SnakeYAML Duplicate Key Checker

When working with YAML files, one common issue that can arise is the duplication of keys within a mapping. This can lead to unexpected behavior and errors, as most YAML parsers, including SnakeYAML, will only retain the last key-value pair for a duplicated key. To mitigate this issue, it's essential to implement a duplicate key checker. In this article, we will delve into the world of SnakeYAML and explore how to create a duplicate key checker to ensure the integrity of your YAML data.

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To understand the importance of a duplicate key checker, let’s first examine how SnakeYAML handles duplicate keys. By default, SnakeYAML will silently ignore duplicate keys, which can lead to data loss and errors. For instance, consider the following YAML file:

key1: value1
key2: value2
key1: value3

In this example, the first occurrence of `key1` with the value `value1` will be overwritten by the second occurrence of `key1` with the value `value3`. To prevent such issues, we need to implement a mechanism that detects and reports duplicate keys.

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One approach to creating a duplicate key checker is to extend the SafeConstructor class provided by SnakeYAML. This class allows us to customize the construction process of YAML objects. By overriding the constructMapping method, we can keep track of the keys encountered during the parsing process and detect any duplicates.

import org.yaml.snakeyaml.constructor.SafeConstructor;
import org.yaml.snakeyaml.nodes.MappingNode;
import org.yaml.snakeyaml.nodes.Node;
import org.yaml.snakeyaml.nodes.ScalarNode;

import java.util.Set;
import java.util.HashSet;

public class DuplicateKeyChecker extends SafeConstructor {
    private Set<String> keys = new HashSet<>();

    @Override
    protected Object constructMapping(MappingNode node) {
        for (Node tuple : node.getValue()) {
            Node keyNode = tuple.getKey();
            String key = constructScalar((ScalarNode) keyNode);
            if (keys.contains(key)) {
                throw new DuplicateKeyException("Duplicate key: " + key);
            }
            keys.add(key);
        }
        return super.constructMapping(node);
    }
}

In this implementation, we maintain a set of keys encountered during the parsing process. For each key-value pair, we check if the key is already present in the set. If it is, we throw a `DuplicateKeyException`. Otherwise, we add the key to the set and continue with the parsing process.

Integrating the Duplicate Key Checker with SnakeYAML

To use the duplicate key checker with SnakeYAML, you need to create a Yaml object with the custom constructor. Here’s an example:

import org.yaml.snakeyaml.Yaml;

public class YAMLParser {
    public static void main(String[] args) {
        Yaml yaml = new Yaml(new DuplicateKeyChecker());
        try {
            yaml.loadAsStream("""
                key1: value1
                key2: value2
                key1: value3
            """);
        } catch (DuplicateKeyException e) {
            System.out.println(e.getMessage());
        }
    }
}

In this example, we create a `Yaml` object with the custom `DuplicateKeyChecker` constructor. When we try to load the YAML stream, the duplicate key checker will detect the duplicate key and throw a `DuplicateKeyException`.

In conclusion, implementing a duplicate key checker is essential when working with YAML files to ensure data integrity and prevent errors. By extending the `SafeConstructor` class and overriding the `constructMapping` method, we can create a custom duplicate key checker that detects and reports duplicate keys.