Fix misleading NPE when request parameter is not Serializable (#16299)

* Fix misleading NPE when request parameter is not Serializable

When a Dubbo service is called with non-serializable parameters, the
invocation deserialization fails silently and `path` becomes null. This
null path is then passed to GroupServiceKeyCache which uses a
ConcurrentHashMap that does not accept null keys, resulting in a
confusing NullPointerException.

Added a null check for `path` in DubboProtocol.getInvoker() to throw
a clear RemotingException with a meaningful message pointing users to
the actual cause (non-serializable parameters).

Fixes #16293

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* Make null path error message generic to avoid misleading diagnosis

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: chensishang

dubbo-rpc/dubbo-rpc-dubbo/src/main/java/org/apache/dubbo/rpc/protocol/dubbo/DubboProtocol.java

@@ -290,6 +290,15 @@ Invoker<?> getInvoker(Channel channel, Invocation inv) throws RemotingException
         boolean isStubServiceInvoke;
         int port = channel.getLocalAddress().getPort();
         String path = (String) inv.getObjectAttachmentWithoutConvert(PATH_KEY);
+        if (path == null) {
+            throw new RemotingException(
+                    channel,
+                    "Service path is missing from the invocation, which indicates the invocation metadata is "
+                            + "missing or corrupted. Possible causes include a request decode failure "
+                            + "(e.g. parameter types that failed to deserialize), an incompatible protocol "
+                            + "version, or a custom codec/invocation implementation that does not set the path, "
+                            + "channel: " + channel.getRemoteAddress() + " --> " + channel.getLocalAddress());
+        }
 
         // if it's stub service on client side(after enable stubevent, usually is set up onconnect or ondisconnect
         // method)

dubbo-rpc/dubbo-rpc-dubbo/src/test/java/org/apache/dubbo/rpc/protocol/dubbo/DubboProtocolTest.java

@@ -19,6 +19,8 @@
 import org.apache.dubbo.common.URL;
 import org.apache.dubbo.common.extension.ExtensionLoader;
 import org.apache.dubbo.common.utils.NetUtils;
+import org.apache.dubbo.remoting.Channel;
+import org.apache.dubbo.remoting.RemotingException;
 import org.apache.dubbo.rpc.Invocation;
 import org.apache.dubbo.rpc.Invoker;
 import org.apache.dubbo.rpc.Protocol;
@@ -38,6 +40,7 @@
 import org.apache.dubbo.rpc.protocol.dubbo.support.Type;
 import org.apache.dubbo.rpc.service.EchoService;
 
+import java.net.InetSocketAddress;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.List;
@@ -281,6 +284,21 @@ public void testReturnNonSerialized() {
         }
     }
 
+    @Test
+    void testGetInvokerThrowsOnNullPath() {
+        DubboProtocol dubboProtocol = DubboProtocol.getDubboProtocol();
+        Channel channel = Mockito.mock(Channel.class);
+        InetSocketAddress localAddress = new InetSocketAddress("127.0.0.1", 20880);
+        InetSocketAddress remoteAddress = new InetSocketAddress("127.0.0.1", 12345);
+        Mockito.when(channel.getLocalAddress()).thenReturn(localAddress);
+        Mockito.when(channel.getRemoteAddress()).thenReturn(remoteAddress);
+
+        Invocation inv = Mockito.mock(Invocation.class);
+        Mockito.when(inv.getObjectAttachmentWithoutConvert("path")).thenReturn(null);
+
+        Assertions.assertThrows(RemotingException.class, () -> dubboProtocol.getInvoker(channel, inv));
+    }
+
     @Disabled
     @Test
     void testRemoteApplicationName() throws Exception {