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();
   }
@@ -30,26 +42,42 @@ 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() {
-    String tmpDir = System.getenv("ROCKSDB_SHAREDLIB_DIR");
-    // loading possibly necessary libraries.
-    for (CompressionType compressionType : CompressionType.values()) {
-      try {
-        if (compressionType.getLibraryName() != null) {
-          System.loadLibrary(compressionType.getLibraryName());
+  public static void loadLibrary() {
+    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 (final CompressionType compressionType : CompressionType.values()) {
+        try {
+          if (compressionType.getLibraryName() != null) {
+            System.loadLibrary(compressionType.getLibraryName());
+          }
+        } catch (UnsatisfiedLinkError e) {
+          // since it may be optional, we ignore its loading failure here.
         }
-      } catch (UnsatisfiedLinkError e) {
-        // since it may be optional, we ignore its loading failure here.
       }
+      try {
+        NativeLibraryLoader.getInstance().loadLibrary(tmpDir);
+      } catch (IOException e) {
+        libraryLoaded.set(LibraryState.NOT_LOADED);
+        throw new RuntimeException("Unable to load the RocksDB shared library"
+            + e);
+      }
+
+      libraryLoaded.set(LibraryState.LOADED);
+      return;
     }
-    try
-    {
-      NativeLibraryLoader.getInstance().loadLibrary(tmpDir);
-    }
-    catch (IOException e)
-    {
-      throw new RuntimeException("Unable to load the RocksDB shared library"
-          + e);
+
+    while (libraryLoaded.get() == LibraryState.LOADING) {
+      try {
+        Thread.sleep(10);
+      } catch(final InterruptedException e) {
+        //ignore
+      }
     }
   }
 
@@ -60,36 +88,55 @@ 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) {
-    for (CompressionType compressionType : CompressionType.values()) {
-      if (compressionType.equals(CompressionType.NO_COMPRESSION)) {
-        continue;
+  public static void loadLibrary(final List<String> paths) {
+    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 (final String path : paths) {
+          try {
+            System.load(path + "/" + Environment.getSharedLibraryFileName(
+                compressionType.getLibraryName()));
+            break;
+          } catch (UnsatisfiedLinkError e) {
+            // since they are optional, we ignore loading fails.
+          }
+        }
       }
-      for (String path : paths) {
+      boolean success = false;
+      UnsatisfiedLinkError err = null;
+      for (final String path : paths) {
         try {
-          System.load(path + "/" + Environment.getSharedLibraryFileName(
-              compressionType.getLibraryName()));
+          System.load(path + "/" +
+              Environment.getJniLibraryFileName("rocksdbjni"));
+          success = true;
           break;
         } catch (UnsatisfiedLinkError e) {
-          // since they are optional, we ignore loading fails.
+          err = e;
         }
       }
+      if (!success) {
+        libraryLoaded.set(LibraryState.NOT_LOADED);
+        throw err;
+      }
+
+      libraryLoaded.set(LibraryState.LOADED);
+      return;
     }
-    boolean success = false;
-    UnsatisfiedLinkError err = null;
-    for (String path : paths) {
+
+    while (libraryLoaded.get() == LibraryState.LOADING) {
       try {
-        System.load(path + "/" +
-            Environment.getJniLibraryFileName("rocksdbjni"));
-        success = true;
-        break;
-      } catch (UnsatisfiedLinkError e) {
-        err = e;
+        Thread.sleep(10);
+      } catch(final InterruptedException e) {
+        //ignore
       }
     }
-    if (!success) {
-      throw err;
-    }
   }
 
   /**