HADOOP-6323. Add comparators to the serialization API. Contributed by Aaron Kimball.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@889877 13f79535-47bb-0310-9956-ffa450edef68

CHANGES.txt

@@ -19,6 +19,9 @@ Trunk (unreleased changes)
     HADOOP-6392. Run namenode and jobtracker on separate EC2 instances.
     (tomwhite)
 
+    HADOOP-6323. Add comparators to the serialization API.
+    (Aaron Kimball via cutting)
+
   IMPROVEMENTS
 
     HADOOP-6283. Improve the exception messages thrown by

src/java/org/apache/hadoop/io/serializer/JavaSerialization.java

@@ -24,6 +24,9 @@ import java.io.ObjectInputStream;
 import java.io.ObjectOutputStream;
 import java.io.OutputStream;
 import java.io.Serializable;
+import java.util.Map;
+
+import org.apache.hadoop.io.RawComparator;
 
 /**
  * <p>
@@ -31,10 +34,10 @@ import java.io.Serializable;
  * </p>
  * @see JavaSerializationComparator
  */
-public class JavaSerialization implements Serialization<Serializable> {
-  
+public class JavaSerialization extends SerializationBase<Serializable> {
+
   static class JavaSerializationDeserializer<T extends Serializable>
-    implements Deserializer<T> {
+    extends DeserializerBase<T> {
 
     private ObjectInputStream ois;
 
@@ -61,11 +64,16 @@ public class JavaSerialization implements Serialization<Serializable> {
     }
 
   }
-  
-  static class JavaSerializationSerializer
-    implements Serializer<Serializable> {
+
+  static class JavaSerializationSerializer<T extends Serializable>
+      extends SerializerBase<T> {
 
     private ObjectOutputStream oos;
+    private Map<String, String> metadata;
+
+    public JavaSerializationSerializer(Map<String, String> metadata) {
+      this.metadata = metadata;
+    }
 
     public void open(OutputStream out) throws IOException {
       oos = new ObjectOutputStream(out) {
@@ -75,7 +83,7 @@ public class JavaSerialization implements Serialization<Serializable> {
       };
     }
 
-    public void serialize(Serializable object) throws IOException {
+    public void serialize(T object) throws IOException {
       oos.reset(); // clear (class) back-references
       oos.writeObject(object);
     }
@@ -84,18 +92,55 @@ public class JavaSerialization implements Serialization<Serializable> {
       oos.close();
     }
 
+    @Override
+    public Map<String, String> getMetadata() throws IOException {
+      return metadata;
+    }
   }
 
-  public boolean accept(Class<?> c) {
+  public boolean accept(Map<String, String> metadata) {
+    String intendedSerializer = metadata.get(SERIALIZATION_KEY);
+    if (intendedSerializer != null &&
+        !getClass().getName().equals(intendedSerializer)) {
+      return false;
+    }
+
+    Class<?> c = getClassFromMetadata(metadata);
     return Serializable.class.isAssignableFrom(c);
   }
 
-  public Deserializer<Serializable> getDeserializer(Class<Serializable> c) {
+  public DeserializerBase<Serializable> getDeserializer(
+      Map<String, String> metadata) {
     return new JavaSerializationDeserializer<Serializable>();
   }
 
-  public Serializer<Serializable> getSerializer(Class<Serializable> c) {
-    return new JavaSerializationSerializer();
+  public SerializerBase<Serializable> getSerializer(
+      Map<String, String> metadata) {
+    return new JavaSerializationSerializer<Serializable>(metadata);
   }
 
