[Java] Add basic Java binding for rocksdb::Env.

Summary: Add basic Java binding for rocksdb::Env.

Test Plan:
make rocksdbjava
make jtest
cd java
./jdb_bench.sh --max_background_compactions=1
./jdb_bench.sh --max_background_compactions=10

Reviewers: sdong, ankgup87, haobo

Reviewed By: haobo

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18903

java/org/rocksdb/Options.java

@@ -24,6 +24,7 @@ public class Options extends RocksObject {
     super();
     cacheSize_ = DEFAULT_CACHE_SIZE;
     newOptions();
+    env_ = RocksEnv.getDefault();
   }
 
   /**
@@ -42,6 +43,24 @@ public class Options extends RocksObject {
     return this;
   }
 
+  /**
+   * Use the specified object to interact with the environment,
+   * e.g. to read/write files, schedule background work, etc.
+   * Default: RocksEnv.getDefault()
+   */
+  public Options setEnv(RocksEnv env) {
+    assert(isInitialized());
+    setEnv(nativeHandle_, env.nativeHandle_);
+    env_ = env;
+    return this;
+  }
+  private native void setEnv(long optHandle, long envHandle);
+
+  public RocksEnv getEnv() {
+    return env_;
+  }
+  private native long getEnvHandle(long handle);
+
   /**
    * Return true if the create_if_missing flag is set to true.
    * If true, the database will be created if it is missing.
@@ -2351,4 +2370,5 @@ public class Options extends RocksObject {
 
   long cacheSize_;
   Filter filter_;
+  RocksEnv env_;
 }

java/org/rocksdb/RocksEnv.java

@@ -0,0 +1,98 @@
+// Copyright (c) 2014, Facebook, Inc.  All rights reserved.
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree. An additional grant
+// of patent rights can be found in the PATENTS file in the same directory.
+
+package org.rocksdb;
+
+/**
+ * A RocksEnv is an interface used by the rocksdb implementation to access
+ * operating system functionality like the filesystem etc.
+ *
+ * All Env implementations are safe for concurrent access from
+ * multiple threads without any external synchronization.
+ */
+public class RocksEnv extends RocksObject {
+  public static final int FLUSH_POOL = 0;
+  public static final int COMPACTION_POOL = 1;
+
+  static {
+    default_env_ = new RocksEnv(getDefaultEnvInternal());
+  }
+  private static native long getDefaultEnvInternal();
+
+  /**
+   * Returns a default environment suitable for the current operating
+   * system.
+   *
+   * The result of getDefault() belongs to rocksdb c++, and calling its
+   * dispose() will be no-op.
+   */
+  public static RocksEnv getDefault() {
+    return default_env_;
+  }
+
+  /**
+   * Sets the number of background worker threads of the flush pool
+   * for this environment.
+   * default number: 1
+   */
+  public RocksEnv setBackgroundThreads(int num) {
+    return setBackgroundThreads(num, FLUSH_POOL);
+  }
+
+  /**
+   * Sets the number of background worker threads of the specified thread
+   * pool for this environment.
+   *
+   * @param num the number of threads
+   * @param poolID the id to specified a thread pool.  Should be either
+   *     FLUSH_POOL or COMPACTION_POOL.
+   * Default number: 1
+   */
+  public RocksEnv setBackgroundThreads(int num, int poolID) {
+    setBackgroundThreads(nativeHandle_, num, poolID);
+    return this;
+  }
+  private native void setBackgroundThreads(
+      long handle, int num, int priority);
+
+  /**
+   * Returns the length of the queue associated with the specified
+   * thread pool.
+   *
+   * @param poolID the id to specified a thread pool.  Should be either
+   *     FLUSH_POOL or COMPACTION_POOL.
+   */
+  public int getThreadPoolQueueLen(int poolID) {
+    return getThreadPoolQueueLen(nativeHandle_, poolID);
+  }
+  private native int getThreadPoolQueueLen(long handle, int poolID);
+
+  /**
+   * Package-private constructor that uses the specified native handle
+   * to construct a RocksEnv.  Note that the newly created RocksEnv
+   * will not take the ownership of the input handle.
+   */
+  RocksEnv(long handle) {
+    super();
+    nativeHandle_ = handle;
+    disOwnNativeHandle();
+  }
+
+  /**
+   * The helper function of dispose() which all subclasses of RocksObject
+   * must implement to release their associated C++ resource.
+   */
+  protected void disposeInternal() {
+    disposeInternal(nativeHandle_);
+  }
+  private native void disposeInternal(long handle);
+
+  /**
+   * The static default RocksEnv.  The ownership of its native handle
+   * belongs to rocksdb c++ and is not able to be released on the Java
+   * side.
+   */
+  static RocksEnv default_env_;
+}

java/org/rocksdb/benchmark/DbBenchmark.java

@@ -526,6 +526,8 @@ public class DbBenchmark {
         (Integer)flags_.get(Flag.max_write_buffer_number));
     options.setMaxBackgroundCompactions(
         (Integer)flags_.get(Flag.max_background_compactions));
+    options.getEnv().setBackgroundThreads(
+        (Integer)flags_.get(Flag.max_background_compactions));
     options.setMaxBackgroundFlushes(
         (Integer)flags_.get(Flag.max_background_flushes));
     options.setCacheSize(