Implement POJO Persister With Gson

Storing data in key-value format has its advantages and disadvantages. For example, you can easily change the contents in a text editor. You can also easily make mistakes like misspelling a key. On a more general note, the Properties library class is particularly useful for those classes that contain a few attributes. It is difficult to use it to, for example, store a collection of objects. Moreover, the resulting data file is flat, which makes storing complex structures awkward (e.g. if there was a list of authors inside the Book class). Furthermore, through the use of Properties, the data types are not specified. It is, therefore, possible to edit the files and, for instance, insert text where numbers should be. A potential solution is to store data in JSON format.

JSON

JSON stands for "JavaScript Object Notation". It is a lightweight format for storing and transporting data, in particular when data is sent from a server to a browser. I recommend watching the (first half of the) following video to learn more about the JSON format.

Google's Gson

Gson is a Java serialization/deserialization library (developed at Google) to convert Java Objects into JSON and back. To use it, add the following line to the dependencies of your project (build.gradle):

implementation group: 'com.google.code.gson', name: 'gson', version: '2.8.6'

Plain Old Java Object (POJO)

POJO is short for Plain Old Java Object; it's a cute way of referring to simple classes like Book and Author that are free of any special restriction or requirement that might be imposed by using a third-part library or a framework.

POJO

The term was coined by Martin Fowler1, Rebecca Parsons and Josh MacKenzie. In their own words: "we wondered why people were so against using regular objects in their systems and concluded that it was because simple objects lacked a fancy name. So we gave them one, and it's caught on very nicely."2

Gson helps us to directly write a POGO to a file and read it back; it literally takes one statement to do so and as such it helps to reduce the amount of code you have to write:

package persistence;
import model.Author;
import com.google.gson.*;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
public class GsonAuthorPersister implements Persister<Author> {
private final String STORE = "Author_gson.txt";
private final Gson gson;
public GsonAuthorPersister() {
gson = new Gson();
}
@Override
public void serialize(Author author) throws IOException {
FileWriter fw = new FileWriter(STORE);
gson.toJson(author, fw);
fw.close();
}
@Override
public Author deserialize() throws IOException {
FileReader fr = new FileReader(STORE);
Author author = gson.fromJson(fr, Author.class);
fr.close();
return author;
}
}

Write and run a simple demo for this and check out the JSON file that stores the Author object.

info

Gson can do a lot! Feel free to explore its "User Guide".


  1. Martin Fowler is known for his famous book "Refactorig" which is pretty popular in the software engineering community.
  2. https://www.martinfowler.com/bliki/POJO.html