[RocksJava] Snapshot - GetSequenceNumber

Summary:
As the C++ part exposes now SequenceNumber retrieval
for Snapshots we want this obviously also in the Java API.

Test Plan:
make rocksdbjava
make jtest
mvn -f rocksjni.pom test

Reviewers: yhchiang, adamretter, ankgup87

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D32571

java/Makefile

@@ -32,6 +32,7 @@ NATIVE_JAVA_CLASSES = org.rocksdb.AbstractComparator\
 	org.rocksdb.TransactionLogIterator\
 	org.rocksdb.TtlDB\
 	org.rocksdb.VectorMemTableConfig\
+	org.rocksdb.Snapshot\
 	org.rocksdb.StringAppendOperator\
 	org.rocksdb.WriteBatch\
 	org.rocksdb.WriteBatch.Handler\

java/org/rocksdb/Snapshot.java

@@ -14,6 +14,17 @@ public class Snapshot extends RocksObject {
     nativeHandle_ = nativeHandle;
   }
 
+  /**
+   * Return the associated sequence number;
+   *
+   * @return the associated sequence number of
+   *     this snapshot.
+   */
+  public long getSequenceNumber() {
+    assert(isInitialized());
+    return getSequenceNumber(nativeHandle_);
+  }
+
   /**
    * Dont release C++ Snapshot pointer. The pointer
    * to the snapshot is released by the database
@@ -21,4 +32,6 @@ public class Snapshot extends RocksObject {
    */
   @Override protected void disposeInternal() {
   }
+
+  private native long getSequenceNumber(long handle);
 }

java/org/rocksdb/test/SnapshotTest.java

@@ -35,6 +35,8 @@ public class SnapshotTest {
       db.put("key".getBytes(), "value".getBytes());
       // Get new Snapshot of database
       Snapshot snapshot = db.getSnapshot();
+      assertThat(snapshot.getSequenceNumber()).isGreaterThan(0);
+      assertThat(snapshot.getSequenceNumber()).isEqualTo(1);
       readOptions = new ReadOptions();
       // set snapshot in ReadOptions
       readOptions.setSnapshot(snapshot);

java/rocksjni/rocksjni.cc

@@ -6,18 +6,18 @@
 // This file implements the "bridge" between Java and C++ and enables
 // calling c++ rocksdb::DB methods from Java side.
 
+#include <jni.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <jni.h>
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "include/org_rocksdb_RocksDB.h"
-#include "rocksjni/portal.h"
 #include "rocksdb/db.h"
 #include "rocksdb/cache.h"
 #include "rocksdb/types.h"
+#include "rocksjni/portal.h"
 
 //////////////////////////////////////////////////////////////////////////////
 // rocksdb::DB::Open

java/rocksjni/snapshot.cc

@@ -0,0 +1,26 @@
+// Copyright (c) 2014, Facebook, Inc.  All rights reserved.
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree. An additional grant
+// of patent rights can be found in the PATENTS file in the same directory.
+//
+// This file implements the "bridge" between Java and C++.
+
+#include <jni.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "include/org_rocksdb_Snapshot.h"
+#include "rocksdb/db.h"
+#include "rocksjni/portal.h"
+
+/*
+ * Class:     org_rocksdb_Snapshot
+ * Method:    getSequenceNumber
+ * Signature: (J)J
+ */
+jlong Java_org_rocksdb_Snapshot_getSequenceNumber(JNIEnv* env,
+    jobject jobj, jlong jsnapshot_handle) {
+  auto* snapshot = reinterpret_cast<rocksdb::Snapshot*>(
+      jsnapshot_handle);
+  return snapshot->GetSequenceNumber();
+}