In this quick tutorial, you'll learn how to write JSON data to a file by using the Jackson API. Jackson is a popular JSON processing library for reading, writing, and parsing JSON data in Java.

Dependencies

To add Jackson to your Gradle project, add the following dependency to build.gradle file:

implementation 'com.fasterxml.jackson.core:jackson-databind:2.10.0'

For Maven, include the below dependency to your pom.xml file:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.10.0</version>
</dependency>

Write Java Map to a JSON File

To write a Java Map to a JSON file, you can use the writeValue() method from ObjectMapper, as shown below:

try {
    // create a map
    Map<String, Object> map = new HashMap<>();
    map.put("name", "John Deo");
    map.put("email", "john.doe@example.com");
    map.put("roles", new String[]{"Member", "Admin"});
    map.put("admin", true);

    // create object mapper instance
    ObjectMapper mapper = new ObjectMapper();

    // convert map to JSON file
    mapper.writeValue(Paths.get("user.json").toFile(), map);

} catch (Exception ex) {
    ex.printStackTrace();
}

The above code will produce the following JSON file:

user.json

{"roles":["Member","Admin"],"name":"John Deo","admin":true,"email":"john.doe@example.com"}

Write Java Object to a JSON File

Let us first create a simple Java class named Book.java that we will use to convert a Java Object to a JSON file:

Book.java

public class Book {

    private String title;
    private String isbn;
    private long year;
    private String[] authors;

    public Book() {
    }

    public Book(String title, String isbn, long year, String[] authors) {
        this.title = title;
        this.isbn = isbn;
        this.year = year;
        this.authors = authors;
    }

    // getters and setters, equals(), toString() .... (omitted for brevity)
}

The following example shows how you can write an instance of the Book class to a JSON file:

try {
    // create book object
    Book book = new Book("Thinking in Java", "978-0131872486", 1998,
            new String[]{"Bruce Eckel"});

    // create object mapper instance
    ObjectMapper mapper = new ObjectMapper();

    // convert book object to JSON file
    mapper.writeValue(Paths.get("book.json").toFile(), book);

} catch (Exception ex) {
    ex.printStackTrace();
}

The above code will produce the following book.json file:

{"title":"Thinking in Java","isbn":"978-0131872486","year":1998,"authors":["Bruce Eckel"]}

Write List of Java Objects to a JSON File

Just like a single Java Object, you can also write a list of Java Objects to a JSON file using the same writeValue() method, as shown below:

try {
    // create books list
    List<Book> books = Arrays.asList(
            new Book("Thinking in Java", "978-0131872486", 1998,
                    new String[]{"Bruce Eckel"}),
            new Book("Head First Java", "0596009208", 2003,
                    new String[]{"Kathy Sierra", "Bert Bates"})
    );

    // create object mapper instance
    ObjectMapper mapper = new ObjectMapper();

    // convert book object to JSON file
    mapper.writeValue(Paths.get("books.json").toFile(), books);

} catch (Exception ex) {
    ex.printStackTrace();
}

The above code will produce the following books.json file:

[{"title":"Thinking in Java","isbn":"978-0131872486","year":1998,"authors":["Bruce Eckel"]},
  {"title":"Head First Java","isbn":"0596009208","year":2003,"authors":["Kathy Sierra","Bert Bates"]}]

Write Java Object to a Pretty Print JSON File

You can use the DefaultPrettyPrinter class to enable the pretty-print JSON output:

try {
    // create book object
    Book book = new Book("Thinking in Java", "978-0131872486", 1998,
            new String[]{"Bruce Eckel"});

    // create object mapper instance
    ObjectMapper mapper = new ObjectMapper();

    // create an instance of DefaultPrettyPrinter
    ObjectWriter writer = mapper.writer(new DefaultPrettyPrinter());

    // convert book object to JSON file
    writer.writeValue(Paths.get("book.json").toFile(), book);

} catch (Exception ex) {
    ex.printStackTrace();
}

Here is how the book.json file looks like after JSON pretty-print is enabled:

{
  "title" : "Thinking in Java",
  "isbn" : "978-0131872486",
  "year" : 1998,
  "authors" : [ "Bruce Eckel" ]
}

For more Jackson examples, check out the How to read and write JSON using Jackson in Java tutorial.

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