How to read a Yaml File and write it in indentation or Json format with SnakeYaml ?

About

SnakeYaml ¹⁾ is a java library that manages yaml document.

Example

Object to read or write must be:

• a java bean (ie getter and setter + public properties)
• a LinkedList
• a HashMap

How to Read a Yaml File

²⁾

Log file:

---
Time: 2001-11-23 15:01:42 -5
User: ed
Warning:
  This is an error message
  for the log file
---
Time: 2001-11-23 15:02:31 -5
User: ed
Warning:
  A slightly different error
  message.
---
Date: 2001-11-23 15:03:17 -5
User: ed
Fatal:
  Unknown variable "bar"
Stack:
  - file: TopClass.py
    line: 23
    code: |
      x = MoreObject("345\n")
  - file: MoreClass.py
    line: 58
    code: |-
      foo = bar

With SnakeYaml, the data object are LinkedList.

• Dependency:

<dependency>
    <groupId>org.yaml</groupId>
    <artifactId>snakeyaml</artifactId>
    <version>1.20</version>
</dependency>

• Code

public void testLoadManyDocuments() throws FileNotFoundException {
    InputStream input = new FileInputStream(new File(
            "src/test/resources/specification/example2_28.yaml"));
    Yaml yaml = new Yaml();
    for (Object data : yaml.loadAll(input)) {
        System.out.println(data);
    }
}

{Time=Fri Nov 23 21:01:42 CET 2001, User=ed, Warning=This is an error message for the log file}
{Time=Fri Nov 23 21:02:31 CET 2001, User=ed, Warning=A slightly different error message.}
{Date=Fri Nov 23 21:03:17 CET 2001, User=ed, Fatal=Unknown variable "bar", Stack=[{file=TopClass.py, line=23, code=x = MoreObject("345\n")
}, {file=MoreClass.py, line=58, code=foo = bar}]}

where:

• curly brace { } are indicating a map
• and [] indicates a sequence

Write

Built with the dumper option to choose the format and styling.

How to write in a Indentation Block format

DumperOptions options = new DumperOptions();
options.setPrettyFlow(true);
options.setDefaultFlowStyle(DumperOptions.FlowStyle.BLOCK);
Yaml yaml = new Yaml(options);

• Then dump your object

StringWriter stringWriter = new StringWriter();
yaml.dump(object, stringWriter);
System.out.println(stringWriter.toString());

How to write in a extended JSON format

The flow style ³⁾ of Yaml is an extended Json format. It means that it adds features such as:

• tag for data type definition
• the possibility to tweak the quotation around the value

The below code shows you how you can simulate a valid json format.

• Instantiate the dump formatting options

DumperOptions options = new DumperOptions();

• Flow is the JSON format extended ⁴⁾

options.setDefaultFlowStyle(DumperOptions.FlowStyle.FLOW);

• No line break (Not json compatible)

options.setWidth(Integer.MAX_VALUE);

• Quote parameter ⁵⁾

options.setDefaultScalarStyle(DumperOptions.ScalarStyle.DOUBLE_QUOTED);

• To delete the yaml implicit tag such as the tag !!timestamp for timestamp ⁶⁾ create the following resolver class.

import org.yaml.snakeyaml.nodes.Tag;
import org.yaml.snakeyaml.resolver.Resolver;

public class YamlNoImplicitTagResolver extends Resolver {

  /**
   * do not resolve float, timestamp, ...
   */
  protected void addImplicitResolvers() {
    // addImplicitResolver(Tag.BOOL, BOOL, "yYnNtTfFoO");
    // addImplicitResolver(Tags.FLOAT, FLOAT, "-+0123456789.");
    // addImplicitResolver(Tag.TIMESTAMP, TIMESTAMP, "0123456789");
    // addImplicitResolver(Tag.INT, INT, "-+0123456789");
    // addImplicitResolver(Tag.MERGE, MERGE, "<");
    // addImplicitResolver(Tag.NULL, NULL, "~nN\0");
    // addImplicitResolver(Tag.NULL, EMPTY, null);
  }

}

• Instantiate snakeyaml

YamlNoImplicitTagResolver noImplicitTagResolver = new YamlNoImplicitTagResolver();
Yaml yaml = new Yaml(
        new Constructor(),
        new Representer(),
        dumperOptions,
        noImplicitTagResolver
);

• Then dump your object

StringWriter stringWriter = new StringWriter();
yaml.dump(object, stringWriter);
System.out.println(stringWriter.toString());