Minor fix in rocksdb jni library.

Summary: Fix a bug in RocksDBSample.java and minor code improvement in rocksjni.cc.

Test Plan:
make jni
make jtest

Reviewers: haobo, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17325

java/RocksDBSample.java

@@ -62,7 +62,7 @@ public class RocksDBSample {
       byte[] enoughArray = new byte[50];
       int len;
       len = db.get(testKey, insufficientArray);
-      assert(len > testKey.length);
+      assert(len > insufficientArray.length);
       len = db.get("asdfjkl;".getBytes(), enoughArray);
       assert(len == RocksDB.NOT_FOUND);
       len = db.get(testKey, enoughArray);

java/org/rocksdb/RocksDB.java

@@ -34,8 +34,10 @@ public class RocksDB implements Closeable {
   }
 
   @Override public void close() throws IOException {
+    if (nativeHandle != 0) {
       close0();
     }
+  }
 
   /**
    * Set the database entry for "key" to "value".

java/rocksjni/rocksjni.cc

@@ -121,6 +121,9 @@ jint Java_org_rocksdb_RocksDB_get___3BI_3BI(
     JNIEnv* env, jobject jdb,
     jbyteArray jkey, jint jkey_len,
     jbyteArray jvalue, jint jvalue_len) {
+  static const int kNotFound = -1;
+  static const int kStatusError = -2;
+
   rocksdb::DB* db = rocksdb::RocksDBJni::getHandle(env, jdb);
 
   jboolean isCopy;
@@ -142,25 +145,30 @@ jint Java_org_rocksdb_RocksDB_get___3BI_3BI(
 
   if (s.IsNotFound()) {
     env->ReleaseByteArrayElements(jvalue, value, JNI_ABORT);
-    return -1;
-  } else if (s.ok()) {
-    int cvalue_len = static_cast<int>(cvalue.size());
-    int length = cvalue_len;
-    // currently we prevent overflowing.
-    if (length > jvalue_len) {
-      length = jvalue_len;
+    return kNotFound;
+  } else if (!s.ok()) {
+    // 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
+    // not stop here, and codes after this throw will still be
+    // executed.
+    rocksdb::RocksDBExceptionJni::ThrowNew(env, s);
+
+    // Return a dummy const value to avoid compilation error, although
+    // java side might not have a chance to get the return value :)
+    return kStatusError;
   }
+
+  int cvalue_len = static_cast<int>(cvalue.size());
+  int length = std::min(jvalue_len, cvalue_len);
+
   memcpy(value, cvalue.c_str(), length);
   env->ReleaseByteArrayElements(jvalue, value, JNI_COMMIT);
   if (cvalue_len > length) {
-      return static_cast<jint>(cvalue.size());
+    return static_cast<jint>(cvalue_len);
   }
   return length;
 }
-  rocksdb::RocksDBExceptionJni::ThrowNew(env, s);
-
-  return -1;
-}
 
 /*
  * Class:     org_rocksdb_RocksDB