How to Resolve `No String-Argument Constructor/Factory Method` Error When Using OpenAPI Spring Boot Generator with Jackson?

How to fix the 'No String-argument constructor/factory method' error while using OpenAPI Spring Boot generator with Jackson?

// Example: Custom Deserializer in Jackson for an Enum\nimport com.fasterxml.jackson.core.JsonParser;\nimport com.fasterxml.jackson.databind.DeserializationContext;\nimport com.fasterxml.jackson.databind.JsonDeserializer;\nimport java.io.IOException;\n\npublic class StatusDeserializer extends JsonDeserializer<Status> {\n    @Override\n    public Status deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {\n        String statusValue = p.getText();\n        return Status.fromValue(statusValue); // Assume a fromValue method exists\n    }\n}

The error message 'No String-argument constructor/factory method' typically occurs when Jackson attempts to deserialize a JSON value into a Java object but cannot find a suitable way to instantiate that object. This issue often arises in Spring Boot applications that utilize OpenAPI specifications for generating models.

// Example of adding a default constructor\npublic class MyClass {\n    private String name;\n\n    // Default constructor\n    public MyClass() {}\n\n    // Constructor with arguments\n    public MyClass(String name) {\n        this.name = name;\n    }\n}

Causes

• The target class does not have a default constructor.
• The target class lacks a factory method that Jackson can use to create an instance from a string.
• The correct data type expected in the JSON does not match the Java class definition.

Solutions

• Add a default no-argument constructor to the target class if it does not exist.
• Create a factory method annotated with `@JsonCreator` to handle instantiation from JSON.
• Implement a custom deserializer using `JsonDeserializer` for complex objects.

Mistake: Forgetting to add a public no-argument constructor in model classes.

Solution: Ensure all model classes have a default constructor to allow Jackson to create instances.

Mistake: Failing to annotate factory methods with `@JsonCreator`.

Solution: Use the `@JsonCreator` annotation to inform Jackson of the method to use for constructing instances.

Mistake: Not matching the expected types in JSON and Java class definitions.

Solution: Double-check your JSON structure and ensure it matches the Java class definitions.