聊聊Dubbo（七）：自定义Filter实践

在现行微服务的趋势下，一次调用的过程中涉及多个服务节点，产生的日志分布在不同的服务器上，虽说可以使用ELK技术将分散的日志，汇总到es中，但是如何将这些日志贯穿起来，则是一个关键问题。

如果需要查看一次调用的全链路日志，则一般的做法是通过在系统边界中产生一个 traceId ，向调用链的后续服务传递 traceId ，后续服务继续使用 traceId 打印日志，并再向其他后续服务传递 traceId ，此过程简称， traceId透传 。

在使用HTTP协议作为服务协议的系统里，可以统一使用一个封装好的http client做traceId透传。但是dubbo实现traceId透传就稍微复杂些了。根据上节讲的 《☆聊聊Dubbo（六）：核心源码-Filter链原理》 ，一般情况下， 会自定义Filter来实现traceId透传 ，但还有两种比较特殊的实现方式： （1）重新实现dubbo内部的相关类；（2）基于RpcContext实现；

1 基于重写实现

1.1 源码分析

Proxy是 Dubbo 使用javassist为consumer 端service生成的动态代理instance。

Implement是provider端的service实现instance。

traceId透传，即要求Proxy 和 Implement具有相同的traceId。Dubbo具有良好的分层特征， transport的对象是RPCInvocation 。

所以，重写的重点逻辑实现，就是Proxy将traceId放入RPCInvocation，交由Client进行序列化和TCP传输，Server反序列化得到RPCInvocation，取出traceId，交由Implement即可。

下面为consumer端 JavassistProxyFactory 的代码分析：

public class JavassistProxyFactory extends AbstractProxyFactory {

    /**
     * Spring容器启动时，该代理工厂类方法会为Consumer生成Service代理类
     * invoker和interfaces都是从Spring配置文件中读取出来 
     */
    @SuppressWarnings("unchecked")
    public <T> T getProxy(Invoker<T> invoker, Class<?>[] interfaces) {
        // 生成Service代理类的每个方法的字节码，都调用了InvokerInvocationHandler.invoke(...)方法，
        // 做实际RpcInvocation包装、序列化、TCP传输、反序列化结果
        return (T) Proxy.getProxy(interfaces).newInstance(new InvokerInvocationHandler(invoker));
    }

    public <T> Invoker<T> getInvoker(T proxy, Class<T> type, URL url) {
        // TODO Wrapper类不能正确处理带$的类名
        final Wrapper wrapper = Wrapper.getWrapper(proxy.getClass().getName().indexOf('$') < 0 ? proxy.getClass() : type);
        return new AbstractProxyInvoker<T>(proxy, type, url) {
            @Override
            protected Object doInvoke(T proxy, String methodName, 
                                      Class<?>[] parameterTypes, 
                                      Object[] arguments) throws Throwable {
                return wrapper.invokeMethod(proxy, methodName, parameterTypes, arguments);
            }
        };
    }
}
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下面为consumer端 InvokerInvocationHandler 的代码分析：

public class InvokerInvocationHandler implements InvocationHandler {

    private final Invoker<?> invoker;
    
    public InvokerInvocationHandler(Invoker<?> handler){
        this.invoker = handler;
    }

    /**
     * 真正调用RPC时，各个Service代理的字节码里调用了这个通用的invoke
     * proxy就是之前生成的代理对象，第二个参数是方法名，第三个参数是参数列表
     * 知道了（1）哪个接口（2）哪个方法（3）参数是什么，就完全可以映射到Provider端实现并获取返回值
     */
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        String methodName = method.getName();
        Class<?>[] parameterTypes = method.getParameterTypes();
        if (method.getDeclaringClass() == Object.class) {
            return method.invoke(invoker, args);
        }
        if ("toString".equals(methodName) && parameterTypes.length == 0) {
            return invoker.toString();
        }
        if ("hashCode".equals(methodName) && parameterTypes.length == 0) {
            return invoker.hashCode();
        }
        if ("equals".equals(methodName) && parameterTypes.length == 1) {
            return invoker.equals(args[0]);
        }
        // 因为到这里，还是consumer端的业务线程，所以在这里取ThreadLocal里的traceId，
        // 再放入RpcInvocation的attachment，那么Provider就可以从收到的RpcInvocation实例取出透传的traceId
        return invoker.invoke(new RpcInvocation(method, args)).recreate();
    }
}
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下面为Provider端 DubboProtocol 的代码分析：

