expose write buffer manager method through Java Api

java/rocksjni/write_buffer_manager.cc

@@ -40,3 +40,44 @@ void Java_org_rocksdb_WriteBufferManager_disposeInternal(
   assert(write_buffer_manager != nullptr);
   delete write_buffer_manager;
 }
+
+/*
+ * Class:     org_rocksdb_WriteBufferManager
+ * Method:    getMemoryUsage
+ * Signature: (J)J
+ */
+jlong Java_org_rocksdb_WriteBufferManager_getMemoryUsage(JNIEnv* /*env*/,
+                                                         jclass,
+                                                         jlong jhandle) {
+  auto* write_buffer_manager =
+      reinterpret_cast<std::shared_ptr<ROCKSDB_NAMESPACE::WriteBufferManager>*>(
+          jhandle);
+  return static_cast<jlong>(write_buffer_manager->memory_usage());
+}
+
+/*
+ * Class:     org_rocksdb_WriteBufferManager
+ * Method:    getMutableMemtableMemoryUsage
+ * Signature: (J)J
+ */
+jlong Java_org_rocksdb_WriteBufferManager_getMutableMemtableMemoryUsage(
+    JNIEnv* /*env*/, jclass, jlong jhandle) {
+  auto* write_buffer_manager =
+      reinterpret_cast <
+      std::shared_ptr<ROCKSDB_NAMESPACE::WriteBufferManager>(jhandle);
+  return static_cast<jlong>(
+      write_buffer_manager->mutable_memtable_memory_usage());
+}
+
+/*
+ * Class:     org_rocksdb_WriteBufferManager
+ * Method:    getBufferSize
+ * Signature: (J)J
+ */
+jlong Java_org_rocksdb_WriteBufferManager_getBufferSize(JNIEnv*, jclass,
+                                                        jlong jhandle) {
+  auto* write_buffer_manager =
+      reinterpret_cast <
+      std::shared_ptr<ROCKSDB_NAMESPACE::WriteBufferManager>(jhandle);
+  return static_cast<jlong>(write_buffer_manager->buffer_size());
+}

java/src/main/java/org/rocksdb/WriteBufferManager.java

@@ -16,18 +16,61 @@ public class WriteBufferManager extends RocksObject {
   /**
    * Construct a new instance of WriteBufferManager.
    *
-   * Check <a href="https://github.com/facebook/rocksdb/wiki/Write-Buffer-Manager">
-   *     https://github.com/facebook/rocksdb/wiki/Write-Buffer-Manager</a>
-   * for more details on when to use it
+   * Check
+   * <a href="https://github.com/facebook/rocksdb/wiki/Write-Buffer-Manager">
+   * https://github.com/facebook/rocksdb/wiki/Write-Buffer-Manager</a> for more
+   * details on when to use it
    *
-   * @param bufferSizeBytes buffer size(in bytes) to use for native write_buffer_manager
-   * @param cache cache whose memory should be bounded by this write buffer manager
+   * @param bufferSizeBytes buffer size(in bytes) to use for native
+   *                        write_buffer_manager
+   * @param cache           cache whose memory should be bounded by this write
+   *                        buffer manager
    */
-  public WriteBufferManager(final long bufferSizeBytes, final Cache cache){
+  public WriteBufferManager(final long bufferSizeBytes, final Cache cache) {
     super(newWriteBufferManager(bufferSizeBytes, cache.nativeHandle_));
   }
 
+  /**
+   * Returns the buffer size of the write buffer manager
+   *
+   * @return buffer size.
+   *
+   */
+  public long getBufferSize() {
+    assert (isOwningHandle());
+    return getBufferSize(this.nativeHandle_);
+  }
+
+  /**
+   * Returns the memory usage of the write buffer manager
+   *
+   * @return memory usage.
+   *
+   */
+  public long getMemoryUsage() {
+    assert (isOwningHandle());
+    return getMemoryUsage(this.nativeHandle_);
+  }
+
+  /**
+   * Returns the mutable memtable memory usage of the write buffer manager
+   *
+   * @return mutable memtable memory usage size.
+   *
+   */
+  public long getMutableMemtableMemoryUsage() {
+    assert (isOwningHandle());
+    return getMutableMemtableMemoryUsage(this.nativeHandle_);
+  }
+
   private native static long newWriteBufferManager(final long bufferSizeBytes, final long cacheHandle);
+
+  private native static long getMemoryUsage(final long handle);
+
+  private native static long getMutableMemtableMemoryUsage(final long handle);
+
+  private native static long getBufferSize(final long handle);
+
   @Override
   protected native void disposeInternal(final long handle);
 }