将 Java 列表写入 JSON 数组的最佳方法是什么

Question

我想使用 Jackson 将 ArrayList 转换为 JsonArray。

Event

是 Java bean 类，有两个字段“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"));

至

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

我能想到的办法是：

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”。

更贪心：

generator.write(out, list1)
    
generator.write(out, list2)

这只会将 list1、list2 转换/添加到单个 JSON 数组中。然后将其写入“out”。

Answer 1

这太复杂了，Jackson 通过其 writer 方法处理列表，就像处理常规对象一样。假设我没有误解你的问题，这应该对你有用：

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));
}

Answer 2

我找不到@atrioom所说的

toByteArray()

，所以我使用

StringWriter

，请尝试：

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(); 
}

或者只是使用

ObjectMapper#writeValueAsString

:

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

Answer 3

在 objectMapper 中，我们有 writeValueAsString() ，它接受对象作为参数。我们可以传递对象列表作为参数来获取字符串。

List<Apartment> aptList =  new ArrayList<Apartment>();
    Apartment aptmt = null;
    for(int i=0;i<5;i++){
         aptmt= new Apartment();
         aptmt.setAptName("Apartment Name : ArrowHead Ranch");
         aptmt.setAptNum("3153"+i);
         aptmt.setPhase((i+1));
         aptmt.setFloorLevel(i+2);
         aptList.add(aptmt);
    }
mapper.writeValueAsString(aptList)

Answer 4

使用 Jackson 最简单的方法是：

List<YourObject> yourList = new ArrayList<>();

// yourList - fill it with the data

final ObjectMapper mapper = new ObjectMapper();
try {      
  System.out.println(mapper.writeValueAsString(yourList));
} catch (JsonProcessingException e) {
  e.printStackTrace();
}