private ExchangeHandler requestHandler = new ExchangeHandlerAdapter() {
        
        public Object reply(ExchangeChannel channel, Object message) throws RemotingException {
            if (message instanceof Invocation) {
                Invocation inv = (Invocation) message;
                Invoker<?> invoker = getInvoker(channel, inv);
                //如果是callback 需要处理高版本调用低版本的问题
                if (Boolean.TRUE.toString().equals(inv.getAttachments().get(IS_CALLBACK_SERVICE_INVOKE))){
                    String methodsStr = invoker.getUrl().getParameters().get("methods");
                    boolean hasMethod = false;
                    if (methodsStr == null || methodsStr.indexOf(",") == -1){
                        hasMethod = inv.getMethodName().equals(methodsStr);
                    } else {
                        String[] methods = methodsStr.split(",");
                        for (String method : methods){
                            if (inv.getMethodName().equals(method)){
                                hasMethod = true;
                                break;
                            }
                        }
                    }
                    if (!hasMethod){
                        logger.warn(new IllegalStateException("The methodName "+inv.getMethodName()+" not found in callback service interface ,invoke will be ignored. please update the api interface. url is:" + invoker.getUrl()) +" ,invocation is :"+inv );
                        return null;
                    }
                }

                // Provider收到报文之后，从线程池中取出一个线程，反序列化出RpcInvocation、并调用实现类的对应方法
                // 所以，此处就是Provider端的实现类的线程，取出traceId，放入ThreadLocal中
                RpcContext.getContext().setRemoteAddress(channel.getRemoteAddress());
                return invoker.invoke(inv);
            }
            throw new RemotingException(channel, "Unsupported request: " + message == null ? null : (message.getClass().getName() + ": " + message) + ", channel: consumer: " + channel.getRemoteAddress() + " --> provider: " + channel.getLocalAddress());
        }
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1.2 具体实现

package com.alibaba.dubbo.rpc.proxy;

/**
 * traceId工具类这个类是新添加的
 */
public class TraceIdUtil {

    private static final ThreadLocal<String> TRACE_ID = new ThreadLocal<String>();

    public static String getTraceId() {
        return TRACE_ID.get();
    }

    public static void setTraceId(String traceId) {
        TRACE_ID.set(traceId);
    }

}

/**
 * InvokerHandler 这个类 是修改的
 */
public class InvokerInvocationHandler implements InvocationHandler {

    private final Invoker<?> invoker;

    public InvokerInvocationHandler(Invoker<?> handler){
        this.invoker = handler;
    }

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        String methodName = method.getName();
        Class<?>[] parameterTypes = method.getParameterTypes();
        if (method.getDeclaringClass() == Object.class) {
            return method.invoke(invoker, args);
        }
        if ("toString".equals(methodName) && parameterTypes.length == 0) {
            return invoker.toString();
        }
        if ("hashCode".equals(methodName) && parameterTypes.length == 0) {
            return invoker.hashCode();
        }
        if ("equals".equals(methodName) && parameterTypes.length == 1) {
            return invoker.equals(args[0]);
        }
        // 这里将cosumer 端的traceId放入RpcInvocation
        RpcInvocation rpcInvocation = new RpcInvocation(method, args);
        rpcInvocation.setAttachment("traceId", TraceIdUtil.getTraceId());
        return invoker.invoke(rpcInvocation).recreate();
    }

}


package com.alibaba.dubbo.rpc.protocol.dubbo;

