在标量值上使用字面样式与 Jackson 进行 YAML 序列化

Question

I am using Jackson to serialize objects to YAML ( jackson-dataformat-yaml library).

I would like to produce literal style output for scalar values (eg 'description' in the following example) like

---
id: 4711
description: |
  FooBar
  HelloWorld

but I only manage to produce quoted scalars like this:

---
id: 4711
description: "FooBar\nHelloWorld"

The code I use to generate the ObjectMapper is (by now) very simple:

    YAMLFactory f = new YAMLFactory();
    f.enable(YAMLGenerator.Feature.SPLIT_LINES); // setting does not matter
    ObjectMapper objectMapperYaml = new ObjectMapper(f);
    String yaml = objectMapperYaml.writeValueAsString(someObject);

I guess there is some possibility to generate literal style scalar values but I don't know how. Any hints are welcome!

Answer 1

If you would be using SNAKEYaml on it's own it would be a matter of setting the corresponding dumper option:

DumperOptions dumperOptions = new DumperOptions();
dumperOptions.setDefaultScalarStyle(ScalarStyle.LITERAL);

Sadly enough it can't be done through a JacksonFeature here.

A quick glance through the source however show that the feature to enable is MINIMIZE_QUOTES , you'll find their algorithm in YAMLGenerator#writeString .

So here's the full class:

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.dataformat.yaml.YAMLFactory;
import com.fasterxml.jackson.dataformat.yaml.YAMLGenerator;

public class NewClass {

    private int id;

    private String description;

    public static void main(String... a) throws JsonProcessingException {
        YAMLFactory f = new YAMLFactory();
        f.enable(YAMLGenerator.Feature.MINIMIZE_QUOTES);
        ObjectMapper objectMapperYaml = new ObjectMapper(f);

        final NewClass someObject = new NewClass();
        someObject.setId(4711);
        someObject.setDescription("Hallo\nWorld!");
        System.out.println(objectMapperYaml.writeValueAsString(someObject));
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }
}

Answer 2

I know it is an year old thread but this was the top link suggested by Google and it needs an update.

The literal style is supported by Jackson since v.2.9. There is an issue though with trailing spaces as in this bug

Example:

YAMLMapper mapper = new YAMLMapper();
mapper.configure(YAMLGenerator.Feature.LITERAL_BLOCK_STYLE, true);
Map map = new HashMap();
map.put("literal_ok", "some value\n wih\n   multiple\n    new lines\nin it");
map.put("can_not_use_literal", "can not\n   use literal    \b because of the trailing spaces");
System.out.println(mapper.writeValueAsString(map));