Java：使用 Gson 库处理 JSON 数据

#Java 笔记

在 build.gradle 中引入依赖：

implementation 'com.google.code.gson:gson:2.8.6'
compileOnly 'org.projectlombok:lombok:1.18.16'
annotationProcessor 'org.projectlombok:lombok:1.18.16'

示例：将对象序列化为json字符串

package com.example.gson;


import lombok.Data;

@Data
public class UserInfo {

    private String name;

}

package com.example.gson;


import com.google.gson.Gson;

public class Main {

    public static void main(String[] args) {

        Gson gson = new Gson();
        UserInfo userInfo = new UserInfo();
        userInfo.setName("李白");
        String jsonStr = gson.toJson(userInfo);

        System.out.println(jsonStr);

    }

}

执行 Main 类，运行结果：

{"name":"李白"}

示例：将 json 字符串反序列化为对象

package com.example.gson;


import com.google.gson.Gson;
import lombok.Data;

public class Main {

    @Data
    public static class UserInfo {
        private String name;
    }

    public static void main(String[] args) {

        Gson gson = new Gson();
        String jsonStr = "{\"name\":\"李白\"}";
        UserInfo userInfo = gson.fromJson(jsonStr, UserInfo.class);
        System.out.println(userInfo);

    }

}

运行结果：

Main.UserInfo(name=李白)

若 fromJson 方法的第一个参数为 null，无法编译通过，因为由多个 fromJson 函数符合条件。

若为空字符串或者 {} 呢？

package com.example.gson;


import com.google.gson.Gson;
import lombok.Data;

public class Main {

    @Data
    public static class UserInfo {
        private String name;
    }

    public static void main(String[] args) {

        Gson gson = new Gson();

        System.out.println(gson.fromJson("", UserInfo.class));
        System.out.println(gson.fromJson(" ", UserInfo.class));

        System.out.println(gson.fromJson("{}", UserInfo.class));
        System.out.println(gson.fromJson("{ } ", UserInfo.class));
        System.out.println(gson.fromJson("{ \"age\": 18}", UserInfo.class));

    }

}

运行结果：

null
null
Main.UserInfo(name=null)
Main.UserInfo(name=null)
Main.UserInfo(name=null)

示例：序列化数字、字符串

package com.example.gson;


import com.google.gson.Gson;
import lombok.Data;

public class Main {

    public static void main(String[] args) {

        Gson gson = new Gson();

        System.out.println(gson.toJson("123"));
        System.out.println(gson.toJson(123));

    }

}

运行结果：

"123"
123

示例：反序列化数字、字符串

package com.example.gson;


import com.google.gson.Gson;

public class Main {

    public static void main(String[] args) {

        Gson gson = new Gson();
        String str = gson.fromJson("\"123\"", String.class);
        System.out.println(str);
        str = gson.fromJson("123", String.class);
        System.out.println(str);

        Long num = gson.fromJson("\"123\"", Long.class);
        System.out.println(num);
        num = gson.fromJson("123", Long.class);
        System.out.println(num);

    }

}

执行结果：

123
123
123
123

示例：反序列化为 List<T>

package com.example.gson;


import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.util.List;

public class Main {

    public static void main(String[] args) {

        Gson gson = new Gson();

        List<Integer> result = gson.fromJson("[1,2,3]", new TypeToken<List<Integer>>(){}.getType());
        for (Integer item: result) {
            System.out.println(item);
        }

    }

}

运行结果：

1
2
3

package com.example.gson;


import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.util.List;

public class Main {

    public static void main(String[] args) {

        Gson gson = new Gson();

        List<Long> result = gson.fromJson("[1,2,3]", new TypeToken<List<Long>>(){}.getType());
        for (Long item: result) {
            System.out.println(item);
        }

    }

}

运行结果：

1
2
3

示例：POJO 中含有 List<T>

package com.example.gson;


import com.google.gson.Gson;
import lombok.Data;

import java.util.Arrays;
import java.util.List;

public class Main {

    @Data
    public static class UserInfo {
        private String name;
        private List<Long> scores;
    }

    public static void main(String[] args) {
        UserInfo libai = new UserInfo();
        libai.setName("李白");
        libai.setScores(Arrays.asList(99L, 98L, 100L));

        Gson gson = new Gson();
        String jsonStr = gson.toJson(libai);
        System.out.println(jsonStr);

        UserInfo libai2 = gson.fromJson(jsonStr, UserInfo.class);
        System.out.println(libai2);

        for (Long score : libai2.scores) {
            System.out.println(score);
        }

    }

}

执行结果：

{"name":"李白","scores":[99,98,100]}
Main.UserInfo(name=李白, scores=[99, 98, 100])
99
98
100

示例：POJO 中含有 byte

import com.google.gson.Gson;
import lombok.Data;

import java.io.Serializable;

public class Main {

    @Data
    public static class SimpleVO implements Serializable {
        private Byte a;
        private Byte[] b;
        private byte c;
    }

    public static void main(String[] args) {
        SimpleVO simpleVO = new SimpleVO();
        simpleVO.setA((byte) 12);
        simpleVO.setB(new Byte[] {1,2,3});

        Gson gson = new Gson();
        System.out.println(gson.toJson(simpleVO));
        System.out.println(gson.fromJson(gson.toJson(simpleVO), SimpleVO.class));
    }

}

执行结果：

{"a":12,"b":[1,2,3],"c":0}
Main.SimpleVO(a=12, b=[1, 2, 3], c=0)

( 本文完 )