As all you Java developers know Hibernate is a popular Java-based Object-Relational Mapping (ORM) framework that is widely used to simplify database operations in Java applications. One of the key features of Hibernate is the Persistence Context, which is an in-memory cache of persistent entities. In this blog post, we will discuss the Hibernate Persistence Context, its states, and the methods to change states in the context. In this blog post, we will explore what Hibernate Persistence Context is, its different states, and the methods available to change these states in a fun and easy-to-understand way.
Hibernate Persistence Context
The Persistence Context is a cache-like mechanism that Hibernate uses to track the state of entities and to coordinate their persistence to the database. Whenever an entity is loaded by Hibernate, it becomes part of the Persistence Context. From that point onwards, Hibernate tracks all changes made to the entity until the Persistence Context is flushed (i.e., the changes are saved to the database) or the entity is detached (i.e., it is no longer being managed by Hibernate).
As a fun analogy to help us understand what the Persistence Context is and what it does. Imagine you are a chef in a restaurant, and the Persistence Context is your kitchen. In your kitchen, you have all the ingredients (entities) you need to prepare a meal (perform database operations). As you start preparing the meal, you take out the necessary ingredients from your fridge (the database), and you start cooking (performing database operations). As you cook, you might need to go back to the fridge to get more ingredients, or you might need to store some ingredients in the fridge to use later. The Persistence Context is like your fridge – it keeps track of the entities you have used, and it can provide you with the same entity if you need it again later.
States of Hibernate Persistence Context
Now that we have a basic understanding of what the Persistence Context is, let's dive into its states. The Persistence Context has four states: transient, persistent, detached, and removed.
- Transient State: When an entity is first created, it is in the transient state. This means that it is not associated with any Persistence Context, and it has not been saved to the database yet.
- Persistent State: When an entity is associated with a Persistence Context and saved to the database, it is in the persistent state. In this state, any changes made to the entity will be automatically saved to the database when the Persistence Context is flushed.
- Detached State: When an entity is no longer associated with a Persistence Context, it is in the detached state. In this state, any changes made to the entity will not be automatically saved to the database. However, if the entity is reattached to a Persistence Context, the changes made to the entity will be saved to the database when the Persistence Context is flushed.
- Removed State: When an entity is associated with a Persistence Context and then removed from the Persistence Context, it is in the removed state. In this state, the entity will be deleted from the database when the Persistence Context is flushed.
Now that we know the different states of the Persistence Context, let's discuss the methods to change states in the context.
- Making an entity persistent: To make an entity persistent, we need to associate it with a Persistence Context and then call the
persist()method on the entity. This will save the entity to the database and put it in the persistent state. - Detaching an entity: To detach an entity from a Persistence Context, we can call the
detach()method on the entity. This will remove the entity from the Persistence Context and put it in the detached state. - Merging a detached entity: To merge a detached entity back into a Persistence Context, we can call the
merge()method on the entity. This will associate the entity with the Persistence Context and put it back in the persistent state. - Removing an entity: To remove an entity from a Persistence Context, we can call the
remove()method on the entity. This will mark the entity as removed and put it in the removed state. The entity will be deleted from the database when the Persistence Context is flushed.