Jackson库中objectMapper用法
ObjectMapper类是Jackson库的主要类。它提供一些功能将转换成Java对象与SON结构互相转换，在项目中遇到过，故记录一下。
在 pom.xml 加入依赖
<dependency> <groupId>com.fasterxml.jackson.core</groupId> <artifactId>jackson-databind</artifactId> <version>2.8.3</version> </dependency>
创建一个实体类RiemannUser：
package com.test.objectMapper;import java.io.Serializable;import java.util.Date;import java.util.List;/** * @author riemann * @date 2019/05/27 22:48 */public class RiemannUser implements Serializable { private static final long serialVersionUID = 1L; private int id; private String message; private Date sendDate; private String nodeName; private List<Integer> intList; public RiemannUser() { super(); } public RiemannUser(int id, String message, Date sendDate) { super(); this.id = id; this.message = message; this.sendDate = sendDate; } public static long getSerialVersionUID() { return serialVersionUID; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getMessage() { return message; } public void setMessage(String message) { this.message = message; } public Date getSendDate() { return sendDate; } public void setSendDate(Date sendDate) { this.sendDate = sendDate; } public String getNodeName() { return nodeName; } public void setNodeName(String nodeName) { this.nodeName = nodeName; } public List<Integer> getIntList() { return intList; } public void setIntList(List<Integer> intList) { this.intList = intList; } @Override public String toString() { return "RiemannUser{" + "id=" + id + ", message='" + message + '\'' + ", sendDate=" + sendDate + ", nodeName='" + nodeName + ''' + ", intList=" + intList + '}'; }}
先创建一个ObjectMapper，然后赋值一些属性：
public static ObjectMapper mapper = new ObjectMapper();static { // 转换为格式化的json mapper.enable(SerializationFeature.INDENT_OUTPUT); // 如果json中有新增的字段并且是实体类类中不存在的，不报错 mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);}
1、对象与json字符串、byte数组
@Testpublic void testObject() throws JsonGenerationException, JsonMappingException, IOException { RiemannUser riemann = new RiemannUser(1,"Hello World", new Date()); mapper.writeValue(new File("D:/test.txt"), riemann);//写到文件中 //mapper.writeValue(System.out, riemann); //写到控制台 String jsonStr = mapper.writeValueAsString(riemann); System.out.println("对象转json字符串: " + jsonStr); byte[] byteArr = mapper.writeValueAsBytes(riemann); System.out.println("对象转为byte数组：" + byteArr); RiemannUser riemannUser = mapper.readValue(jsonStr, RiemannUser.class); System.out.println("json字符串转为对象：" + riemannUser); RiemannUser riemannUser2 = mapper.readValue(byteArr, RiemannUser.class); System.out.println("byte数组转为对象：" + riemannUser2);}
运行结果：
对象转json字符串: {“id” : 1,”message” : “Hello World”,”sendDate” : 1558971056693,”nodeName” : null,”intList” : null}对象转为byte数组：[B@31610302json字符串转为对象：RiemannUser{id=1, message=’Hello World’, sendDate=Mon May 27 23:30:56 CST 2019, nodeName=’null’, intList=null}byte数组转为对象：RiemannUser{id=1, message=’Hello World’, sendDate=Mon May 27 23:30:56 CST 2019, nodeName=’null’, intList=null}
2、list集合与json字符串
@Testpublic void testList() throws JsonGenerationException, JsonMappingException, IOException { List<RiemannUser> riemannList = new ArrayList<>(); riemannList.add(new RiemannUser(1,"a",new Date())); riemannList.add(new RiemannUser(2,"b",new Date())); riemannList.add(new RiemannUser(3,"c",new Date())); String jsonStr = mapper.writeValueAsString(riemannList); System.out.println("集合转为字符串：" + jsonStr); List<RiemannUser> riemannLists = mapper.readValue(jsonStr, List.class); System.out.println("字符串转集合：" + riemannLists);}
运行结果：
