Protocol Buffers - map

Overview

The map data type is one of the composite datatypes of Protobuf. It translates to a java.util.Map interface in Java.

Continuing with our theater example from Protocol Buffers - String chapter, following is the syntax that we need to have to instruct Protobuf that we will be creating a repeated −

theater.proto

syntax = "proto3";
package theater;
option java_package = "com.tutorialspoint.theater";

message Theater {
   map<string, int32> movieTicketPrice = 9;
}

Now our message class contains a map of movie and their ticket price. Note that although we have "string -> int" map, we can as well have number, bool, and custom data types. However, note that we cannot have a nested map. It also has a position which is what Protobuf uses while serialization and deserialization. Each attribute of a member needs to have a unique number assigned.

Creating Java Classes from Proto File

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

protoc  --java_out=. theater.proto

This will create a TheaterOuterClass.java class in com > tutorialspoint > theater folder in current directory. We're using this class in our application similar to as done in Protocol Buffers - Basic App chapter.

Using Java Classes created from Proto File

TheaterWriter.java

package com.tutorialspoint.theater;

import java.io.FileOutputStream;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import com.tutorialspoint.theater.TheaterOuterClass.Theater;

public class TheaterWriter{
   public static void main(String[] args) throws IOException {
      Map<String, Integer> ticketPrice = new HashMap<>();
	    
      ticketPrice.put("Avengers Endgame", 700);
      ticketPrice.put("Captain America", 200);
      ticketPrice.put("Wonder Woman 1984", 400);
	    
      Theater theater = Theater.newBuilder()
         .putAllMovieTicketPrice(ticketPrice)
         .build();
		
      String filename = "theater_protobuf_output";
      System.out.println("Saving theater information to file: " + filename);
		
      try(FileOutputStream output = new FileOutputStream(filename)){
         theater.writeTo(output);
      }
      System.out.println("Saved theater information 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.IOException;

import com.tutorialspoint.theater.TheaterOuterClass.Theater;
import com.tutorialspoint.theater.TheaterOuterClass.Theater.Builder;

public class TheaterReader{
   public static void main(String[] args) throws IOException {
      Builder theaterBuilder = Theater.newBuilder();

      String filename = "theater_protobuf_output";
      System.out.println("Reading from file " + filename);
        
      try(FileInputStream input = new FileInputStream(filename)) {
         Theater theater = theaterBuilder.mergeFrom(input).build();
         System.out.println(theater);
      }
   }
}

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\protobuf-tutorial-1.0.jar com.tutorialspoint.theater.TheaterWriter

Saving theater information to file: theater_protobuf_output
Saved theater information with following data to disk:
movieTicketPrice {
   key: "Avengers Endgame"
   value: 700
}
movieTicketPrice {
   key: "Captain America"
   value: 200
}
movieTicketPrice {
   key: "Wonder Woman 1984"
   value: 400
}

Deserialize the Serialized Object

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

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

Reading from file theater_protobuf_output
movieTicketPrice {
   key: "Avengers Endgame"
   value: 700
}
movieTicketPrice {
   key: "Captain America"
   value: 200
}
movieTicketPrice {
   key: "Wonder Woman 1984"
   value: 400
}

So, as we see, we are able to read the serialized map by deserializing the binary data to Theater object. In the next chapter Protocol Buffers - Nested Class, we will look at the nested classes.