+  @SuppressWarnings("unchecked")
+  @Override
+  public RawComparator<Serializable> getRawComparator(
+      Map<String, String> metadata) {
+    Class<?> klazz = getClassFromMetadata(metadata);
+    if (null == klazz) {
+      throw new IllegalArgumentException(
+          "Cannot get comparator without " + SerializationBase.CLASS_KEY
+          + " set in metadata");
+    }
+
+    if (Serializable.class.isAssignableFrom(klazz)) {
+      try {
+        return (RawComparator<Serializable>) new JavaSerializationComparator();
+      } catch (IOException ioe) {
+        throw new IllegalArgumentException(
+            "Could not instantiate JavaSerializationComparator for type "
+            + klazz.getName(), ioe);
+      }
+    } else {
+      throw new IllegalArgumentException("Class " + klazz.getName()
+          + " is incompatible with JavaSerialization");
+    }
+  }
 }

src/java/org/apache/hadoop/io/serializer/LegacySerialization.java

@@ -21,6 +21,7 @@ package org.apache.hadoop.io.serializer;
 import java.util.Map;
 
 import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.io.RawComparator;
 
 /**
  * <p>
@@ -82,4 +83,14 @@ class LegacySerialization<T> extends SerializationBase<T> {
     return new LegacyDeserializer<T>(getDeserializer(c));
   }
 
+  @Override
+  public RawComparator<T> getRawComparator(Map<String, String> metadata) {
+    // Since this method is being added to an API meant to provide legacy
+    // compatability with deprecated serializers, leaving this as an incomplete
+    // stub.
+
+    throw new UnsupportedOperationException(
+        "LegacySerialization does not provide raw comparators");
+  }
+
 }

src/java/org/apache/hadoop/io/serializer/SerializationBase.java

@@ -22,6 +22,7 @@ import java.util.HashMap;
 import java.util.Map;
 
 import org.apache.hadoop.conf.Configured;
+import org.apache.hadoop.io.RawComparator;
 
 /**
  * <p>
@@ -88,4 +89,16 @@ public abstract class SerializationBase<T> extends Configured
       throw new IllegalArgumentException(e);
     }
   }
+
+  /** Provide a raw comparator for the specified serializable class.
+   * Requires a serialization-specific metadata entry to name the class
+   * to compare (e.g., "Serialized-Class" for JavaSerialization and
+   * WritableSerialization).
+   * @param metadata a set of string mappings providing serialization-specific
+   * arguments that parameterize the data being serialized/compared.
+   * @return a {@link RawComparator} for the given metadata.
+   * @throws UnsupportedOperationException if it cannot instantiate a RawComparator
+   * for this given metadata.
+   */
+  public abstract RawComparator<T> getRawComparator(Map<String,String> metadata);
 }

src/java/org/apache/hadoop/io/serializer/WritableSerialization.java

@@ -26,8 +26,12 @@ import java.io.OutputStream;
 import java.util.Map;
 
 import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.io.RawComparator;
 import org.apache.hadoop.io.Writable;
+import org.apache.hadoop.io.WritableComparable;
+import org.apache.hadoop.io.WritableComparator;
 import org.apache.hadoop.util.ReflectionUtils;
+import org.apache.hadoop.util.StringUtils;
 
 /**
  * A {@link SerializationBase} for {@link Writable}s that delegates to
@@ -35,7 +39,6 @@ import org.apache.hadoop.util.ReflectionUtils;
  * {@link Writable#readFields(java.io.DataInput)}.
  */
 public class WritableSerialization extends SerializationBase<Writable> {
-  
   static class WritableDeserializer extends DeserializerBase<Writable> {
 
     private Class<?> writableClass;
@@ -79,9 +82,25 @@ public class WritableSerialization extends SerializationBase<Writable> {
     
     private Map<String, String> metadata;
     private DataOutputStream dataOut;
+    private Class<?> serializedClass;
     
-    public WritableSerializer(Map<String, String> metadata) {
+    public WritableSerializer(Configuration conf,
+        Map<String, String> metadata) {
       this.metadata = metadata;
+
+      // If this metadata specifies a serialized class, memoize the
+      // class object for this.
+      String className = this.metadata.get(CLASS_KEY);
+      if (null != className) {
+        try {
+          this.serializedClass = conf.getClassByName(className);
+        } catch (ClassNotFoundException cnfe) {
+          throw new RuntimeException(cnfe);
+        }
+      } else {
+        throw new UnsupportedOperationException("the "
+            + CLASS_KEY + " metadata is missing, but is required.");
+      }
     }
     