集合转为字符串：[ {“id” : 1,”message” : “a”,”sendDate” : 1558971833351,”nodeName” : null,”intList” : null}, {“id” : 2,”message” : “b”,”sendDate” : 1558971833351,”nodeName” : null,”intList” : null}, {“id” : 3,”message” : “c”,”sendDate” : 1558971833351,”nodeName” : null,”intList” : null} ]字符串转集合：[{id=1, message=a, sendDate=1558971833351, nodeName=null, intList=null}, {id=2, message=b, sendDate=1558971833351, nodeName=null, intList=null}, {id=3, message=c, sendDate=1558971833351, nodeName=null, intList=null}]
3、map与json字符串
@Testpublic void testMap() { Map<String, Object> testMap = new HashMap<>(); testMap.put("name", "riemann"); testMap.put("age", 27); testMap.put("date", new Date()); testMap.put("user", new RiemannUser(1, "Hello World", new Date())); String jsonStr = null; try { jsonStr = mapper.writeValueAsString(testMap); System.out.println("Map转为字符串：" + jsonStr); Map<String, Object> testMapDes = null; try { testMapDes = mapper.readValue(jsonStr, Map.class); System.out.println("字符串转Map：" + testMapDes); } catch (IOException e) { e.printStackTrace(); } } catch (JsonProcessingException e) { e.printStackTrace(); }}
Map转为字符串：{“date” : 1558972169132,”name” : “riemann”,”user” : {“id” : 1,”message” : “Hello World”,”sendDate” : 1558972169134,”nodeName” : null,”intList” : null},”age” : 27}字符串转Map：{date=1558972169132, name=riemann, user={id=1, message=Hello World, sendDate=1558972169134, nodeName=null, intList=null}, age=27}
4、修改转换时的日期格式：
@Testpublic void testOther() throws IOException { // 修改时间格式 mapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")); RiemannUser riemannUser = new RiemannUser(1,"Hello World",new Date()); riemannUser.setIntList(Arrays.asList(1,2,3)); String jsonStr = mapper.writeValueAsString(riemannUser); System.out.println("对象转为字符串：" + jsonStr);}
运行结果：
对象转为字符串：{“id” : 1,”message” : “Hello World”,”sendDate” : “2019-05-27 23:53:55″,”nodeName” : null,”intList” : [ 1, 2, 3 ]}
objectMapper的一些坑
相信做过Java 开发对这个类应该不陌生,没错,这个类是jackson提供的，主要是用来把对象转换成为一个json字符串返回到前端,
现在大部分数据交换都是以json来传输的,所以这个很重要,那你到底又对这个类有着有多少了解呢，下面我说一下我遇到的一些坑
首先，先把我要说的几个坑需要设置的属性贴出来先
ObjectMapper objectMapper = new ObjectMapper();//序列化的时候序列对象的所有属性objectMapper.setSerializationInclusion(Include.ALWAYS);//反序列化的时候如果多了其他属性,不抛出异常objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);//如果是空对象的时候,不抛异常objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);//取消时间的转化格式,默认是时间戳,可以取消,同时需要设置要表现的时间格式objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);objectMapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"))
简单说一下这个类的基本用法，以下采用代码块加截图的形式来说明和部分文字件数
package com.shiro.test; import java.text.SimpleDateFormat;import java.util.Date; import com.fasterxml.jackson.annotation.JsonInclude.Include;import com.fasterxml.jackson.databind.DeserializationFeature;import com.fasterxml.jackson.databind.ObjectMapper;import com.fasterxml.jackson.databind.SerializationFeature; public class Main2 {public static void main(String[] args) throws Exception{ObjectMapper objectMapper = new ObjectMapper();//序列化的时候序列对象的所有属性objectMapper.setSerializationInclusion(Include.ALWAYS);//取消时间的转化格式,默认是时间戳,可以取消,同时需要设置要表现的时间格式objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);objectMapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));Person person = new Person(1, "zxc", new Date());//这是最简单的一个例子,把一个对象转换为json字符串String personJson = objectMapper.writeValueAsString(person);System.out.println(personJson);//默认为true,会显示时间戳objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, true);personJson = objectMapper.writeValueAsString(person);System.out.println(personJson);}}
输出的信息如下
objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false)的作用