/**
 * dubbo protocol support 重新实现DubboProtocol
 *
 */
public class DubboProtocol extends AbstractProtocol {


    private ExchangeHandler requestHandler = new ExchangeHandlerAdapter() {

        public Object reply(ExchangeChannel channel, Object message) throws RemotingException {
            if (message instanceof Invocation) {
                Invocation inv = (Invocation) message;
                Invoker<?> invoker = getInvoker(channel, inv);
                //如果是callback 需要处理高版本调用低版本的问题
                if (Boolean.TRUE.toString().equals(inv.getAttachments().get(IS_CALLBACK_SERVICE_INVOKE))){
                    String methodsStr = invoker.getUrl().getParameters().get("methods");
                    boolean hasMethod = false;
                    if (methodsStr == null || methodsStr.indexOf(",") == -1){
                        hasMethod = inv.getMethodName().equals(methodsStr);
                    } else {
                        String[] methods = methodsStr.split(",");
                        for (String method : methods){
                            if (inv.getMethodName().equals(method)){
                                hasMethod = true;
                                break;
                            }
                        }
                    }
                    if (!hasMethod){
                        logger.warn(new IllegalStateException("The methodName "+inv.getMethodName()+" not found in callback service interface ,invoke will be ignored. please update the api interface. url is:" + invoker.getUrl()) +" ,invocation is :"+inv );
                        return null;
                    }
                }
                RpcContext.getContext().setRemoteAddress(channel.getRemoteAddress());
                // 这里将收到的consumer端的traceId放入provider端的thread local
                TraceIdUtil.setTraceId(inv.getAttachment("traceId"));
                return invoker.invoke(inv);
            }
            throw new RemotingException(channel, "Unsupported request: " + message == null ? null : (message.getClass().getName() + ": " + message) + ", channel: consumer: " + channel.getRemoteAddress() + " --> provider: " + channel.getLocalAddress());
        }
    }
}
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2 基于RpcContext实现

在具体讲解自定义filter来实现透传traceId的方案前，我们先来研究下RpcContext对象。 其RpcContext本质上是个ThreadLocal对象，其维护了一次rpc交互的上下文信息 。

/*
 * Copyright 1999-2011 Alibaba Group.
 *  
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *  
 *      http://www.apache.org/licenses/LICENSE-2.0
 *  
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.dubbo.rpc;

import java.net.InetSocketAddress;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

import com.alibaba.dubbo.common.Constants;
import com.alibaba.dubbo.common.URL;
import com.alibaba.dubbo.common.utils.NetUtils;

/**
 * Thread local context. (API, ThreadLocal, ThreadSafe)
 * 
 * 注意：RpcContext是一个临时状态记录器，当接收到RPC请求，或发起RPC请求时，RpcContext的状态都会变化。
 * 比如：A调B，B再调C，则B机器上，在B调C之前，RpcContext记录的是A调B的信息，在B调C之后，RpcContext记录的是B调C的信息。
 * 
 * @see com.alibaba.dubbo.rpc.filter.ContextFilter
 * @author qian.lei
 * @author william.liangf
 * @export
 */
public class RpcContext {
	
	private static final ThreadLocal<RpcContext> LOCAL = new ThreadLocal<RpcContext>() {
		@Override
		protected RpcContext initialValue() {
			return new RpcContext();
		}
	};

	/**
	 * get context.
	 * 
	 * @return context
	 */
	public static RpcContext getContext() {
	    return LOCAL.get();
	}
	
	/**
	 * remove context.
	 * 
	 * @see com.alibaba.dubbo.rpc.filter.ContextFilter
	 */
	public static void removeContext() {
	    LOCAL.remove();
	}

    private Future<?> future;

    private List<URL> urls;

    private URL url;

    private String methodName;

    private Class<?>[] parameterTypes;

    private Object[] arguments;

	private InetSocketAddress localAddress;

	private InetSocketAddress remoteAddress;

    private final Map<String, String> attachments = new HashMap<String, String>();

    private final Map<String, Object> values = new HashMap<String, Object>();

    // now we don't use the 'values' map to hold these objects
    // we want these objects to be as generic as possible
    private Object request;
    private Object response;

	@Deprecated
    private List<Invoker<?>> invokers;
    
