
public class WebInterceptor implements HandlerInterceptor {
    private static final Logger logger = LoggerFactory.getLogger(WebInterceptor.class);
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object object) throws Exception {
        if (logger.isDebugEnabled()) {
            StringBuffer sb = new StringBuffer();
            sb.append("{");
            Enumeration<String> headers = request.getHeaderNames();
            int i = 0;
            while (headers.hasMoreElements()) {
                String header = headers.nextElement();
                if (i > 0)
                    sb.append(", ");
                sb.append(header + ": " + request.getHeader(header));
                i++;
            }
            sb.append("}");
            logger.debug("Pre handling request: {}, headers: {}", getRequestInfo(request, true), sb.toString());
        }
        return true;
    }
    private static String getRequestInfo(HttpServletRequest request, boolean requestDetails) {
        StringBuffer sb = new StringBuffer();
        sb.append(request.getMethod()).append(" ");
        sb.append(request.getRequestURI());
        if (requestDetails) {
            Enumeration<String> e = request.getParameterNames();
            sb.append("{");
            int i = 0;
            while (e.hasMoreElements()) {
                String name = e.nextElement();
                String val = request.getParameter(name);
                if (val != null && !val.isEmpty()) {
                    if (i > 0)
                        sb.append(", ");
                    sb.append(name).append(": ").append(val);
                    i++;
                }
            }
            sb.append("}");
        }
        return sb.toString();
    }
    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object object,
            ModelAndView modelAndView) throws Exception {
    }
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object object,
            Exception exception) throws Exception {
        String requestInfo = getRequestInfo(request, false);
        if (logger.isDebugEnabled()) {
            logger.debug("Complete request: {}", requestInfo);
        }
    }
}

WrapperOutputStream.java

package com.zuolin.filter;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import javax.servlet.ServletOutputStream;
public class WrapperOutputStream extends ServletOutputStream{
    private ByteArrayOutputStream bos;
    public WrapperOutputStream(ByteArrayOutputStream bos) {
        this.bos = bos;
    }
    @Override
    public void write(int b) throws IOException {
        bos.write(b);
    }
}

WrapperResponse.java

package com.zuolin.filter;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;
public class WrapperResponse extends HttpServletResponseWrapper{
    private ByteArrayOutputStream buffer;
    private ServletOutputStream out;
    public WrapperResponse(HttpServletResponse response) throws UnsupportedEncodingException {
        super(response);
        buffer = new ByteArrayOutputStream();
        out = new WrapperOutputStream(buffer);
    }
    @Override
    public ServletOutputStream getOutputStream() throws IOException {
        return out;
    }
    @Override
    public void flushBuffer() throws IOException {
        if (out != null) {
            out.flush();
        }
    }
    public byte[] getContent() throws IOException {
        flushBuffer();
        return buffer.toByteArray();
    }
}

ResponseFilter.java

package com.zuolin.filter;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletResponse;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class ResponseFilter implements Filter {
    private static final Logger logger = LoggerFactory.getLogger(ResponseFilter.class);
    @Override
    public void destroy() {
    }
    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain filterChain) throws IOException, ServletException {
        WrapperResponse wrapperResponse = new WrapperResponse((HttpServletResponse) response);
        filterChain.doFilter(request, wrapperResponse);
        byte[] content = wrapperResponse.getContent();
        if (logger.isDebugEnabled() && content != null && content.length > 0) {
            logger.debug("Response Content: {}", new String(content));
        }
        ServletOutputStream out = response.getOutputStream();
        out.write(content);
        out.flush();
    }
    @Override
    public void init(FilterConfig paramFilterConfig) throws ServletException {
    }
}

然后记得在web.xml或spring的配置文件中加上以上类的定义就可以直接使用了。

这种方式有个缺陷，就是要写很多代码，而且一般人看不懂这里的流程，因为使用的是流。

###2. 使用spring中的advice

这种就是spring里面的面向切面编程的使用了，这种方式只能依赖于spring的aspect存在，实现方法为：
（1）声明一个类，比如叫WebRequestAroundAdvice；
（2）定义切入点；
（3）定义切入点的处理方法。
（4）相关代码如下：

WebRequestAroundAdvice.java

package com.zuolin.interceptor;
import java.util.Enumeration;
import javax.servlet.http.HttpServletRequest;
import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.web.context.request.RequestContextHolder;
import org.springframework.web.context.request.ServletRequestAttributes;
import com.alibaba.fastjson.JSONObject;
@Aspect
public class WebRequestAroundAdvice {
    private static final Logger logger = LoggerFactory.getLogger(WebRequestAroundAdvice.class);
    @Pointcut( value = "execution(* com.zuolin.controller.*.*(..))" )
    public void pointcut(){}
    @Around("pointcut()")
    public Object handle(ProceedingJoinPoint joinPoint) throws Throwable{
        preHandle();
        Object retVal = joinPoint.proceed();
        postHandle(retVal);
        return retVal;
    }
    private void preHandle() {
        if (logger.isDebugEnabled()) {
            HttpServletRequest request = ( (ServletRequestAttributes) RequestContextHolder.getRequestAttributes() ).getRequest();
            StringBuffer sb = new StringBuffer();
            sb.append("{");
            Enumeration<String> headers = request.getHeaderNames();
            int i = 0;
            while (headers.hasMoreElements()) {
                String header = headers.nextElement();
                if (i > 0)
                    sb.append(", ");
                sb.append(header + ": " + request.getHeader(header));
                i++;
            }
            sb.append("}");
            logger.debug("Pre handling request: {}, headers: {}", getRequestInfo(request, true), sb.toString());
        }
    }
    private void postHandle(Object retVal) {
        if (logger.isDebugEnabled()) {
            HttpServletRequest request = ( (ServletRequestAttributes) RequestContextHolder.getRequestAttributes() ).getRequest();
            logger.debug("Post handling request: {}, response: {}", getRequestInfo(request, false), JSONObject.toJSONString(retVal));
        }
    }
    private String getRequestInfo(HttpServletRequest request, boolean requestDetails) {
        StringBuffer sb = new StringBuffer();
        sb.append(request.getMethod()).append(" ");
        sb.append(request.getRequestURI());
        if (requestDetails) {
            Enumeration<String> e = request.getParameterNames();
            sb.append(" ").append("{");
            int i = 0;
            while (e.hasMoreElements()) {
                String name = e.nextElement();
                String val = request.getParameter(name);
                if (val != null && !val.isEmpty()) {
                    if (i > 0)
                        sb.append(", ");
                    sb.append(name).append(": ").append(val);
                    i++;
                }
            }
            sb.append("}");
        }
        return sb.toString();
    }
}

然后在spring的配置文件中配置这个bean就可以直接使用了。

这种方式的优点就是代码很简洁，只要稍微学过一点AOP都能很容易理解，缺点就是依赖于spring。

这里只使用了spring的around advice，当然，使用其它的几种advice的组合也可以实现相同的效果，或者混合使用拦截器和advice也可以，关键在于你。

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