Changed the exception's message. Added unit test.

Signed-off-by: Dmitry Kryukov <[email protected]>

libs/core/src/main/java/org/opensearch/core/common/io/stream/Writeable.java

@@ -83,7 +83,7 @@ public static <R extends Reader<?>> void registerReader(final byte ordinal, fina
 
         public static void registerClassAlias(final Class<?> classInstance, final Class<?> classGeneric) {
             if (WRITER_CUSTOM_CLASS_MAP.putIfAbsent(classInstance, classGeneric) != null) {
-                throw new IllegalArgumentException("Streamable custom class already registered [" + classInstance + "]");
+                throw new IllegalArgumentException("Streamable custom class already registered [" + classInstance.getName() + "]");
             }
         }
 
@@ -96,7 +96,7 @@ public static <W extends Writer<?>> W getWriter(final Class<?> clazz) {
         }
 
         /**
-         * Returns the ristered reader keyed by the unique ordinal
+         * Returns the registered reader keyed by the unique ordinal
          */
         @SuppressWarnings("unchecked")
         public static <R extends Reader<?>> R getReader(final byte b) {

libs/core/src/test/java/org/opensearch/core/common/io/stream/WriteableTests.java

@@ -0,0 +1,19 @@
+package org.opensearch.core.common.io.stream;
+
+import org.junit.Assert;
+import org.opensearch.test.OpenSearchTestCase;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class WriteableTests extends OpenSearchTestCase {
+
+    public void testRegisterClassAlias() {
+        Writeable.WriteableRegistry.registerClassAlias(StringBuilder.class, AtomicInteger.class);
+        try {
+            Writeable.WriteableRegistry.registerClassAlias(StringBuilder.class, AtomicInteger.class);
+            Assert.fail("expected exception not thrown");
+        } catch (IllegalArgumentException illegalArgumentException) {
+            Assert.assertEquals("Streamable custom class already registered [java.lang.StringBuilder]", illegalArgumentException.getMessage());
+        }
+    }
+}