Jackson - Hello World

Introduction

本篇內容展示Jackson的基本功能-將物件轉為json與xml格式。由於內容簡單，看程式碼就知道該怎麼做，不多做說明。

Ivy.xml

相依函式庫使用以下四個項目:

<dependency org="com.fasterxml.jackson.core" name="jackson-core" rev="latest.integration" conf="default->default(*)" />
<dependency org="com.fasterxml.jackson.core" name="jackson-annotations" rev="latest.integration" conf="default->default(*)" />
<dependency org="com.fasterxml.jackson.core" name="jackson-databind" rev="latest.integration" conf="default->default(*)" />
<dependency org="com.fasterxml.jackson.dataformat" name="jackson-dataformat-xml" rev="latest.integration" conf="default->default(*)" />

Source Object

User類別是此範例中，要轉為json與xml格式的目標。在mID與mPasswd成員變數上，使用了JsonProperty annotation去描述結果要呈現的欄位名稱。以json為例，mID顯示的欄位名稱即為id；以xml為例，mID的標籤名稱為id:

package org.tonylin.practice.jackson;
 
import com.fasterxml.jackson.annotation.JsonProperty;
 
public class User {
	@JsonProperty(value="id")
	private String mID;
 
	@JsonProperty(value="passwd")
	private String mPasswd;
 
	public String getmID() {
		return mID;
	}
 
	public void setID(String aID) {
		this.mID = aID;
	}
 
	public String getPasswd() {
		return mPasswd;
	}
 
	public void setPasswd(String aPasswd) {
		this.mPasswd = aPasswd;
	}
}

這裡必須特別注意的是，Jackson會優先使用get method為欄位名稱。下面這個範例所輸出的ss值為test而不是test2，即使對ss加上@JsonProperty(value=“ss”)也是test:

	public String ss = "test2";
 
	public String getSS(){
		return "test";
	}

一般的member宣告會使用m開頭，如果@JsonProperty的value和get method不同，就會導致同時輸出method欄位與@JsonProperty宣告的欄位:

	@JsonProperty(value="s1")
	private String mss = "test2";
 
	public String getSS(){
		return "test";
	}

Output:

"ss":"test","s1":"test2"

Convert to json

	@Test
	public void testJsonGenerator() throws IOException {
		ObjectMapper mapper = new ObjectMapper();
 
		JsonGenerator generator = mapper.getFactory().createGenerator(System.out);
 
		User u = new User();
		u.setID("user");
		u.setPasswd("123456");
 
		generator.writeObject(u);
	}

Output:

{"id":"user","passwd":"123456"}

Convert to xml

	@Test
	public void testXmlGenerator() throws IOException {
		XmlMapper mapper = new XmlMapper();
		JsonGenerator generator = mapper.getFactory().createGenerator(System.out);
 
		User u = new User();
		u.setID("user");
		u.setPasswd("123456");
 
		generator.writeObject(u);
	}

Output:

<User xmlns=""><id>user</id><passwd>123456</passwd></User>