有没有办法在没有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.

但是，我看到的所有方法都涉及写入文件然后读回来，看起来真的很低效。我想知道是否还有直接这样做？

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?

这是一个我想做的实例的例子

Here's an example of an instance where I'd like to do it

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"
}

到目前为止每个例子我都是我们遇到使用 ObjectMapper 涉及写入文件然后再读回来。

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

是否有杰克逊版的Java HashMap 或地图这与我实施的方式类似吗？

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

将地图传递给 ObjectMapper.writeValueAsString（Object value）

它比使用 StringWriter 更有效，< a href =http://jackson.codehaus.org/1.7.9/javadoc/org/codehaus/jackson/map/ObjectMapper.html#writeValueAsString%28java.lang.Object%29>根据文档：

可用于将任何Java值序列化为String的方法。在功能上等同于使用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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