Config utility class for JSON handling.

- Config class, which can also be used as CodecContext
  to easily deal with ONOS related JSON Strings

Change-Id: Ie6930f892d214b27725d98580c4ff83098e1c9ca
diff --git a/core/api/src/main/java/org/onosproject/net/config/BaseConfig.java b/core/api/src/main/java/org/onosproject/net/config/BaseConfig.java
new file mode 100644
index 0000000..33d485f
--- /dev/null
+++ b/core/api/src/main/java/org/onosproject/net/config/BaseConfig.java
@@ -0,0 +1,77 @@
+/*
+ * Copyright 2016-present Open Networking Laboratory
+ *
+ * Licensed 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.onosproject.net.config;
+
+import static org.slf4j.LoggerFactory.getLogger;
+
+import java.io.IOException;
+
+import org.onlab.osgi.DefaultServiceDirectory;
+import org.onlab.osgi.ServiceDirectory;
+import org.onosproject.codec.CodecContext;
+import org.onosproject.codec.CodecService;
+import org.onosproject.codec.JsonCodec;
+import org.slf4j.Logger;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.common.base.Throwables;
+
+/**
+ * {@link Config} with access to {@link CodecService}.
+ *
+ * @param <S> type of subject
+ */
+public abstract class BaseConfig<S>
+    extends Config<S>
+    implements CodecContext {
+
+    // might need to make it non-final for unit testing
+    private static final ServiceDirectory SERVICES = new DefaultServiceDirectory();
+    private static final Logger log = getLogger(BaseConfig.class);
+
+    @Override
+    public <T> JsonCodec<T> codec(Class<T> entityClass) {
+        return getService(CodecService.class).getCodec(entityClass);
+    }
+
+    @Override
+    public <T> T getService(Class<T> serviceClass) {
+        return SERVICES.get(serviceClass);
+    }
+
+    @Override
+    public ObjectMapper mapper() {
+        return mapper;
+    }
+
+    /**
+     * Decodes the specified entity from JSON using codec
+     * registered to this context.
+     *
+     * @param json    JSON String to decode
+     * @param entityClass entity class
+     * @param <T> entity class type
+     * @return decoded entity
+     */
+    protected <T> T decode(String json, Class<T> entityClass) {
+        try {
+            return decode(mapper().readTree(json), entityClass);
+        } catch (IOException e) {
+            log.error("Exception caught.", e);
+            throw Throwables.propagate(e);
+        }
+    }
+
+}