如何使用Jackson JsonGenerator将列表添加到JSON中。

Posted 2021-04-28

我正试图用Jackson JsonGenerator将一个对象列表写成JSON。我想知道这是否可能。

我有一个hashmap，形成了Key,List。

我正在使用Jackson JsonGenerator创建一个JSON，希望能够简单地添加一个对象列表，而不是单独处理列表中的每个对象。

这可能吗？

    JsonFactory factory = new JsonFactory();
    StringWriter sw = new StringWriter();
    JsonGenerator generator = factory.createGenerator(sw);
    generator.setPrettyPrinter(new DefaultPrettyPrinter());

        generator.writeStartObject();               // {
        generator.writeFieldName("tags");           // {"tags":
        generator.writeStartArray();                // {"tags": [

    for ( String key : hm.keySet() ) {

        log.info("Record to Convert to JSON: " + key);

        generator.writeStartObject();               // {
        generator.writeStringField("tagId", key);   // { "tagId": "SAGE.esn.tag_name"
        generator.writeFieldName("data");           // { "tagId": "SAGE.esn.tag_name", "data"
        generator.writeStartArray();                // { "tagId": "SAGE.esn.tag_name", "data" : [
//          generator.writeStartObject();               // { "tagId": "SAGE.esn.tag_name", "data" : [{

        generator.write(hm.get(key));  

        generator.writeStringField("ts", data[0].trim()); // { "tagId": "NST_MD1_CRANK_CASE", "data" : [{"ts":"2017-11-15T19:55:00"
        generator.writeEndObject(); // { "tagId": "SAGE.esn.tag_name", "data" : [{"ts":"2017-11-15T19:55:00","v" : "100","q":"3"}
        generator.writeEndArray();  // { "tagId": "SAGE.esn.tag_name", "data" : [{"ts":"2017-11-15T19:55:00","v" : "100","q":"3"}]
        generator.writeEndObject(); // { "tagId": "SAGE.esn.tag_name", "data" : [{"ts":"2017-11-15T19:55:00","v" : "100","q":"3"}]}

    }

        generator.writeEndArray(); // {"tags": [ { "tagId": "SAGE.esn.tag_name", "data" : [{"ts":"2017-11-15T19:55:00","v" : "100","q":"3"}]} ]
        generator.writeEndObject();  // {"tags": [ { "tagId": "SAGE.esn.tag_name", "data" : [{"ts":"2017-11-15T19:55:00","v" : "100","q":"3"}]} ]}
        generator.flush();
        generator.close();

答案

是的，你几乎已经做到了。你应该只是通过你的列表迭代。

for ( String key : hm.keySet() ) {

    log.info("Record to Convert to JSON: " + key);

    generator.writeStartObject();               // {
    generator.writeStringField("tagId", key);   // { "tagId": "NST_MD1_CRANK_CASE"
    generator.writeFieldName("data");           // { "tagId": "NST_MD1_CRANK_CASE", "data"
    generator.writeStartArray();                // { "tagId": "NST_MD1_CRANK_CASE", "data" : [
    generator.writeStartObject();               // { "tagId": "NST_MD1_CRANK_CASE", "data" : [{

    List<String> data = hm.get(key);

    generator.writeStringField("ts", data.get(0).trim()); // { "tagId": "NST_MD1_CRANK_CASE", "data" : [{"ts":"2017-11-15T19:55:00"
    generator.writeStringField("v", data.get(1).trim());  // { "tagId": "NST_MD1_CRANK_CASE", "data" : [{"ts":"2017-11-15T19:55:00", "v":"100"
    generator.writeStringField("q", data.get(2).trim());  // { "tagId": "NST_MD1_CRANK_CASE", "data" : [{"ts":"2017-11-15T19:55:00", "v":"100", "q":"3"
    generator.writeEndObject(); // { "tagId": "NST_MD1_CRANK_CASE", "data" : [{"ts":"2017-11-15T19:55:00", "v":"100", "q":"3"}
    generator.writeEndArray();  // { "tagId": "NST_MD1_CRANK_CASE", "data" : [{"ts":"2017-11-15T19:55:00", "v":"100", "q":"3"}]
    generator.writeEndObject(); // { "tagId": "NST_MD1_CRANK_CASE", "data" : [{"ts":"2017-11-15T19:55:00", "v":"100", "q":"3"}]}
}

然后，如果你准备一些数据，

Map<String, List<String>> hm = new HashMap<>();
hm.put("NST_MD1_CRANK_CASE", Arrays.asList("2017-11-15T19:55:00", "100","3"));
hm.put("SAGE.esn.tag_name", Arrays.asList("2017-11-15T19:55:00", "100","3"));

并打印出其中的内容 StringWriter将你选择的数据输出到控制台，你会得到以下结果。

{
  "tags" : [ {
    "tagId" : "NST_MD1_CRANK_CASE",
    "data" : [ {
      "ts" : "2017-11-15T19:55:00",
      "v" : "100",
      "q" : "3"
    } ]
  }, {
    "tagId" : "SAGE.esn.tag_name",
    "data" : [ {
      "ts" : "2017-11-15T19:55:00",
      "v" : "100",
      "q" : "3"
    } ]
  } ]
}

如果你需要把数据写成一个数组，你可以用这个。

for ( String key : hm.keySet() ) {

    log.info("Record to Convert to JSON: " + key);

    generator.writeStartObject();               // {
    generator.writeStringField("tagId", key);   // { "tagId": "NST_MD1_CRANK_CASE"
    generator.writeFieldName("data");           // { "tagId": "NST_MD1_CRANK_CASE", "data"
    generator.writeStartArray();                // { "tagId": "NST_MD1_CRANK_CASE", "data" : [

    List<String> data = hm.get(key);
    for (String s : data) {
        generator.writeString(s);
    }

    generator.writeEndArray();  // { "tagId": "NST_MD1_CRANK_CASE", "data" : ["2017-11-15T19:55:00", "100", "3"]
    generator.writeEndObject(); // { "tagId": "NST_MD1_CRANK_CASE", "data" : ["2017-11-15T19:55:00", "100", "3"]}
}

那么同样的输入数据的输出是：

{
  "tags" : [ {
    "tagId" : "NST_MD1_CRANK_CASE",
    "data" : [ "2017-11-15T19:55:00", "100", "3" ]
  }, {
    "tagId" : "SAGE.esn.tag_name",
    "data" : [ "2017-11-15T19:55:00", "100", "3" ]
  } ]
}