Jackson - best way writes a java list to a json array

I want to use jackson to convert a ArrayList to a JsonArray.

Event.java : this 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: 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());
}

I am looking for something like:

generator.write(out, list)  

this directly convert 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"

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));
}

I can't find toByteArray() as @atrioom said, so I use StringWriter, please try:

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(); 
}

Or just use ObjectMapper#writeValueAsString:

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

In objectMapper we have writeValueAsString() which accepts object as parameter. We can pass object list as parameter get the string back.

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)