@JsonIgnoreProperties转换实体时忽略json中不存在的字段
json转换成的实体类加注解@JsonIgnoreProperties(ignoreUnknown = true),注意这是类级别的注解。
@JsonIgnore注解用来忽略某些字段,可以用在Field或者Getter方法上,用在Setter方法时,和Filed效果一样。这个注解只能用在POJO存在的字段要忽略的情况,不能满足现在需要的情况。
@JsonIgnoreProperties(ignoreUnknown = true),将这个注解写在类上之后,就会忽略类中不存在的字段,可以满足当前的需要。这个注解还可以指定要忽略的字段。使用方法如下:
@JsonIgnoreProperties({ "internalId", "secretKey" })
指定的字段不会被序列化和反序列化。
附上一个转换的工具类
1. package com.*******.****.drp.util;
2. import java.io.IOException;
3.
4. import org.codehaus.jackson.JsonGenerationException;
5. import org.codehaus.jackson.JsonParseException;
6. import org.codehaus.jackson.JsonParser;
7. import org.codehaus.jackson.map.DeserializationConfig;
8. import org.codehaus.jackson.map.JsonMappingException;
9. import org.codehaus.jackson.map.ObjectMapper;
10.
11. import lombok.extern.slf4j.Slf4j;
12. /**
13. * @Description:
14. * @author :******| paranoia_zk@yeah.net
15. * @date :2017年6月8日 上午10:32:04
16. */
17. @Slf4j
18. public class JacksonUtil {
19. private final static ObjectMapper objectMapper = new ObjectMapper();
20.
21. static {
22. true);
23. true);
24. true);
25. true);
26. true);
27. true);
28. false);
29. }
30.
31. public static String encode(Object obj) {
32. try {
33. return objectMapper.writeValueAsString(obj);
34. catch (JsonGenerationException e) {
35. "encode(Object)", e); //$NON-NLS-1$
36. catch (JsonMappingException e) {
37. "encode(Object)", e); //$NON-NLS-1$
38. catch (IOException e) {
39. "encode(Object)", e); //$NON-NLS-1$
40. }
41. return null;
42. }
43.
44. /**
45. * 将json string反序列化成对象
46. *
47. * @param json
48. * @param valueType
49. * @return
50. */
51. public static <T> T decode(String json, Class<T> valueType) {
52. try {
53. return objectMapper.readValue(json, valueType);
54. catch (JsonParseException e) {
55. "decode(String, Class<T>)", e);
56. catch (JsonMappingException e) {
57. "decode(String, Class<T>)", e);
58. catch (IOException e) {
59. "decode(String, Class<T>)", e);
60. }
61. return null;
62. }
63.
64. }
概述
使用jackson annotations简化和增强的json解析与生成。
Jackson-2.x通用annotations列表:https://github.com/FasterXML/jackson-annotations/wiki/Jackson-Annotations
Jackson-1.x通用annotations列表:http://wiki.fasterxml.com/JacksonAnnotations
准备工作
基于JDK1.7,依赖Jackson框架核心类库:
jackson-core-2.5.3.jar
jackson-annotations-2.5.3.jar
jackson-databind-2.5.3.jar
Jackson - Annotations
想要了解更多内容,请查看annotations列表。下面只列出一些常用的Json注解。
@JsonProperty
它关联json字段到java属性。可以标记属性,也可以用来标记属性的getter/setter方法。当标记属性时,可以对属性字段重命名。当标记方法时,可以把json字段关联到java属性的getter或setter方法。
@JsonCreator
json反序列化为java对象时,该注解用于定义构造函数。当从json创建java时,@JsonCreator注解的构造函数被会调用,如果没有@JsonCreator注解,则默认调用java类的无参构造函数,此时,如果java类中只有有参构造函数,而无默认的无参构造函数,在反序列化时会抛出这样的异常:com.fasterxml.jackson.databind.JsonMappingException,所以,当我们不使用@JsonCreator指定反序列化的构造函数,而又在java类中重载了构造函数时,一定要记得编写类的无参构造函数。
@JsonAnyGetter和@JsonAnySetter
用于标记类方法,设置和读取json字段作为键值对存储到map中,这两个注解标记的方法不会处理任何java类中已经定义过的属性变量,只对java中未定义的json字段作处理。
@JsonIgnoreProperties和@JsonIgnore:
用于标记属性,在json与java之间相互转化时,将忽略被此注解标记的属性。@JsonIgnoreProperties是类级别注解,可以忽略多个属性,@JsonIgnore用来标注单个属性。
@JsonTypeInfo和@JsonSubTypes
用于维持java类的子类信息,将子类对象类型信息嵌入到json中,以便反序列化创建具体的对象。
Example
下面通过例子来演示注解的使用
example.1
读取company.json,反序列化json,创建java对象,并遍历信息
company.json
1. {
2. "name" : "Oracle",
3. "HQ" : "California",
4. "birthDate" : "1977-01-01",
5. "departments" : [ {
6. "name" : "development",
7. "employee_number" : 5000,
8. "projectManager" : "jack",
9. "product" : "oracle_db"
10. }, {
11. "name" : "test",
12. "employee_number" : 500,
13. "projectManager" : "rose",
14. "product" : "oracle_test"
15. } ]
16. }
Company.java
1. package com.jackson.json.annotation;
2.