	@Deprecated
    private Invoker<?> invoker;

	@Deprecated
    private Invocation invocation;
    
	protected RpcContext() {
	}

    /**
     * Get the request object of the underlying RPC protocol, e.g. HttpServletRequest
     *
     * @return null if the underlying protocol doesn't provide support for getting request
     */
    public Object getRequest() {
        return request;
    }

    /**
     * Get the request object of the underlying RPC protocol, e.g. HttpServletRequest
     *
     * @return null if the underlying protocol doesn't provide support for getting request or the request is not of the specified type
     */
    @SuppressWarnings("unchecked")
    public <T> T getRequest(Class<T> clazz) {
        return (request != null && clazz.isAssignableFrom(request.getClass())) ? (T) request : null;
    }


    public void setRequest(Object request) {
        this.request = request;
    }

    /**
     * Get the response object of the underlying RPC protocol, e.g. HttpServletResponse
     *
     * @return null if the underlying protocol doesn't provide support for getting response
     */
    public Object getResponse() {
        return response;
    }

    /**
     * Get the response object of the underlying RPC protocol, e.g. HttpServletResponse
     *
     * @return null if the underlying protocol doesn't provide support for getting response or the response is not of the specified type
     */
    @SuppressWarnings("unchecked")
    public <T> T getResponse(Class<T> clazz) {
        return (response != null && clazz.isAssignableFrom(response.getClass())) ? (T) response : null;
    }

    public void setResponse(Object response) {
        this.response = response;
    }

    /**
     * is provider side.
     * 
     * @return provider side.
     */
    public boolean isProviderSide() {
        URL url = getUrl();
        if (url == null) {
            return false;
        }
        InetSocketAddress address = getRemoteAddress();
        if (address == null) {
            return false;
        }
        String host;
        if (address.getAddress() == null) {
            host = address.getHostName();
        } else {
            host = address.getAddress().getHostAddress();
        }
        return url.getPort() != address.getPort() || 
                ! NetUtils.filterLocalHost(url.getIp()).equals(NetUtils.filterLocalHost(host));
    }

    /**
     * is consumer side.
     * 
     * @return consumer side.
     */
    public boolean isConsumerSide() {
        URL url = getUrl();
        if (url == null) {
            return false;
        }
        InetSocketAddress address = getRemoteAddress();
        if (address == null) {
            return false;
        }
        String host;
        if (address.getAddress() == null) {
            host = address.getHostName();
        } else {
            host = address.getAddress().getHostAddress();
        }
        return url.getPort() == address.getPort() && 
                NetUtils.filterLocalHost(url.getIp()).equals(NetUtils.filterLocalHost(host));
    }

    /**
     * get future.
     * 
     * @param <T>
     * @return future
     */
    @SuppressWarnings("unchecked")
    public <T> Future<T> getFuture() {
        return (Future<T>) future;
    }

    /**
     * set future.
     * 
     * @param future
     */
    public void setFuture(Future<?> future) {
        this.future = future;
    }

    public List<URL> getUrls() {
        return urls == null && url != null ? (List<URL>) Arrays.asList(url) : urls;
    }

    public void setUrls(List<URL> urls) {
        this.urls = urls;
    }

    public URL getUrl() {
        return url;
    }

    public void setUrl(URL url) {
        this.url = url;
    }

    /**
     * get method name.
     * 
     * @return method name.
     */
    public String getMethodName() {
        return methodName;
    }

    public void setMethodName(String methodName) {
        this.methodName = methodName;
    }

    /**
     * get parameter types.
     * 
     * @serial
     */
    public Class<?>[] getParameterTypes() {
        return parameterTypes;
    }

    public void setParameterTypes(Class<?>[] parameterTypes) {
        this.parameterTypes = parameterTypes;
    }

    /**
     * get arguments.
     * 
     * @return arguments.
     */
    public Object[] getArguments() {
        return arguments;
    }

    public void setArguments(Object[] arguments) {
        this.arguments = arguments;
    }

