在工作中实际使用到Java处理JSON的情况，且有很大部分都使用的是开源工具Jackson实现的。
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
Student st=new Student(); //Java Object
ObjectMapper mapper = new ObjectMapper();
java.text.DateFormat myFormat = new java.text.SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
mapper.getSerializationConfig().setDateFormat(myFormat);
try {
//返回字符串
String res = mapper.writeValueAsString(st);
System.out.println(res);
//输出格式化后的字符串(有性能损耗)
res = mapper.defaultPrettyPrintingWriter().writeValueAsString(st);
System.out.println(res);
mapper.writeValue(new File("D:\\st.json"), st); //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.
mapper.getSerializationConfig().setSerializationInclusion(Inclusion.NON_NULL);
res = mapper.writeValueAsString(st);
System.out.println(res);
} catch (Exception e) {
e.printStackTrace();
}
2.JSON反序列化为Java对象
String json = "{\"error\":0,\"data\":{\"name\":\"ABC\",\"age\":20,\"phone\":{\"home\":\"abc\",\"mobile\":\"def\"},\"friends\":[{\"name\":\"DEF\",\"phone\":{\"home\":\"hij\",\"mobile\":\"klm\"}},{\"name\":\"GHI\",\"phone\":{\"home\":\"nop\",\"mobile\":\"qrs\"}}]},\"other\":{\"nickname\":[]}}";
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);
try {
//转换为HashMap对象
HashMap jsonMap = mapper.readValue(json, HashMap.class);
Map<String, Map<String, Object>> maps = objectMapper.readValue(json, Map.class);
System.out.println(maps.get("error"));//0
System.out.println((Object) (maps.get("data").get("phone")));//{home=abc, mobile=def}
} catch (Exception e) {
e.printStackTrace();
}
二.Jackson支持三种使用方式
1.Data Binding：最方便使用
(1)Full Data Binding
/*
* Full Data Binding
*/
public static void fullDataBinding() {
ObjectMapper mapper = new ObjectMapper();
try {
Model model = mapper.readValue(MODEL_BINDING, Model.class);
//readValue到一个实体类中.
System.out.println(model.getName()); //name1
System.out.println(model.getType()); //1
} catch (Exception e) {
e.printStackTrace();
}
}
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
/*
* Raw Data Binding
*/
public static void rawDataBinding() {
ObjectMapper mapper = new ObjectMapper();
try {
HashMap map = mapper.readValue(MODEL_BINDING, HashMap.class);//readValue到一个原始数据类型.
System.out.println(map.get("name")); //name1
System.out.println(map.get("type")); //1
} catch (Exception e) {
e.printStackTrace();
}
}
(3)generic Data Binding
/*
* generic Data Binding
*/
public static void genericDataBinding() {
ObjectMapper mapper = new ObjectMapper();
try {
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()); //name3
System.out.println(model.getType()); //3
} catch (Exception e) {
e.printStackTrace();
}
}
2.Tree Model：最灵活
/*
* Tree Model：最灵活
*/
public static void treeModelBinding() {
ObjectMapper mapper = new ObjectMapper();
try {
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));
} catch (Exception e) {
e.printStackTrace();
}
}
3.Streaming API。最佳性能
见官方文档例子。
进一步学习资料:
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。