An introduction to Kryo

Employee.java

/* A simple Employee class */
public class Employee  { 
   private String name; 
   private Date birthDate; 
} 

EmployeeSerializer.java

/*This is a cusotm EmployeeSerializer class which will be used to writed to a file and read it back*/
public class EmployeeSerializer extends Serializer<Employee> { 
  
   @Override 
   public void write(Kryo kryo, Output output, Employee employee) { 
       output.writeString(employee.getName()); 
       output.writeLong(employee.getBirthDate().getTime()); 
   } 

   @Override 
   public Employee read(Kryo kryo, Input input, Class<? extends Employee> aClass) { 
       // we need default constructor 
       Employee employee = new Employee(); 
       employee.setName(input.readString()); 
       employee.setBirthDate(new Date(input.readLong())); 
       return employee; 

   } 

} 

KryoTest.java

package com.example;

import com.esotericsoftware.kryo.Kryo;
import com.esotericsoftware.kryo.io.Input;
import com.esotericsoftware.kryo.io.Output;
import com.esotericsoftware.kryo.serializers.JavaSerializer;
import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.util.Date;

public class KryoTest {

    private Kryo kryo;
    private String fileName = "test_file.dat";
    private Output output;
    private Input input;

    @Before
    public void setUp() throws FileNotFoundException {
        kryo = new Kryo();
        output = new Output(new FileOutputStream(fileName));
        input = new Input(new FileInputStream(fileName));
    }

    @Test
    public void testKryoSerializeAndDeserializeString(){
        Object obj = "Hello World!!!";
        kryo.writeClassAndObject(output, obj);
        output.close();
        Object readObj = kryo.readClassAndObject(input);
        input.close();
        Assert.assertEquals(readObj, obj);
    }

    @Test
    public void testKryoSerializeAndDeserializeMultiObj(){
        String string = "Hello World!!!";
        int value = 55;
        Date date = new Date(651943079000L);
        kryo.writeObject(output, string);
        kryo.writeObject(output, value);
        // Date class need to be registered.
        kryo.register(Date.class);
        kryo.writeObject(output, date);
        output.close();
        String readString = kryo.readObject(input, String.class);
        int readValue = kryo.readObject(input, int.class);
        Date readDate = kryo.readObject(input,Date.class);
        input.close();
        Assert.assertEquals(string, readString);
        Assert.assertEquals(value, readValue);
        Assert.assertEquals(date, readDate);
    }

    @Test
    public void testSerializeAndDeserializeEmployee(){
        // registering class
        kryo.register(Employee.class);
        kryo.register(Date.class);
        Employee employee = new Employee("Rose", new Date(651943079000L));
        kryo.writeObject(output, employee);
        output.close();
        Employee readEmployee = kryo.readObject(input, Employee.class);
        input.close();
        Assert.assertEquals(employee.getName(), readEmployee.getName());
        Assert.assertEquals(employee.getBirthDate(), readEmployee.getBirthDate());
    }

    @Test
    public void testCustomSerializerAndDeserializeEmployee(){
        Employee employee = new Employee("Rose", new Date(651943079000L));
        kryo.register(Employee.class, new EmployeeSerializer());
        kryo.writeObject(output, employee);
        output.close();
        Employee readEmployee = kryo.readObject(input, Employee.class);
        input.close();
        Assert.assertEquals(employee.getName(), readEmployee.getName());
        Assert.assertEquals(employee.getBirthDate(), readEmployee.getBirthDate());
    }

    @Test
    public void testJavaSerializerAndDeserializeEStudent(){
        Student student = new Student("Rose", "Geller", new Date(951943079000L), new String[]{"Science", "History"}, "A", "GOVT");
        kryo.register(Student.class, new JavaSerializer());
        kryo.writeObject(output, student);
        output.close();

        Student readStudent = kryo.readObject(input, Student.class);
        input.close();
        Assert.assertEquals(student.getFirstName(), readStudent.getFirstName());
    }

}

MultiObjectTest.java

/* In this method we are going write multiple objects to a file and read them back */
@Test 
public void testKryoSerializeAndDeserializeMultiObj(){ 
   String string = "Hello World!!!"; 
   int value = 55; 
   Date date = new Date(651943079000L); 
   // register Date.class with kryo.register else it will throw an exception
   kryo.register(Date.class); 
  
   kryo.writeObject(output, string); 
   kryo.writeObject(output, value); 
   kryo.writeObject(output, date); 
   // close output resource
   output.close(); 
   String readString = kryo.readObject(input, String.class); 
   int readValue = kryo.readObject(input, int.class); 
   Date readDate = kryo.readObject(input,Date.class); 
   // close the input resource
   input.close(); 
  
