Jackson的基本用法

Jackson 是当前用的比较广泛的,用来序列化和反序列化 json 的 Java 的开源框架。Jackson 社 区相对比较活跃,更新速度也比较快, 从 Github 中的统计来看,Jackson 是最流行的 json 解析器之一 。 Spring MVC 的默认 json 解析器便是 Jackson。 Jackson 优点很多。 Jackson 所依赖的 jar 包较少 ,简单易用。与其他 Java 的 json 的框架 Gson 等相比, Jackson 解析大的 json 文件速度比较快;Jackson 运行时占用内存比较低,性能比较好;Jackson 有灵活的 API,可以很容易进行扩展和定制。
Jackson 的 1.x 版本的包名是 org.codehaus.jackson ,当升级到 2.x 版本时,包名变为 com.fasterxml.jackson,本文讨论的内容是基于 Jackson 的 2.9.1 版本。

Jackson 的核心模块由三部分组成

  • jackson-core,核心包,提供基于"流模式"解析的相关 API,它包括 JsonPaser 和 JsonGenerator。 Jackson 内部实现正是通过高性能的流模式 API 的 JsonGenerator 和 JsonParser 来生成和解析 json。
  • jackson-annotations,注解包,提供标准注解功能;
  • jackson-databind ,数据绑定包, 提供基于"对象绑定" 解析的相关 API ( ObjectMapper ) 和"树模型" 解析的相关 API (JsonNode);基于"对象绑定" 解析的 API 和"树模型"解析的 API 依赖基于"流模式"解析的 API。
在 pom.xml 的 Jackson 的配置信息
 
com.fasterxml.jackson.core 
jackson-databind 
2.9.1 

jackson-databind 依赖 jackson-core 和 jackson-annotations,当添加 jackson-databind 之后, jackson-core 和 jackson-annotations 也随之添加到 Java 项目工程中。在添加相关依赖包之后,就可以使用 Jackson。

ObjectMapper 的 使用

ObjectMapper mapper = new ObjectMapper(); 
Person person = new Person(); 
person.setName("Tom"); 
person.setAge(40); 
String jsonString = mapper.writerWithDefaultPrettyPrinter() 
.writeValueAsString(person); 
Person deserializedPerson = mapper.readValue(jsonString, Person.class);

mapper.writerWithDefaultPrettyPrinter();可以让输出的json具有格式。

ObjectMapper 通过 writeValue 系列方法 将 java 对 象序列化 为 json,并 将 json 存 储成不同的格式,String(writeValueAsString),Byte Array(writeValueAsString),Writer, File,OutStream 和 DataOutput。
ObjectMapper 通过 readValue 系列方法从不同的数据源像 String , Byte Array, Reader,File,URL, InputStream 将 json 反序列化为 java 对象。

信息配置

在调用 writeValue 或调用 readValue 方法之前,往往需要设置 ObjectMapper 的相关配置信息。这些配置信息应用 java 对象的所有属性上。示例如下:

//在反序列化时忽略在 json 中存在但 Java 对象不存在的属性 
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES,
   false); 
//在序列化时日期格式默认为 yyyy-MM-dd'T'HH:mm:ss.SSSZ 
mapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS,false) 
//在序列化时忽略值为 null 的属性 
mapper.setSerializationInclusion(Include.NON_NULL); 
//忽略值为默认值的属性 
mapper.setDefaultPropertyInclusion(Include.NON_DEFAULT);

更多配置信息可以查看 Jackson 的 DeserializationFeature,SerializationFeature 和 Include。

Jackson 的 注解的使用

Jackson 根据它的默认方式序列化和反序列化 java 对象,若根据实际需要,灵活的调整它的默认方式,可以使用 Jackson 的注解。常用的注解及用法如下。


image.png

Jackson ObjectMapper Example

ObjectMapper objectMapper = new ObjectMapper();

String carJson =
    "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";

try {
    Car car = objectMapper.readValue(carJson, Car.class);

    System.out.println("car brand = " + car.getBrand());
    System.out.println("car doors = " + car.getDoors());
} catch (IOException e) {
    e.printStackTrace();
}

public class Car {
    private String brand = null;
    private int doors = 0;

    public String getBrand() { return this.brand; }
    public void   setBrand(String brand){ this.brand = brand;}

    public int  getDoors() { return this.doors; }
    public void setDoors (int doors) { this.doors = doors; }
}

