如何在java中解析嵌套的JSON对象

Question

以下是示例JSON字符串。我想解析这个嵌套的JSON对象，即使嵌套的对象具有相同的名称。有时，我们可能会有多个级别的嵌套对象。我厌倦了Jackson的嵌套对象解析，但这对我不起作用。在解析对象之后，我想把它转换成一个不同的format.Please来帮助我解析这个JSON。提前谢谢。

 {
      "operator": "and",
      "predicates": [
        {
          "operator": "and",
          "predicates": [
            {
              "columnName": "userName",
              "datatype": "string",
              "input": "text",
              "operand": "equal",
              "value": "xxxx"
            },
            {
              "columnName": "Age",
              "datatype": "number",
              "input": "number",
              "operand": "greater_or_equal",
              "value": "21"
            }
          ]
        },
        {
          "operator": "and",
          "predicates": [
            {
              "columnName": "userName",
              "datatype": "string",
              "input": "text",
              "operand": "not_equal",
              "value": "nnn"
            },
            {
              "columnName": "Birthday",
              "datatype": "date",
              "input": "date_picker",
              "operand": "in",
              "value": "2020-07-23,2020-07-24"
            }
          ]
        }
      ]
    }

下面是用java编写的代码

import java.util.Map;
import com.fasterxml.jackson.annotation.JsonProperty;

public class Predicates {
    
    private String columnName;
    private String datatype;
    private String input;
    private String operator;
    private String value;
    private String operand;
    
    /**
     * 
     */
    
    private List<Predicates> predicates;
    
    @JsonProperty("predicates")
    private void unpackNested(Map<String,Object> predicates) {
        this.columnName = (String)predicates.get("columnName");
        this.datatype = (String)predicates.get("datatype");
        this.input = (String)predicates.get("input");
        this.operator = (String)predicates.get("operator");
        this.value = (String)predicates.get("value");
        this.operand = (String)predicates.get("operand");
    }
    
    
    public String getColumnName() {
        return columnName;
    }
    public void setColumnName(String columnName) {
        this.columnName = columnName;
    }
    public String getDatatype() {
        return datatype;
    }
    public void setDatatype(String datatype) {
        this.datatype = datatype;
    }
    public String getInput() {
        return input;
    }
    public void setInput(String input) {
        this.input = input;
    }
    public String getOperator() {
        return operator;
    }
    public void setOperator(String operator) {
        this.operator = operator;
    }
    public String getValue() {
        return value;
    }
    public void setValue(String value) {
        this.value = value;
    }

    public String getOperand() {
        return operand;
    }

    public void setOperand(String operand) {
        this.operand = operand;
    }
    public List<Predicates> getPredicates() {
        return predicates;
    }
    public void setPredicates(List<Predicates> predicates) {
        this.predicates = predicates;
    }
}

解析

ObjectMapper mapper = new ObjectMapper();
Predicates pr = mapper.readValue(json, Predicates.class);

Answer 1

我不知道您的unpackNested方法想要达到什么目的。Jackson已经可以将属性从JSON绑定到对象，并且可以很好地处理递归属性。

我只是删除了您的unpackNested方法，并在您提供的输入上运行了您的代码：

ObjectMapper mapper = new ObjectMapper();
Predicates pr = mapper.readValue(json, Predicates.class);

对象pr包含完整的json，包括嵌套的子谓词。(我用jackson databind 2.11.2运行了代码)。

如果未自动检测到您的属性，请使用@JsonProperty注释您的getters

class Predicate {
  // ..snip..

  @JsonProperty("input")
  public String getInput() {
    return input;
  }

  @JsonProperty("predicates")
  public List<Predicates> getPredicates() {
    return predicates;
  }
}

但除此之外，不需要采取额外的步骤。Jackson已经可以解包嵌套的对象了，只需移除你的(奇怪的) unpackNested方法，让Jackson来做它的工作。