What is the best way to write a Java list to a JSON array

Question

I want to use Jackson to convert an ArrayList to a JsonArray.

Event is the Java bean class with two fields "field1", "field2" mapped as JsonProperty.

My goal is:

Convert

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

To

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

The way I can think of is:

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());
}

I am looking for something like:

generator.write(out, list)

This directly converts the list to JSON array format and then write it to outputstream "out".

Even greedier:

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

This will just convert/add in the list1, list2 into a single JSON array. then write it to "out".

Answer 1

This is overly complicated, Jackson handles lists via its writer methods just as well as it handles regular objects. This should work just fine for you, assuming I have not misunderstood your question:

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));
}