   // verify the contents 
   Assert.assertEquals(string, readString); 
   Assert.assertEquals(value, readValue); 
   Assert.assertEquals(date, readDate); 

} 

pom_sample.xml

<dependencies> 
        <dependency> 
            <groupId>com.esotericsoftware</groupId> 
            <artifactId>kryo</artifactId> 
            <version>5.3.0</version> 
        </dependency> 
        <dependency> 
            <groupId>org.projectlombok</groupId> 
            <artifactId>lombok</artifactId> 
            <version>1.18.24</version> 
            <scope>provided</scope> 
        </dependency> 
        <dependency> 
            <groupId>junit</groupId> 
            <artifactId>junit</artifactId> 
            <version>4.13.1</version> 
            <scope>test</scope> 
        </dependency> 
</dependencies> 

Sample2.java

/*This is a sample on how to implement KryoSerializable methods */
public class Employee implements KryoSerializable { 

    public void write(Kryo kryo, Output output) { 
        output.writeString(name); 
        // ... 

    } 

    public void read(Kryo kryo, Input input) { 
        name = input.readString(); 
        // ... 

    } 
} 

Setup.java

/* This method is executed before each test */
@Before 
public void setUp() throws FileNotFoundException { 
   kryo = new Kryo(); 
   output = new Output(new FileOutputStream(fileName)); 
   input = new Input(new FileInputStream(fileName)); 
} 

Student.java

/* Student class with more fields */
@AllArgsConstructor 
@NoArgsConstructor 
@Getter 
@Setter 
public class Student implements Serializable { 
   private String firstName; 
   private String lastName; 
   private Date dateOfBirth; 
   private String[] courses; 
   private String classGroup; 
   private String schoolName; 

} 

Test1.java

/* In this we are going to write an object to a file and read the same file and verify the objects are same */ 
@Test 
public void testKryoSerializeAndDeserializeString(){ 
   Object obj = "Hello World!!!"; 
   kryo.writeClassAndObject(output, obj); 
   output.close(); 
   Object readObj = kryo.readClassAndObject(input); 
   input.close(); 
   Assert.assertEquals(readObj, obj); 
} 

TestCustomEmployee.java

/* In this test case we are going to serialize and deserialize Employee class with custom serializer we created */
@Test 
public void testCustomSerializerAndDeserializeEmployee(){ 
   Employee employee = new Employee("Rose", new Date(651943079000L)); 
   // register Employee class and its serialzer with kryo.
   kryo.register(Employee.class, new EmployeeSerializer()); 
   kryo.writeObject(output, employee); 
   // close output resource
   output.close(); 
  
   Employee readEmployee = kryo.readObject(input, Employee.class); 
   // close input resource
   input.close(); 
   // verify contents
   Assert.assertEquals(employee.getName(), readEmployee.getName()); 
   Assert.assertEquals(employee.getBirthDate(), readEmployee.getBirthDate()); 

} 

TestEmployee.java

/* In this method we are going to serialze and Deserialize the Employee class. */
@Test 
public void testSerializeAndDeserializeEmployee(){ 
   // register Employee and Date
   kryo.register(Employee.class); 
   kryo.register(Date.class); 

   Employee employee = new Employee("Rose", new Date(651943079000L)); 
   kryo.writeObject(output, employee); 
   // close output resource
   output.close(); 

   Employee readEmployee = kryo.readObject(input, Employee.class); 
   // close input resource
   input.close(); 
   // verify contents
   Assert.assertEquals(employee.getName(), readEmployee.getName()); 
   Assert.assertEquals(employee.getBirthDate(), readEmployee.getBirthDate()); 
} 

TestStudent.java

/*In this testcase we are going to serialze and deserialize student class using JavaSerializer. */
@Test 
public void testJavaSerializerAndDeserializeEStudent(){ 

   Student student = new Student("Rose", "Geller", new Date(951943079000L), new String[]{"Science", "History"}, "A", "GOVT"); 
   // register student class with kryo.
   kryo.register(Student.class, new JavaSerializer()); 
   
   kryo.writeObject(output, student); 
   // close output resource 
   output.close(); 

   Student readStudent = kryo.readObject(input, Student.class); 
   // close input resource
   input.close(); 
   // verify contents
   Assert.assertEquals(student.getFirstName(), readStudent.getFirstName()); 

}