无法理解 ObjectMapper writeValueAsString(Object value) 方法的行为

Question

I am using ObjectMapper to get value as String from JsonNode but see this weird behavior. I am missing something here. Can someone explain why is the Actual result has double quotes in it? I expect this test to pass but the assertion fails:

Unit Test:

@Test
public void testGetValueAsString() throws JsonProcessingException {
  ObjectMapper fasterXmlMapper = new com.fasterxml.jackson.databind.ObjectMapper();
  JsonNode node = new TextNode("{}");
  String valueAsString = fasterXmlMapper.writeValueAsString(node);
  assertEquals("{}", valueAsString);
}

Test Run Output:

expected:<[{}]> but was:<["{}"]>
Expected :{}
Actual   :"{}"

Here I am referring to following library and classes:

I am using jackson-databind-2.9.9.jar

com.fasterxml.jackson.databind.JsonNode and com.fasterxml.jackson.databind.ObjectMapper

Answer 1

According to the documentation a TextNode is

/** * Value node that contains a text value. */

So TextNode wraps its argument in "" . In order to make your test running you either need to chose a different subclass of Node or adjust the assertion to assertEquals("\"{}\"", valueAsString); .

Answer 2

You can read on json.org that string is:

A string is a sequence of zero or more Unicode characters, wrapped in double quotes , using backslash escapes. A character is represented as a single character string. A string is very much like a C or Java string.

And this is what com.fasterxml.jackson.databind.node.TextNode does, wraps {} in double quotes which gives "{}" .

If you want to have an empty JSON Object you can create com.fasterxml.jackson.databind.node.ObjectNode instance:

JsonNode node = mapper.getNodeFactory().objectNode();