Designing clear and consistent error responses in a REST API is often harder than it looks. Without a shared standard, each application ends up inventing its own ad-hoc format, which quickly leads to inconsistency and confusion. RFC 7807 - Problem Details for HTTP APIs solves this by defining a simple, extensible JSON structure for error messages.

Problem4J brings this specification into the Java ecosystem, offering a practical way to model, throw, and handle API errors using Problem objects. It helps you enforce a consistent error contract across your services, while staying flexible enough for custom exceptions and business-specific details.

An example of application/problem+json response on HTTP API would look as follows.

{
  "status" : 400,
  "title" : "Bad Request",
  "detail" : "Validation failed",
  "errors" : [ {
    "field" : "email",
    "error" : "must be a well-formed email address"
  }, {
    "field" : "age",
    "error" : "must be greater than or equal to 18"
  } ]
}

It focuses on a minimal core API with optional integrations for popular Java frameworks.

• problem4j-core - core Problem model
• problem4j-jackson - JSON (de)serialization support
• problem4j-spring - Spring & Spring Boot integration

📘 https://problem4j.github.io

The documentation includes usage guides, examples, and module-specific details.

Problem4J aims to be small, explicit, and framework-friendly.