java11-vavr-exception-handling's Introduction

java11-vavr-exception-handling

Simple project to show how to handle exceptions using vavr.

preface

Please refer my other github projects:

project description

Suppose we have outer repository (to backup database) that we cannot modify. What is more - find method throws EntityNotFoundException:

public class LegacyBackupUserRepository {
    User find(int id) {
        return MockBackupDatabase.users.find(User.withId(id))
                .getOrElseThrow(EntityNotFoundException::new);
    }
}

in our application we have also a repository that:
- has find method that will lookup into our database
- has backupLookup method that will call find method from repository mentioned above (LegacyBackupUserRepository)
- has findOrBackup method that will try to lookup in our database and:
  - if user is found returns it
  - if user is not found - try to lookup in backup database

we want to model our UserRepository in a right way, so:

Option is perfect for modelling existence:

Option<User> find(int id) {
    return MockDatabase.users.find(User.withId(id));
}

Try is perfect to cope with third-party libraries that throws exceptions

Try<User> backupLookup(int id) {
    return Try.of(() -> legacyBackupUserRepository.find(id));
}

Either is perfect as a tailored error container

Either<List<DatabaseUserFindReport>, User> findOrBackup(int id) {
    Option<User> normalUser = find(id);
    if (normalUser.isDefined()) {
        return Either.right(normalUser.get());
    }
    
    Try<User> backupUser = backupLookup(id);
    if (backupUser.isSuccess()) {
        return Either.right(backupUser.get());
    }
    
    return Either.left(List.of(
            new DatabaseUserFindReport(id, DatabaseType.NORMAL),
            new DatabaseUserFindReport(id, DatabaseType.BACKUP)
    ));

}

summary: we return user if found otherwise list databases that we looked-up into.