Dubbo Provider Filter链是怎么构建的

从调用栈中方法 org.apache.dubbo.rpc.protocol.ProtocolFilterWrapper#export 代码来看，buildInvokerChain会构建provider的Filter链：

public  Exporter export(Invoker invoker) throws RpcException {
    if (UrlUtils.isRegistry(invoker.getUrl())) {
        return protocol.export(invoker);
    }
    // invoker就是直接调用对应方法的代码，JavassistProxyFactory
    return protocol.export(buildInvokerChain(invoker, SERVICE_FILTER_KEY, CommonConstants.PROVIDER));
}

从上述代码来看，buildInvokerChain就是构建Filter链的逻辑，通过SPI方式进行加载，FIlter配置路径为META-INF/dubbo/internal/org.apache.dubbo.rpc.Filter，配置内容如下：

echo=org.apache.dubbo.rpc.filter.EchoFilter
generic=org.apache.dubbo.rpc.filter.GenericFilter
genericimpl=org.apache.dubbo.rpc.filter.GenericImplFilter
token=org.apache.dubbo.rpc.filter.TokenFilter
accesslog=org.apache.dubbo.rpc.filter.AccessLogFilter
activelimit=org.apache.dubbo.rpc.filter.ActiveLimitFilter
classloader=org.apache.dubbo.rpc.filter.ClassLoaderFilter
context=org.apache.dubbo.rpc.filter.ContextFilter
consumercontext=org.apache.dubbo.rpc.filter.ConsumerContextFilter
exception=org.apache.dubbo.rpc.filter.ExceptionFilter
executelimit=org.apache.dubbo.rpc.filter.ExecuteLimitFilter
deprecated=org.apache.dubbo.rpc.filter.DeprecatedFilter
compatible=org.apache.dubbo.rpc.filter.CompatibleFilter
timeout=org.apache.dubbo.rpc.filter.TimeoutFilter

注意，上述配置的Filter是包括Consumer和Provider的，初始化provider只会用到provider侧的Filter配置，具体Filter实现类会通过group来区分PROVIDER或者CONSUMER，对应的buildInvokerChain逻辑如下：

private static  Invoker buildInvokerChain(final Invoker invoker, String key, String group) {
    Invoker last = invoker;
    List filters = ExtensionLoader.getExtensionLoader(Filter.class).getActivateExtension(invoker.getUrl(), key, group);

    if (!filters.isEmpty()) {
        for (int i = filters.size() - 1; i >= 0; i--) {
            final Filter filter = filters.get(i);
            final Invoker next = last;
            last = new Invoker() {
                @Override
                public Result invoke(Invocation invocation) throws RpcException {
                    return filter.invoke(next, invocation);
                }
            };
        }
    }
    return last;
}

buildInvokerChain逻辑会按照顺序进行构建Filter过滤链，由于FIlter实现类未设置@Activate注解的顺序，因此Filter过滤链的前后顺序就是在META-INF/dubbo/internal/org.apache.dubbo.rpc.Filter文件配置的顺序，这点要注意。