Jackson Streaming API To Read And Write JSON

最新推荐文章于 2024-08-14 16:51:34 发布

转载最新推荐文章于 2024-08-14 16:51:34 发布 · 1.2k 阅读

本文介绍如何使用 Jackson 的 Streaming API 进行 JSON 数据的读写操作。通过具体示例展示了 JsonGenerator 如何写出 JSON 数据，以及 JsonParser 如何解析 JSON 数据。此外还介绍了在处理 JSON 数组时的一些技巧。

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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/