使用Jackson在Java中处理JSON

前一段工作中总会遇到使用Java处理JSON的情况，大部分都使用的是开源工具Jackson实现的。

因此总结一下发上来，希望对看到的人有所帮助。

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一、入门

Jackson中有个ObjectMapper类很是实用，用于Java对象与JSON的互换。

1、Java对象转换为JSON：

User user=new User(); //Java Object

ObjectMapper mapper = new ObjectMapper();

mapper.writeValueAsString(user); //返回字符串

//输出格式化后的字符串(有性能损耗)

mapper.defaultPrettyPrintingWriter().writeValueAsString(user);

mapper.writeValue(new File("c:\\user.json"), user); //指定文件写入

 

//设置序列化配置(全局),设置序列化时不输出空值.

sharedMapper.getSerializationConfig().setSerializationInclusion(Inclusion.NON_NULL);

 

2、JSON反序列化为Java对象：

ObjectMapper mapper = new ObjectMapper();

//解析器支持解析单引号

mapper.configure(Feature.ALLOW_SINGLE_QUOTES,true);

//解析器支持解析结束符

mapper.configure(Feature.ALLOW_UNQUOTED_CONTROL_CHARS,true);

HashMap jsonMap = mapper.readValue(json,HashMap.class); //转换为HashMap对象

 

二、Jackson支持3种使用方式:

1、Data Binding：最方便使用.

(1)Full Data Binding：

private static final String MODEL_BINDING = "{\"name\":\"name1\",\"type\":1}";
	public void fullDataBinding() throws Exception{
		ObjectMapper mapper = new ObjectMapper();
		Model user = mapper.readValue(MODEL_BINDING, Model.class);//readValue到一个实体类中.
		System.out.println(user.getName());
		System.out.println(user.getType());
	}

Model类：

private static class Model{
		private String name;
		private int type;
		
		public String getName() {
			return name;
		}
		public void setName(String name) {
			this.name = name;
		}
		public int getType() {
			return type;
		}
		public void setType(int type) {
			this.type = type;
		}
	}

(2)Raw Data Binding：

/**
	Concrete Java types that Jackson will use for simple data binding are:
	JSON Type   	Java Type
	object			LinkedHashMap<String,Object>
	array			ArrayList<Object>
	string			String
	number(no fraction)	Integer, Long or BigInteger (smallest applicable)
	number(fraction)	Double(configurable to use BigDecimal)
	true|false		Boolean
	null			null
	*/
	public void rawDataBinding() throws Exception{
		ObjectMapper mapper = new ObjectMapper();
		HashMap map = mapper.readValue(MODEL_BINDING,HashMap.class);//readValue到一个原始数据类型.
		System.out.println(map.get("name"));
		System.out.println(map.get("type"));
	}

 (3)generic Data Binding：

private static final String GENERIC_BINDING = "{\"key1\":{\"name\":\"name2\",\"type\":2},\"key2\":{\"name\":\"name3\",\"type\":3}}";
	public void genericDataBinding() throws Exception{
		ObjectMapper mapper = new ObjectMapper();
		HashMap<String,Model> modelMap = mapper.readValue(GENERIC_BINDING,new TypeReference<HashMap<String,Model>>(){});//readValue到一个范型数据中.
		Model model = modelMap.get("key2");
		System.out.println(model.getName());
		System.out.println(model.getType());
	}

2、Tree Model：最灵活。

private static final String TREE_MODEL_BINDING = "{\"treekey1\":\"treevalue1\",\"treekey2\":\"treevalue2\",\"children\":[{\"childkey1\":\"childkey1\"}]}";
	public void treeModelBinding() throws Exception{
		ObjectMapper mapper = new ObjectMapper();
		JsonNode rootNode = mapper.readTree(TREE_MODEL_BINDING);
		//path与get作用相同,但是当找不到该节点的时候,返回missing node而不是Null.
		String treekey2value = rootNode.path("treekey2").getTextValue();//
		System.out.println("treekey2value:" + treekey2value);
		JsonNode childrenNode = rootNode.path("children");
		String childkey1Value = childrenNode.get(0).path("childkey1").getTextValue();
		System.out.println("childkey1Value:"+childkey1Value);
		
		//创建根节点
		ObjectNode root = mapper.createObjectNode();
		//创建子节点1
		ObjectNode node1 = mapper.createObjectNode();
		node1.put("nodekey1",1);
		node1.put("nodekey2",2);
		//绑定子节点1
		root.put("child",node1);
		//数组节点
		ArrayNode arrayNode = mapper.createArrayNode();
		arrayNode.add(node1);
		arrayNode.add(1);
		//绑定数组节点
		root.put("arraynode", arrayNode);
		//JSON读到树节点
		JsonNode valueToTreeNode = mapper.valueToTree(TREE_MODEL_BINDING);
		//绑定JSON节点
		root.put("valuetotreenode",valueToTreeNode);
		//JSON绑定到JSON节点对象
		JsonNode bindJsonNode = mapper.readValue(GENERIC_BINDING, JsonNode.class);//绑定JSON到JSON节点对象.
		//绑定JSON节点
		root.put("bindJsonNode",bindJsonNode);
		System.out.println(mapper.writeValueAsString(root));
	}

3、Streaming API：最佳性能。

见官方文档例子。

 

参考资料:

1、http://wiki.fasterxml.com/JacksonInFiveMinutes Jackson官方教程示例

2、http://wiki.fasterxml.com/JacksonJavaDocs Jackson在线API文档

3、http://hjg1988.iteye.com/blog/561368 JSON工具性能比较:json-lib和jackson进行Java对象到json字符串序列化。

 

评论

2 楼 di1984HIT 2017-12-31  

xuexile!!

1 楼 yangchengfyj 2015-05-31  

mapper.writeValueAsString(model)

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我使用这个方法的时候 获取的对象输出后。发现少字段！怎么解决！