Flat Buffers - Nested Table

Overview

Here, we will see how to create a nested table in Flat Buffers. It is equivalent to a nested Java class.

Continuing with our theater example from Flat Buffers - String chapter, following is the syntax that we need to have to instruct FlatBuffers that we will be creating a nested table −

theater.fbs

namespace com.tutorialspoint.theater;

table Theater {
   owner: TheaterOwner;
}

table TheaterOwner {
	name:string;
	address:string;
}
root_type Theater;

Now our Theater table contains a nested table, i.e., information about the owner of the theater.

Creating Java Classes from fbs File

To use FlatBuffers, we will now have to use flatc binary to create the required classes from this ".fbs" file. Let us see how to do that −

flatc --java theater.fbs

This will create a Theater and TheaterOwner classes in com > tutorialspoint > theater folder in current directory. We're using this class in our application similar to as done in Flat Buffers - Schema chapter.

Using Java Classes created from fbs File

TheaterWriter.java

package com.tutorialspoint.theater;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

import com.google.flatbuffers.FlatBufferBuilder;

public class TheaterWriter {
   public static void main(String[] args) throws FileNotFoundException, IOException {
      // create a flat buffer builder
      // it will be used to create Theater FlatBuffer
      FlatBufferBuilder builder = new FlatBufferBuilder(1024);
      
      // create offset for TheaterOwner
      int ownerName = builder.createString("Mery");
      int ownerAddress = builder.createString("Avenue 4");
      int owner = TheaterOwner.createTheaterOwner(builder, ownerName, ownerAddress);
      
      // create theater FlatBuffers using startTheater() method
      Theater.startTheater(builder);
      // add details to the Theater FlatBuffer
      Theater.addOwner(builder, owner);      
      
      // mark end of data being entered in Greet FlatBuffer
      int theater = Theater.endTheater(builder);

      // finish the builder
      builder.finish(theater);

      // get the bytes to be stored
      byte[] data = builder.sizedByteArray();

      String filename = "theater_flatbuffers_output";
      System.out.println("Saving theater to file: " + filename);
      // write the builder content to the file named theater_flatbuffers_output
      try(FileOutputStream output = new FileOutputStream(filename)){
         output.write(data);
      }
      System.out.println("Saved theater with following data to disk: \n" + theater);
   }
}	

Next, we will have a reader to read the theater information −

TheaterReader.java

package com.tutorialspoint.theater;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.ByteBuffer;

public class TheaterReader {
   public static void main(String[] args) throws FileNotFoundException, IOException {

      String filename = "theater_flatbuffers_output";
      System.out.println("Reading from file " + filename);
      try(FileInputStream input = new FileInputStream(filename)) {
         // get the serialized data
         byte[] data = input.readAllBytes();
         ByteBuffer buf = ByteBuffer.wrap(data);
         // read the root object in serialized data
         Theater theater = Theater.getRootAsTheater(buf);
         // print theater values 
         System.out.println("Owner Details: ");
         TheaterOwner owner = theater.owner();
         System.out.println("Name: " + owner.name());
         System.out.println("Address: " + owner.address());        
      }
   }
}

Compile the project

Now that we have set up the reader and the writer, let us compile the project.

mvn clean install

Serialize the Java Object

Now, post compilation, let us execute the writer first −

java -cp .\target\flatbuffers-tutorial-1.0.jar com.tutorialspoint.theater.TheaterWriter

Saving theater to file: theater_flatbuffers_output
Saved theater with following data to disk:
56

Deserialize the Serialized Object

Now, let us execute the reader to read from the same file −

java -cp .\target\flatbuffers-tutorial-1.0.jar com.tutorialspoint.theater.TheaterReader

Reading from file theater_flatbuffers_output
Owner Details:
Name: Mery
Address: Avenue 4

So, as we see, we are able to read the serialized nested table/object by deserializing the binary data to the Theater object.