Add duplicate class check

java-client-serverless/src/main/java/co/elastic/clients/elasticsearch/ElasticsearchClient.java

@@ -112,7 +112,9 @@
 import co.elastic.clients.transport.endpoints.BinaryResponse;
 import co.elastic.clients.transport.endpoints.BooleanResponse;
 import co.elastic.clients.transport.endpoints.EndpointWithResponseMapperAttr;
+import co.elastic.clients.util.DuplicateResourceFinder;
 import co.elastic.clients.util.ObjectBuilder;
+
 import java.io.IOException;
 import java.lang.reflect.Type;
 import java.util.function.Function;
@@ -123,6 +125,12 @@
  */
 public class ElasticsearchClient extends ApiClient<ElasticsearchTransport, ElasticsearchClient> {
 
+	static {
+		// Make sure we don't have several versions of this class. This may happen if
+		// several flavors of the client (Stack and Serverless) are present in the classpath.
+		DuplicateResourceFinder.ensureClassUniqueness(ElasticsearchClient.class);
+	}
+
 	public ElasticsearchClient(ElasticsearchTransport transport) {
 		super(transport, null);
 	}

java-client/src/main/java/co/elastic/clients/util/DuplicateResourceFinder.java

@@ -0,0 +1,80 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. 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 co.elastic.clients.util;
+
+import java.io.IOException;
+import java.net.URL;
+import java.util.ArrayList;
+import java.util.Enumeration;
+import java.util.List;
+
+public class DuplicateResourceFinder {
+
+    private static volatile boolean ENABLED = true;
+
+    /**
+     * Disables the resource uniqueness checks. Use with caution, as it will mask problems that may hit later.
+     */
+    public static void enableCheck(boolean enabled) {
+        ENABLED = enabled;
+    }
+
+    /**
+     * Ensure a class is only defined once in this class' classpath
+     */
+    public static void ensureClassUniqueness(Class<?> clazz) {
+        String name = clazz.getName();
+        String resource = clazz.getName().replace('.', '/') + ".class";
+        ensureResourceUniqueness(resource, name, DuplicateResourceFinder.class.getClassLoader());
+    }
+
+    public static void ensureResourceUniqueness(String path) {
+        ensureResourceUniqueness(path, path, DuplicateResourceFinder.class.getClassLoader());
+    }
+
+    private static void ensureResourceUniqueness(String path, String name, ClassLoader classLoader) {
+        if (!ENABLED) {
+            return;
+        }
+
+        // With Java9 modules, will work only with exported classes/resources. This is actually
+        // what we want, as non-exported classes/resources will not conflict.
+        List<URL> list = new ArrayList<>();
+        try {
+            Enumeration<URL> resources = classLoader.getResources(path);
+            while (resources.hasMoreElements()) {
+                list.add(resources.nextElement());
+            }
+        } catch (IOException ioe) {
+            // Ignore
+        }
+
+        if (list.size() > 1) {
+            StringBuilder sb = new StringBuilder("Several versions of ")
+                .append(name)
+                .append(" were found. This can cause conflicts, please fix the classpath:\n");
+            for (URL url: list) {
+                sb.append("    ").append(url.toString()).append("\n");
+            }
+            sb.append("    See the Java API client's troubleshooting documentation for more information.\n");
+            throw new RuntimeException(sb.toString());
+        }
+    }
+}

java-client/src/test/java/co/elastic/clients/util/DuplicateResourceFinderTest.java

@@ -0,0 +1,40 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. 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 co.elastic.clients.util;
+
+import com.fasterxml.jackson.databind.ext.CoreXMLSerializers;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+class DuplicateResourceFinderTest extends Assertions {
+
+    @Test
+    public void testDuplicateCheck() {
+
+        Exception e = assertThrows(RuntimeException.class, () -> {
+            DuplicateResourceFinder.ensureClassUniqueness(CoreXMLSerializers.class);
+        });
+        assertTrue(e.getMessage().contains("Several versions of"));
+
+        // Disabling the test should not throw an exception
+        DuplicateResourceFinder.enableCheck(false);
+        DuplicateResourceFinder.ensureClassUniqueness(CoreXMLSerializers.class);
+    }
+}

java-client/src/test/java/com/fasterxml/jackson/databind/ext/CoreXMLSerializers.java

@@ -0,0 +1,25 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. 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 com.fasterxml.jackson.databind.ext;
+
+// A "duplicate" of a class that exists in the project's dependencies.
+// It's not actually used by the project, so the duplicate won't harm and is used in DuplicateResourceFinderTest
+public class CoreXMLSerializers {
+}