    /**
     * set local address.
     * 
     * @param address
     * @return context
     */
	public RpcContext setLocalAddress(InetSocketAddress address) {
	    this.localAddress = address;
	    return this;
	}

	/**
	 * set local address.
	 * 
	 * @param host
	 * @param port
	 * @return context
	 */
    public RpcContext setLocalAddress(String host, int port) {
        if (port < 0) {
            port = 0;
        }
        this.localAddress = InetSocketAddress.createUnresolved(host, port);
        return this;
    }

	/**
	 * get local address.
	 * 
	 * @return local address
	 */
	public InetSocketAddress getLocalAddress() {
		return localAddress;
	}

	public String getLocalAddressString() {
        return getLocalHost() + ":" + getLocalPort();
    }
    
	/**
	 * get local host name.
	 * 
	 * @return local host name
	 */
	public String getLocalHostName() {
		String host = localAddress == null ? null : localAddress.getHostName();
		if (host == null || host.length() == 0) {
		    return getLocalHost();
		}
		return host;
	}

    /**
     * set remote address.
     * 
     * @param address
     * @return context
     */
    public RpcContext setRemoteAddress(InetSocketAddress address) {
        this.remoteAddress = address;
        return this;
    }
    
    /**
     * set remote address.
     * 
     * @param host
     * @param port
     * @return context
     */
    public RpcContext setRemoteAddress(String host, int port) {
        if (port < 0) {
            port = 0;
        }
        this.remoteAddress = InetSocketAddress.createUnresolved(host, port);
        return this;
    }

	/**
	 * get remote address.
	 * 
	 * @return remote address
	 */
	public InetSocketAddress getRemoteAddress() {
		return remoteAddress;
	}
	
	/**
	 * get remote address string.
	 * 
	 * @return remote address string.
	 */
	public String getRemoteAddressString() {
	    return getRemoteHost() + ":" + getRemotePort();
	}
	
	/**
	 * get remote host name.
	 * 
	 * @return remote host name
	 */
	public String getRemoteHostName() {
		return remoteAddress == null ? null : remoteAddress.getHostName();
	}

    /**
     * get local host.
     * 
     * @return local host
     */
    public String getLocalHost() {
        String host = localAddress == null ? null : 
            localAddress.getAddress() == null ? localAddress.getHostName() 
                    : NetUtils.filterLocalHost(localAddress.getAddress().getHostAddress());
        if (host == null || host.length() == 0) {
            return NetUtils.getLocalHost();
        }
        return host;
    }

    /**
     * get local port.
     * 
     * @return port
     */
    public int getLocalPort() {
        return localAddress == null ? 0 : localAddress.getPort();
    }

    /**
     * get remote host.
     * 
     * @return remote host
     */
    public String getRemoteHost() {
        return remoteAddress == null ? null : 
            remoteAddress.getAddress() == null ? remoteAddress.getHostName() 
                    : NetUtils.filterLocalHost(remoteAddress.getAddress().getHostAddress());
    }

    /**
     * get remote port.
     * 
     * @return remote port
     */
    public int getRemotePort() {
        return remoteAddress == null ? 0 : remoteAddress.getPort();
    }

    /**
     * get attachment.
     * 
     * @param key
     * @return attachment
     */
    public String getAttachment(String key) {
        return attachments.get(key);
    }

    /**
     * set attachment.
     * 
     * @param key
     * @param value
     * @return context
     */
    public RpcContext setAttachment(String key, String value) {
        if (value == null) {
            attachments.remove(key);
        } else {
            attachments.put(key, value);
        }
        return this;
    }

    /**
     * remove attachment.
     * 
     * @param key
     * @return context
     */
    public RpcContext removeAttachment(String key) {
        attachments.remove(key);
        return this;
    }

    /**
     * get attachments.
     * 
     * @return attachments
     */
    public Map<String, String> getAttachments() {
        return attachments;
    }

    /**
     * set attachments
     * 
     * @param attachment
     * @return context
     */
    public RpcContext setAttachments(Map<String, String> attachment) {
        this.attachments.clear();
        if (attachment != null && attachment.size() > 0) {
            this.attachments.putAll(attachment);
        }
        return this;
    }
    
    public void clearAttachments() {
        this.attachments.clear();
    }

    /**
     * get values.
     * 
     * @return values
     */
    public Map<String, Object> get() {
        return values;
    }

