Merge pull request #471 from fyrz/RocksJava-Fix-NativeLibraryLoader

[RocksJava] Fix native library loader

java/org/rocksdb/NativeLibraryLoader.java

@@ -13,6 +13,7 @@ import org.rocksdb.util.Environment;
 public class NativeLibraryLoader {
   //singleton
   private static final NativeLibraryLoader instance = new NativeLibraryLoader();
+  private static boolean initialized = false;
 
   private static final String sharedLibraryName = Environment.getJniLibraryName("rocksdb");
   private static final String tempFilePrefix = "librocksdbjni";
@@ -41,10 +42,11 @@ public class NativeLibraryLoader {
    *
    * @throws java.io.IOException if a filesystem operation fails.
    */
-  public void loadLibraryFromJar(final String tmpDir)
+  public synchronized void loadLibraryFromJar(final String tmpDir)
       throws IOException {
+    if (!initialized) {
       final File temp;
-    if(tmpDir == null || tmpDir.equals("")) {
+      if (tmpDir == null || tmpDir.equals("")) {
         temp = File.createTempFile(tempFilePrefix, tempFileSuffix);
       } else {
         temp = new File(tmpDir, sharedLibraryName);
@@ -57,7 +59,8 @@ public class NativeLibraryLoader {
       }
 
       // attempt to copy the library from the Jar file to the temp destination
-    try(final InputStream is = getClass().getClassLoader().getResourceAsStream(sharedLibraryName)) {
+      try (final InputStream is = getClass().getClassLoader().
+          getResourceAsStream(sharedLibraryName)) {
         if (is == null) {
           throw new RuntimeException(sharedLibraryName + " was not found inside JAR.");
         } else {
@@ -66,6 +69,8 @@ public class NativeLibraryLoader {
       }
 
       System.load(temp.getAbsolutePath());
+      initialized = true;
+    }
   }
   /**
    * Private constructor to disallow instantiation