Add a jni library for rocksdb which supports Open, Get, Put, and Close.

Summary:
This diff contains a simple jni library for rocksdb which supports open, get,
put and closeusing default options (including Options, ReadOptions, and
WriteOptions.)  In the usual case, Java developers can use the c++ rocksdb
library in the way similar to the following:

    RocksDB db = RocksDB.open(path_to_db);
    ...
    db.put("hello".getBytes(), "world".getBytes();
    byte[] value = db.get("hello".getBytes());
    ...
    db.close();

Specifically, this diff has the following major classes:

* RocksDB: a Java wrapper class which forwards the operations
  from the java side to c++ rocksdb library.
* RocksDBException: ncapsulates the error of an operation.
  This exception type is used to describe an internal error from
  the c++ rocksdb library.

This diff also include a simple java sample code calling c++ rocksdb library.

To build the rocksdb jni library, simply run make jni, and make jtest will try to
build and run the sample code.

Note that if the rocksdb is not built with the default glibc that Java uses,
java will try to load the wrong glibc during the run time.  As a result,
the sample code might not work properly during the run time.

Test Plan:
* make jni
* make jtest

Reviewers: haobo, dhruba, sdong, igor, ljin

Reviewed By: dhruba

CC: leveldb, xjin

Differential Revision: https://reviews.facebook.net/D17109

java/org/rocksdb/RocksDB.java

@@ -0,0 +1,101 @@
+// Copyright (c) 2013, 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;
+
+import java.lang.*;
+import java.util.*;
+import java.io.Closeable;
+import java.io.IOException;
+
+/**
+ * A RocksDB is a persistent ordered map from keys to values.  It is safe for
+ * concurrent access from multiple threads without any external synchronization.
+ * All methods of this class could potentially throw RocksDBException, which
+ * indicates sth wrong at the rocksdb library side and the call failed.
+ */
+public class RocksDB implements Closeable {
+  public static final int NOT_FOUND = -1;
+  /**
+   * The factory constructor of RocksDB that opens a RocksDB instance given
+   * the path to the database.
+   *
+   * @param path the path to the rocksdb.
+   * @param status an out value indicating the status of the Open().
+   * @return a rocksdb instance on success, null if the specified rocksdb can
+   *     not be opened.
+   */
+  public static RocksDB open(String path) throws RocksDBException {
+    RocksDB db = new RocksDB();
+    db.open0(path);
+    return db;
+  }
+
+  @Override public void close() throws IOException {
+    close0();
+  }
+
+  /**
+   * Set the database entry for "key" to "value".
+   *
+   * @param key the specified key to be inserted.
+   * @param value the value associated with the specified key.
+   */
+  public void put(byte[] key, byte[] value) throws RocksDBException {
+    put(key, key.length, value, value.length);
+  }
+
+  /**
+   * Get the value associated with the specified key.
+   *
+   * @param key the key to retrieve the value.
+   * @param value the out-value to receive the retrieved value.
+   * @return The size of the actual value that matches the specified
+   *     {@code key} in byte.  If the return value is greater than the
+   *     length of {@code value}, then it indicates that the size of the
+   *     input buffer {@code value} is insufficient and partial result will
+   *     be returned.  RocksDB.NOT_FOUND will be returned if the value not
+   *     found.
+   */
+  public int get(byte[] key, byte[] value) throws RocksDBException {
+    return get(key, key.length, value, value.length);
+  }
+
+  /**
+   * The simplified version of get which returns a new byte array storing
+   * the value associated with the specified input key if any.  null will be
+   * returned if the specified key is not found.
+   *
+   * @param key the key retrieve the value.
+   * @return a byte array storing the value associated with the input key if
+   *     any.  null if it does not find the specified key.
+   *
+   * @see RocksDBException
+   */
+  public byte[] get(byte[] key) throws RocksDBException {
+    return get(key, key.length);
+  }
+
+  /**
+   * Private constructor.
+   */
+  private RocksDB() {
+    nativeHandle = -1;
+  }
+
+  // native methods
+  private native void open0(String path) throws RocksDBException;
+  private native void put(
+      byte[] key, int keyLen,
+      byte[] value, int valueLen) throws RocksDBException;
+  private native int get(
+      byte[] key, int keyLen,
+      byte[] value, int valueLen) throws RocksDBException;
+  private native byte[] get(
+      byte[] key, int keyLen) throws RocksDBException;
+  private native void close0();
+
+  private long nativeHandle;
+}