从 JSON 数组创建 POJO (BufferReader)

Question

我正在尝试解组以下 JSON

[{
    "myId": "12851cb3087f51b4fb392b1fea36eef9508",
    "secondaryId": "787CFD4A-6B1D-4415-AD56-D075B535B890",
    "my_key": "keyABCD",
    "email": ""
}, {
    "myId": "12851cb3087f51b4fb392b1fea36eef9508",
    "secondaryId": "BFACD2F0-F5EF-4F05-AA6B-00E18CA907EF",
    "my_key": "keyABCD",
    "email": ""
}, {
    "myId": "12851cb3087f51b4fb392b1fea36eef9508",
    "secondaryId": "567DE8C0-B5B5-4961-B97A-A2DD374AEED1",
    "my_key": "keyABCD",
    "email": ""
}, {
    "myId": "12851cb3087f51b4fb392b1fea36eef9508",
    "secondaryId": "78a52d90-be6c-4d80-b79d-0e256028ba01",
    "my_key": "keyABCD",
    "email": "test@email.com"
}, {
    "myId": "12851cb3087f51b4fb392b1fea36eef9508",
    "secondaryId": "aeb148e7-fc88-4a71-8baa-63b6528e463e",
    "my_key": "keyABCD",
    "email": ""
}]

并且已经有一个具有上述 json 的 bufferreader (myBufferedReader)。 POJO

import lombok.AllArgsConstructor;
import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

@ToString
@AllArgsConstructor
public class MyPOJO {
    @Getter @Setter private String myId;
    @Getter @Setter private String secondaryId;
    @Getter @Setter private String my_key;
    @Getter @Setter private String email;

}

关于使用下面的映射器 -

ObjectMapper mapper = new ObjectMapper();
List<MyPOJO> eventList = mapper.readValue(myBufferedReader.readLine(),mapper.getTypeFactory().constructCollectionType(List.class, MyPOJO.class));

出现错误。请帮忙。 - （不存在像默认构造一样的创建者）：无法从对象值反序列化（没有基于委托或属性的创建者）

如果上述方法不正确，请建议从 bufferreader 读取并创建使用 json 映射的 pojo 类列表的最佳方法。

问候， 丹

Answer 1

通过在 POJO 类的顶部添加 @NoArgConstructor 添加您的默认构造函数，即空构造函数。 然后只需将缓冲区读取器 JSON 字符串转换为 POJO 列表，如下所示：

List<MyPOJO> eventList = mapper.readValue(myBufferedReader, new TypeReference<List<MyPOJO>>(){});

Answer 2

好吧，我不是万事通Java JSON。我有一个工具，每当我需要解析 JSON 时都会使用它。请注意，有多种工具可以解析 JSON String's，但我只会针对我使用的版本发布解决方案。我个人不会涉足 Java 的组件 Annotations，因为它们增加了如此巨大的复杂性，并且不会增加代码的任何价值。我不是来证明我的观点的，但我不会进入 Java Beans。

有一个名为 GSON 的库（据说）可以将 JSON String's 直接映射到 Java Object's（POJO's正如你所说的那样）。不幸的是，我不熟悉 GSON 工具，它实际上可能能够按照您的要求使用 Annotations “自动”构建class MyPOJO。

这是我的解决方案，它仅使用标准 JSON 解析库，我在下面将其称为 javax.json.*。您必须使用 Google 搜索 来检索 JSON JAR。

import java.io.*;
import javax.json.*;

public class S
{
    // I am just going to create the class for parsing this
    // If there is a GSON way to do this "Automatically", then you
    // should not use this Answer I have written to Stack Overflow

    public static class MyPOJO
    {
        public final String myId;
        public final String secondaryId;
        public final String myKey;
        public final String email;

        public MyPOJO(String myId, String secondaryId, String myKey, String email)
        { this.myId=myId; this.secondaryId=secondaryId; this.myKey=myKey; this.email=email; }

        public String toString()
        {
            return
                "myId:         " + myId         + '\n' +
                "seoondaryId:  " + secondaryId  + '\n' +
                "myKey:        " + myKey        + '\n' +
                "email:        " + email        + "\n\n";
        }
    }

    public static void main(String[] argv) throws IOException
    {
        // This reads the 'input.json' file into the Json parser using
        // a simple java.io.FileReader
        Reader r = new FileReader("input.json");

        // This reads the file, and retrieves the JsonArray that you
        // have provided in your original post.
        JsonArray ja = Json
            .createReader(r)
            .readArray();
        
        for (JsonObject jo : ja.getValuesAs(JsonObject.class))
        {
            String myId         = jo.getString("myId");
            String secondaryId  = jo.getString("secondaryId");
            String myKey        = jo.getString("my_key");
            String email        = jo.getString("email");

            // What *I* would do is to get rid of the Component Annotation, and simply 
            // use a Constructor.  I don't strongly believe in Java's Annotation Classes,
            // and I never use them.  If you can find an AUTOMATED WAY to do all of this,
            // YOU SHOULD ... - if you are willing to learn it all.
            // I HAVE NOT! :)
            
            // If there is an easy way to **DIRECTLY MAP** a JSON Object to a specified
            // class - and I believe that the GSON library is capable of directly mapping
            // JSON Object's to GSON Java POJO's (Java Object's), but I have not used them
            // before.  My own personal belief is that if it were easier, then learning the
            // GSON JAR Library and Java Documentation (JavaDoc) for GSON.

            // Here, though, a Constructor is what I would prefer myself.
            MyPOJO mp = new MyPOJO(myId, secondaryId, myKey, email);
            System.out.println(mp.toString());
        }
    }
}

上面的类输出到Shell Terminal的如下：

@cloudshell:~$ java S
myId:         12851cb3087f51b4fb392b1fea36eef9508
seoondaryId:  787CFD4A-6B1D-4415-AD56-D075B535B890
myKey:        keyABCD
email:        


myId:         12851cb3087f51b4fb392b1fea36eef9508
seoondaryId:  BFACD2F0-F5EF-4F05-AA6B-00E18CA907EF
myKey:        keyABCD
email:        


myId:         12851cb3087f51b4fb392b1fea36eef9508
seoondaryId:  567DE8C0-B5B5-4961-B97A-A2DD374AEED1
myKey:        keyABCD
email:        


myId:         12851cb3087f51b4fb392b1fea36eef9508
seoondaryId:  78a52d90-be6c-4d80-b79d-0e256028ba01
myKey:        keyABCD
email:        test@email.com


myId:         12851cb3087f51b4fb392b1fea36eef9508
seoondaryId:  aeb148e7-fc88-4a71-8baa-63b6528e463e
myKey:        keyABCD
email: