杰克逊 - 最好的方法是将一个java列表写入json数组

Question

我想使用jackson将ArrayList转换为JsonArray.

Event.java:这是java bean类,有两个字段"field1","field2"映射为JsonProperty.

我的目标是:

兑换

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)  

这会直接将列表转换为json数组格式,然后将其写入outputstream"out".

甚至更贪婪:

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.然后把它写成"out"

Answer 1

这过于复杂,Jackson通过其编写器方法处理列表以及处理常规对象.假设我没有误解你的问题,这对你来说应该没问题:

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));
}

Answer 2

我无法找到toByteArray()@atrioom说,所以我用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object作为参数.我们可以传递对象列表作为参数获取字符串.

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)