     @Override
@@ -95,6 +114,10 @@ public class WritableSerialization extends SerializationBase<Writable> {
 
     @Override
     public void serialize(Writable w) throws IOException {
+      if (serializedClass != w.getClass()) {
+        throw new IOException("Type mismatch in serialization: expected "
+            + serializedClass + "; received " + w.getClass());
+      }
       w.write(dataOut);
     }
 
@@ -112,8 +135,10 @@ public class WritableSerialization extends SerializationBase<Writable> {
 
   @Override
   public boolean accept(Map<String, String> metadata) {
-    if (getClass().getName().equals(metadata.get(SERIALIZATION_KEY))) {
-      return true;
+    String intendedSerializer = metadata.get(SERIALIZATION_KEY);
+    if (intendedSerializer != null &&
+        !getClass().getName().equals(intendedSerializer)) {
+      return false;
     }
     Class<?> c = getClassFromMetadata(metadata);
     return c == null ? false : Writable.class.isAssignableFrom(c);
@@ -121,7 +146,7 @@ public class WritableSerialization extends SerializationBase<Writable> {
 
   @Override
   public SerializerBase<Writable> getSerializer(Map<String, String> metadata) {
-    return new WritableSerializer(metadata);
+    return new WritableSerializer(getConf(), metadata);
   }
   
   @Override
@@ -130,4 +155,17 @@ public class WritableSerialization extends SerializationBase<Writable> {
     return new WritableDeserializer(getConf(), c);
   }
 
+  @Override
+  @SuppressWarnings("unchecked")
+  public RawComparator<Writable> getRawComparator(Map<String, String> metadata) {
+    Class<?> klazz = getClassFromMetadata(metadata);
+    if (null == klazz) {
+      throw new IllegalArgumentException(
+          "Cannot get comparator without " + SerializationBase.CLASS_KEY
+          + " set in metadata");
+    }
+
+    return (RawComparator) WritableComparator.get(
+        (Class<WritableComparable>)klazz);
+  }
 }

src/java/org/apache/hadoop/io/serializer/avro/AvroComparator.java

@@ -0,0 +1,48 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hadoop.io.serializer.avro;
+
+import org.apache.avro.Schema;
+import org.apache.avro.io.BinaryData;
+import org.apache.hadoop.io.RawComparator;
+
+/**
+ * <p>
+ * A {@link RawComparator} that uses Avro to extract data from the
+ * source stream and compare their contents without explicit
+ * deserialization.
+ */
+public class AvroComparator<T extends Comparable<T>>
+    implements RawComparator<T> {
+
+  private final Schema schema;
+
+  public AvroComparator(final Schema s) {
+    this.schema = s;
+  }
+
+  public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {
+    return BinaryData.compare(b1, s1, b2, s2, schema);
+  }
+
+  public int compare(T t1, T t2) {
+    return t1.compareTo(t2);
+  }
+
+}

src/java/org/apache/hadoop/io/serializer/avro/AvroGenericSerialization.java

@@ -50,9 +50,8 @@ public class AvroGenericSerialization extends AvroSerialization<Object> {
   }
 
   @Override
-  protected Schema getSchema(Object t, Map<String, String> metadata) {
-    String jsonSchema = metadata.get(AVRO_SCHEMA_KEY);
-    return jsonSchema != null ? Schema.parse(jsonSchema) : GenericData.get().induce(t);
+  protected Schema getSchema(Map<String, String> metadata) {
+    return Schema.parse(metadata.get(AVRO_SCHEMA_KEY));
   }
 
   @Override

src/java/org/apache/hadoop/io/serializer/avro/AvroReflectSerialization.java

@@ -85,8 +85,9 @@ public class AvroReflectSerialization extends AvroSerialization<Object>{
   }
 
   @Override
-  protected Schema getSchema(Object t, Map<String, String> metadata) {
-    return ReflectData.get().getSchema(t.getClass());
+  protected Schema getSchema(Map<String, String> metadata) {
+    Class<?> c = getClassFromMetadata(metadata);
+    return ReflectData.get().getSchema(c);
   }
 
