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

Question

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"

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)); } 

Answer 2

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));