JackSon学习笔记(一)

概述

Jackson框架是基于Java平台的一套数据处理工具，被称为“最好的Java Json解析器”。 
Jackson框架包含了3个核心库：streaming,databind,annotations.Jackson还包含了其它数据处理类库，此外不作说明。
Jackson版本： 1.x (目前版本从1.1~1.9)与2.x。1.x与2.x从包的命名上可以看出来，1.x的类库中，包命名以：org.codehaus.jackson.xxx开头，而2.x类库中包命令：com.fastxml.jackson.xxx开头

Jackson Home Page：https://github.com/FasterXML/jackson
Jackson Wiki：http://wiki.fasterxml.com/JacksonHome
Jackson doc: https://github.com/FasterXML/jackson-docs
Jackson Download Page：http://wiki.fasterxml.com/JacksonDownload

准备工作

本文所有程序都基于JDK1.7,依赖jackon的三个核心类库：
jackson-core-2.5.3.jar
jackson-annotations-2.5.3.jar
jackson-databind-2.5.3.jar

Jackson处理Json

Jackson提供了三种可选的Json处理方法：流式API(Streaming API) 、树模型(Tree Model)、数据绑定(Data Binding)。从使用角度来看，比较一下这三种处理Json的方式的特性：

Streaming API：是效率最高的处理方式(开销低、读写速度快，但程序编写复杂度高)
Tree Model：是最灵活的处理方式
Data Binding：是最常用的处理方式

下面我们通过例子程序分别使用DataBinding,TreeModel,Streaming的方式来创建和解析Json字符串

1.DataBinding处理Json

Jackson支持Java对象与Json之间的相互转化。java对象序列化为json字符串，json字符串也可以反序列化为相同的java对象。

 

(1)java对象转化成json:

Province.java

package com.jackson.json.databinding;  
 
public class Province {
    public String name;
    public int population;
    public String[] city;
}  

Country.java

package com.jackson.json.databinding;  
 
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Date;
import java.util.HashMap;
import java.util.List;
import java.util.Map;  
 
public class Country {
    // 注意：被序列化的bean的private属性字段需要创建getter方法或者属性字段应该为public
    private String country_id;
    private Date birthDate;
    private List<String> nation = new ArrayList<String>();
    private String[] lakes;
    private List<Province> provinces = new ArrayList<Province>();
    private Map<String, Integer> traffic = new HashMap<String, Integer>();  
 
    public Country() {
        // TODO Auto-generated constructor stub
    }  
 
    public Country(String countryId) {
        this.country_id = countryId;
    }  
 
    public String getCountry_id() {
        return country_id;
    }  
 
    public void setCountry_id(String country_id) {
        this.country_id = country_id;
    }  
 
    public Date getBirthDate() {
        return birthDate;
    }  
 
    public void setBirthDate(Date birthDate) {
        this.birthDate = birthDate;
    }  
 
    public List<String> getNation() {
        return nation;
    }  
 
    public void setNation(List<String> nation) {
        this.nation = nation;
    }  
 
    public String[] getLakes() {
        return lakes;
    }  
 
    public void setLakes(String[] lakes) {
        this.lakes = lakes;
    }  
 
    public Integer get(String key) {
        return traffic.get(key);
    }  
 
    public Map<String, Integer> getTraffic() {
        return traffic;
    }  
 
    public void setTraffic(Map<String, Integer> traffic) {
        this.traffic = traffic;
    }  
 
    public void addTraffic(String key, Integer value) {
        traffic.put(key, value);
    }  
 
    public List<Province> getProvinces() {
        return provinces;
    }  
 
    public void setProvinces(List<Province> provinces) {
        this.provinces = provinces;
    }  
 
    @Override
    public String toString() {
        return "Country [country_id=" + country_id + ", birthDate=" + birthDate
                + ", nation=" + nation + ", lakes=" + Arrays.toString(lakes)
                + ", province=" + provinces + ", traffic=" + traffic + "]";
    }  
 
