jackson的使用

1、jackson能做什么？

Jackson 被称为 "Java JSON 库 "或 "Java 的最佳 JSON 解析器"。不仅如此，Jackson 还是一套适用于 Java（和 JVM 平台）的数据处理工具，除了能处理json格式数据以外，
还能处理csv、xml、（java）properties等格式数据。

2、使用方法

1. 处理json数据

• maven中加入依赖

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>maven_demo</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
        <jackson.version>2.13.3</jackson.version>
    </properties>
    <dependencies>
    <!--<dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-annotations</artifactId>
        <version>${jackson.version}</version>
    </dependency>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-core</artifactId>
        <version>${jackson.version}</version>
    </dependency>-->
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>${jackson.version}</version>
        </dependency>
    </dependencies>
</project>

• 序列化

import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.type.CollectionType;
import org.junit.Test;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Test1 {
    static ObjectMapper objectMapper = new ObjectMapper();
    static {
        // 序列化时空值属性不序列化
        objectMapper.setSerializationInclusion(JsonInclude.Include.NON_EMPTY);
        // jackson在反序列化时默认如果遇到未知属性（没有映射到属性并且没有setter或者handler来处理它）时会报错，可以通过以下配置解决
        objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES,false);
    }
    @Test
    public void test1() throws JsonProcessingException {
        MyBean myBean = new MyBean();
        myBean.setId(1);
        myBean.setName("My name");
        // bean
        String s = objectMapper.writeValueAsString(myBean);
        System.out.println(s);
        List<MyBean> list = new ArrayList<>();
        list.add(myBean);
        // list
        String s1 = objectMapper.writeValueAsString(list);
        System.out.println(s1);
        // map
        Map<String,Object> map = new HashMap<>();
        map.put("id",2);
        map.put("name","your name");
        String s2 = objectMapper.writeValueAsString(map);
        System.out.println(s2);
    }
}

class MyBean {
    public Integer id;
    public String name;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "MyBean{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

• 反序列化

import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.type.CollectionType;
import org.junit.Test;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Test1 {
    static ObjectMapper objectMapper = new ObjectMapper();
    static {
        // 序列化时空值属性不序列化
        objectMapper.setSerializationInclusion(JsonInclude.Include.NON_EMPTY);
        // jackson在反序列化时默认如果遇到未知属性（没有映射到属性并且没有setter或者handler来处理它）时会报错，可以通过以下配置解决
        objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES,false);
    }
    @Test
    public void test2() throws JsonProcessingException {
        String jsonStr = "{\"id\":1,\"name\":\"My name\"}";

        MyBean myBean = objectMapper.readValue(jsonStr, MyBean.class);
        System.out.println(myBean);
        String jsonStr1 = "[{\"id\":1,\"name\":\"My name\"}]";
        // 指定返回class为List.class的话，list中的类型是map类型
        List list = objectMapper.readValue(jsonStr1, List.class);
        System.out.println(list);
        // 使用TypeReference指定反序列化后的类型
        List<MyBean> list1 = objectMapper.readValue(jsonStr1, new TypeReference<List<MyBean>>() {});
        System.out.println(list1.get(0));

        // 使用TypeFactory构建反序列化后的类型
        CollectionType collectionType = objectMapper.getTypeFactory().constructCollectionType(List.class, MyBean.class);
        List<MyBean> list2 = objectMapper.readValue(jsonStr1, collectionType);
        System.out.println(list2.get(0));

    }
}

class MyBean {
    public Integer id;
    public String name;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "MyBean{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}