AVRO - Deserialization By Generating Class

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As described earlier, one can read an Avro schema into a program either by generating a class corresponding to the schema or by using the parsers library. This chapter describes how to read the schema by generating a class and Deserialize the data using Avro.

Deserialization by Generating a Class

The serialized data is stored in the file emp.avro. You can deserialize and read it using Avro.

Serialized Data is Stored

Follow the procedure given below to deserialize the serialized data from a file.

Step 1

Create an object of DatumReader interface using SpecificDatumReader class.

DatumReader<emp>empDatumReader = new SpecificDatumReader<emp>(emp.class);

Step 2

Instantiate DataFileReader for emp class. This class reads serialized data from a file. It requires the Dataumeader object, and path of the file where the serialized data is existing, as a parameters to the constructor.

DataFileReader<emp> dataFileReader = new DataFileReader(new File("/path/to/emp.avro"), empDatumReader);

Step 3

Print the deserialized data, using the methods of DataFileReader.

  • The hasNext() method will return a boolean if there are any elements in the Reader.

  • The next() method of DataFileReader returns the data in the Reader.

while(dataFileReader.hasNext()){

   em=dataFileReader.next(em);
   System.out.println(em);
}

Example – Deserialization by Generating a Class

The following complete program shows how to deserialize the data in a file using Avro.

import java.io.File;
import java.io.IOException;

import org.apache.avro.file.DataFileReader;
import org.apache.avro.io.DatumReader;
import org.apache.avro.specific.SpecificDatumReader;

public class Deserialize {
   public static void main(String args[]) throws IOException{
	
      //DeSerializing the objects
      DatumReader<emp> empDatumReader = new SpecificDatumReader<emp>(emp.class);
		
      //Instantiating DataFileReader
      DataFileReader<emp> dataFileReader = new DataFileReader<emp>(new
         File("/home/Hadoop/Avro_Work/with_code_genfile/emp.avro"), empDatumReader);
      emp em=null;
		
      while(dataFileReader.hasNext()){
      
         em=dataFileReader.next(em);
         System.out.println(em);
      }
   }
}

Browse into the directory where the generated code is placed. In this case, at home/Hadoop/Avro_work/with_code_gen.

$ cd home/Hadoop/Avro_work/with_code_gen/

Now, copy and save the above program in the file named DeSerialize.java. Compile and execute it as shown below −

$ javac Deserialize.java
$ java Deserialize

Output

{"name": "omar", "id": 1, "salary": 30000, "age": 21, "address": "Hyderabad"}
{"name": "ram", "id": 2, "salary": 40000, "age": 30, "address": "Hyderabad"}
{"name": "robbin", "id": 3, "salary": 35000, "age": 25, "address": "Hyderabad"}
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