}  

JavaBeanSerializeToJson.java

package com.jackson.json.databinding;  
 
import java.io.File;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.List;  
 
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;  
 
public class JavaBeanSerializeToJson {  
 
    public static void convert() throws Exception {
        // 使用ObjectMapper来转化对象为Json
        ObjectMapper mapper = new ObjectMapper();
        // 添加功能，让时间格式更具有可读性
        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
        mapper.setDateFormat(dateFormat);  
 
        Country country = new Country("China");
        country.setBirthDate(dateFormat.parse("1949-10-01"));
        country.setLakes(new String[] { "Qinghai Lake", "Poyang Lake",
                "Dongting Lake", "Taihu Lake" });  
 
        List<String> nation = new ArrayList<String>();
        nation.add("Han");
        nation.add("Meng");
        nation.add("Hui");
        nation.add("WeiWuEr");
        nation.add("Zang");
        country.setNation(nation);  
 
        Province province = new Province();
        province.name = "Shanxi";
        province.population = 37751200;
        Province province2 = new Province();
        province2.name = "ZheJiang";
        province2.population = 55080000;
        List<Province> provinces = new ArrayList<Province>();
        provinces.add(province);
        provinces.add(province2);
        country.setProvinces(provinces);  
 
        country.addTraffic("Train(KM)", 112000);
        country.addTraffic("HighWay(KM)", 4240000);
        // 为了使JSON视觉上的可读性，增加一行如下代码，注意，在生产中不需要这样，因为这样会增大Json的内容
        mapper.configure(SerializationFeature.INDENT_OUTPUT, true);
        // 配置mapper忽略空属性
        mapper.setSerializationInclusion(Include.NON_EMPTY);
        // 默认情况，Jackson使用Java属性字段名称作为 Json的属性名称,也可以使用Jackson annotations(注解)改变Json属性名称
        mapper.writeValue(new File("country.json"), country);
    }  
 
    public static void main(String[] args) throws Exception {
        convert();
    }  
 
}

程序运行后生成country.json，内容如下：

{
  "country_id" : "China",
  "birthDate" : "1949-10-01",
  "nation" : [ "Han", "Meng", "Hui", "WeiWuEr", "Zang" ],
  "lakes" : [ "Qinghai Lake", "Poyang Lake", "Dongting Lake", "Taihu Lake" ],
  "provinces" : [ {
    "name" : "Shanxi",
    "population" : 37751200
  }, {
    "name" : "ZheJiang",
    "population" : 55080000
  } ],
  "traffic" : {
    "HighWay(KM)" : 4240000,
    "Train(KM)" : 112000
  }
}  

(2)Json字符串反序列化为java对象：

package com.jackson.json.databinding;  
 
import java.io.File;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Iterator;
import java.util.List;  
 
import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;  
 
/**
 * 将Json字符串反序列化为Java对象
 */
public class JsonDeserializeToJava {  
 
    public static void main(String[] args) throws Exception {
        //ObjectMapper类用序列化与反序列化映射器
        ObjectMapper mapper = new ObjectMapper();
        File json = new File("country.json");
        //当反序列化json时，未知属性会引起的反序列化被打断，这里我们禁用未知属性打断反序列化功能，
        //因为，例如json里有10个属性，而我们的bean中只定义了2个属性，其它8个属性将被忽略
        mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);  
 
        //从json映射到java对象，得到country对象后就可以遍历查找,下面遍历部分内容，能说明问题就可以了
        Country country = mapper.readValue(json, Country.class);
        System.out.println("country_id:"+country.getCountry_id());
        //设置时间格式，便于阅读
        SimpleDateFormat dateformat = new SimpleDateFormat("yyyy-MM-dd");
        String birthDate = dateformat.format(country.getBirthDate());
        System.out.println("birthDate:"+birthDate);  
 
        List<Province> provinces = country.getProvinces();
        for (Province province : provinces) {
            System.out.println("province:"+province.name + "\n" + "population:"+province.population);
        }
    }
}  

程序运行结果：

country_id:China
birthDate:1949-10-01
province:Shanxi
population:37751200
province:ZheJiang
population:55080000 