   @Override

src/java/org/apache/hadoop/io/serializer/avro/AvroSerialization.java

@@ -28,6 +28,7 @@ import org.apache.avro.io.BinaryDecoder;
 import org.apache.avro.io.BinaryEncoder;
 import org.apache.avro.io.DatumReader;
 import org.apache.avro.io.DatumWriter;
+import org.apache.hadoop.io.RawComparator;
 import org.apache.hadoop.io.serializer.DeserializerBase;
 import org.apache.hadoop.io.serializer.SerializationBase;
 import org.apache.hadoop.io.serializer.SerializerBase;
@@ -50,7 +51,7 @@ public abstract class AvroSerialization<T> extends SerializationBase<T> {
   /**
    * Return an Avro Schema instance for the given class and metadata.
    */
-  protected abstract Schema getSchema(T t, Map<String, String> metadata);
+  protected abstract Schema getSchema(Map<String, String> metadata);
 
   /**
    * Create and return Avro DatumWriter for the given metadata.
@@ -68,10 +69,13 @@ public abstract class AvroSerialization<T> extends SerializationBase<T> {
     private DatumWriter<T> writer;
     private BinaryEncoder encoder;
     private OutputStream outStream;
+    private Schema schema;
 
     AvroSerializer(Map<String, String> metadata) {
       this.metadata = metadata;
-      writer = getWriter(metadata);
+      this.writer = getWriter(metadata);
+      this.schema = getSchema(this.metadata);
+      writer.setSchema(this.schema);
     }
 
     @Override
@@ -88,7 +92,6 @@ public abstract class AvroSerialization<T> extends SerializationBase<T> {
 
     @Override
     public void serialize(T t) throws IOException {
-      writer.setSchema(getSchema(t, metadata));
       writer.write(t, encoder);
     }
 
@@ -127,4 +130,19 @@ public abstract class AvroSerialization<T> extends SerializationBase<T> {
 
   }
 
+  @Override
+  @SuppressWarnings("unchecked")
+  /**
+   * Provides a raw comparator for Avro-encoded serialized data.
+   * Requires that {@link AvroSerialization#AVRO_SCHEMA_KEY} be provided
+   * in the metadata argument.
+   * @param metadata the Avro-serialization-specific parameters being
+   * provided that detail the schema for the data to deserialize and compare.
+   * @return a RawComparator parameterized for the specified Avro schema.
+   */
+  public RawComparator<T> getRawComparator(Map<String, String> metadata) {
+    Schema schema = Schema.parse(metadata.get(AVRO_SCHEMA_KEY));
+    return new AvroComparator(schema);
+  }
+
 }

src/java/org/apache/hadoop/io/serializer/avro/AvroSpecificSerialization.java

@@ -24,6 +24,7 @@ import org.apache.avro.Schema;
 import org.apache.avro.generic.GenericDatumReader;
 import org.apache.avro.io.DatumReader;
 import org.apache.avro.io.DatumWriter;
+import org.apache.avro.specific.SpecificData;
 import org.apache.avro.specific.SpecificDatumReader;
 import org.apache.avro.specific.SpecificDatumWriter;
 import org.apache.avro.specific.SpecificRecord;
@@ -55,8 +56,9 @@ public class AvroSpecificSerialization
   }
 
   @Override
-  protected Schema getSchema(SpecificRecord t, Map<String, String> metadata) {
-    return t.getSchema();
+  protected Schema getSchema(Map<String, String> metadata) {
+    Class<?> c = getClassFromMetadata(metadata);
+    return SpecificData.get().getSchema(c);
   }
 
