Jackson学习笔记(三)
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
{
"name" : "Oracle",
"HQ" : "California",
"birthDate" : "1977-01-01",
"departments" : [ {
"name" : "development",
"employee_number" : 5000,
"projectManager" : "jack",
"product" : "oracle_db"
}, {
"name" : "test",
"employee_number" : 500,
"projectManager" : "rose",
"product" : "oracle_test"
} ]
}Company.java
package com.jackson.json.annotation;
import java.util.Date;
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.annotation.JsonProperty;
public class Company {
private String name;
@JsonProperty("HQ") //java属性headquarters序列化到json字段的名称为HQ
private String headquarters;
private Department[] departments;
@JsonIgnore //在序列化与反序列化时，忽略birthDate属性
private Date birthDate;
public Date getBirthDate() {
return birthDate;
}
@JsonCreator
public Company(@JsonProperty("name") String name) {
this.name = name;
}
public String getName() {
return name;
}
public String getHeadquarters() {
return headquarters;
}
public Department[] getDepartments() {
return departments;
}
public void setDepartments(Department[] departments) {
this.departments = departments;
}
}
Department.java
package com.jackson.json.annotation;
import java.util.HashMap;
import java.util.Map;
import com.fasterxml.jackson.annotation.JsonAnyGetter;
import com.fasterxml.jackson.annotation.JsonAnySetter;
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
public class Department {
private String name;
private String pm;
private Map<String, Object> otherProperties = new HashMap<String, Object>(); //otherProperties用来存放Department中未定义的json字段
@JsonCreator //指定json反序列化创建Department对象时调用此构造函数
public Department(@JsonProperty("name") String name){
this.name = name;
}
@JsonProperty("projectManager") //将company.json中projectManager字段关联到getPm方法
public String getPm() {
return pm;
}
public String getName() {
return name;
}
public Object get(String key) {
return otherProperties.get(key);
}
@JsonAnyGetter //得到所有Department中未定义的json字段的
public Map<String, Object> any() {
return otherProperties;
}
@JsonAnySetter
public void set(String key, Object value) {
otherProperties.put(key, value);
}
}
DeserializationExample.java
package com.jackson.json.annotation;
import java.io.File;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
public class DeserializationExample {
public static void main(String[] args) throws Exception {
ObjectMapper mapper = new ObjectMapper();
mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES); //禁止未知属性打断反序列化
Company company = mapper.readValue(new File("company_back.json"), Company.class);
System.out.print("company_name:"+company.getName()+"\t");
System.out.print("headquarters:"+company.getHeadquarters()+"\t");
System.out.println("birthDate:"+company.getBirthDate()); //birthDate被标记为@JsonIgnore，所以此处得到的值应该为null
Department[] departments = company.getDepartments();
for (Department department : departments) {
System.out.print("department_name:" + department.getName()+"\t");
System.out.print("employee_number:" + department.getPm()+"\t");
//Department中未定义的字段product,employee_number
System.out.print("product:"+department.get("product")+"\t");
System.out.println("projectManager:"+department.get("employee_number"));
}
}
}
程序运行控制台打印结果如下：
company_name:Oracle headquarters:California birthDate:null department_name:development employee_number:jack product:oracle_db projectManager:5000 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
package com.jackson.json.databinding.list;
import java.util.List;
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
public class Zoo {
public String name;
public String city;
public List<Animal> animals;
@JsonCreator
public Zoo(@JsonProperty("name") String name, @JsonProperty("city") String city) {
this.name = name;
this.city = city;
}
public void setAnimals(List<Animal> animals) {
this.animals = animals;
}
@Override
public String toString() {
return "Zoo [name=" + name + ", city=" + city + ", animals=" + animals
+ "]";
}
}
Animal.java
package com.jackson.json.databinding.list;
import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonSubTypes.Type;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.annotation.JsonTypeInfo.As;
import com.fasterxml.jackson.annotation.JsonTypeInfo.Id;
@JsonTypeInfo(use=Id.CLASS,include=As.PROPERTY,property="@class")
@JsonSubTypes({@Type(value=Lion.class,name="lion"),@Type(value=Elephant.class,name="elephant")})
public abstract class Animal {
String name;
String type;
}
Lion.java
package com.jackson.json.databinding.list;
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
public class Lion extends Animal {
private String name;
@JsonCreator
public Lion(@JsonProperty("name") String name) {
this.name = name;
}
public String getName() {
return name;
}
public String getType() {
return "carnivorous";
}
@Override
public String toString() {
return "Lion [name=" + name + ", getName()=" + getName()
+ ", getType()=" + getType() + "]";
}
}
Elephant.java
package com.jackson.json.databinding.list;
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
public class Elephant extends Animal {
private String name;
@JsonCreator
public Elephant(@JsonProperty("name") String name) {
this.name = name;
}
public String getName() {
return name;
}
public String getType() {
return "herbivorous";
}
@Override
public String toString() {
return "Elephant [getName()=" + getName() + ", getType()=" + getType()
+ "]";
}
}
SerializeExmaple.java
package com.jackson.json.databinding.list;
import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
public class SerializeExample {
public static void main(String[] args) throws Exception {
Zoo zoo = new Zoo("SH Wild Park", "ShangHai");
Lion lion = new Lion("Samba");
Elephant elephant = new Elephant("Manny");
List<Animal> animals = new ArrayList<Animal>();
animals.add(lion);
animals.add(elephant);
zoo.setAnimals(animals);
ObjectMapper mapper = new ObjectMapper();
mapper.configure(SerializationFeature.INDENT_OUTPUT, true);
mapper.writeValue(new File("zoo.json"), zoo);
}
}
生成zoo.json内容如下：
{
"name" : "SH Wild Park",
"city" : "ShangHai",
"animals" : [ {
"@class" : "com.jackson.json.databinding.list.Lion",
"name" : "Samba",
"type" : "carnivorous"
}, {
"@class" : "com.jackson.json.databinding.list.Elephant",
"name" : "Manny",
"type" : "herbivorous"
} ]
}
反序列化，DeserializeExmaple.java
package com.jackson.json.databinding.list;
import java.io.File;
import com.fasterxml.jackson.databind.ObjectMapper;
public class DeserializeExample {
public static void main(String[] args) throws Exception {
ObjectMapper mapper = new ObjectMapper();
Zoo zoo = mapper.readValue(new File("zoo.json"), Zoo.class);
System.out.println(zoo);
}
}
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
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
package com.jackson.json.databinding.list;
import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
public class SerializeExample {
public static void main(String[] args) throws Exception {
Zoo zoo = new Zoo("SH Wild Park", "ShangHai");
Lion lion = new Lion("Samba");
Elephant elephant = new Elephant("Manny");
List<Animal> animals = new ArrayList<Animal>();
animals.add(lion);
animals.add(elephant);
zoo.setAnimals(animals);
ObjectMapper mapper = new ObjectMapper();
mapper.configure(SerializationFeature.INDENT_OUTPUT, true);
mapper.writerFor(new TypeReference<List<Animal>>() {
}).writeValue(new File("animal.json"), animals);
}
}
生成 animal.json,内容如下：
[ {
"@class" : "com.jackson.json.databinding.list.Lion",
"name" : "Samba",
"type" : "carnivorous"
}, {
"@class" : "com.jackson.json.databinding.list.Elephant",
"name" : "Manny",
"type" : "herbivorous"
} ]
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。