[Java] Add RocksObject, the base class of all java objects with a c++ pointer.

Summary:
Add RocksObject, the base class of all java objects which has a c++ pointer.
While the finalizer of a RocksObject will release its c++ resource, it is
suggested to call its RocksObject.dispose() to manually release its
c++ resource.

Existing RocksDB java classes are now extending RocksObject.

Test Plan:
make rocksdbjava
make jtest
make jdb_bench

Reviewers: haobo, dhruba, sdong, ankgup87, rsumbaly, swapnilghike, zzbennett

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18411

java/org/rocksdb/RocksObject.java

@@ -0,0 +1,35 @@
+// 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.
+
+package org.rocksdb;
+
+/**
+ * RocksObject is the base-class of all RocksDB related class that has
+ * a pointer to some c++ rocksdb object.  Although RocksObject
+ * will release its c++ resource on its finalize() once it has been
+ * garbage-collected, it is suggested to call dispose() manually to
+ * release its c++ resource once an instance of RocksObject is no
+ * longer used.
+ */
+public abstract class RocksObject {
+  protected RocksObject() {
+    nativeHandle_ = 0;
+  }
+
+  /**
+   * Release the c++ object pointed by the native handle.
+   */
+  public abstract void dispose();
+
+  protected boolean isInitialized() {
+    return (nativeHandle_ != 0);
+  }
+
+  @Override protected void finalize() {
+    dispose();
+  }
+
+  protected long nativeHandle_;
+}