[Bug] Service startup fails when a weakly dependent ZooKeeper is down

[jliuhust]

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Apache Dubbo Component

Java SDK (apache/dubbo)

Dubbo Version

Dubbo version: 3.3.6 (issue also exists in 3.2.5+)

Steps to reproduce this issue

We have a Dubbo service that subscribes to multiple ZooKeeper registries. One of them is a weakly dependent ZooKeeper (not critical for our service). We expect the service to start successfully even if this weakly dependent ZooKeeper is down.
However, in Dubbo 3.3.6, the service fails to start when the weakly dependent ZooKeeper is unavailable. We tested and found that:
Dubbo 3.2.0 ~ 3.2.4: ✅ Starts successfully (as expected)
Dubbo 3.2.5 and later: ❌ Startup fails

Steps to Reproduce:
Configure a Dubbo service with multiple ZooKeeper registries
Mark one ZooKeeper as weakly dependent (with check=false)
Shut down the weakly dependent ZooKeeper
Start the Dubbo service

What you expected to happen

The service should start successfully, ignoring the unavailable weakly dependent ZooKeeper.

Anything else

Configuration Example:
<dubbo:registry id="registry_zk" protocol="zookeeper" address="127.0.0.1:2181" check="false" use-as-metadata-center="false" use-as-config-center="false"/>
<dubbo:reference registry="registry_zk" id="someId" interface="com.SomeService" version="1.0.0" check="false"/>

Error Log:
java.lang.NullPointerException: Cannot invoke "org.apache.dubbo.registry.Registry.getUrl()" because "this.registry" is null
at org.apache.dubbo.registry.ListenerRegistryWrapper.getUrl(ListenerRegistryWrapper.java:44)
at org.apache.dubbo.registry.integration.RegistryDirectory.subscribe(RegistryDirectory.java:147)
at org.apache.dubbo.registry.integration.RegistryProtocol.doCreateInvoker(RegistryProtocol.java:667)
at org.apache.dubbo.registry.integration.InterfaceCompatibleRegistryProtocol.getInvoker(InterfaceCompatibleRegistryProtocol.java:58)
at org.apache.dubbo.registry.client.migration.MigrationInvoker.refreshInterfaceInvoker(MigrationInvoker.java:482)
at org.apache.dubbo.registry.client.migration.MigrationInvoker.migrateToApplicationFirstInvoker(MigrationInvoker.java:255)
at org.apache.dubbo.registry.client.migration.MigrationRuleHandler.refreshInvoker(MigrationRuleHandler.java:90)
at org.apache.dubbo.registry.client.migration.MigrationRuleHandler.doMigrate(MigrationRuleHandler.java:71)
at org.apache.dubbo.registry.client.migration.MigrationRuleListener.onRefer(MigrationRuleListener.java:285)
at org.apache.dubbo.registry.integration.RegistryProtocol.interceptInvoker(RegistryProtocol.java:630)
at org.apache.dubbo.registry.integration.RegistryProtocol.doRefer(RegistryProtocol.java:594)
at org.apache.dubbo.registry.integration.RegistryProtocol.refer(RegistryProtocol.java:575)
at org.apache.dubbo.rpc.protocol.ProtocolListenerWrapper.refer(ProtocolListenerWrapper.java:83)
at org.apache.dubbo.rpc.protocol.ProtocolSecurityWrapper.refer(ProtocolSecurityWrapper.java:112)
at org.apache.dubbo.qos.protocol.QosProtocolWrapper.refer(QosProtocolWrapper.java:85)
at org.apache.dubbo.rpc.cluster.filter.ProtocolFilterWrapper.refer(ProtocolFilterWrapper.java:69)
at org.apache.dubbo.rpc.protocol.ProtocolSerializationWrapper.refer(ProtocolSerializationWrapper.java:55)
at org.apache.dubbo.rpc.protocol.InvokerCountWrapper.refer(InvokerCountWrapper.java:48)
at org.apache.dubbo.rpc.Protocol$Adaptive.refer(Protocol$Adaptive.java)
at org.apache.dubbo.config.ReferenceConfig.createInvoker(ReferenceConfig.java:672)
at org.apache.dubbo.config.ReferenceConfig.createProxy(ReferenceConfig.java:502)
at org.apache.dubbo.config.ReferenceConfig.init(ReferenceConfig.java:383)
at org.apache.dubbo.config.ReferenceConfig.get(ReferenceConfig.java:244)
at org.apache.dubbo.config.ReferenceConfigBase.get(ReferenceConfigBase.java:399)
at org.apache.dubbo.config.utils.SimpleReferenceCache.destroyReference(SimpleReferenceCache.java:294)
at org.apache.dubbo.config.utils.SimpleReferenceCache.destroy(SimpleReferenceCache.java:248)
at org.apache.dubbo.config.utils.SimpleReferenceCache.destroy(SimpleReferenceCache.java:271)
at org.apache.dubbo.config.deploy.DefaultModuleDeployer.lambda$referServices$6(DefaultModuleDeployer.java:577)
at java.base/java.util.concurrent.ConcurrentHashMap$ValuesView.forEach(ConcurrentHashMap.java:4783)
at org.apache.dubbo.config.deploy.DefaultModuleDeployer.referServices(DefaultModuleDeployer.java:539)
at org.apache.dubbo.config.deploy.DefaultModuleDeployer.startSync(DefaultModuleDeployer.java:186)
at org.apache.dubbo.config.deploy.DefaultModuleDeployer.start(DefaultModuleDeployer.java:159)
at org.apache.dubbo.config.spring.context.DubboDeployApplicationListener.onContextRefreshedEvent(DubboDeployApplicationListener.java:167)
at org.apache.dubbo.config.spring.context.DubboDeployApplicationListener.onApplicationEvent(DubboDeployApplicationListener.java:153)
at org.apache.dubbo.config.spring.context.DubboDeployApplicationListener.onApplicationEvent(DubboDeployApplicationListener.java:52)
at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:178)
at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:171)
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:145)
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:430)
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:387)
at org.springframework.context.support.AbstractApplicationContext.finishRefresh(AbstractApplicationContext.java:952)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:594)
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:732)
at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:409)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:308)

Do you have a (mini) reproduction demo?

• Yes, I have a minimal reproduction demo to help resolve this issue more effectively!

Are you willing to submit a pull request to fix on your own?

• Yes I am willing to submit a pull request on my own!

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