2.Tree Model处理Json

(1)tree model生成json:

package com.jackson.json.treemodel;  
 
import java.io.File;
import java.io.FileWriter;  
 
import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.node.ArrayNode;
import com.fasterxml.jackson.databind.node.JsonNodeFactory;
import com.fasterxml.jackson.databind.node.ObjectNode;  
 
public class SerializationExampleTreeModel {  
 
    public static void main(String[] args) throws Exception {
        //创建一个节点工厂,为我们提供所有节点
        JsonNodeFactory factory = new JsonNodeFactory(false);
        //创建一个json factory来写tree modle为json
        JsonFactory jsonFactory = new JsonFactory();
        //创建一个json生成器
        JsonGenerator generator = jsonFactory.createGenerator(new FileWriter(new File("country2.json")));
        //注意，默认情况下对象映射器不会指定根节点，下面设根节点为country
        ObjectMapper mapper = new ObjectMapper();
        ObjectNode country = factory.objectNode();  
 
        country.put("country_id", "China");
        country.put("birthDate", "1949-10-01");  
 
        //在Java中，List和Array转化为json后对应的格式符号都是"obj:[]"
        ArrayNode nation = factory.arrayNode();
        nation.add("Han").add("Meng").add("Hui").add("WeiWuEr").add("Zang");
        country.set("nation", nation);  
 
        ArrayNode lakes = factory.arrayNode();
        lakes.add("QingHai Lake").add("Poyang Lake").add("Dongting Lake").add("Taihu Lake");
        country.set("lakes", lakes);  
 
        ArrayNode provinces = factory.arrayNode();
        ObjectNode province = factory.objectNode();
        ObjectNode province2 = factory.objectNode();
        province.put("name","Shanxi");
        province.put("population", 37751200);
        province2.put("name","ZheJiang");
        province2.put("population", 55080000);
        provinces.add(province).add(province2);
        country.set("provinces", provinces);  
 
        ObjectNode traffic = factory.objectNode();
        traffic.put("HighWay(KM)", 4240000);
        traffic.put("Train(KM)", 112000);
        country.set("traffic", traffic);  
 
        mapper.configure(SerializationFeature.INDENT_OUTPUT, true);
        mapper.writeTree(generator, country);
    }  
 
} 

程序运行生成country2.json,内容如下：

{"country_id":"China","birthDate":"1949-10-01","nation":["Han","Meng","Hui","WeiWuEr","Zang"],"lakes":["QingHai Lake","Poyang Lake","Dongting Lake","Taihu Lake"],"provinces":[{"name":"Shanxi","population":37751200},{"name":"ZheJiang","population":55080000}],"traffic":{"HighWay(KM)":4240000,"Train(KM)":112000}} 

(2) json字符串反序列化为tree mode

DeserializationExampleTreeModel1.java,请注意观察程序中不同的JsonNode的类型变化

package com.jackson.json.treemodel;  
 
import java.io.File;
import java.util.Iterator;  
 
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;  
 
public class DeserializationExampleTreeModel1 {  
 
    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        // Jackson提供一个树节点被称为"JsonNode",ObjectMapper提供方法来读json作为树的JsonNode根节点
        JsonNode node = mapper.readTree(new File("country2.json"));
        // 看看根节点的类型
        System.out.println("node JsonNodeType:"+node.getNodeType());
        // 是不是一个容器
        System.out.println("node is container Node ? "+node.isContainerNode());
        // 得到所有node节点的子节点名称
        System.out.println("---------得到所有node节点的子节点名称-------------------------");
        Iterator<String> fieldNames = node.fieldNames();
        while (fieldNames.hasNext()) {
            String fieldName = fieldNames.next();
            System.out.print(fieldName+" ");
        }
        System.out.println("\n-----------------------------------------------------");
        // as.Text的作用是有值返回值，无值返回空字符串
        JsonNode country_id = node.get("country_id");
        System.out.println("country_id:"+country_id.asText() + " JsonNodeType:"+country_id.getNodeType());  
 
