Minor fixes to RocksJava Native Library initialization (#1287)

* [bugfix] Make sure the Native Library is initialized. Closes https://github.com/facebook/rocksdb/issues/989

* [bugfix] Just load the native libraries once

java/src/main/java/org/rocksdb/FlushOptions.java

@@ -5,6 +5,9 @@ package org.rocksdb;
  * {@link org.rocksdb.RocksDB}.
  */
 public class FlushOptions extends RocksObject {
+  static {
+    RocksDB.loadLibrary();
+  }
 
   /**
    * Construct a new instance of FlushOptions.

java/src/main/java/org/rocksdb/RocksDB.java

@@ -7,6 +7,9 @@ package org.rocksdb;
 
 import java.util.*;
 import java.io.IOException;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicReference;
+
 import org.rocksdb.util.Environment;
 
 /**
@@ -19,6 +22,15 @@ public class RocksDB extends RocksObject {
   public static final byte[] DEFAULT_COLUMN_FAMILY = "default".getBytes();
   public static final int NOT_FOUND = -1;
 
+  private enum LibraryState {
+    NOT_LOADED,
+    LOADING,
+    LOADED
+  }
+
+  private static AtomicReference<LibraryState> libraryLoaded
+      = new AtomicReference<>(LibraryState.NOT_LOADED);
+
   static {
     RocksDB.loadLibrary();
   }
@@ -30,10 +42,16 @@ public class RocksDB extends RocksObject {
    * java.io.tmpdir, however, you can override this temporary location by
    * setting the environment variable ROCKSDB_SHAREDLIB_DIR.
    */
-  public static synchronized void loadLibrary() {
+  public static void loadLibrary() {
-    String tmpDir = System.getenv("ROCKSDB_SHAREDLIB_DIR");
+    if (libraryLoaded.get() == LibraryState.LOADED) {
+      return;
+    }
+
+    if (libraryLoaded.compareAndSet(LibraryState.NOT_LOADED,
+        LibraryState.LOADING)) {
+      final String tmpDir = System.getenv("ROCKSDB_SHAREDLIB_DIR");
       // loading possibly necessary libraries.
-    for (CompressionType compressionType : CompressionType.values()) {
+      for (final CompressionType compressionType : CompressionType.values()) {
         try {
           if (compressionType.getLibraryName() != null) {
             System.loadLibrary(compressionType.getLibraryName());
@@ -42,15 +60,25 @@ public class RocksDB extends RocksObject {
           // since it may be optional, we ignore its loading failure here.
         }
       }
-    try
+      try {
-    {
         NativeLibraryLoader.getInstance().loadLibrary(tmpDir);
-    }
+      } catch (IOException e) {
-    catch (IOException e)
+        libraryLoaded.set(LibraryState.NOT_LOADED);
-    {
         throw new RuntimeException("Unable to load the RocksDB shared library"
             + e);
       }
+
+      libraryLoaded.set(LibraryState.LOADED);
+      return;
+    }
+
+    while (libraryLoaded.get() == LibraryState.LOADING) {
+      try {
+        Thread.sleep(10);
+      } catch(final InterruptedException e) {
+        //ignore
+      }
+    }
   }
 
   /**
@@ -60,12 +88,18 @@ public class RocksDB extends RocksObject {
    * @param paths a list of strings where each describes a directory
    *     of a library.
    */
-  public static synchronized void loadLibrary(final List<String> paths) {
+  public static void loadLibrary(final List<String> paths) {
-    for (CompressionType compressionType : CompressionType.values()) {
+    if (libraryLoaded.get() == LibraryState.LOADED) {
+      return;
+    }
+
+    if (libraryLoaded.compareAndSet(LibraryState.NOT_LOADED,
+        LibraryState.LOADING)) {
+      for (final CompressionType compressionType : CompressionType.values()) {
         if (compressionType.equals(CompressionType.NO_COMPRESSION)) {
           continue;
         }
-      for (String path : paths) {
+        for (final String path : paths) {
           try {
             System.load(path + "/" + Environment.getSharedLibraryFileName(
                 compressionType.getLibraryName()));
@@ -77,7 +111,7 @@ public class RocksDB extends RocksObject {
       }
       boolean success = false;
       UnsatisfiedLinkError err = null;
-    for (String path : paths) {
+      for (final String path : paths) {
         try {
           System.load(path + "/" +
               Environment.getJniLibraryFileName("rocksdbjni"));
@@ -88,8 +122,21 @@ public class RocksDB extends RocksObject {
         }
       }
       if (!success) {
+        libraryLoaded.set(LibraryState.NOT_LOADED);
         throw err;
       }
+
+      libraryLoaded.set(LibraryState.LOADED);
+      return;
+    }
+
+    while (libraryLoaded.get() == LibraryState.LOADING) {
+      try {
+        Thread.sleep(10);
+      } catch(final InterruptedException e) {
+        //ignore
+      }
+    }
   }
 
   /**