项目使用dubbo 、zookeeper服务报 Could not initialize class org.I0Itec.zkclient.ZkClient

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在使用Dubbo和Zookeeper作为服务注册中心的项目中，启动时遇到Caused by: java.lang.NoClassDefFoundError，原因是ZkClient类初始化失败。问题排查发现可能是由于缺少相关jar包或者日志框架冲突导致。检查pom.xml文件，确实引入了ZkClient的依赖，但同时发现排除了log4j，推测Dubbo内部依赖log4j。移除对log4j的排除后，重新启动项目，问题解决。

项目中使用了dubbo 服务，用zookeeper 做注册中心，用的logback 作为日志框架，在启动项目时候报错如下：

Caused by: java.lang.NoClassDefFoundError: Could not initialize class org.I0Itec.zkclient.ZkClient
    at com.alibaba.dubbo.remoting.zookeeper.zkclient.ZkclientZookeeperClient.<init>(ZkclientZookeeperClient.java:25) ~[dubbo-2.5.3.jar:2.5.3]
    at com.alibaba.dubbo.remoting.zookeeper.zkclient.ZkclientZookeeperTransporter.connect(ZkclientZookeeperTransporter.java:10) ~[dubbo-2.5.3.jar:2.5.3]
    at com.alibaba.dubbo.remoting.zookeeper.ZookeeperTransporter$Adpative.connect(ZookeeperTransporter$Adpative.java) ~[na:2.5.3]
    at com.alibaba.dubbo.registry.zookeeper.ZookeeperRegistry.<init>(ZookeeperRegistry.java:71) ~[dubbo-2.5.3.jar:2.5.3]

看第一句 Caused by: java.lang.NoClassDefFoundError: Could not initialize class org.I0Itec.zkclient.ZkClient
这句说是不能找到初始化ZKClient 类，查了网上资源，说是缺少 jar包，看项目pom，确实引入了该jar

        <dependency>
            <groupId>com.101tec</groupId>
            <artifactId>zkclient</artifactId>
            <version>0.3</version>
        </dependency>

再网上翻启动日志, 看到一行

java.lang.NoClassDefFoundError: org/apache/log4j/Logger
    at org.I0Itec.zkclient.ZkClient.<clinit>(ZkClient.java:57) ~[zkclient-0.3.jar:0.3]
    at com.alibaba.dubbo.remoting.zookeeper.zkclient.ZkclientZookeeperClient.<init>(ZkclientZookeeperClient.java:25) ~[dubbo-2.5.3.jar:2.5.3]
    at com.alibaba.dubbo.remoting.zookeeper.zkclient.ZkclientZookeeperTransporter.connect(ZkclientZookeeperTransporter.java:10) ~[dubbo-2.5.3.jar:2.5.3]
    at com.alibaba.dubbo.remoting.zookeeper.ZookeeperTransporter$Adpative.connect(ZookeeperTransporter$Adpative.java) ~[na:2.5.3]

说是缺少log4j 包，这么说dubbo内部使用的log4j, 而我在我最后pom里加入了排除log4j

<exclusions>
                <exclusion>
                    <groupId>org.slf4j</groupId>
                    <artifactId>slf4j-log4j12</artifactId>
                </exclusion>
                <exclusion>
                    <groupId>log4j</groupId>
                    <artifactId>log4j</artifactId>
                </exclusion>
            </exclusions>