3. import java.util.Date;
4.
5. import com.fasterxml.jackson.annotation.JsonCreator;
6. import com.fasterxml.jackson.annotation.JsonIgnore;
7. import com.fasterxml.jackson.annotation.JsonProperty;
8.
9. public class Company {
10. private String name;
11. @JsonProperty("HQ") //java属性headquarters序列化到json字段的名称为HQ
12. private String headquarters;
13. private Department[] departments;
14. @JsonIgnore //在序列化与反序列化时,忽略birthDate属性
15. private Date birthDate;
16.
17. public Date getBirthDate() {
18. return birthDate;
19. }
20.
21. @JsonCreator
22. public Company(@JsonProperty("name") String name) {
23. this.name = name;
24. }
25.
26. public String getName() {
27. return name;
28. }
29.
30. public String getHeadquarters() {
31. return headquarters;
32. }
33.
34. public Department[] getDepartments() {
35. return departments;
36. }
37.
38. public void setDepartments(Department[] departments) {
39. this.departments = departments;
40. }
41.
42. }
Department.java
1. package com.jackson.json.annotation;
2.
3. import java.util.HashMap;
4. import java.util.Map;
5.
6. import com.fasterxml.jackson.annotation.JsonAnyGetter;
7. import com.fasterxml.jackson.annotation.JsonAnySetter;
8. import com.fasterxml.jackson.annotation.JsonCreator;
9. import com.fasterxml.jackson.annotation.JsonProperty;
10.
11. public class Department {
12. private String name;
13. private String pm;
14. private Map<String, Object> otherProperties = new HashMap<String, Object>(); //otherProperties用来存放Department中未定义的json字段
15.
16. @JsonCreator //指定json反序列化创建Department对象时调用此构造函数
17. public Department(@JsonProperty("name") String name){
18. this.name = name;
19. }
20.
21. @JsonProperty("projectManager") //将company.json中projectManager字段关联到getPm方法
22. public String getPm() {
23. return pm;
24. }
25.
26. public String getName() {
27. return name;
28. }
29.
30. public Object get(String key) {
31. return otherProperties.get(key);
32. }
33.
34. @JsonAnyGetter //得到所有Department中未定义的json字段的
35. public Map<String, Object> any() {
36. return otherProperties;
37. }
38.
39. @JsonAnySetter
40. public void set(String key, Object value) {
41. otherProperties.put(key, value);
42. }
43.
44. }
DeserializationExample.java
1. package com.jackson.json.annotation;
2.
3. import java.io.File;
4.
5. import com.fasterxml.jackson.databind.DeserializationFeature;
6. import com.fasterxml.jackson.databind.ObjectMapper;
7.
8. public class DeserializationExample {
9. public static void main(String[] args) throws Exception {
10. new ObjectMapper();
11. //禁止未知属性打断反序列化
12.
13. new File("company_back.json"), Company.class);
14. "company_name:"+company.getName()+"\t");
15. "headquarters:"+company.getHeadquarters()+"\t");
16. "birthDate:"+company.getBirthDate()); //birthDate被标记为@JsonIgnore,所以此处得到的值应该为null
17.
