关于解决Request method 'HEAD' not supported

该异常一般是在线上生产环境中产生的请求异常。

产生该异常的缘由：

实际生产中的head请求一般由搜索引擎发出 。  

搜索引擎先发送HEAD 请求是为了获得页面的更新时间(即response header 中的Last-Modified 域），
用于判断自从上一次该页面被收入索引库以后有没有被更新过，如果判断页面没有被更新过就忽略该页面，
否则就再用GET 方法获取一次最新的内容并更新到索引库中。

引用：https://www.cnblogs.com/xuan52rock/p/4495233.html

解决方式：

import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;

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

/**
 * Head请求处理过滤器
 * 实际生产中的head请求一般由搜索引擎发出
 * 搜索引擎先发送HEAD 请求是为了获得页面的更新时间(即response header 中的Last-Modified 域），
 * 用于判断自从上一次该页面被收入索引库以后有没有被更新过，如果判断页面没有被更新过就忽略该页面，
 * 否则就再用GET 方法获取一次最新的内容并更新到索引库中
 * 可解决Request method 'HEAD' not supported异常
 * @author wangjiao
 *
 */
public class HttpHeadReqFilter implements Filter{
	@Override
	public void init(FilterConfig arg0) throws ServletException {
		// TODO Auto-generated method stub
	}
	
	@Override
	public void destroy() {
		// TODO Auto-generated method stub
	}
	
	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		// TODO Auto-generated method stub
		HttpServletRequest httpServletRequest = (HttpServletRequest) request;
        if ("HEAD".equals(httpServletRequest.getMethod())) {
        	//对"HEAD"请求进行处理
            HttpServletResponse httpServletResponse = (HttpServletResponse) response;
            //设置Last-Modified即页面更新时间
            httpServletResponse.setDateHeader("Last-Modified", System.currentTimeMillis()); 
            NoBodyResponseWrapper noBodyResponseWrapper = new NoBodyResponseWrapper(httpServletResponse);
            //强制使用没有返回体(noBodyResponseWrapper)的"GET"请求
            chain.doFilter(new ForceGetRequestWrapper(httpServletRequest), noBodyResponseWrapper);
            noBodyResponseWrapper.setContentLength();
        } else {
            chain.doFilter(request, response);
        }
	}
	

    private class ForceGetRequestWrapper extends HttpServletRequestWrapper {
        public ForceGetRequestWrapper(HttpServletRequest request) {
            super(request);
        }

        public String getMethod() {
            return "GET";
        }
    }
    
    /**
     * 无响应体的包装类
     * @author wangjiao
     *
     */
    private class NoBodyResponseWrapper extends HttpServletResponseWrapper {
        private final NoBodyOutputStream noBodyOutputStream = new NoBodyOutputStream();
        private PrintWriter writer;

        public NoBodyResponseWrapper(HttpServletResponse response) {
            super(response);
        }

        public ServletOutputStream getOutputStream() throws IOException {
            return noBodyOutputStream;
        }

        public PrintWriter getWriter() throws UnsupportedEncodingException {
            if (writer == null) {
                writer = new PrintWriter(new OutputStreamWriter(noBodyOutputStream, getCharacterEncoding()));
            }

            return writer;
        }

        void setContentLength() {
            super.setContentLength(noBodyOutputStream.getContentLength());
        }
    }
    
    private class NoBodyOutputStream extends ServletOutputStream {
        private int contentLength = 0;

        int getContentLength() {
            return contentLength;
        }

        public void write(int b) {
            contentLength++;
        }

        public void write(byte buf[], int offset, int len) throws IOException {
            contentLength += len;
        }
    }

}