聊聊Dubbo（七）：自定義Filter實踐

摘要： 在現行微服務的趨勢下，一次呼叫的過程中涉及多個服務節點，產生的日誌分佈在不同的伺服器上，雖說可以使用ELK技術將分散的日誌，彙總到es中，但是如何將這些日誌貫穿起來，則是一個關鍵問題。 如果需要檢視一次呼叫的全鏈路日誌，則一般的做法是通過在系統邊界中產生一個traceId，向呼叫...

在現行微服務的趨勢下，一次呼叫的過程中涉及多個服務節點，產生的日誌分佈在不同的伺服器上，雖說可以使用ELK技術將分散的日誌，彙總到es中，但是如何將這些日誌貫穿起來，則是一個關鍵問題。

如果需要檢視一次呼叫的全鏈路日誌，則一般的做法是通過在系統邊界中產生一個 traceId ，向呼叫鏈的後續服務傳遞 traceId ，後續服務繼續使用 traceId 列印日誌，並再向其他後續服務傳遞 traceId ，此過程簡稱， traceId透傳 。

在使用HTTP協議作為服務協議的系統裡，可以統一使用一個封裝好的http client做traceId透傳。但是dubbo實現traceId透傳就稍微複雜些了。根據上節講的 ofollow,noindex">《☆聊聊Dubbo（六）：核心原始碼-Filter鏈原理》 ，一般情況下， 會自定義Filter來實現traceId透傳 ，但還有兩種比較特殊的實現方式： （1）重新實現dubbo內部的相關類；（2）基於RpcContext實現；

1 基於重寫實現

1.1 原始碼分析

Proxy是 Dubbo 使用javassist為consumer 端service生成的動態代理instance。

Implement是provider端的service實現instance。

traceId透傳，即要求Proxy 和 Implement具有相同的traceId。Dubbo具有良好的分層特徵， transport的物件是RPCInvocation 。

所以，重寫的重點邏輯實現，就是Proxy將traceId放入RPCInvocation，交由Client進行序列化和TCP傳輸，Server反序列化得到RPCInvocation，取出traceId，交由Implement即可。

下面為consumer端 JavassistProxyFactory 的程式碼分析：

public class JavassistProxyFactory extends AbstractProxyFactory {

/**
* Spring容器啟動時，該代理工廠類方法會為Consumer生成Service代理類
* invoker和interfaces都是從Spring配置檔案中讀取出來 
*/
@SuppressWarnings("unchecked")
public <T> T getProxy(Invoker<T> invoker, Class<?>[] interfaces) {
// 生成Service代理類的每個方法的位元組碼，都呼叫了InvokerInvocationHandler.invoke(...)方法，
// 做實際RpcInvocation包裝、序列化、TCP傳輸、反序列化結果
return (T) Proxy.getProxy(interfaces).newInstance(new InvokerInvocationHandler(invoker));
}

public <T> Invoker<T> getInvoker(T proxy, Class<T> type, URL url) {
// TODO Wrapper類不能正確處理帶$的類名
final Wrapper wrapper = Wrapper.getWrapper(proxy.getClass().getName().indexOf('$') < 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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下面為consumer端 InvokerInvocationHandler 的程式碼分析：

public class InvokerInvocationHandler implements InvocationHandler {

private final Invoker<?> invoker;

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

/**
* 真正呼叫RPC時，各個Service代理的位元組碼裡呼叫了這個通用的invoke
* proxy就是之前生成的代理物件，第二個引數是方法名，第三個引數是引數列表
* 知道了（1）哪個介面（2）哪個方法（3）引數是什麼，就完全可以對映到Provider端實現並獲取返回值
*/
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]);
}
// 因為到這裡，還是consumer端的業務執行緒，所以在這裡取ThreadLocal裡的traceId，
// 再放入RpcInvocation的attachment，那麼Provider就可以從收到的RpcInvocation例項取出透傳的traceId
return invoker.invoke(new RpcInvocation(method, args)).recreate();
}
}
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下面為Provider端 DubboProtocol 的程式碼分析：

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 需要處理高版本呼叫低版本的問題
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收到報文之後，從執行緒池中取出一個執行緒，反序列化出RpcInvocation、並呼叫實現類的對應方法
// 所以，此處就是Provider端的實現類的執行緒，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 具體實現

package com.alibaba.dubbo.rpc.proxy;

/**
 * traceId工具類這個類是新新增的
 */
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 這個類 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]);
}
// 這裡將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 重新實現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 需要處理高版本呼叫低版本的問題
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());
// 這裡將收到的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 基於RpcContext實現

在具體講解自定義filter來實現透傳traceId的方案前，我們先來研究下RpcContext物件。 其RpcContext本質上是個ThreadLocal物件，其維護了一次rpc互動的上下文資訊 。

/*
 * 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是一個臨時狀態記錄器，當接收到RPC請求，或發起RPC請求時，RpcContext的狀態都會變化。
 * 比如：A調B，B再調C，則B機器上，在B調C之前，RpcContext記錄的是A調B的資訊，在B調C之後，RpcContext記錄的是B調C的資訊。
 * 
 * @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;
}