18. Department[] departments = company.getDepartments();
19.
20. for (Department department : departments) {
21. "department_name:" + department.getName()+"\t");
22. "employee_number:" + department.getPm()+"\t");
23. //Department中未定义的字段product,employee_number
24. "product:"+department.get("product")+"\t");
25. "projectManager:"+department.get("employee_number"));
26. }
27. }
28.
29. }
程序运行控制台打印结果如下:
1. company_name:Oracle headquarters:California birthDate:null
2. department_name:development employee_number:jack product:oracle_db projectManager:5000
3. department_name:test employee_number:rose product:oracle_test projectManager:500
example.2
下面例子演示,当java对象中包含List<Object>属性时,如何序列化与反序列化。
当java对象中含List<Object>时,如果Object一个抽象类或接口,这里就会出现java多态的现象,比如,List<Animal>,Animal是个抽象类,并且有多个子类时,由于List中保存的Animal没有明确指向具体的子类或实现类,json反序列化java对象时就会抛出提示:Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException:Can not construct instance of Animal, problem: abstract types either need to be mapped to concrete types, have custom deserializer, or be instantiated with additional type information
@JsonTypeInfo与@JsonSubTypes就是解决此类问题,通过注解,可以在序列化时,保存具体的类型信息到json中,当json反序列到java对象时,就可以根据具体类型信息创建正确的java对象。
Zoo.java
1. package com.jackson.json.databinding.list;
2.
3. import java.util.List;
4.
5. import com.fasterxml.jackson.annotation.JsonCreator;
6. import com.fasterxml.jackson.annotation.JsonProperty;
7.
8. public class Zoo {
9. public String name;
10. public String city;
11. public List<Animal> animals;
12.
13. @JsonCreator
14. public Zoo(@JsonProperty("name") String name, @JsonProperty("city") String city) {
15. this.name = name;
16. this.city = city;
17. }
18.
19. public void setAnimals(List<Animal> animals) {
20. this.animals = animals;
21. }
22.
23. @Override
24. public String toString() {
25. return "Zoo [name=" + name + ", city=" + city + ", animals=" + animals
26. "]";
27. }
28.
29. }
Animal.java
1. package com.jackson.json.databinding.list;
2.
3. import com.fasterxml.jackson.annotation.JsonSubTypes;
4. import com.fasterxml.jackson.annotation.JsonSubTypes.Type;
5. import com.fasterxml.jackson.annotation.JsonTypeInfo;
6. import com.fasterxml.jackson.annotation.JsonTypeInfo.As;
7. import com.fasterxml.jackson.annotation.JsonTypeInfo.Id;
8.
9. @JsonTypeInfo(use=Id.CLASS,include=As.PROPERTY,property="@class")
10. @JsonSubTypes({@Type(value=Lion.class,name="lion"),@Type(value=Elephant.class,name="elephant")})
11. public abstract class Animal {
12. String name;
13. String type;
14. }
Lion.java
1. package com.jackson.json.databinding.list;
2.
3. import com.fasterxml.jackson.annotation.JsonCreator;
4. import com.fasterxml.jackson.annotation.JsonProperty;
5.
6. public class Lion extends Animal {
7.
8. private String name;
9.
10. @JsonCreator
11. public Lion(@JsonProperty("name") String name) {
12. this.name = name;
13. }
14.
15. public String getName() {
16. return name;
17. }
18.
19. public String getType() {
20. return "carnivorous";
21. }
22.
23. @Override
24. public String toString() {
25. return "Lion [name=" + name + ", getName()=" + getName()
26. ", getType()=" + getType() + "]";
27. }
28.
29. }
Elephant.java
1. package com.jackson.json.databinding.list;
2.
3. import com.fasterxml.jackson.annotation.JsonCreator;
4. import com.fasterxml.jackson.annotation.JsonProperty;
5.
6. public class Elephant extends Animal {
7. private String name;
8.
9. @JsonCreator
10. public Elephant(@JsonProperty("name") String name) {
11. this.name = name;
12. }
13.
