Jackson学习笔记(三)<转>

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()+"	");
        System.out.print("headquarters:"+company.getHeadquarters()+"	");
        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()+"	");
            System.out.print("employee_number:" + department.getPm()+"	");
            //Department中未定义的字段product,employee_number
            System.out.print("product:"+department.get("product")+"	");
            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。

转自 http://blog.csdn.net/java_huashan/article/details/46428971