Introduce an S3 object monitor.

service/src/test/java/org/whispersystems/textsecuregcm/util/S3ObjectMonitorTest.java

@@ -0,0 +1,49 @@
+package org.whispersystems.textsecuregcm.util;
+
+import com.amazonaws.services.s3.AmazonS3;
+import com.amazonaws.services.s3.model.ObjectMetadata;
+import com.amazonaws.services.s3.model.S3Object;
+import org.junit.jupiter.api.Test;
+
+import java.time.Duration;
+import java.util.UUID;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.function.Consumer;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+class S3ObjectMonitorTest {
+
+  @Test
+  void refresh() {
+    final AmazonS3 s3Client = mock(AmazonS3.class);
+    final ObjectMetadata metadata = mock(ObjectMetadata.class);
+    final S3Object s3Object = mock(S3Object.class);
+
+    final String bucket = "s3bucket";
+    final String objectKey = "greatest-smooth-jazz-hits-of-all-time.zip";
+
+    //noinspection unchecked
+    final Consumer<S3Object> listener = mock(Consumer.class);
+
+    final S3ObjectMonitor objectMonitor = new S3ObjectMonitor(
+        s3Client,
+        bucket,
+        objectKey, mock(ScheduledExecutorService.class),
+        Duration.ofMinutes(1),
+        listener);
+
+    when(metadata.getETag()).thenReturn(UUID.randomUUID().toString());
+    when(s3Object.getObjectMetadata()).thenReturn(metadata);
+    when(s3Client.getObjectMetadata(bucket, objectKey)).thenReturn(metadata);
+    when(s3Client.getObject(bucket, objectKey)).thenReturn(s3Object);
+
+    objectMonitor.refresh();
+    objectMonitor.refresh();
+
+    verify(listener).accept(s3Object);
+  }
+}