Added initial sketch of JSON codec abstraction and related codec tracking service.

+/*
+ * 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.onlab.onos.codec;
+
+import java.util.Set;
+
+/**
+ * Service for registering and retrieving JSON codecs for various entities.
+ */
+public interface CodecService {
+
+    /**
+     * Returns the set of classes with currently registered codecs.
+     *
+     * @return set of entity classes
+     */
+    Set<Class<?>> getCodecs();
+
+    /**
+     * Returns the JSON codec for the specified entity class.
+     *
+     * @param entityClass entity class
+     * @return JSON codec; null if no codec available for the class
+     */
+    JsonCodec getCodec(Class<?> entityClass);
+
+    /**
+     * Registers the specified JSON codec for the given entity class.
+     *
+     * @param entityClass entity class
+     * @param codec       JSON codec
+     */
+    void registerCodec(Class<?> entityClass, JsonCodec codec);
+
+    /**
+     * Unregisters the JSON codec for the specified entity class.
+     *
+     * @param entityClass entity class
+     */
+    void unregisterCodec(Class<?> entityClass);
+
+}

+/*
+ * 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.onlab.onos.codec;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.node.ArrayNode;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Abstraction of a codec capable for encoding/decoding arbitrary objects to/from JSON.
+ */
+public abstract class JsonCodec<T> {
+
+    /**
+     * Encodes the specified entity into JSON.
+     *
+     * @param entity entity to encode
+     * @param mapper object mapper
+     * @return JSON node
+     * @throws java.lang.UnsupportedOperationException if the codec does not
+     *                                                 support encode operations
+     */
+    public abstract ObjectNode encode(T entity, ObjectMapper mapper);
+
+    /**
+     * Decodes the specified entity from JSON.
+     *
+     * @param json JSON to decode
+     * @return decoded entity
+     * @throws java.lang.UnsupportedOperationException if the codec does not
+     *                                                 support decode operations
+     */
+    public abstract T decode(ObjectNode json);
+
+    /**
+     * Encodes the collection of the specified entities.
+     *
+     * @param entities collection of entities to encode
+     * @param mapper   object mapper
+     * @return JSON array
+     * @throws java.lang.UnsupportedOperationException if the codec does not
+     *                                                 support encode operations
+     */
+    public ArrayNode encode(Iterable<T> entities, ObjectMapper mapper) {
+        ArrayNode result = mapper.createArrayNode();
+        for (T entity : entities) {
+            result.add(encode(entity, mapper));
+        }
+        return result;
+    }
+
+    /**
+     * Decodes the specified JSON array into a collection of entities.
+     *
+     * @param json JSON array to decode
+     * @return collection of decoded entities
+     * @throws java.lang.UnsupportedOperationException if the codec does not
+     *                                                 support decode operations
+     */
+    public List<T> decode(ArrayNode json) {
+        List<T> result = new ArrayList<>();
+        for (JsonNode node : json) {
+            result.add(decode((ObjectNode) node));
+        }
+        return result;
+    }
+
+}

+/*
+ * 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.
+ */
+
+/**
+ * Base JSON codec abstraction and a service for tracking various JSON codecs.
+ */
+package org.onlab.onos.codec;
\ No newline at end of file

+/*
+ * 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.onlab.onos.codec;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.node.ArrayNode;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+import com.google.common.collect.ImmutableList;
+import org.junit.Test;
+
+import java.util.List;
+import java.util.Objects;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * Test of the base JSON codec abstraction.
+ */
+public class JsonCodecTest {
+
+    private static class Foo {
+        final String name;
+
+        Foo(String name) {
+            this.name = name;
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hash(name);
+        }
+
+        @Override
+        public boolean equals(Object obj) {
+            if (this == obj) {
+                return true;
+            }
+            if (obj == null || getClass() != obj.getClass()) {
+                return false;
+            }
+            final Foo other = (Foo) obj;
+            return Objects.equals(this.name, other.name);
+        }
+    }
+
+    private static class FooCodec extends JsonCodec<Foo> {
+        @Override
+        public ObjectNode encode(Foo entity, ObjectMapper mapper) {
+            return mapper.createObjectNode().put("name", entity.name);
+        }
+
+        @Override
+        public Foo decode(ObjectNode json) {
+            return new Foo(json.get("name").asText());
+        }
+    }
+
+    @Test
+    public void encode() {
+        Foo f1 = new Foo("foo");
+        Foo f2 = new Foo("bar");
+        FooCodec codec = new FooCodec();
+        ImmutableList<Foo> entities = ImmutableList.of(f1, f2);
+        ArrayNode json = codec.encode(entities, new ObjectMapper());
+        List<Foo> foos = codec.decode(json);
+        assertEquals("incorrect encode/decode", entities, foos);
+    }
+
+}
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