Invoking Subclass Methods With Jdbi.onDemand

by ADMIN 45 views

Introduction

Jdbi is a popular Java library for database interactions. It provides a flexible and extensible way to interact with databases. However, when using Jdbi's onDemand feature, subclass methods may not be invoked as expected. In this article, we will discuss the issue and provide a solution to ensure that subclass methods are invoked correctly.

The Problem

The problem arises when using Jdbi's onDemand feature to attach a DAO (Data Access Object) to a database handle. The onDemand feature creates a proxy instance of the DAO, which may not have the subclass methods available. This can lead to a NullPointerException when trying to invoke the subclass methods.

The Code

The code provided by David Stryker is a good example of the issue. The JdbiDaoPickerExtensionFactory class is an ExtensionFactory that attaches a PostgresDaoPicker instance to the database handle. The PostgresDaoPicker class has a pickDao method that returns the subclass instance if it exists.

public class JdbiDaoPickerExtensionFactory implements ExtensionFactory {
    private final PostgresDaoPicker postgresDaoPicker;

    public JdbiDaoPickerExtensionFactory(PostgresDaoPicker postgresDaoPicker) {
        this.postgresDaoPicker = requireNonNull(postgresDaoPicker, "postgresDaoPicker is null");
    }

    @Override
    public boolean accepts(Class<?> extensionType) {
        // Only accept the extensionType if we're running Postgres, and there is a Postgres overloading
        return postgresDaoPicker.getOverloadingSubclass(extensionType).isPresent();
    }

    @Override
    public Set<FactoryFlag> getFactoryFlags() {
        return EnumSet.of(FactoryFlag.DONT_USE_PROXY);
    }

    @Override
    public <E> E attach(Class<E> extensionType, HandleSupplier handleSupplier) {
        return handleSupplier.getHandle().attach(postgresDaoPicker.pickDao(extensionType));
    }
}

The Solution

To solve this issue, we need to ensure that the subclass methods are invoked correctly when using Jdbi's onDemand feature. One way to achieve this is to use the Jdbi.withExtension method instead of Jdbi.onDemand. The withExtension method allows us to specify the extension factory to use, which can be our custom JdbiDaoPickerExtensionFactory class.

Jdbi jdbi = Jdbi.create("jdbc:postgresql://localhost:5432/mydb");
jdbi.withExtension(PostgresDaoPicker.class, new JdbiDaoPickerExtensionFactory(new PostgresDaoPicker()));

Alternative Solution

Another solution is to use the Jdbi.onDemand method with a custom ExtensionFactory that returns the subclass instance. We can create a custom ExtensionFactory class that extends the JdbiDaoPickerExtensionFactory class and overrides the attach method to return the subclass instance.

public class CustomExtensionFactory extends JdbiDaoPickerExtensionFactory {
    public CustomExtensionFactory(PostgresDaoPicker postgresDaoPicker) {
        super(postgresDaoPicker);
    }

    @Override
    public <E> E attach(Class<E> extension, HandleSupplier handleSupplier) {
        return handleSupplier.getHandle().attach(postgresDaoPicker.pickDao(extensionType));
    }
}

We can then use the custom ExtensionFactory class with the Jdbi.onDemand method.

Jdbi jdbi = Jdbi.create("jdbc:postgresql://localhost:5432/mydb");
jdbi.onDemand(PostgresDaoPicker.class, new CustomExtensionFactory(new PostgresDaoPicker()));

Conclusion

In conclusion, invoking subclass methods with Jdbi's onDemand feature can be challenging. However, by using the Jdbi.withExtension method or creating a custom ExtensionFactory class, we can ensure that the subclass methods are invoked correctly. The custom ExtensionFactory class can be used to return the subclass instance, which can then be used to invoke the subclass methods.

Best Practices

When working with Jdbi, it's essential to follow best practices to ensure that your code is efficient and scalable. Here are some best practices to keep in mind:

  • Use the Jdbi.withExtension method instead of Jdbi.onDemand when possible.
  • Create a custom ExtensionFactory class to return the subclass instance.
  • Use the JdbiDaoPickerExtensionFactory class as a base class for your custom ExtensionFactory class.
  • Override the attach method in your custom ExtensionFactory class to return the subclass instance.
  • Use the Jdbi.onDemand method with caution and only when necessary.

Introduction

In our previous article, we discussed the issue of invoking subclass methods with Jdbi's onDemand feature. We provided a solution using the Jdbi.withExtension method and creating a custom ExtensionFactory class. In this article, we will answer some frequently asked questions (FAQs) related to invoking subclass methods with Jdbi's onDemand feature.

Q: What is the difference between Jdbi's onDemand and withExtension methods?

A: The onDemand method creates a proxy instance of the DAO, which may not have the subclass methods available. The withExtension method, on the other hand, allows us to specify the extension factory to use, which can be our custom JdbiDaoPickerExtensionFactory class. This ensures that the subclass methods are invoked correctly.

Q: Why do I need to create a custom ExtensionFactory class?

A: Creating a custom ExtensionFactory class allows us to return the subclass instance, which can then be used to invoke the subclass methods. This is necessary because the onDemand method creates a proxy instance of the DAO, which may not have the subclass methods available.

Q: Can I use the JdbiDaoPickerExtensionFactory class as a base class for my custom ExtensionFactory class?

A: Yes, you can use the JdbiDaoPickerExtensionFactory class as a base class for your custom ExtensionFactory class. This is a good practice because it allows you to inherit the common functionality of the JdbiDaoPickerExtensionFactory class and override the attach method to return the subclass instance.

Q: How do I override the attach method in my custom ExtensionFactory class?

A: To override the attach method in your custom ExtensionFactory class, you need to create a new method with the same signature as the attach method in the JdbiDaoPickerExtensionFactory class. In this new method, you can return the subclass instance using the pickDao method of the PostgresDaoPicker class.

Q: Can I use the onDemand method with a custom ExtensionFactory class?

A: Yes, you can use the onDemand method with a custom ExtensionFactory class. To do this, you need to create a custom ExtensionFactory class that extends the JdbiDaoPickerExtensionFactory class and overrides the attach method to return the subclass instance. You can then use the custom ExtensionFactory class with the onDemand method.

Q: What are the best practices for working with Jdbi's onDemand feature?

A: Here are some best practices to keep in mind when working with Jdbi's onDemand feature:

  • Use the Jdbi.withExtension method instead of Jdbi.onDemand when possible.
  • Create a custom ExtensionFactory class to return the subclass instance.
  • Use the JdbiDaoPickerExtensionFactory class as a base class for your custom ExtensionFactory class.
  • the attach method in your custom ExtensionFactory class to return the subclass instance.
  • Use the Jdbi.onDemand method with caution and only when necessary.

Conclusion

In conclusion, invoking subclass methods with Jdbi's onDemand feature can be challenging. However, by using the Jdbi.withExtension method and creating a custom ExtensionFactory class, we can ensure that the subclass methods are invoked correctly. By following the best practices outlined in this article, you can ensure that your code is efficient, scalable, and easy to maintain.

Additional Resources

For more information on Jdbi and its onDemand feature, please refer to the following resources: