Apache Kafka: ...StringDeserializer is not an instance of ...Deserializer

This might be the problem with Kafka classloading.
Setting classloader to null might help.

...
Thread currentThread = Thread.currentThread();    
ClassLoader savedClassLoader = currentThread.getContextClassLoader();

currentThread.setContextClassLoader(null);
KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);

currentThread.setContextClassLoader(savedClassLoader);
...

There is full explanation:
https://stackoverflow.com/a/50981469/1673775

Not sure if this is what finally fixed your error, but note that when using spring-kafka-test (version 2.1.x, starting with version 2.1.5) with the 1.1.x kafka-clients jar, you will need to override certain transitive dependencies as follows:

<dependency>
    <groupId>org.springframework.kafka</groupId>
    <artifactId>spring-kafka</artifactId>
    <version>${spring.kafka.version}</version>
</dependency>

<dependency>
    <groupId>org.springframework.kafka</groupId>
    <artifactId>spring-kafka-test</artifactId>
    <version>${spring.kafka.version}</version>
    <scope>test</scope>
</dependency>

<dependency>
    <groupId>org.apache.kafka</groupId>
    <artifactId>kafka-clients</artifactId>
    <version>1.1.1</version>
</dependency>

<dependency>
    <groupId>org.apache.kafka</groupId>
    <artifactId>kafka-clients</artifactId>
    <version>1.1.1</version>
    <classifier>test</classifier>
</dependency>

<dependency>
    <groupId>org.apache.kafka</groupId>
    <artifactId>kafka_2.11</artifactId>
    <version>1.1.1</version>
    <scope>test</scope>
</dependency>

<dependency>
    <groupId>org.apache.kafka</groupId>
    <artifactId>kafka_2.11</artifactId>
    <version>1.1.1</version>
    <classifier>test</classifier>
    <scope>test</scope>
</dependency>

so it could've been a problem with your transitive dependency for sure

Your Custom class need to implement, org.apache.kafka.common.serialization.Deserializer.

like

import org.apache.kafka.common.header.Headers;
import org.apache.kafka.common.serialization.Serializer;
import org.apache.kafka.common.serialization.Deserializer;
import org.codehaus.jackson.map.ObjectMapper;

import java.io.Serializable;
import java.util.Map;

//Developed by Arun Singh
public class Employee implements Serializable, Serializer, **Deserializer** {

@Override
    public Object deserialize(String s, byte[] bytes) {
        ObjectMapper mapper = new ObjectMapper();
        Employee employee = null;
        try {
            //employee = mapper.readValue(bytes, Employee.class);
            employee = mapper.readValue(bytes.toString(), Employee.class);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return employee;
    }

    @Override
    public Object deserialize(String topic, Headers headers, byte[] data) {
        ObjectMapper mapper = new ObjectMapper();
        Employee employee = null;
        try {
            //employee = mapper.readValue(bytes, Employee.class);
            employee = mapper.readValue(data.toString(), Employee.class);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return employee;
    }

    public void close() {

    }
}