Jackson Streaming API To Read And Write JSON

Jackson supports>Streaming
APIs document for detail explanation on the benefit of using streaming API.

Jackson’s streaming processing is high-performance, fast and convenient, but it’s also difficult to use, because you need to handle each and every detail of JSON data.

In this tutorial, we show you how to use following Jackson streaming APIs to read and write JSON data.

JsonGenerator

 – Write to JSON.

JsonParser

 – Parse JSON.

1. JsonGenerator

In this example, you use “JsonGenerator”>

import java.io.File;
import java.io.IOException;
import org.codehaus.jackson.JsonEncoding;
import org.codehaus.jackson.JsonFactory;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.map.JsonMappingException;
 
public class JacksonStreamExample {
public static void main(String[] args) {
 
try {
 
JsonFactory jfactory = new JsonFactory();
 
/*** write to file ***/
JsonGenerator jGenerator = jfactory.createJsonGenerator(new File(
"c:\\user.json"), JsonEncoding.UTF8);
jGenerator.writeStartObject(); // {
 
jGenerator.writeStringField("name", "mkyong"); // "name" : "mkyong"
jGenerator.writeNumberField("age", 29); // "age" : 29
 
jGenerator.writeFieldName("messages"); // "messages" :
jGenerator.writeStartArray(); // [
 
jGenerator.writeString("msg 1"); // "msg 1"
jGenerator.writeString("msg 2"); // "msg 2"
jGenerator.writeString("msg 3"); // "msg 3"
 
jGenerator.writeEndArray(); // ]
 
jGenerator.writeEndObject(); // }
 
jGenerator.close();
 
} catch (JsonGenerationException e) {
 
e.printStackTrace();
 
} catch (JsonMappingException e) {
 
e.printStackTrace();
 
} catch (IOException e) {
 
e.printStackTrace();
 
}
 
}
 
}

As result, following new file named “file.json”>

{
"name":"mkyong",
"age":29,
"messages":["msg 1","msg 2","msg 3"]
}

2. JsonParser

On the other hand, use 

JsonParser

 to>
Token>

{
"name":"mkyong"
}

Token 1 = “{“
Token 2 = “name”
Token 3 = “mkyong”
Token 4 = “}”

See full example.

import java.io.File;
import java.io.IOException;
import org.codehaus.jackson.JsonFactory;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.JsonParser;
import org.codehaus.jackson.JsonToken;
import org.codehaus.jackson.map.JsonMappingException;
 
public class JacksonStreamExample {
public static void main(String[] args) {
 
try {
 
JsonFactory jfactory = new JsonFactory();
 
/*** read from file ***/
JsonParser jParser = jfactory.createJsonParser(new File("c:\\user.json"));
 
// loop until token equal to "}"
while (jParser.nextToken() != JsonToken.END_OBJECT) {
 
String fieldname = jParser.getCurrentName();
if ("name".equals(fieldname)) {
 
// current token is "name",
// move to next, which is "name"'s value
jParser.nextToken();
System.out.println(jParser.getText()); // display mkyong
 
}
 
if ("age".equals(fieldname)) {
 
// current token is "age",
// move to next, which is "name"'s value
jParser.nextToken();
System.out.println(jParser.getIntValue()); // display 29
 
}
 
if ("messages".equals(fieldname)) {
 
jParser.nextToken(); // current token is "[", move next
 
// messages is array, loop until token equal to "]"
while (jParser.nextToken() != JsonToken.END_ARRAY) {
 
// display msg1, msg2, msg3
System.out.println(jParser.getText());
 
}
 
}
 
}
jParser.close();
 
} catch (JsonGenerationException e) {
 
e.printStackTrace();
 
} catch (JsonMappingException e) {
 
e.printStackTrace();
 
} catch (IOException e) {
 
e.printStackTrace();
 
}
 
}
 
}

Warning

The array parsing is a bit tricky, read code comments for explanation.

Output

mkyong
29
msg 1
msg 2
msg 3

Conclusion

In summary, for performance critical application, use Steaming API, otherwise, just use normal Jackson data binding.

Reference:http://www.mkyong.com/java/jackson-streaming-api-to-read-and-write-json/