write Java List to JSON Array via Jackson

Question

I am using Jackson for JSON serialization. I am trying to convert a Java List (containing string values) to a JSON array. I tried the following approaches (issues given below for each)

1. write array elements using JsonGenerator's writeString

final JsonGenerator generator = factory.createGenerator(output, JsonEncoding.UTF8);
  generator.writeStartArray();
        for (String arg: argsList) {
            generator.writeStartObject();
            log.info("arg value is {}", arg);
            generator.writeString(arg);
            generator.writeEndObject();
        }
        generator.writeEndArray();

Exception

Can not write a string, expecting field name (context: Object)

I get the exception from "generator.writeString(arg)". I cannot use writeStringField.

object mapper

ObjectMapper mapper = new ObjectMapper();
mapper.writeValue(out, argsList);
            final byte[] argsBytes = out.toByteArray();
            generator.writeFieldName("args");
            generator.writeObjectField("args", argsBytes)

This creates the array as a String and not an array within the JSON object (which is what I am trying to achieve). Any suggestions would be welcome.

End state (trying to achieve):

{
"args":["abc","def","ghi","jkl","mno"]
}

So, you want a JSON object with a single key named args, containing your list? Is that right? Why don't you create a class, with a single field named args, containing your list, and serialize an instance of that class? — JB Nizet
– JB Nizet, Commented Mar 17, 2017 at 18:19
will try that - I was hoping I could achieve the same through the generator. — ali haider
– ali haider, Commented Mar 17, 2017 at 18:24
You can achieve every valid json through the generator. If if you do try a serializer (which is a fine suggestion), please at least finish this question so we don't have just another "Don't do that, use a serializer" question/answer. — weston
– weston, Commented Mar 17, 2017 at 18:42
I had to remove the writeStart/endObject which fixed the issue - thanks everyone — ali haider
– ali haider, Commented Mar 17, 2017 at 18:52

weston · Accepted Answer · 2017-03-17 18:46:08Z

9

By starting/ending an object around each array entry what you are doing is trying to make invalid json:

{
    "args":[{"abc"},{"def"},{"ghi"},{"jkl"},{"mno"}]
}

And the generator is rightly stopping you from doing this.

Just write the strings directly into the array:

final JsonGenerator generator = factory.createGenerator(output, JsonEncoding.UTF8);
generator.writeStartArray();
for (String arg: argsList) {
    generator.writeString(arg);
}
generator.writeEndArray();

edited Mar 17, 2017 at 18:46

answered Mar 17, 2017 at 18:40

weston

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