        JsonNode birthDate = node.get("birthDate");
        System.out.println("birthDate:"+birthDate.asText()+" JsonNodeType:"+birthDate.getNodeType());  
 
        JsonNode nation = node.get("nation");
        System.out.println("nation:"+ nation+ " JsonNodeType:"+nation.getNodeType());  
 
        JsonNode lakes = node.get("lakes");
        System.out.println("lakes:"+lakes+" JsonNodeType:"+lakes.getNodeType());  
 
        JsonNode provinces = node.get("provinces");
        System.out.println("provinces JsonNodeType:"+provinces.getNodeType());  
 
        boolean flag = true;
        for (JsonNode provinceElements : provinces) {
            //为了避免provinceElements多次打印，用flag控制打印，能体现provinceElements的JsonNodeType就可以了
            if(flag){
                System.out.println("provinceElements JsonNodeType:"+provinceElements.getNodeType());
                System.out.println("provinceElements is container node? "+provinceElements.isContainerNode());
                flag = false;
            }
            Iterator<String> provinceElementFields = provinceElements.fieldNames();
            while (provinceElementFields.hasNext()) {
                String fieldName = (String) provinceElementFields.next();
                String province;
                if ("population".equals(fieldName)) {
                    province = fieldName + ":" + provinceElements.get(fieldName).asInt();
                }else{
                    province = fieldName + ":" + provinceElements.get(fieldName).asText();
                }
                System.out.println(province);
            }
        }
    }
}  

程序运行后打印结果如下：

node JsonNodeType:OBJECT
node is container Node ? true
---------得到所有node节点的子节点名称-------------------------
country_id birthDate nation lakes provinces traffic
-----------------------------------------------------
country_id:China JsonNodeType:STRING
birthDate:1949-10-01 JsonNodeType:STRING
nation:["Han","Meng","Hui","WeiWuEr","Zang"] JsonNodeType:ARRAY
lakes:["QingHai Lake","Poyang Lake","Dongting Lake","Taihu Lake"] JsonNodeType:ARRAY
provinces JsonNodeType:ARRAY
provinceElements JsonNodeType:OBJECT
provinceElements is container node? true
name:Shanxi
population:37751200
name:ZheJiang
population:55080000 

在来看一下DeserializationExampleTreeModel2.java,本例中使用JsonNode.path的方法，path方法类似于DeserializationExampleTreeModel1.java中使用的get方法，

但当node不存在时,get方法返回null,而path返回MISSING类型的JsonNode

 

package com.jackson.json.treemodel;  
 
import java.io.File;
import java.io.IOException;
import java.util.Iterator;  
 
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;  
 
public class DeserializationExampleTreeModle2 {  
 
    public static void main(String[] args) throws JsonProcessingException, IOException{
        ObjectMapper mapper = new ObjectMapper();
        JsonNode node = mapper.readTree(new File("country2.json"));
        //path方法获取JsonNode时，当对象不存在时，返回MISSING类型的JsonNode
        JsonNode missingNode = node.path("test");
        if(missingNode.isMissingNode()){
            System.out.println("JsonNodeType : " + missingNode.getNodeType());
        }  
 
        System.out.println("country_id:"+node.path("country_id").asText());  
 
        JsonNode provinces = node.path("provinces");
        for (JsonNode provinceElements : provinces) {
            Iterator<String> provincesFields = provinceElements.fieldNames();
            while (provincesFields.hasNext()) {
                String fieldName = (String) provincesFields.next();
                String province;
                if("name".equals(fieldName)){
                    province = fieldName +":"+ provinceElements.path(fieldName).asText();
                }else{
                    province = fieldName +":"+ provinceElements.path(fieldName).asInt();
                }
                System.out.println(province);
            }
        }
    }  
 
}

程序运行打印结果：

JsonNodeType : MISSING
country_id:China
name:Shanxi
population:37751200
name:ZheJiang
population:55080000  