    /**
     * set value.
     * 
     * @param key
     * @param value
     * @return context
     */
    public RpcContext set(String key, Object value) {
        if (value == null) {
            values.remove(key);
        } else {
            values.put(key, value);
        }
        return this;
    }

    /**
     * remove value.
     * 
     * @param key
     * @return value
     */
    public RpcContext remove(String key) {
        values.remove(key);
        return this;
    }

    /**
     * get value.
     * 
     * @param key
     * @return value
     */
    public Object get(String key) {
        return values.get(key);
    }

    public RpcContext setInvokers(List<Invoker<?>> invokers) {
        this.invokers = invokers;
        if (invokers != null && invokers.size() > 0) {
            List<URL> urls = new ArrayList<URL>(invokers.size());
            for (Invoker<?> invoker : invokers) {
                urls.add(invoker.getUrl());
            }
            setUrls(urls);
        }
        return this;
    }

    public RpcContext setInvoker(Invoker<?> invoker) {
        this.invoker = invoker;
        if (invoker != null) {
            setUrl(invoker.getUrl());
        }
        return this;
    }

    public RpcContext setInvocation(Invocation invocation) {
        this.invocation = invocation;
        if (invocation != null) {
            setMethodName(invocation.getMethodName());
            setParameterTypes(invocation.getParameterTypes());
            setArguments(invocation.getArguments());
        }
        return this;
    }

    /**
     * @deprecated Replace to isProviderSide()
     */
    @Deprecated
    public boolean isServerSide() {
        return isProviderSide();
    }
    
    /**
     * @deprecated Replace to isConsumerSide()
     */
    @Deprecated
    public boolean isClientSide() {
        return isConsumerSide();
    }
    
    /**
     * @deprecated Replace to getUrls()
     */
    @Deprecated
    @SuppressWarnings({ "unchecked", "rawtypes" })
    public List<Invoker<?>> getInvokers() {
        return invokers == null && invoker != null ? (List)Arrays.asList(invoker) : invokers;
    }

    /**
     * @deprecated Replace to getUrl()
     */
    @Deprecated
    public Invoker<?> getInvoker() {
        return invoker;
    }

    /**
     * @deprecated Replace to getMethodName(), getParameterTypes(), getArguments()
     */
    @Deprecated
    public Invocation getInvocation() {
        return invocation;
    }
    
    /**
     * 异步调用 ，需要返回值，即使步调用Future.get方法，也会处理调用超时问题.
     * @param callable
     * @return 通过future.get()获取返回结果.
     */
    @SuppressWarnings("unchecked")
	public <T> Future<T> asyncCall(Callable<T> callable) {
    	try {
	    	try {
	    		setAttachment(Constants.ASYNC_KEY, Boolean.TRUE.toString());
				final T o = callable.call();
				//local调用会直接返回结果.
				if (o != null) {
					FutureTask<T> f = new FutureTask<T>(new Callable<T>() {
						public T call() throws Exception {
							return o;
						}
					});
					f.run();
					return f;
				} else {
					
				}
			} catch (Exception e) {
				throw new RpcException(e);
			} finally {
				removeAttachment(Constants.ASYNC_KEY);
			}
    	} catch (final RpcException e) {
			return new Future<T>() {
				public boolean cancel(boolean mayInterruptIfRunning) {
					return false;
				}
				public boolean isCancelled() {
					return false;
				}
				public boolean isDone() {
					return true;
				}
				public T get() throws InterruptedException, ExecutionException {
					throw new ExecutionException(e.getCause());
				}
				public T get(long timeout, TimeUnit unit)
						throws InterruptedException, ExecutionException,
						TimeoutException {
					return get();
				}
			};
		}
    	return ((Future<T>)getContext().getFuture());
    }
    