/**
* 非同步呼叫 ，需要返回值，即使步呼叫Future.get方法，也會處理呼叫超時問題.
* @param callable
* @return 通過future.get()獲取返回結果.
*/
@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呼叫會直接返回結果.
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呼叫，只發送請求，不接收返回結果.
 * @param callable
 */
public void asyncCall(Runnable runable) {
try {
setAttachment(Constants.RETURN_KEY, Boolean.FALSE.toString());
runable.run();
} catch (Throwable e) {
//FIXME 異常是否應該放在future中？
throw new RpcException("oneway call error ." + e.getMessage(), e);
} finally {
removeAttachment(Constants.RETURN_KEY);
}
}
}
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注：RpcContext裡的attachments資訊會填入到RpcInvocation物件中, 一起傳遞過去。

因此有人就建議可以簡單的把traceId注入到RpcContext中，這樣就可以簡單的實現traceId的透傳了，事實是否如此，先讓我們來一起實踐一下。

定義Dubbo介面類：

public interface IEchoService {
String echo(String name);
}
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編寫服務端程式碼（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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編寫客戶端程式碼（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");
 
// *) 設定traceId
RpcContext.getContext().setAttachment("traceId", "100001");
System.out.println(RpcContext.getContext().getAttachments());
// *) 第一呼叫
service.echo("lilei");
 
// *) 第二次呼叫
System.out.println(RpcContext.getContext().getAttachments());
service.echo("hanmeimei");
}
}
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執行的結果如下：

服務端輸出:
name = lilei, traceId = 100001
name = hanmeimei, traceId = null
 
客戶端輸出:
{traceId=100001}
{}
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從服務端的輸出資訊中，我們可以驚喜的發現，traceId確實傳遞過去了，但是隻有第一次有，第二次沒有。而從客戶端對RpcContext的內容輸出，也印證了這個現象，同時產生這個現象的本質原因是 RpcContext物件的attachment在一次rpc互動後被清空了 。

給RpcContext的clearAttachments方法, 設定斷點後復現. 我們可以找到如下呼叫堆疊：

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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其最直接的呼叫為Dubbo自帶的ConsumerContextFilter，讓我們來分析其程式碼：

@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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確實在finally程式碼片段中， 我們發現RpcContext在每次rpc呼叫後, 都會清空attachment物件 。

既然我們找到了本質原因，那麼解決方法， 可以在每次呼叫的時候，重新設定下traceId ，比如像這樣（看著感覺吃像相對難看了一點）：

// *) 第一呼叫
RpcContext.getContext().setAttachment("traceId", "100001");
service.echo("lilei");
 
// *) 第二次呼叫
RpcContext.getContext().setAttachment("traceId", "100001");
service.echo("hanmeimei");
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3 基於Filter實現

先引入一個工具類：

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) ) {
// *) 從RpcContext裡獲取traceId並儲存
TraceIdUtils.setTraceId(traceId);
} else {
// *) 互動前重新設定traceId, 避免資訊丟失
RpcContext.getContext().setAttachment("traceId", TraceIdUtils.getTraceId());
}
// *) 實際的rpc呼叫
return invoker.invoke(invocation);
}
}
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然後我們定義一個Filter類：

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) ) {
// *) 從RpcContext裡獲取traceId並儲存
TraceIdUtils.setTraceId(traceId);
} else {
// *) 互動前重新設定traceId, 避免資訊丟失
RpcContext.getContext().setAttachment("traceId", TraceIdUtils.getTraceId());
}
// *) 實際的rpc呼叫
return invoker.invoke(invocation);
}
}
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在resource目錄下, 新增META-INF/dubbo目錄, 繼而新增com.alibaba.dubbo.rpc.Filter檔案：

編輯（com.alibaba.dubbo.rpc.Filter檔案）內容如下：

traceIdFilter=com.test.dubbo.TraceIdFilter
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然後我們給dubbo的producer和consumer都配置對應的filter項：

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

客戶端:
<dubbo:reference interface="com.test.dubbo.IEchoService" id="echoService" version="1.0.0"
filter="traceIdFilter"/>
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服務端的測試程式碼小改為如下：

@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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客戶端的測試程式碼片段為：

// *) 第一呼叫
RpcContext.getContext().setAttachment("traceId", "100001");
service.echo("lilei");
 
// *) 第二次呼叫
service.echo("hanmeimei");
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同樣的程式碼, 測試結果如下：

服務端輸出:
name = lilei, traceId = 100001
name = hanmeimei, traceId = 100001
 
客戶端輸出:
{traceId=100001}
{}
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符合預期，感覺這個方案就非常優雅了。 RpcContext的attachment依舊被清空(ConsumerContextFilter在自定義的Filter後執行) ，但是每次rpc互動前，traceId會被重新注入，保證跟蹤線索透傳成功。