从Reader读取对象

ObjectMapper objectMapper = new ObjectMapper();

String carJson =
        "{ \"brand\" : \"Mercedes\", \"doors\" : 4 }";
Reader reader = new StringReader(carJson);

Car car = objectMapper.readValue(reader, Car.class);

从File中读取对象

ObjectMapper objectMapper = new ObjectMapper();

File file = new File("data/car.json");

Car car = objectMapper.readValue(file, Car.class);

从URL中读取对象

ObjectMapper objectMapper = new ObjectMapper();

URL url = new URL("file:data/car.json");

Car car = objectMapper.readValue(url, Car.class);

从InputStream读取对象

ObjectMapper objectMapper = new ObjectMapper();

InputStream input = new FileInputStream("data/car.json");

Car car = objectMapper.readValue(input, Car.class);

从字节数组中读取对象

ObjectMapper objectMapper = new ObjectMapper();

String carJson =
        "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";

byte[] bytes = carJson.getBytes("UTF-8");

Car car = objectMapper.readValue(bytes, Car.class);

从JSON数组字符串中读取对象数组

String jsonArray = "[{\"brand\":\"ford\"}, {\"brand\":\"Fiat\"}]";

ObjectMapper objectMapper = new ObjectMapper();

Car[] cars2 = objectMapper.readValue(jsonArray, Car[].class);

从JSON数组字符串中读取对象列表

String jsonArray ="[{\"brand \":\"ford \"},{\"brand \":\"Fiat \"}]";

ObjectMapper objectMapper = new ObjectMapper();

List  cars1 = objectMapper.readValue(jsonArray,new TypeReference >(){});

从JSON字符串中读取映射为map

String jsonObject =“{\”brand \“:\”ford \“,\”doors \“:5}”;

ObjectMapper objectMapper = new ObjectMapper();
Map  jsonMap = objectMapper.readValue(jsonObject,
new TypeReference >(){});

树模型

String carJson =
        "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";

ObjectMapper objectMapper = new ObjectMapper();

try {

    JsonNode jsonNode = objectMapper.readValue(carJson, JsonNode.class);

} catch (IOException e) {
    e.printStackTrace();
}

JSON字符串被解析为JsonNode对象而不是Car对象,只需将JsonNode.class第二个参数传递给readValue()方法而不是Car.class本教程前面的示例中使用的方法。
该ObjectMapper班也有一个特殊的readTree(),它总是返回一个方法 JsonNode。以下是JsonNode使用该ObjectMapper readTree()方法将JSON解析为a的示例:

String carJson =
        "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";

ObjectMapper objectMapper = new ObjectMapper();

try {

    JsonNode jsonNode = objectMapper.readTree(carJson);

} catch (IOException e) {
    e.printStackTrace();
}

JsonNode类

String carJson =
        "{ \"brand\" : \"Mercedes\", \"doors\" : 5," +
        "  \"owners\" : [\"John\", \"Jack\", \"Jill\"]," +
        "  \"nestedObject\" : { \"field\" : \"value\" } }";

ObjectMapper objectMapper = new ObjectMapper();


try {

    JsonNode jsonNode = objectMapper.readValue(carJson, JsonNode.class);

    JsonNode brandNode = jsonNode.get("brand");
    String brand = brandNode.asText();
    System.out.println("brand = " + brand);

    JsonNode doorsNode = jsonNode.get("doors");

    /*
    也可替换成下面
    JsonNode doorsNode = jsonNode.path("doors");
    区别:在于如果要读取的key在Json串中不存在时,get方法会null,而path会返回MissingNode实例对象,在链路方法情况下保证不会抛出异常。
    */
    int doors = doorsNode.asInt();
    System.out.println("doors = " + doors);

    JsonNode array = jsonNode.get("owners");
    JsonNode jsonNode = array.get(0);
    String john = jsonNode.asText();
    System.out.println("john  = " + john);

    JsonNode child = jsonNode.get("nestedObject");
    JsonNode childField = child.get("field");
    String field = childField.asText();
    System.out.println("field = " + field);