	/**
	 * oneway调用，只发送请求，不接收返回结果.
	 * @param callable
	 */
	public void asyncCall(Runnable runable) {
    	try {
    		setAttachment(Constants.RETURN_KEY, Boolean.FALSE.toString());
    		runable.run();
		} catch (Throwable e) {
			//FIXME 异常是否应该放在future中？
			throw new RpcException("oneway call error ." + e.getMessage(), e);
		} finally {
			removeAttachment(Constants.RETURN_KEY);
		}
    }
}
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注：RpcContext里的attachments信息会填入到RpcInvocation对象中, 一起传递过去。

因此有人就建议可以简单的把traceId注入到RpcContext中，这样就可以简单的实现traceId的透传了，事实是否如此，先让我们来一起实践一下。

定义Dubbo接口类：

public interface IEchoService {
    String echo(String name);
}
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编写服务端代码（Provider）：

@Service("echoService")
public class EchoServiceImpl implements IEchoService {
 
    @Override
    public String echo(String name) {
        String traceId = RpcContext.getContext().getAttachment("traceId");
        System.out.println("name = " + name + ", traceId = " + traceId);
        return name;
    }
 
    public static void main(String[] args) {
        ClassPathXmlApplicationContext applicationContext =
                new ClassPathXmlApplicationContext("spring-dubbo-test-producer.xml");
 
        System.out.println("server start");
        while (true) {
            try {
                Thread.sleep(1000L);
            } catch (InterruptedException e) {
            }
        }
    } 
}
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编写客户端代码（Consumer）：

public class EchoServiceConsumer {
 
    public static void main(String[] args) {
        ClassPathXmlApplicationContext applicationContext =
                new ClassPathXmlApplicationContext("spring-dubbo-test-consumer.xml");
 
        IEchoService service = (IEchoService) applicationContext
                .getBean("echoService");
 
        // *) 设置traceId
        RpcContext.getContext().setAttachment("traceId", "100001");
        System.out.println(RpcContext.getContext().getAttachments());
        // *) 第一调用
        service.echo("lilei");
 
        // *) 第二次调用
        System.out.println(RpcContext.getContext().getAttachments());
        service.echo("hanmeimei");
    }
}
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执行的结果如下：

服务端输出:
name = lilei, traceId = 100001
name = hanmeimei, traceId = null
 
客户端输出:
{traceId=100001}
{}
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从服务端的输出信息中，我们可以惊喜的发现，traceId确实传递过去了，但是只有第一次有，第二次没有。而从客户端对RpcContext的内容输出，也印证了这个现象，同时产生这个现象的本质原因是 RpcContext对象的attachment在一次rpc交互后被清空了 。

给RpcContext的clearAttachments方法, 设置断点后复现. 我们可以找到如下调用堆栈：

java.lang.Thread.State: RUNNABLE
    at com.alibaba.dubbo.rpc.RpcContext.clearAttachments(RpcContext.java:438)
    at com.alibaba.dubbo.rpc.filter.ConsumerContextFilter.invoke(ConsumerContextFilter.java:50)
    at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
    at com.alibaba.dubbo.rpc.protocol.InvokerWrapper.invoke(InvokerWrapper.java:53)
    at com.alibaba.dubbo.rpc.cluster.support.FailoverClusterInvoker.doInvoke(FailoverClusterInvoker.java:77)
    at com.alibaba.dubbo.rpc.cluster.support.AbstractClusterInvoker.invoke(AbstractClusterInvoker.java:227)
    at com.alibaba.dubbo.rpc.cluster.support.wrapper.MockClusterInvoker.invoke(MockClusterInvoker.java:72)
    at com.alibaba.dubbo.rpc.proxy.InvokerInvocationHandler.invoke(InvokerInvocationHandler.java:52)
    at com.alibaba.dubbo.common.bytecode.proxy0.echo(proxy0.java:-1)
    at com.test.dubbo.EchoServiceConsumer.main(EchoServiceConsumer.java:20)
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其最直接的调用为Dubbo自带的ConsumerContextFilter，让我们来分析其代码：

