Jackson - 将java列表写入json数组的最佳方式

json java jackson

2022-08-31 16:38:49

我想使用jackson将ArrayList转换为JsonArray。

事件.java：这是java bean类，其中两个字段“field1”，“field2”映射为JsonProperty。

我的目标是：

转换

ArrayList<Event> list = new ArrayList<Event>();
    list.add(new Event("a1","a2"));
    list.add(new Event("b1","b2"));

自

[
{"field1":"a1", "field":"a2"},
{"field1":"b1", "field":"b2"}
]

我能想到的方法是：writeListToJsonArray（）：

public void writeListToJsonArray() throws IOException {  
    ArrayList<Event> list = new ArrayList<Event>();
    list.add(new Event("a1","a2"));
    list.add(new Event("b1","b2"));

    OutputStream out = new ByteArrayOutputStream();

    JsonFactory jfactory = new JsonFactory();
    JsonGenerator jGenerator = jfactory.createJsonGenerator(out, JsonEncoding.UTF8);
    ObjectMapper mapper = new ObjectMapper();
    jGenerator.writeStartArray(); // [

    for (Event event : list) {
        String e = mapper.writeValueAsString(event);
        jGenerator.writeRaw(usage);
        // here, big hassles to write a comma to separate json objects, when the last object in the list is reached, no comma 
    }

    jGenerator.writeEndArray(); // ]

    jGenerator.close();

    System.out.println(out.toString());
}

我正在寻找类似的东西：

generator.write(out, list)

这将直接将列表转换为json数组格式，然后将其写入输出流“out”。

更贪婪：

generator.write(out, list1)

generator.write(out, list2)

这将只是将list1，list2转换/添加到单个json数组中。然后将其写到“out”

答案 1

这太复杂了，Jackson通过其编写器方法处理列表，就像它处理常规对象一样。这应该对你来说很好，假设我没有误解你的问题：

public void writeListToJsonArray() throws IOException {  
    final List<Event> list = new ArrayList<Event>(2);
    list.add(new Event("a1","a2"));
    list.add(new Event("b1","b2"));

    final ByteArrayOutputStream out = new ByteArrayOutputStream();
    final ObjectMapper mapper = new ObjectMapper();

    mapper.writeValue(out, list);

    final byte[] data = out.toByteArray();
    System.out.println(new String(data));
}

答案 2

我找不到@atrioom说，所以我使用，请尝试：toByteArray()StringWriter

public void writeListToJsonArray() throws IOException {  

    //your list
    final List<Event> list = new ArrayList<Event>(2);
    list.add(new Event("a1","a2"));
    list.add(new Event("b1","b2"));


    final StringWriter sw =new StringWriter();
    final ObjectMapper mapper = new ObjectMapper();
    mapper.writeValue(sw, list);
    System.out.println(sw.toString());//use toString() to convert to JSON

    sw.close(); 
}

或者直接使用：ObjectMapper#writeValueAsString

    final ObjectMapper mapper = new ObjectMapper();
    System.out.println(mapper.writeValueAsString(list));