Pool table instances to make PureJavaSnappy thread safe (#271)

https://github.com/xerial/snappy-java/issues/270

Co-authored-by: BO8979 <BO8979@W1971362.northamerica.cerner.net>

src/main/java/org/xerial/snappy/pure/PureJavaSnappy.java

@@ -1,26 +1,41 @@
 package org.xerial.snappy.pure;
 
-import org.xerial.snappy.SnappyApi;
-
-import java.io.IOException;
-import java.nio.ByteBuffer;
-
 import static org.xerial.snappy.pure.UnsafeUtil.getAddress;
 import static sun.misc.Unsafe.ARRAY_BYTE_BASE_OFFSET;
 
+import java.io.IOException;
+import java.lang.ref.SoftReference;
+import java.nio.ByteBuffer;
+import java.util.concurrent.ConcurrentLinkedDeque;
+
+import org.xerial.snappy.SnappyApi;
+
 /**
  * A pure-java Snappy implementation using https://github.com/airlift/aircompressor
  */
 public class PureJavaSnappy implements SnappyApi
 {
-    private final short[] table = new short[SnappyRawCompressor.MAX_HASH_TABLE_SIZE];
+    /**
+     * Using a {@link ConcurrentLinkedDeque}, with values constantly popped and pushed from the head, leads to the fewest
+     * {@code short[]} instances remaining live over time.
+     */
+    private final static ConcurrentLinkedDeque<SoftReference<short[]>> CACHED_TABLES = new ConcurrentLinkedDeque<>();
+
     private final static int MAX_OUTPUT_LENGTH = Integer.MAX_VALUE;
 
     @Override
     public long rawCompress(long inputAddr, long inputSize, long destAddr)
             throws IOException
     {
-        return SnappyRawCompressor.compress(null, inputAddr, inputSize, null, destAddr, MAX_OUTPUT_LENGTH, table);
+        final short[] table = getTable();
+        try
+        {
+            return SnappyRawCompressor.compress(null, inputAddr, inputSize, null, destAddr, MAX_OUTPUT_LENGTH, table);
+        }
+        finally
+        {
+            returnTable(table);
+        }
     }
 
     @Override
@@ -76,16 +91,24 @@ public class PureJavaSnappy implements SnappyApi
         // collected in a block, and technically, the JVM is allowed to eliminate these locks.
         synchronized (input) {
             synchronized (compressed) {
-                int written = SnappyRawCompressor.compress(
-                        inputBase,
-                        inputAddress,
-                        inputLimit,
-                        outputBase,
-                        outputAddress,
-                        outputLimit,
-                        table);
-                compressed.position(compressed.position() + written);
-                return written;
+                final short[] table = getTable();
+                try
+                {
+                    int written = SnappyRawCompressor.compress(
+                            inputBase,
+                            inputAddress,
+                            inputLimit,
+                            outputBase,
+                            outputAddress,
+                            outputLimit,
+                            table);
+                    compressed.position(compressed.position() + written);
+                    return written;
+                }
+                finally
+                {
+                    returnTable(table);
+                }
             }
         }
     }
@@ -99,7 +122,15 @@ public class PureJavaSnappy implements SnappyApi
         long outputAddress = ARRAY_BYTE_BASE_OFFSET + outputOffset;
         long outputLimit = outputAddress + MAX_OUTPUT_LENGTH;
 
-        return SnappyRawCompressor.compress(input, inputAddress, inputLimit, output, outputAddress, outputLimit, table);
+        final short[] table = getTable();
+        try
+        {
+            return SnappyRawCompressor.compress(input, inputAddress, inputLimit, output, outputAddress, outputLimit, table);
+        }
+        finally
+        {
+            returnTable(table);
+        }   
     }
 
     @Override
@@ -241,4 +272,38 @@ public class PureJavaSnappy implements SnappyApi
     {
         System.arraycopy(src, offset, dest, dOffset, byteLength);
     }
+
+    private static short[] getTable()
+    {
+        SoftReference<short[]> existingRef;
+        while((existingRef = CACHED_TABLES.poll()) != null)
+        {
+            short[] table = existingRef.get();
+            if (table != null)
+            {
+                //purge oldest entries have lost references
+                SoftReference<short[]> entry;
+                boolean lastEmpty = true;
+                while (lastEmpty && (entry = CACHED_TABLES.peekLast()) != null)
+                {
+                    if (entry.get() == null)
+                    {
+                        CACHED_TABLES.removeLastOccurrence(entry);
+                    }
+                    else
+                    {
+                        lastEmpty = false;
+                    }
+                }
+
+                return table;
+            }
+        }
+        return new short[SnappyRawCompressor.MAX_HASH_TABLE_SIZE];
+    }
+
+    private static void returnTable(short[] table)
+    {
+        CACHED_TABLES.addFirst(new SoftReference<short[]>(table));
+    }
 }