3.Stream处理Json

(1)stream生成json

 

package com.jackson.json.streaming;  
 
import java.io.File;
import java.io.FileWriter;
import java.io.Exception;  
 
import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonGenerator;  
 
public class StreamGeneratorJson {  
 
    public static void main(String[] args) throws Exception {
        JsonFactory factory = new JsonFactory();
        //从JsonFactory创建一个JsonGenerator生成器的实例
        JsonGenerator generator = factory.createGenerator(new FileWriter(new File("country3.json")));  
 
        generator.writeStartObject();
        generator.writeFieldName("country_id");
        generator.writeString("China");
        generator.writeFieldName("provinces");
        generator.writeStartArray();
        generator.writeStartObject();
        generator.writeStringField("name", "Shanxi");
        generator.writeNumberField("population", 33750000);
        generator.writeEndObject();
        generator.writeEndArray();
        generator.writeEndObject();  
 
        generator.close();
    }  
 
}

程序运行后生成country3.json文件内容：

{"country_id":"China","provinces":[{"name":"Shanxi","population":33750000}]}  

(2)stream解析json:
现在adgcountry3.json，我们用Streaming API的方式来解析上面的Json,并查找json中population的值。

 

package com.jackson.json.streaming;  
 
import java.io.File;
import java.io.IOException;  
 
import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;  
 
/*Jackson API提供了token对每个Json对象，例如，Json开始符号“{”是token指向的第一个解析的对象，
 key:value键值对是另一个单独的对象。这个API很强大，但也需要编写大量代码。不推荐使用，平时更多的是使用DataBinding和TreeModel来处理json
 */
public class StreamParserJson {
    public static void main(String[] args) throws JsonParseException,
            IOException {
        JsonFactory factory = new JsonFactory();
        // 从JsonFactory创建JsonParser解析器的实例
        JsonParser parser = factory.createParser(new File("country3.json"));  
 
        while (!parser.isClosed()) {
            // 得到一个token,第一次遍历时，token指向json文件中第一个符号"{"
            JsonToken token = parser.nextToken();
            if (token == null) {
                break;
            }
            // 我们只查找 country3.json中的"population"字段的值，能体现解析的流程就可以了
            // 当key是provinces时，我们进入provinces,查找population
            if (JsonToken.FIELD_NAME.equals(token)
                    && "provinces".equals(parser.getCurrentName())) {
                token = parser.nextToken();
                if (!JsonToken.START_ARRAY.equals(token)) {
                    break;
                }
                // 此时，token指向的应该是"{"
                token = parser.nextToken();
                if (!JsonToken.START_OBJECT.equals(token)) {
                    break;
                }
                while (true) {
                    token = parser.nextToken();
                    if (token == null) {
                        break;
                    }
                    if (JsonToken.FIELD_NAME.equals(token)
                            && "population".equals(parser.getCurrentName())) {
                        token = parser.nextToken();
                        System.out.println(parser.getCurrentName() + " : "
                                + parser.getIntValue());
                    }
                }
            }
        }
    }  
 
}  

程序运行后，在控制台打印结果如下：

population : 33750000 

总结

上面的例子中，分别用3种方式处理Json，我的体会大致如下：

 

Stream API方式是开销最低、效率最高，但编写代码复杂度也最高，在生成Json时，需要逐步编写符号和字段拼接json,在解析Json时，需要根据token指向也查找json值，生成和解析json都不是很方便，代码可读性也很低。

Databinding处理Json是最常用的json处理方式，生成json时，创建相关的java对象，并根据json内容结构把java对象组装起来，最后调用writeValue方法即可生成json,

解析时，就更简单了，直接把json映射到相关的java对象，然后就可以遍历java对象来获取值了。

TreeModel处理Json，是以树型结构来生成和解析json，生成json时，根据json内容结构，我们创建不同类型的节点对象，组装这些节点生成json。解析json时，它不需要绑定json到java bean，根据json结构，使用path或get方法轻松查找内容。

 

 

原文地址：http://blog.csdn.net/java_huashan/article/details/46375857

 

学习参考：http://www.cnblogs.com/lee0oo0/articles/2652528.html