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HQprivate 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;}@JsonCreatorpublic 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;}@JsonAnySetterpublic 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nullDepartment[] 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_numberSystem.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:Oracleheadquarters:CaliforniabirthDate:nulldepartment_name:developmentemployee_number:jackproduct:oracle_dbprojectManager:5000department_name:testemployee_number:roseproduct:oracle_testprojectManager: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;}@Overridepublic 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";}@Overridepublic 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";}@Overridepublic 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。