} catch (IOException e) {
    e.printStackTrace();
}

将Object转换为JsonNode

ObjectMapper objectMapper = new ObjectMapper();

Car car = new Car();
car.brand = "Cadillac";
car.doors = 4;

JsonNode carJsonNode = objectMapper.valueToTree(car);

将JsonNode转换为Object

ObjectMapper objectMapper = new ObjectMapper();

String carJson = "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";

JsonNode carJsonNode = objectMapper.readTree(carJson);

Car car = objectMapper.treeToValue(carJsonNode);

使用Jackson ObjectMapper读取和编写YAML

1.示例1(只是yaml字符串和对象的互转,不涉及yaml文件的处理)

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.dataformat.yaml.YAMLFactory;

import java.io.IOException;

public class YamlJacksonExample {

    public static void main(String[] args) {
        ObjectMapper objectMapper = new ObjectMapper(new YAMLFactory());

        Employee employee = new Employee("John Doe", "[email protected]");

        String yamlString = null;
        try {
            yamlString = objectMapper.writeValueAsString(employee);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
            // normally, rethrow exception here - or don't catch it at all.
        }

    }
}

该yamlString变量包含Employee在执行此代码后序列化为YAML数据格式的对象。
以下是Employee再次将YAML文本读入对象的示例:

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.dataformat.yaml.YAMLFactory;

import java.io.IOException;

public class YamlJacksonExample {

    public static void main(String[] args) {
        ObjectMapper objectMapper = new ObjectMapper(new YAMLFactory());

        Employee employee = new Employee("John Doe", "[email protected]");

        String yamlString = null;
        try {
            yamlString = objectMapper.writeValueAsString(employee);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
            // normally, rethrow exception here - or don't catch it at all.
        }

        try {
            Employee employee2 = objectMapper.readValue(yamlString, Employee.class);

            System.out.println("Done");
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}
  1. 示例2 (yaml文件的读取和写入)
    2.1定义Employee实体类
package com.example.jackjson;

import lombok.Data;

/**
 * @author: GuanBin
 * @date: Created in 上午10:18 2020/6/15
 */
@Data
public class Employee {

    public Employee() {
    }

    public Employee(String name, String email) {
        this.name = name;
        this.email = email;
    }

    String name;

    String email;
}

2.2创建要读取的yml EmployeeYaml.yml文件,并初始化一条数据

name: test
email: [email protected]

2.3创建要写入的yml文件,EmployeeYamlOutput.yml (空文件)
2.4 测试类

package com.example.jackjson;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.dataformat.yaml.YAMLFactory;
import com.fasterxml.jackson.dataformat.yaml.YAMLGenerator;

import java.io.File;
import java.io.IOException;

/**
 * @author: GuanBin
 * @date: Created in 上午10:17 2020/6/15
 */
public class YamlJacksonExample {
    public static void main(String[] args) {


        try {
            //从yaml文件读取数据
            reaedYamlToEmployee();
            //写入yaml文件
            reaedEmployeeToYaml();
        } catch (Exception e) {
            e.printStackTrace();
        }

    }


    /**
     * 从yaml文件读取数据
     * @throws IOException
     */
    private static void reaedYamlToEmployee() throws IOException {
        ObjectMapper mapper = new ObjectMapper(new YAMLFactory());
        Employee employee = mapper.readValue(new File("src/test/java/com/example/jackjson/EmployeeYaml.yml"), Employee.class);
        System.out.println(employee.getName() + "********" + employee.getEmail());

    }

    /**
     * 写入yaml文件
     * @throws IOException
     */
    private static void reaedEmployeeToYaml() throws IOException {
        //去掉三个破折号
        ObjectMapper  mapper = new ObjectMapper(new YAMLFactory().disable(YAMLGenerator.Feature.WRITE_DOC_START_MARKER));
        //禁用掉把时间写为时间戳
        mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);

        Employee employee = new Employee("test2", "[email protected]");
        mapper.writeValue(new File("src/test/java/com/example/jackjson/EmployeeYamlOutput.yml"), employee);
    }
}

读取文件的打印输出

test********[email protected]

Process finished with exit code 0

写入文件的输出


image.png

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