Discard the reference to the underlying buffer during close(), and throw ClosedDatabaseException on subsequent database access.

Currently clients can continue to access the database even after it has been closed. This prevents that, and should also help ensure the underlying database is garbage collected.

Author: Andrew Snare

src/main/java/com/maxmind/db/ClosedDatabaseException.java

@@ -0,0 +1,15 @@
+package com.maxmind.db;
+
+import java.io.IOException;
+
+/**
+ * Signals that the underlying database has been closed.
+ */
+public class ClosedDatabaseException extends IOException {
+
+    private static final long serialVersionUID = 1L;
+
+    ClosedDatabaseException() {
+        super("The MaxMind DB has been closed.");
+    }
+}

src/main/java/com/maxmind/db/Reader.java

@@ -6,6 +6,7 @@
 import java.io.InputStream;
 import java.net.InetAddress;
 import java.nio.ByteBuffer;
+import java.util.concurrent.atomic.AtomicReference;
 
 import com.fasterxml.jackson.databind.JsonNode;
 
@@ -21,7 +22,7 @@ public final class Reader implements Closeable {
 
     private final int ipV4Start;
     private final Metadata metadata;
-    private final BufferHolder bufferHolder;
+    private final AtomicReference<BufferHolder> bufferHolderReference;
 
     /**
      * The file mode to use when opening a MaxMind DB.
@@ -81,9 +82,9 @@ public Reader(File database, FileMode fileMode) throws IOException {
     }
 
     private Reader(BufferHolder bufferHolder, String name) throws IOException {
-        this.bufferHolder = bufferHolder;
+        this.bufferHolderReference = new AtomicReference<BufferHolder>(bufferHolder);
 
-        ByteBuffer buffer = this.bufferHolder.get();
+        ByteBuffer buffer = bufferHolder.get();
         int start = this.findMetadataStart(buffer, name);
 
         Decoder metadataDecoder = new Decoder(buffer, start);
@@ -102,14 +103,22 @@ private Reader(BufferHolder bufferHolder, String name) throws IOException {
      *             if a file I/O error occurs.
      */
     public JsonNode get(InetAddress ipAddress) throws IOException {
-        ByteBuffer buffer = this.bufferHolder.get();
+        ByteBuffer buffer = getBufferHolder().get();
         int pointer = this.findAddressInTree(buffer, ipAddress);
         if (pointer == 0) {
             return null;
         }
         return this.resolveDataPointer(buffer, pointer);
     }
 
+    private BufferHolder getBufferHolder() throws ClosedDatabaseException {
+        BufferHolder bufferHolder = bufferHolderReference.get();
+        if (bufferHolder == null) {
+            throw new ClosedDatabaseException();
+        }
+        return bufferHolder;
+    }
+
     private int findAddressInTree(ByteBuffer buffer, InetAddress address)
             throws InvalidDatabaseException {
         byte[] rawAddress = address.getAddress();
@@ -241,6 +250,6 @@ Metadata getMetadata() {
      */
     @Override
     public void close() throws IOException {
-        this.bufferHolder.close();
+        bufferHolderReference.getAndSet(null).close();
     }
 }

src/test/java/com/maxmind/db/ReaderTest.java

@@ -37,15 +37,14 @@ public void test() throws IOException, URISyntaxException {
                 Reader reader = new Reader(new File(file));
                 try {
                     this.testMetadata(reader, ipVersion, recordSize);
+                    if (ipVersion == 4) {
+                        this.testIpV4(reader, file);
+                    } else {
+                        this.testIpV6(reader, file);
+                    }
                 } finally {
                     reader.close();
                 }
-
-                if (ipVersion == 4) {
-                    this.testIpV4(reader, file);
-                } else {
-                    this.testIpV6(reader, file);
-                }
             }
         }
     }
@@ -290,6 +289,19 @@ private void testBrokenDataPointer(Reader reader) throws IOException,
         reader.get(InetAddress.getByName("1.1.1.16"));
     }
 
+    @Test
+    public void testClosedReaderThrowsException() throws IOException, URISyntaxException {
+        URI file = ReaderTest.class.getResource(
+                "/maxmind-db/test-data/MaxMind-DB-test-decoder.mmdb").toURI();
+        Reader reader = new Reader(new File(file));
+
+        this.thrown.expect(ClosedDatabaseException.class);
+        this.thrown.expectMessage("The MaxMind DB has been closed.");
+
+        reader.close();
+        reader.get(InetAddress.getByName("1.1.1.16"));
+    }
+
     private void testMetadata(Reader reader, int ipVersion, long recordSize) {
 
         Metadata metadata = reader.getMetadata();