[Bug] Fix RpcContext compatibility between Object and String attachments

dubbo-rpc/dubbo-rpc-api/src/main/java/org/apache/dubbo/rpc/RpcContext.java

@@ -19,7 +19,6 @@
 import org.apache.dubbo.common.Experimental;
 import org.apache.dubbo.common.URL;
 import org.apache.dubbo.common.threadlocal.InternalThreadLocal;
-import org.apache.dubbo.common.utils.StringUtils;
 
 import java.net.InetSocketAddress;
 import java.util.HashMap;
@@ -533,11 +532,16 @@ public int getRemotePort() {
      * @return attachment
      */
     public String getAttachment(String key) {
-        String client = CLIENT_ATTACHMENT.get().getAttachment(key);
-        if (StringUtils.isEmpty(client)) {
-            return SERVER_ATTACHMENT.get().getAttachment(key);
+        Object value = CLIENT_ATTACHMENT.get().getObjectAttachment(key);
+        if (value == null) {
+            value = SERVER_ATTACHMENT.get().getObjectAttachment(key);
         }
-        return client;
+
+        if (value != null && !(value instanceof String)) {
+            return String.valueOf(value);
+        }
+
+        return (String) value;
     }
 
     /**
@@ -574,6 +578,7 @@ public RpcContext setAttachment(String key, Object value) {
     public RpcContext setObjectAttachment(String key, Object value) {
         // TODO compatible with previous
         CLIENT_ATTACHMENT.get().setObjectAttachment(key, value);
+
         return this;
     }
 

dubbo-rpc/dubbo-rpc-api/src/test/java/org/apache/dubbo/rpc/RpcContextCompatibilityTest.java

@@ -0,0 +1,52 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.dubbo.rpc;
+
+import java.util.Map;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class RpcContextCompatibilityTest {
+
+    @Test
+    void testAttachmentCompatibility() {
+        RpcContext context = RpcContext.getServiceContext();
+
+        String key = "test_long_key";
+        Long value = 12345L;
+        context.setObjectAttachment(key, value);
+
+        Assertions.assertEquals(value, context.getObjectAttachment(key));
+
+        Object legacyValue = context.getAttachment(key);
+        Assertions.assertEquals("12345", legacyValue, "Legacy API should return String representation");
+
+        Map<String, String> allAttachments = context.getAttachments();
+
+        Assertions.assertDoesNotThrow(
+                () -> {
+                    for (Map.Entry<String, String> entry : allAttachments.entrySet()) {
+                        String k = entry.getKey();
+
+                        String v = entry.getValue();
+                        Assertions.assertNotNull(v);
+                    }
+                },
+                "Iterating legacy attachments should not throw ClassCastException");
+    }
+}