AryamannSingh7 opened a new pull request, #16356:
URL: https://github.com/apache/dubbo/pull/16356
## What is the purpose of the change?
Fixes #16178.
When a service subscribes to multiple registries and one of them is **weakly
dependent** (`check="false"`) and **unavailable at startup**, the application
fails to start with:
```
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:...)
```
This is a regression: Dubbo `3.2.0`–`3.2.4` start successfully, `3.2.5`+
fail (also reproduced on `3.3.x`).
### Root cause
Under `check=false`, when a registry cannot be created (the registry is
down), `AbstractRegistryFactory#getRegistry` swallows the exception and returns
`null`. `RegistryFactoryWrapper` still wraps this `null` into a
`ListenerRegistryWrapper`, so a null delegate is an **expected state** —
`ListenerRegistryWrapper` already guards `register` / `unregister` /
`subscribe` with `if (registry != null)`.
However:
- `getUrl` / `isAvailable` / `destroy` / `unsubscribe` /
`isServiceDiscovery` / `lookup` were **not** guarded.
- `RegistryDirectory#subscribe` computes a metrics cluster name via
`registry.getUrl().getParameter(...)` unconditionally. That line was introduced
in #12582 (released in `3.2.5`), which is exactly why the NPE appears from
`3.2.5` onward.
### What changed
1. **`ListenerRegistryWrapper`** — complete the existing null-safety:
`getUrl` returns `null`, `isAvailable` / `isServiceDiscovery` return `false`,
`destroy` / `unsubscribe` become no-ops, and `lookup` returns an empty list
when the delegate registry is `null`.
2. **`RegistryDirectory#subscribe`** — guard the metrics cluster-name
computation against a `null` registry URL (no cluster name is reported when the
registry is unavailable). `RegistryEvent#toSubscribeEvent` already tolerates a
`null` name.
### Verification
Added `ListenerRegistryWrapperTest#testNullRegistryIsTolerated`, which
reproduces the exact NPE **without** the fix and passes **with** it.
`:dubbo-registry-api` tests and `spotless:check` pass locally (JDK 21).
## Checklist
- [x] Make sure there is a [GitHub
issue](https://github.com/apache/dubbo/issues/16178) field for the change.
- [x] Write a pull request description that is detailed enough to understand
what the pull request does, how, and why.
- [x] Write necessary unit-test to verify your logic correction.
- [x] Make sure GitHub actions can pass.
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