package com.shiro.test; import java.text.SimpleDateFormat;import java.util.Date; import com.fasterxml.jackson.annotation.JsonInclude.Include;import com.fasterxml.jackson.databind.DeserializationFeature;import com.fasterxml.jackson.databind.ObjectMapper;import com.fasterxml.jackson.databind.SerializationFeature; public class Main2 {public static void main(String[] args) throws Exception{ObjectMapper objectMapper = new ObjectMapper();//序列化的时候序列对象的所有属性objectMapper.setSerializationInclusion(Include.ALWAYS);//如果是空对象的时候,不抛异常,也就是对应的属性没有get方法objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);Person person = new Person(1, "zxc", new Date()); String personJson = objectMapper.writeValueAsString(person);System.out.println(personJson);//默认是true,即会抛异常objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, true);personJson = objectMapper.writeValueAsString(person);System.out.println(personJson);}}
对应的person类此时为
package com.shiro.test; import java.util.Date; public class Person { private Integer id;private String name;private Date birthDate;//public Integer getId() {//return id;//}//public void setId(Integer id) {//this.id = id;//}//public String getName() {//return name;//}//public void setName(String name) {//this.name = name;//}//public Date getBirthDate() {//return birthDate;//}//public void setBirthDate(Date birthDate) {//this.birthDate = birthDate;//}@Overridepublic String toString() {return "Person [id=" + id + ", name=" + name + ", birthDate=" + birthDate + "]";}public Person(Integer id, String name, Date birthDate) {super();this.id = id;this.name = name;this.birthDate = birthDate;}public Person() {// TODO Auto-generated constructor stub}}
结果如下
package com.shiro.test; import com.fasterxml.jackson.annotation.JsonInclude.Include;import com.fasterxml.jackson.databind.DeserializationFeature;import com.fasterxml.jackson.databind.ObjectMapper; public class Main2 {public static void main(String[] args) throws Exception{ObjectMapper objectMapper = new ObjectMapper();//序列化的时候序列对象的所有属性objectMapper.setSerializationInclusion(Include.ALWAYS);//反序列化的时候如果多了其他属性,不抛出异常objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);//Person person = new Person(1, "zxc", new Date()); //String personJson = objectMapper.writeValueAsString(person);//System.out.println(personJson);//注意,age属性是不存在在person对象中的String personStr = "{\"id\":1,\"name\":\"zxc\",\"age\":\"zxc\"}";Person person = objectMapper.readValue(personStr, Person.class);System.out.println(person);//默认为trueobjectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, true);person = objectMapper.readValue(personStr, Person.class);System.out.println(person);}}
执行后的结果如下
这些便是这几个属性的作用所以，由于第一个比较简单我就这样说一下吧
Include.ALWAYS 是序列化对像所有属性
Include.NON_NULL 只有不为null的字段才被序列化
Include.NON_EMPTY 如果为null或者 空字符串和空集合都不会被序列化
然后再说一下如何把一个对象集合转换为一个 Java里面的数组
package com.shiro.test; import java.util.ArrayList;import java.util.Date;import java.util.List; import com.fasterxml.jackson.annotation.JsonInclude.Include;import com.fasterxml.jackson.core.type.TypeReference;import com.fasterxml.jackson.databind.JavaType;import com.fasterxml.jackson.databind.ObjectMapper; public class Main2 {public static void main(String[] args) throws Exception{ObjectMapper objectMapper = new ObjectMapper();//序列化的时候序列对象的所有属性objectMapper.setSerializationInclusion(Include.NON_DEFAULT);Person person1 = new Person(1, "zxc", new Date());Person person2 = new Person(2, "ldh", new Date());List<Person> persons = new ArrayList<>();persons.add(person1);persons.add(person2);//先转换为json字符串String personStr = objectMapper.writeValueAsString(persons);//反序列化为List<user> 集合,1需要通过 TypeReference 来具体传递值List<Person> persons2 = objectMapper.readValue(personStr, new TypeReference<List<Person>>() {});for(Person person : persons2) {System.out.println(person);}//2,通过 JavaType 来进行处理返回JavaType javaType = objectMapper.getTypeFactory().constructParametricType(List.class, Person.class);List<Person> persons3 = objectMapper.readValue(personStr, javaType);for(Person person : persons3) {System.out.println(person);}}}
以上为个人经验，希望能给大家一个参考，也希望大家多多支持。
穷则思变，差则思勤！没有比人更高的山没有比脚更长的路。
