GH-44927: [Java][Flight] Add middleware for auth2 bearer tokens

java/flight/flight-core/src/main/java/org/apache/arrow/flight/auth2/ClientBearerMiddleware.java

@@ -0,0 +1,77 @@
+/*
+ * 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.arrow.flight.auth2;
+
+import java.util.Objects;
+import org.apache.arrow.flight.CallHeaders;
+import org.apache.arrow.flight.CallInfo;
+import org.apache.arrow.flight.CallStatus;
+import org.apache.arrow.flight.FlightClientMiddleware;
+
+/** Middleware for capturing and setting bearer tokens. */
+public class ClientBearerMiddleware implements FlightClientMiddleware {
+
+  private final Factory factory;
+
+  private ClientBearerMiddleware(Factory factory) {
+    this.factory = Objects.requireNonNull(factory);
+  }
+
+  @Override
+  public void onHeadersReceived(CallHeaders incomingHeaders) {
+    final String bearerValue =
+        AuthUtilities.getValueFromAuthHeader(incomingHeaders, Auth2Constants.BEARER_PREFIX);
+    if (bearerValue != null) {
+      factory.setBearerValue(bearerValue);
+    }
+  }
+
+  @Override
+  public void onBeforeSendingHeaders(CallHeaders outgoingHeaders) {
+    final String bearerValue = factory.getBearerValue();
+    if (bearerValue != null) {
+      outgoingHeaders.insert(
+          Auth2Constants.AUTHORIZATION_HEADER, Auth2Constants.BEARER_PREFIX + bearerValue);
+    }
+  }
+
+  @Override
+  public void onCallCompleted(CallStatus status) {}
+
+  public static class Factory implements FlightClientMiddleware.Factory {
+
+    private volatile String bearerValue;
+
+    public Factory() {}
+
+    @Override
+    public FlightClientMiddleware onCallStarted(CallInfo info) {
+      return new ClientBearerMiddleware(this);
+    }
+
+    String getBearerValue() {
+      return bearerValue;
+    }
+
+    void setBearerValue(String bearerValue) {
+      // Avoid volatile write if the value hasn't changed
+      if (!Objects.equals(this.bearerValue, bearerValue)) {
+        this.bearerValue = bearerValue;
+      }
+    }
+  }
+}