ObjectMapper not printing correct json in file

Question

ArrayList<ActionOutput> res = manager.getRes();
objectMapper.configure(SerializationFeature.INDENT_OUTPUT, true);
objectMapper.writeValue(new File(args[1]), res);

I expect my outputFile to look EXACTLY like this:

[
  {
    "key1": "value",
    "key2": [],
    "key3": null
  },
  {
    "key1": "value",
    "key2": [],
    "key3": null
  }
]

but it looks like this:

[ {                   <-- I need a newline after [
  "key1" : "value",
  "key2" : [ ],
  "key3" : null
}, {                  <-- I need a newline after },
  "key1" : "value",
  "key2" : [ ],       <-- no whitespace in empty array: [] instead of [ ]
  "key3" : null       <-- no whitespace after keys: "key3": null instead of "key3" : null
  } ]                 <-- I need a newline after }

How can I achieve this? I tried also to use pretty printer on objectMapper, but the result is the same.

Answer 1

You can customize indentation options of the DefaultPrettyPrinter using its methods indentObjectsWith() and indentArraysWith() . Both expect an instance of Indenter interface. One of its implementations shipped with Jackson is DefaultIndenter .

DefaultPrettyPrinter printer = new DefaultPrettyPrinter();
printer.indentObjectsWith(new DefaultIndenter());
printer.indentArraysWith(new DefaultIndenter());

When we have customized the DefaultPrettyPrinter , ObjectMapper should be instructed to use it for serialization.

ObjectMapper mapper = new ObjectMapper();
mapper.setDefaultPrettyPrinter(printer);

Finally, produce the JSON:

ArrayList<ActionOutput> res = manager.getRes();

mapper
    .writerWithDefaultPrettyPrinter()
    .writeValue(new File(args[1]), res)

Here's a small demo illustrating that indentations would be set correctly.

Consider the following POJO:

public class ActionOutput {
    private String key1;
    private List<String> key2;
    private String key3;
    
    // all-args constructor, getters
}

Serialization example:

List<ActionOutput> actions = List.of(
    new ActionOutput("value", List.of(), null),
    new ActionOutput("value", List.of(), null)
);
        
String jsonRes = mapper
    .writerWithDefaultPrettyPrinter()
    .writeValueAsString(actions);
    
System.out.println(jsonRes);

Output:

[
  {
    "key1" : "value",
    "key2" : [ ],
    "key3" : null
  },
  {
    "key1" : "value",
    "key2" : [ ],
    "key3" : null
  }
]