prabhashkr commented on code in PR #21483:
URL: https://github.com/apache/kafka/pull/21483#discussion_r2888212620


##########
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/internals/secured/assertion/AssertionSupplierFactory.java:
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@@ -0,0 +1,133 @@
+/*
+ * 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.kafka.common.security.oauthbearer.internals.secured.assertion;
+
+import org.apache.kafka.common.security.oauthbearer.JwtRetrieverException;
+import 
org.apache.kafka.common.security.oauthbearer.internals.secured.ConfigurationUtils;
+import org.apache.kafka.common.utils.Time;
+import org.apache.kafka.common.utils.Utils;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.Optional;
+
+import static 
org.apache.kafka.common.config.SaslConfigs.SASL_OAUTHBEARER_ASSERTION_ALGORITHM;
+import static 
org.apache.kafka.common.config.SaslConfigs.SASL_OAUTHBEARER_ASSERTION_FILE;
+import static 
org.apache.kafka.common.config.SaslConfigs.SASL_OAUTHBEARER_ASSERTION_PRIVATE_KEY_FILE;
+import static 
org.apache.kafka.common.config.SaslConfigs.SASL_OAUTHBEARER_ASSERTION_PRIVATE_KEY_PASSPHRASE;
+import static 
org.apache.kafka.common.security.oauthbearer.internals.secured.assertion.AssertionUtils.layeredAssertionJwtTemplate;
+
+/**
+ * Factory class for creating JWT assertion suppliers used in OAuth 2.0 client 
authentication.
+ *
+ * <p>
+ * This factory supports two methods of obtaining JWT assertions for client 
authentication:
+ * <ul>
+ *   <li><b>File-based assertions:</b> Pre-generated JWT assertions read from 
a file specified by
+ *       {@link 
org.apache.kafka.common.config.SaslConfigs#SASL_OAUTHBEARER_ASSERTION_FILE}.
+ *       This is useful for testing or when assertions are managed 
externally.</li>
+ *   <li><b>Locally-generated assertions:</b> JWTs dynamically created and 
signed using a private key
+ *       specified by {@link 
org.apache.kafka.common.config.SaslConfigs#SASL_OAUTHBEARER_ASSERTION_PRIVATE_KEY_FILE}.
+ *       This is the recommended approach for production use.</li>
+ * </ul>
+ * </p>
+ *
+ * <p>
+ * The created supplier can be invoked repeatedly to obtain assertions as 
needed (for example, when
+ * refreshing tokens). For file-based assertions, the assertion is cached and 
reloaded automatically when the file changes on disk.
+ * For locally-generated assertions, a new assertion with updated timestamps 
is created on each invocation.
+ * </p>
+ */
+public class AssertionSupplierFactory {
+    private static final Logger LOG = 
LoggerFactory.getLogger(AssertionSupplierFactory.class);
+
+    /**
+     * Creates a closeable supplier that provides JWT assertions based on the 
provided configuration.
+     *
+     * <p>
+     * If {@link 
org.apache.kafka.common.config.SaslConfigs#SASL_OAUTHBEARER_ASSERTION_FILE} is 
configured,
+     * the supplier will read assertions from that file. Otherwise, it will 
create assertions dynamically
+     * using the configured private key and algorithm.
+     * </p>
+     *
+     * <p>
+     * <b>Important:</b> The returned {@link CloseableSupplier} must be closed 
when no longer needed
+     * to properly release resources such as file handles and cryptographic 
resources.

Review Comment:
   Simplified the Javadoc.



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