@Activate(
    group = {"consumer"},
    order = -10000
)
public class ConsumerContextFilter implements Filter {
    public ConsumerContextFilter() {
    }
 
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        RpcContext.getContext().setInvoker(invoker).setInvocation(invocation)
                .setLocalAddress(NetUtils.getLocalHost(), 0)
                .setRemoteAddress(invoker.getUrl().getHost(), invoker.getUrl().getPort());
        if(invocation instanceof RpcInvocation) {
            ((RpcInvocation)invocation).setInvoker(invoker);
        }
 
        Result var3;
        try {
            var3 = invoker.invoke(invocation);
        } finally {
            RpcContext.getContext().clearAttachments();
        }
 
        return var3;
    }
}
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确实在finally代码片段中， 我们发现RpcContext在每次rpc调用后, 都会清空attachment对象 。

既然我们找到了本质原因，那么解决方法， 可以在每次调用的时候，重新设置下traceId ，比如像这样（看着感觉吃像相对难看了一点）：

// *) 第一调用
RpcContext.getContext().setAttachment("traceId", "100001");
service.echo("lilei");
 
// *) 第二次调用
RpcContext.getContext().setAttachment("traceId", "100001");
service.echo("hanmeimei");
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3 基于Filter实现

先引入一个工具类：

package com.test.dubbo;
 
public class TraceIdFilter implements Filter {
 
    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        String traceId = RpcContext.getContext().getAttachment("traceId");
        if ( !StringUtils.isEmpty(traceId) ) {
            // *) 从RpcContext里获取traceId并保存
            TraceIdUtils.setTraceId(traceId);
        } else {
            // *) 交互前重新设置traceId, 避免信息丢失
            RpcContext.getContext().setAttachment("traceId", TraceIdUtils.getTraceId());
        }
        // *) 实际的rpc调用
        return invoker.invoke(invocation);
    }
}
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然后我们定义一个Filter类：

package com.test.dubbo;
 
public class TraceIdFilter implements Filter {
 
    @Override
    public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
        String traceId = RpcContext.getContext().getAttachment("traceId");
        if ( !StringUtils.isEmpty(traceId) ) {
            // *) 从RpcContext里获取traceId并保存
            TraceIdUtils.setTraceId(traceId);
        } else {
            // *) 交互前重新设置traceId, 避免信息丢失
            RpcContext.getContext().setAttachment("traceId", TraceIdUtils.getTraceId());
        }
        // *) 实际的rpc调用
        return invoker.invoke(invocation);
    }
}
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在resource目录下, 添加META-INF/dubbo目录, 继而添加com.alibaba.dubbo.rpc.Filter文件：

编辑（com.alibaba.dubbo.rpc.Filter文件）内容如下：

traceIdFilter=com.test.dubbo.TraceIdFilter
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然后我们给dubbo的producer和consumer都配置对应的filter项：

服务端:
<dubbo:service interface="com.test.dubbo.IEchoService" ref="echoService" version="1.0.0"
        filter="traceIdFilter"/>

客户端:
<dubbo:reference interface="com.test.dubbo.IEchoService" id="echoService" version="1.0.0"
        filter="traceIdFilter"/>
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服务端的测试代码小改为如下：

@Service("echoService")
public class EchoServiceImpl implements IEchoService {
 
    @Override
    public String echo(String name) {
        String traceId = TraceIdUtils.getTraceId();
        System.out.println("name = " + name + ", traceId = " + traceId);
        return name;
    }
 
}
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客户端的测试代码片段为：

// *) 第一调用
RpcContext.getContext().setAttachment("traceId", "100001");
service.echo("lilei");
 
// *) 第二次调用
service.echo("hanmeimei");
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同样的代码, 测试结果如下：

服务端输出:
name = lilei, traceId = 100001
name = hanmeimei, traceId = 100001
 
客户端输出:
{traceId=100001}
{}
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符合预期，感觉这个方案就非常优雅了。 RpcContext的attachment依旧被清空(ConsumerContextFilter在自定义的Filter后执行) ，但是每次rpc交互前，traceId会被重新注入，保证跟踪线索透传成功。