writeValueAsString
- Java course
- Intermediate
- Lesson 78/116
- 5 min
Learn to convert objects to JSON with writeValueAsString.
Serialization with writeValueAsString
To convert a Java object to structured text, use mapper.writeValueAsString(obj). The returned String is in a structured format. A User u = new User("Ayşe", 30); instance serializes as {"name":"Ayşe","age":30}.
Fields become keys by default. To make the output pretty, use mapper.enable(SerializationFeature.INDENT_OUTPUT). To skip null values, add Jackson's null-skipping annotation on the class.
Formatting the output
Default output is single-line compact text. For readability, mapper.enable(SerializationFeature.INDENT_OUTPUT) gives indented output. To skip null fields, add Jackson's null-skipping annotation on the class.
For special types like dates, mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS) is used. These settings are made on a single ObjectMapper instance and affect every serialization it does.
Practice Convert a User object to structured text
Define record User(String name, int age) {}. Add throws Exception to main. Create an ObjectMapper mapper = new ObjectMapper();, create a user with User u = new User("Mehmet", 25);, produce text with String text = mapper.writeValueAsString(u); and print it with System.out.println(text);.
Starter code
import com.fasterxml.jackson.databind.ObjectMapper;
// Define the User record
public class Main {
public static void main(String[] args) throws Exception {
// ObjectMapper, User instance, writeValueAsString and println
}
}Sample solution
import com.fasterxml.jackson.databind.ObjectMapper;
record User(String name, int age) {}
public class Main {
public static void main(String[] args) throws Exception {
ObjectMapper mapper = new ObjectMapper();
User u = new User("Mehmet", 25);
String text = mapper.writeValueAsString(u);
System.out.println(text);
}
}Expected output
{"name":"Mehmet","age":25}
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does writeValueAsString return?
- byte[]
- Object
- List
- String Correct answer
Explanation: writeValueAsString returns structured text as a String.
2 Complete the serialization call
String text = mapper.____(u);String text = mapper.writeValueAsString(u);Explanation: writeValueAsString converts the User to structured text.
3 Match the serialization concepts
| Term | Meaning |
|---|---|
writeValueAsString |
Object to text |
INDENT_OUTPUT |
Indented output |
@JsonInclude |
Skipping null values |
User u = new User(...) |
Object to serialize |