   @Override

src/test/core/org/apache/hadoop/io/serializer/TestRawComparators.java

@@ -0,0 +1,175 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hadoop.io.serializer;
+
+import static org.apache.hadoop.io.TestGenericWritable.CONF_TEST_KEY;
+import static org.apache.hadoop.io.TestGenericWritable.CONF_TEST_VALUE;
+import junit.framework.TestCase;
+
+import java.io.DataInput;
+import java.io.DataOutput;
+import java.io.IOException;
+import java.io.Serializable;
+import java.util.HashMap;
+import java.util.Map;
+
+import org.apache.avro.Schema;
+import org.apache.avro.util.Utf8;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.io.DataInputBuffer;
+import org.apache.hadoop.io.DataOutputBuffer;
+import org.apache.hadoop.io.RawComparator;
+import org.apache.hadoop.io.Text;
+import org.apache.hadoop.io.WritableComparable;
+import org.apache.hadoop.io.serializer.avro.AvroSerialization;
+import org.apache.hadoop.io.serializer.avro.AvroGenericSerialization;
+import org.apache.hadoop.util.GenericsUtil;
+
+/**
+ * Test the getRawComparator API of the various serialization systems.
+ */
+public class TestRawComparators extends TestCase {
+
+  private Configuration conf;
+
+  public void setUp() {
+    conf = new Configuration();
+  }
+
+  /** A WritableComparable that is guaranteed to use the
+   * generic WritableComparator.
+   */
+  public static class FooWritable implements WritableComparable<FooWritable> {
+
+    public long val;
+
+    public FooWritable() {
+      this.val = 0;
+    }
+
+    public FooWritable(long v) {
+      this.val = v;
+    }
+
+    public void write(DataOutput out) throws IOException {
+      out.writeLong(val);
+    }
+
+    public void readFields(DataInput in) throws IOException {
+      val = in.readLong();
+    }
+
+    public int compareTo(FooWritable other) {
+      return new Long(val).compareTo(other.val);
+    }
+  }
+
+  @SuppressWarnings("unchecked")
+  private void runComparisonTest(Object low, Object high) throws Exception {
+    Map<String, String> metadata =
+          SerializationBase.getMetadataFromClass(GenericsUtil.getClass(low));
+    runComparisonTest(low, high, metadata);
+  }
+
+  @SuppressWarnings("unchecked")
+  private void runComparisonTest(Object low, Object high,
+      Map<String, String> metadata) throws Exception {
+
+    DataOutputBuffer out1 = new DataOutputBuffer();
+    DataOutputBuffer out2 = new DataOutputBuffer();
+    DataInputBuffer in1 = new DataInputBuffer();
+    DataInputBuffer in2 = new DataInputBuffer();
+
+    SerializationFactory factory = new SerializationFactory(conf);
+
+    // Serialize some data to two byte streams.
+    SerializerBase serializer = factory.getSerializer(metadata);
+    assertNotNull("Serializer is null!", serializer);
+
+    serializer.open(out1);
+    serializer.serialize(low);
+    serializer.close();
+
+    serializer.open(out2);
+    serializer.serialize(high);
+    serializer.close();
+
+    // Shift that data into an input buffer.
+    in1.reset(out1.getData(), out1.getLength());
+    in2.reset(out2.getData(), out2.getLength());
+
+    // Get the serialization and then the RawComparator;
+    // use these to compare the data in the input streams and
+    // assert that the low stream (1) is less than the high stream (2).
+
+    SerializationBase serializationBase = factory.getSerialization(metadata);
+    assertNotNull("Null SerializationBase!", serializationBase);
+
+    RawComparator rawComparator = serializationBase.getRawComparator(metadata);
+    assertNotNull("Null raw comparator!", rawComparator);
+    int actual = rawComparator.compare(in1.getData(), 0, in1.getLength(),
+        in2.getData(), 0, in2.getLength());
+    assertTrue("Did not compare FooWritable correctly", actual < 0);
+  }
+
+  public void testBasicWritable() throws Exception {
+    // Test that a WritableComparable can be used with this API
+    // correctly.
+
+    FooWritable low = new FooWritable(10);
+    FooWritable high = new FooWritable(42);
+
+    runComparisonTest(low, high);
+  }
+
+  public void testTextWritable() throws Exception {
+    // Test that a Text object (which uses Writable serialization, and
+    // has its own RawComparator implementation) can be used with this
+    // API correctly.
+
+    Text low = new Text("aaa");
+    Text high = new Text("zzz");
+
+    runComparisonTest(low, high);
+  }
+
+  public void testAvroComparator() throws Exception {
+    // Test a record created via an Avro schema that doesn't have a fixed
+    // class associated with it.
+
+    Schema s1 = Schema.create(Schema.Type.INT);
+
+    // Create a metadata mapping containing an Avro schema and a request to use
+    // Avro generic serialization.
+    Map<String, String> metadata = new HashMap<String, String>();
+    metadata.put(AvroSerialization.AVRO_SCHEMA_KEY, s1.toString());
+    metadata.put(SerializationBase.SERIALIZATION_KEY,
+       AvroGenericSerialization.class.getName());
+
+    runComparisonTest(new Integer(42), new Integer(123), metadata);
+
+    // Now test it with a string record type.
+    Schema s2 = Schema.create(Schema.Type.STRING);
+    metadata.put(AvroSerialization.AVRO_SCHEMA_KEY, s2.toString());
+    runComparisonTest(new Utf8("baz"), new Utf8("meep"), metadata);
+
+  }
+
+}

