Add StringDecoder for kafka messages

- Add StringDecoder for decoding kafka key/value messages

Refactor StringDecoder

spring-integration-kafka/src/main/java/org/springframework/integration/kafka/serializer/common/StringDecoder.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2002-2014 the original author or authors.
+ *
+ * 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.springframework.integration.kafka.serializer.common;
+
+import java.util.Properties;
+
+import kafka.serializer.Decoder;
+import kafka.utils.VerifiableProperties;
+
+
+/**
+ * String Decoder for Kafka message key/value decoding.
+ * The Default decoder returns the same byte array it takes in.
+ *
+ * @author Soby Chacko
+ * @author Ilayaperumal Gopinathan
+ */
+public class StringDecoder implements Decoder<String> {
+
+	private final kafka.serializer.StringDecoder stringDecoder;
+
+	public StringDecoder() {
+		this("UTF8");
+	}
+
+	public StringDecoder(final String encoding) {
+		final Properties props = new Properties();
+		props.put("serializer.encoding", encoding);
+		this.stringDecoder = new kafka.serializer.StringDecoder(new VerifiableProperties(props));
+	}
+
+	@Override
+	public String fromBytes(byte[] bytes) {
+		return this.stringDecoder.fromBytes(bytes);
+	}
+
+}