[RocksJava] Fix native library loader

Summary:
Prior to this the native library loader instance didn`t
care about a state. So if library loading was called multiple
times, multiple copies of the shared object were put into
the tmp folder and loaded into the JVM.

This changed within this commit to the following behavior:

- library loading is now synchronized
- library is loaded within the first call
- if loading was successful the library loaded sets a flag
- every subsequent call checks for a boolean flag indicating if there was
  already a successful attempt

Test Plan:
- Execute example and watch tmp folder while the example is running

- After this patch only one shared object will be in the tmp folder

Usual tests:
- make rocksdbjava jtest
- mvn -f rocksjni.pom package

Reviewers: adamretter, ankgup87, yhchiang

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D32133

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";
@@ -41,31 +42,35 @@ 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 {
-    final File temp;
-    if(tmpDir == null || tmpDir.equals("")) {
-      temp = File.createTempFile(tempFilePrefix, tempFileSuffix);
-    } else {
-      temp = new File(tmpDir, sharedLibraryName);
-    }
-
-    if (!temp.exists()) {
-      throw new RuntimeException("File " + temp.getAbsolutePath() + " does not exist.");
-    } else {
-      temp.deleteOnExit();
-    }
+    if (!initialized) {
+      final File temp;
+      if (tmpDir == null || tmpDir.equals("")) {
+        temp = File.createTempFile(tempFilePrefix, tempFileSuffix);
+      } else {
+        temp = new File(tmpDir, sharedLibraryName);
+      }
 
-    // attempt to copy the library from the Jar file to the temp destination
-    try(final InputStream is = getClass().getClassLoader().getResourceAsStream(sharedLibraryName)) {
-      if (is == null) {
-        throw new RuntimeException(sharedLibraryName + " was not found inside JAR.");
+      if (!temp.exists()) {
+        throw new RuntimeException("File " + temp.getAbsolutePath() + " does not exist.");
       } else {
-        Files.copy(is, temp.toPath(), StandardCopyOption.REPLACE_EXISTING);
+        temp.deleteOnExit();
+      }
+
+      // attempt to copy the library from the Jar file to the temp destination
+      try (final InputStream is = getClass().getClassLoader().
+          getResourceAsStream(sharedLibraryName)) {
+        if (is == null) {
+          throw new RuntimeException(sharedLibraryName + " was not found inside JAR.");
+        } else {
+          Files.copy(is, temp.toPath(), StandardCopyOption.REPLACE_EXISTING);
+        }
       }
-    }
 
-    System.load(temp.getAbsolutePath());
+      System.load(temp.getAbsolutePath());
+      initialized = true;
+    }
   }
   /**
    * Private constructor to disallow instantiation