为什么https被redirect成了http？

全站HTTPS并不是配置证书CA，改改路径URL那么简单！

SSL卸载（SSL Offloading）:把SSL配置在负载均衡器上，然后通过其处理握手之后将decode的https数据转发给后台的Web服务器。
如下构成：

            |
            |(https)
            |
         Load Balancer
      /        |        \
     /         |         \
    /(http)   |(http)    \(http)
WebServer1  WebServer2  WebServer3

HTTPS的加密传输将只限于客户端发起请求到负载均衡器之间的公网阶段，内网的通讯仍然使用非加密的HTTP传输。每个Web服务器中所有的处理请求都认为是来自http，所有相对路径的 sendredirect 将都会被转发到http！

比如在Filter或Interceptor里的sendRedirect：

response.sendRedirect(request.getContextPath() + "/admin/welcome.do?flag=timeout");

或者Spring的Controller里的“redirect:”：

return "redirect:/admin/welcome.do";

通过HttpServletResponseWrapper可以通过Filter拦截SendRedirect请求并固定跳转到HTTPS。

web.xml

<filter>
  <filter-name>AbsoluteSendRedirectFilter</filter-name>
  <filter-class>com.rensanning.core.filter.AbsoluteSendRedirectFilter</filter-class>
</filter>
<filter-mapping>
  <filter-name>AbsoluteSendRedirectFilter</filter-name>
  <url-pattern>/*</url-pattern>
</filter-mapping>

AbsoluteSendRedirectFilter.java

public class AbsoluteSendRedirectFilter extends OncePerRequestFilter {

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        RedirectResponseWrapper redirectResponseWrapper = new RedirectResponseWrapper(request, response);
        filterChain.doFilter(request, redirectResponseWrapper);
    }

}

RedirectResponseWrapper.java

public class RedirectResponseWrapper extends HttpServletResponseWrapper {

    private final HttpServletRequest request;

    public RedirectResponseWrapper(final HttpServletRequest inRequest, final HttpServletResponse response) {
        super(response);
        this.request = inRequest;
    }

    @Override
    public void sendRedirect(final String pLocation) throws IOException {

        if (StringUtils.isBlank(pLocation)) {
            super.sendRedirect(pLocation);
            return;
        }

        try {
            final URI uri = new URI(pLocation);
            if (uri.getScheme() != null) {
                super.sendRedirect(pLocation);
                return;
            }
        } catch (URISyntaxException ex) {
            super.sendRedirect(pLocation);
        }

        // !!! FIX Scheme  !!!
        String finalurl = "https://" + this.request.getServerName();
        if (request.getServerPort() != 80 && request.getServerPort() != 443) {
            finalurl += ":" + request.getServerPort();
        }
        finalurl += pLocation;

        super.sendRedirect(finalurl);
    }

}

如果采用Spring的话可以设置：redirectHttp10Compatible=false。

<bean id="viewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver">
  <property name="viewClass" value="org.springframework.web.servlet.view.JstlView" />
  <property name="prefix" value="/" />
  <property name="suffix" value=".jsp" />
  <property name="redirectHttp10Compatible" value="false" />
</bean>

参考：
http://www.exampit.com/blog/javahunter/5-8-2016-Why-does-https-become-http-on-a-sendredirect
http://*.com/questions/3401113/spring-mvc-redirect-prefix-always-redirects-to-http-how-do-i-make-it-stay