src/test/core/org/apache/hadoop/io/serializer/TestWritableSerialization.java

@@ -22,10 +22,17 @@ import static org.apache.hadoop.io.TestGenericWritable.CONF_TEST_KEY;
 import static org.apache.hadoop.io.TestGenericWritable.CONF_TEST_VALUE;
 import junit.framework.TestCase;
 
+import java.io.IOException;
+import java.util.Map;
+
 import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.io.DataOutputBuffer;
 import org.apache.hadoop.io.Text;
+import org.apache.hadoop.io.TestGenericWritable.Foo;
+import org.apache.hadoop.io.TestGenericWritable.Bar;
 import org.apache.hadoop.io.TestGenericWritable.Baz;
 import org.apache.hadoop.io.TestGenericWritable.FooGenericWritable;
+import org.apache.hadoop.util.GenericsUtil;
 
 public class TestWritableSerialization extends TestCase {
 
@@ -53,4 +60,56 @@ public class TestWritableSerialization extends TestCase {
     assertEquals(baz, result);
     assertNotNull(result.getConf());
   }
+
+  @SuppressWarnings("unchecked")
+  public void testReuseSerializer() throws IOException {
+    // Test that we can write multiple objects of the same type
+    // through the same serializer.
+
+    DataOutputBuffer out = new DataOutputBuffer();
+    SerializationFactory factory = new SerializationFactory(
+        new Configuration());
+
+    // Create a few Foo objects and serialize them.
+    Foo foo = new Foo();
+    Foo foo2 = new Foo();
+    Map<String, String> metadata = SerializationBase.getMetadataFromClass(
+        GenericsUtil.getClass(foo));
+
+    SerializerBase fooSerializer = factory.getSerializer(metadata);
+    fooSerializer.open(out);
+    fooSerializer.serialize(foo);
+    fooSerializer.serialize(foo2);
+    fooSerializer.close();
+
+    out.reset();
+
+    // Create a new serializer for Bar objects
+    Bar bar = new Bar();
+    Baz baz = new Baz(); // Baz inherits from Bar.
+    metadata = SerializationBase.getMetadataFromClass(
+        GenericsUtil.getClass(bar));
+    // Check that we can serialize Bar objects.
+    SerializerBase barSerializer = factory.getSerializer(metadata);
+    barSerializer.open(out);
+    barSerializer.serialize(bar); // this should work.
+    try {
+      // This should not work. We should not allow subtype serialization.
+      barSerializer.serialize(baz);
+      fail("Expected IOException serializing baz via bar serializer.");
+    } catch (IOException ioe) {
+      // Expected.
+    }
+
+    try {
+      // This should not work. Disallow unrelated type serialization.
+      barSerializer.serialize(foo);
+      fail("Expected IOException serializing foo via bar serializer.");
+    } catch (IOException ioe) {
+      // Expected.
+    }
+
+    barSerializer.close();
+    out.reset();
+  }
 }