14. public String getName() {
15. return name;
16. }
17.
18. public String getType() {
19. return "herbivorous";
20. }
21.
22. @Override
23. public String toString() {
24. return "Elephant [getName()=" + getName() + ", getType()=" + getType()
25. "]";
26. }
27.
28. }
SerializeExmaple.java
1. package com.jackson.json.databinding.list;
2.
3. import java.io.File;
4. import java.io.IOException;
5. import java.util.ArrayList;
6. import java.util.List;
7.
8. import com.fasterxml.jackson.databind.ObjectMapper;
9. import com.fasterxml.jackson.databind.SerializationFeature;
10.
11. public class SerializeExample {
12. public static void main(String[] args) throws Exception {
13. new Zoo("SH Wild Park", "ShangHai");
14. new Lion("Samba");
15. new Elephant("Manny");
16. new ArrayList<Animal>();
17. animals.add(lion);
18. animals.add(elephant);
19. zoo.setAnimals(animals);
20.
21. new ObjectMapper();
22. true);
23. new File("zoo.json"), zoo);
24. }
25.
26. }
生成zoo.json内容如下:
1. {
2. "name" : "SH Wild Park",
3. "city" : "ShangHai",
4. "animals" : [ {
5. "@class" : "com.jackson.json.databinding.list.Lion",
6. "name" : "Samba",
7. "type" : "carnivorous"
8. }, {
9. "@class" : "com.jackson.json.databinding.list.Elephant",
10. "name" : "Manny",
11. "type" : "herbivorous"
12. } ]
13. }
反序列化,DeserializeExmaple.java
1. package com.jackson.json.databinding.list;
2.
3. import java.io.File;
4.
5. import com.fasterxml.jackson.databind.ObjectMapper;
6.
7. public class DeserializeExample {
8.
9. public static void main(String[] args) throws Exception {
10. new ObjectMapper();
11. new File("zoo.json"), Zoo.class);
12. System.out.println(zoo);
13. }
14. }
这里对反序列化后的Zoo对象不作详细遍历了,只打印toString看一下,结果如下,正确创建了Lion和Elephant对象
1. Zoo [name=SH Wild Park, city=ShangHai, animals=[Lion [name=Samba, getName()=Samba, getType()=carnivorous], Elephant [getName()=Manny, getType()=herbivorous]]]
我们还可以用另一个种方法来替代JsonSubTypes注释,现在,我们对Animal.java类稍作修改,隐去@JsonSubTypes注解,保留@JsonInfoType注解.这一次,我们直接序列化List<Animal>
SerializeExample2.java
1. package com.jackson.json.databinding.list;
2.
3. import java.io.File;
4. import java.io.IOException;
5. import java.util.ArrayList;
6. import java.util.List;
7.
8. import com.fasterxml.jackson.core.type.TypeReference;
9. import com.fasterxml.jackson.databind.ObjectMapper;
10. import com.fasterxml.jackson.databind.SerializationFeature;
11.
12. public class SerializeExample {
13. public static void main(String[] args) throws Exception {
14. new Zoo("SH Wild Park", "ShangHai");
15. new Lion("Samba");
16. new Elephant("Manny");
17. new ArrayList<Animal>();
18. animals.add(lion);
19. animals.add(elephant);
20. zoo.setAnimals(animals);
21.
22. new ObjectMapper();
23. true);
24. new TypeReference<List<Animal>>() {
25. new File("animal.json"), animals);
26.
27. }
28.
29. }
生成 animal.json,内容如下:
1. [ {
2. "@class" : "com.jackson.json.databinding.list.Lion",
3. "name" : "Samba",
4. "type" : "carnivorous"
5. }, {
6. "@class" : "com.jackson.json.databinding.list.Elephant",
7. "name" : "Manny",
8. "type" : "herbivorous"
9. } ]
总结
当采用数据绑定(DataBinding)方式处理json时,适当的使用Jackson Annotations可以帮助我们更好的解决问题,特别体现在序列化List时,解决多态的问题,以保证反序化到java对象时的正确性。@JsonAnyGetter,@JsonAnySetter,@JsonProperty都是很常用的注解,可以帮助我们更简洁的处理java对象与json之间的相互转化。