hessian隐式传参

背景

web应用的项目：分表现层portal端和业务服务层service端，使用dubbo框架，rpc使用hessian。dubbo里的dubbo协议由于是socket长链接能够attachment隐式带参数，hessian协议则不能attachment参数。现有需求须要在每一个服务调用都多传一个参数，因而考虑不在改动接口的状况下，也在hessian协议下进行隐式传参。

同事说hessian也是走http，能够在发起请求的时候把参数添加到http头信息，服务端接收的时候再解析出来。还有就是hack一下hessian的二进制协议在序列化和反序列化的时候作点手脚。因而源码看起、google百度相关的hessian源码搭配更快地看懂源码。途中就找在远在2007年的一个帖子：Hessian源码分析和Hack --让Hessian携带远程调用端的信息，里面就说到隐式传参的解决方案。因此本文不算原创。

方案1：使用http头信息传参

1. 在Hessian客户端HessianProxy调用方法发送请求中添加http头信息：
   public Object invoke(Object proxy, Method method, Object []args) -->
   protected HessianConnection sendRequest(String methodName, Object []args) -->
   protected void addRequestHeaders(HessianConnection conn) 里添加头信息

   protected void addRequestHeaders(HessianConnection conn)
        {
        conn.addHeader("Content-Type", "x-application/hessian");
        conn.addHeader("Accept-Encoding", "deflate");
        //从ThreadLocal里获取参数
        Map<String,String> context=HessianContext.getContext();
        if(context!=null)
        {
    	    for(Map.Entry<String, String> entry:context.entrySet())
    	        conn.addHeader(entry.getKey(), entry.getValue());
        }
   
        String basicAuth = _factory.getBasicAuth();
   
        if (basicAuth != null)
                conn.addHeader("Authorization", basicAuth);
        }

2.在dubbo的HessianProtocol中，skeleton.invoke(request.getInputStream(), response.getOutputStream()); 前提取http头信息

private class HessianHandler implements HttpHandler 
{
    public void handle(HttpServletRequest request, HttpServletResponse response)
            throws IOException, ServletException {
        String uri = request.getRequestURI();
        HessianSkeleton skeleton = skeletonMap.get(uri);
        if (! request.getMethod().equalsIgnoreCase("POST")) {
            response.setStatus(500);
        } else {
            RpcContext.getContext().setRemoteAddress(request.getRemoteAddr(), request.getRemotePort());
            try {
            	Enumeration paramName=request.getHeaderNames();
            	String name;
            	Map<String,String> data=new HashMap<String, String>();
            	while(paramName.hasMoreElements())
            	{
            		name=(String)paramName.nextElement();
            		data.put(name, request.getHeader(name));
            	}
            	HessianContext.setContext(data);
                skeleton.invoke(request.getInputStream(), response.getOutputStream());
                HessianContext.removeContext();
            } catch (Throwable e) {
                throw new ServletException(e);
            }
        }
    }
}

3.这种用法须要同时升级hessian和dubbo包，可是不破坏hessian协议能够和原有的hessian兼容。在portal端调用service端方法先后须要HessianContext.setContext(...)和HessianContext.removeContext()，service端用HessianContext.getContext()获取参数。

方案2：hessian多序列化反序列化一个参数

1. hessian的HessianOutput和Hessian2Output在HessianProxy的sendRequest方法里用call方法拼接请求body。在call方法的拼接参数后再write一个参数

   public void call(String method, Object []args)
        throws IOException
      {
        int length = args != null ? args.length : 0;
   
        startCall(method, length);
   
        for (int i = 0; i < length; i++)
          writeObject(args[i]);
   
        //再write一个参数
        writeObject(HessianContext.getContext());
   
        completeCall();
      }

2. 在HessianSkeleton里invoke(Object service,AbstractHessianInput in,AbstractHessianOutput out)里读取参数后再读取一个参数

   for (int i = 0; i < args.length; i++) {
      // XXX: needs Marshal object
      values[i] = in.readObject(args[i]);
    }
    HessianContext.setContext((Map<String, Object>) in.readObject());
    Object result = null;
   
    try {
      result = method.invoke(service, values);
    } catch (Exception e) {
      Throwable e1 = e;

3. 这个方案只需升级hessian，可是破坏hessian的二进制规则，不能兼容原有的hessian，须要全部项目都升级，且不能外部使用。

HessianContext

HessianContext用ThreadLocal存放参数，代码简单直接上代码：

public class HessianContext {

	private static final ThreadLocal<HessianContext> _THREADLOCAL = new ThreadLocal<HessianContext>();
	private Map<String,Object> data;
	
	private HessianContext(){}
	
	
	public static void setContext(Map<String,Object> data)
	{
		if(data==null)
			return;
		
		HessianContext context=_THREADLOCAL.get();
		if(context==null)
		{
			context=new HessianContext();
			_THREADLOCAL.set(context);
		}
		context.data=data;
	}
	
	public static Map<String,Object> getAndRemoveContext()
	{
		Map<String,Object> data=getContext();
		removeContext();
		return data;
	}
	
	public static Map<String,Object> getContext()
	{
		HessianContext context=_THREADLOCAL.get();
		return context==null?null:context.data;
	}
	
	public static void removeContext()
	{
		_THREADLOCAL.remove();
	}
}