How to write list of maps using Jackson Streaming API

Question

Going through some reading, I saw below example for Jackson Streaming API. They are writing for one object (kind of one map). Can any one please help me on how to write multiple objects. Like a List of Maps.

jsonFactory jfactory = new JsonFactory();

/*** write to file ***/
JsonGenerator jGenerator = jfactory.createJsonGenerator(new File(
        "c:\\user.json"), JsonEncoding.UTF8);
jGenerator.writeStartObject(); // {

jGenerator.writeStringField("name", "mkyong"); // "name" : "mkyong"
jGenerator.writeNumberField("age", 29); // "age" : 29

jGenerator.writeFieldName("messages"); // "messages" :
jGenerator.writeStartArray(); // [

jGenerator.writeString("msg 1"); // "msg 1"
jGenerator.writeString("msg 2"); // "msg 2"
jGenerator.writeString("msg 3"); // "msg 3"

jGenerator.writeEndArray(); // ]

jGenerator.writeEndObject(); // }

jGenerator.close();

I tried looping and got below result.

{“Item1label1”:”Item1value1”,”Item1label2”:”Item1value2"} {“Item2label1”:”Item2value1”,”Item2label2”:”Item2value2"}   

But this is not a valid JSON. As something is missing in between two objects. What code has to be made to make it valid JSON.

Answer 1

Below is an example of writing a large array of objects (HashMap here but could be any other) with writePOJOField method. I use jackson-core and jackson-databind (see https://github.com/FasterXML/jackson).

// given
ByteArrayOutputStream stream = new ByteArrayOutputStream();
JsonFactory jfactory = new JsonFactory(new ObjectMapper());
JsonGenerator jGenerator = jfactory.createGenerator(stream, JsonEncoding.UTF8);

// when
jGenerator.writeStartObject();
jGenerator.writeStringField("name", "Tom");
jGenerator.writeNumberField("age", 25);

jGenerator.writeFieldName("objects");
jGenerator.writeStartArray();
IntStream.range(0, 3).forEach(it -> {
    try {
        jGenerator.writePOJO(new HashMap<String, String>() {{
            put("key1", "value1");
            put("key2", "value2");
        }});
    } catch (IOException e) {
        e.printStackTrace();
        throw new RuntimeException(e);
    }
});
jGenerator.writeEndArray();

jGenerator.writeFieldName("strings");
jGenerator.writeStartArray();
jGenerator.writeString("textX");
jGenerator.writeString("textY");
IntStream.range(0, 3).forEach(it -> {
    try {
        jGenerator.writeString("text-" + it);
    } catch (IOException e) {
        e.printStackTrace();
        throw new RuntimeException(e);
    }
});
jGenerator.writeEndArray();

jGenerator.writeNullField("nullField");
jGenerator.writeEndObject();
jGenerator.close();