Minor fix in rocksdb jni library, RocksDB now does not implement Closeable.

Summary:
* [java] RocksDB now does not implement Closeable.
* [java] RocksDB.close() is now synchronized.
* [c++] Fix a bug in rocksjni.cc that does not release a java reference before
        throwing an exception.

Test Plan:
make jni
make jtest

Reviewers: haobo, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17355

Author: yhchiang

java/RocksDBSample.java

@@ -21,9 +21,10 @@ public static void main(String[] args) {
     String db_path = args[0];
 
     System.out.println("RocksDBSample");
+    RocksDB db = null;
 
     try {
-      RocksDB db = RocksDB.open(db_path);
+      db = RocksDB.open(db_path);
       db.put("hello".getBytes(), "world".getBytes());
       byte[] value = db.get("hello".getBytes());
       System.out.format("Get('hello') = %s\n",
@@ -67,13 +68,11 @@ public static void main(String[] args) {
       assert(len == RocksDB.NOT_FOUND);
       len = db.get(testKey, enoughArray);
       assert(len == testValue.length);
-      try {
-        db.close();
-      } catch (IOException e) {
-        System.err.println(e);
-      }
     } catch (RocksDBException e) {
       System.err.println(e);
     }
+    if (db != null) {
+      db.close();
+    }
   }
 }

java/org/rocksdb/RocksDB.java

@@ -16,7 +16,7 @@
  * 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 class RocksDB {
   public static final int NOT_FOUND = -1;
   /**
    * The factory constructor of RocksDB that opens a RocksDB instance given
@@ -33,8 +33,8 @@ public static RocksDB open(String path) throws RocksDBException {
     return db;
   }
 
-  @Override public void close() throws IOException {
-    if (nativeHandle != 0) {
+  public synchronized void close() {
+    if (nativeHandle_ != 0) {
       close0();
     }
   }
@@ -80,11 +80,15 @@ public byte[] get(byte[] key) throws RocksDBException {
     return get(key, key.length);
   }
 
+  @Override protected void finalize() {
+    close();
+  }
+
   /**
    * Private constructor.
    */
   private RocksDB() {
-    nativeHandle = -1;
+    nativeHandle_ = 0;
   }
 
   // native methods
@@ -99,5 +103,5 @@ private native byte[] get(
       byte[] key, int keyLen) throws RocksDBException;
   private native void close0();
 
-  private long nativeHandle;
+  private long nativeHandle_;
 }

java/rocksjni/portal.h

@@ -29,7 +29,7 @@ class RocksDBJni {
   // that stores the pointer to rocksdb::DB.
   static jfieldID getHandleFieldID(JNIEnv* env) {
     static jfieldID fid = env->GetFieldID(
-        getJClass(env), "nativeHandle", "J");
+        getJClass(env), "nativeHandle_", "J");
     assert(fid != nullptr);
     return fid;
   }

java/rocksjni/rocksjni.cc

@@ -147,6 +147,7 @@ jint Java_org_rocksdb_RocksDB_get___3BI_3BI(
     env->ReleaseByteArrayElements(jvalue, value, JNI_ABORT);
     return kNotFound;
   } else if (!s.ok()) {
+    env->ReleaseByteArrayElements(jvalue, value, JNI_ABORT);
     // Here since we are throwing a Java exception from c++ side.
     // As a result, c++ does not know calling this function will in fact
     // throwing an exception.  As a result, the execution flow will
@@ -164,10 +165,7 @@ jint Java_org_rocksdb_RocksDB_get___3BI_3BI(
 
   memcpy(value, cvalue.c_str(), length);
   env->ReleaseByteArrayElements(jvalue, value, JNI_COMMIT);
-  if (cvalue_len > length) {
-    return static_cast<jint>(cvalue_len);
-  }
-  return length;
+  return static_cast<jint>(cvalue_len);
 }
 
 /*