How does query caching work in Hibernate framework?

The query caching function in the Hibernate framework can improve query performance and avoid repeated execution of queries by caching query results. Its working principle is two-level caching, including session level and global level, and caching is enabled through the @Cacheable annotation. Cached data is shared by all Sessions until explicitly cleared or expired. Methods to explicitly clear the cache include session.clear() or session.evict(), and transparent clearing is performed automatically when the query results change.

Query cache in Hibernate framework

Overview

Query cache is Hibernate A feature provided by the framework that improves query performance by caching query results. When subsequent queries hit the cache, Hibernate returns the results directly from the cache instead of re-executing the query.

Working principle

Hibernate’s query cache is a two-level cache, including:

• First-level cache ( Session level): A temporary, thread-isolated cache that contains all Hibernate entities in the current Session.
• Second level cache (global level): An optional, globally available cache used to persist query results.

When Hibernate executes a query, it first checks the first level cache. If the query results are not in the first level cache, it will execute the query and cache the results in the first level cache.

If query caching is enabled, Hibernate will also cache query results into the second-level cache. Results in the second-level cache will be shared by all Sessions until explicitly cleared or the cache expires.

Practical Case:

Suppose we have an Employee entity, and we frequently execute queries that find specific employees:

Query query = session.createQuery("from Employee where id = :id");
query.setParameter("id", employeeId);
List<Employee> employees = query.list();

In order to cache the results of this query, we can use @Cacheable Note:

@Entity
@Cacheable
public class Employee {
    // ...
}

In this way, when we execute the same query, Hibernate will first look up the results from the cache. It will only execute the query and cache the results if there are no results in the cache.

Clear the cache

There are two main ways to clear the Hibernate cache:

• Explicit clearing: Use session.clear() or session.evict() method.
• Transparent clearing: Hibernate will automatically clear the cache when the query results change.

Performance Impact

Query caching can significantly improve query performance, especially for frequently executed queries. However, there are a few things to note:

• Cache can take up a lot of memory.
• If the cached data is updated frequently, you need to disable the cache or clear the cache frequently.

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