有没有办法在不写入文件的情况下使用 Jackson 将 Map 转换为 JSON 表示? [英] Is there any way to convert a Map to a JSON representation using Jackson without writing to a file?

问题描述

我正在尝试使用 Jackson 将 HashMap 转换为 JSON 表示.

I'm trying to use Jackson to convert a HashMap to a JSON representation.

然而，我见过的所有方法都涉及写入文件然后读回，这似乎非常低效.我想知道是否有办法直接做到这一点?

However, all the ways I've seen involve writing to a file and then reading it back, which seems really inefficient. I was wondering if there was anyway to do it directly?

这是我想做的一个例子

public static Party readOneParty(String partyName) {
  Party localParty = new Party();
  if(connection==null) {
    connection = new DBConnection();
  } try {
    String query = "SELECT * FROM PureServlet WHERE PARTY_NAME=?";
    ps = con.prepareStatement(query);
    ps.setString(1, partyName);
    resultSet = ps.executeQuery();
    meta = resultSet.getMetaData();
    String columnName, value;
    resultSet.next();
    for(int j=1;j<=meta.getColumnCount();j++) { // necessary to start at j=1 because of MySQL index starting at 1
      columnName = meta.getColumnLabel(j);
      value = resultSet.getString(columnName);
      localParty.getPartyInfo().put(columnName, value); // this is the hashmap within the party that keeps track of the individual values. The column Name = label, value is the value
    }
  }
}

public class Party {

  HashMap <String,String> partyInfo = new HashMap<String,String>();

  public HashMap<String,String> getPartyInfo() throws Exception {
    return partyInfo;
  }
}

输出看起来像这样

"partyInfo": {
  "PARTY_NAME": "VSN",
  "PARTY_ID": "92716518",
  "PARTY_NUMBER": "92716518"
}

到目前为止，我遇到的每个使用 ObjectMapper 的示例都涉及写入文件然后读取它.

So far every example I've come across of using ObjectMapper involves writing to a file and then reading it back.

是否有 Java 的 HashMap 或 Map 的 Jackson 版本，其工作方式与我已实现的方式类似?

Is there a Jackson version of Java's HashMap or Map that'll work in a similar way to what I have implemented?

推荐答案

Pass your Map to ObjectMapper.writeValueAsString(Object value)

Pass your Map to ObjectMapper.writeValueAsString(Object value)

它比使用 StringWriter 更高效，根据文档:

It's more efficient than using StringWriter, according to the docs:

可用于将任何 Java 值序列化为字符串的方法.功能上相当于用StringWriter调用writeValue(Writer,Object)并构造String，但效率更高.

Method that can be used to serialize any Java value as a String. Functionally equivalent to calling writeValue(Writer,Object) with StringWriter and constructing String, but more efficient.

示例

import org.codehaus.jackson.map.ObjectMapper;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public class Example {

    public static void main(String[] args) throws IOException {
        Map<String,String> map = new HashMap<>();
        map.put("key1","value1");
        map.put("key2","value2");

        String mapAsJson = new ObjectMapper().writeValueAsString(map);
        System.out.println(mapAsJson);
    }
}

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