The Flyweight pattern is a useful tool for optimizing code when working with large numbers of objects. The key idea is to reuse existing objects instead of creating new ones, which can save both time and memory.
In Spring Boot development, the Flyweight pattern can be used to improve performance by caching common objects and reusing them across requests. For example, if you have a service that returns a list of users, you can cache the list and reuse it instead of fetching it from the database each time.
To implement the Flyweight pattern in Spring Boot, you can use the Spring Cache abstraction. This allows you to declaratively specify which objects should be cached and how they should be invalidated.
In this article, we'll take a look at how to use the Flyweight pattern in Spring Boot development. We'll start by looking at the problem that the pattern can solve. We'll then look at how to implement the pattern using the Spring Cache abstraction. Finally, we'll look at a few more advanced topics related to the Flyweight pattern.
Consider a simple Spring Boot application that exposes a REST API for managing users. The API has a GET endpoint that returns a list of users. The endpoint looks like this:
@GetMapping("/users")
public List<User> getUsers() {
return userService.getUsers();
}
The getUsers() method returns a list of users from a UserService. The UserService is a simple service that fetches users from a database:
@Service
public class UserService {
@Autowired
private UserRepository userRepository;
public List<User> getUsers() {
return userRepository.findAll();
}
}
The UserService uses a UserRepository to fetch users from a database. The UserRepository is a simple CRUD repository:
@Repository
public interface UserRepository extends CrudRepository<User, Long> {
}
Now, imagine that we have a large number of users in our database. When we make a request to the /users endpoint, it takes a long time to fetch all of the users from the database. This can cause our application to become slow and unresponsive.
One way to solve this problem is to cache the list of users. That way, when we make a request to the /users endpoint, we can simply return the cached list of users instead of fetching it from the database each time.
We can implement the Flyweight pattern in our example application by using the Spring Cache abstraction. First, we need to add the spring-boot-starter-cache dependency to our pom.xml:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-cache</artifactId>
</dependency>
Next, we need to configure the UserService to use caching. We can do this by annotating the getUsers() method with the @Cacheable annotation:
@Service
public class UserService {
@Autowired
private UserRepository userRepository;
@Cacheable(value = "users")
public List<User> getUsers() {
return userRepository.findAll();
}
}
The @Cacheable annotation tells Spring to cache the results of the getUsers() method. The value attribute specifies the name of the cache. In this case, we've named the cache users.
Now, when we make a request to the /users endpoint, the getUsers() method will return the cached list of users. The first time the method is invoked, it will fetch the users from the database and cache them. Subsequent invocations will simply return the cached list of users.
We can also configure the @Cacheable annotation to control how the cached results are invalidated. For example, we can use the key attribute to specify the key of the cached object:
@Cacheable(value = "users", key = "#id")
public User getUserById(Long id) {
return userRepository.findById(id).orElse(null);
}
In this example, we've specified that the cached results should be keyed by the id parameter. This means that the cached results will be invalidated whenever the id changes.
We can also use the @CacheEvict annotation to explicitly evict an object from the cache:
@CacheEvict(value = "users", key = "#id")
public void deleteUserById(Long id) {
userRepository.deleteById(id);
}
In this example, we've annotated the deleteUserById() method with the @CacheEvict annotation. This tells Spring to evict the cached results of the getUserById() method when the deleteUserById() method is invoked.
There are a few advanced topics related to the Flyweight pattern that are worth mentioning.
By default, the Spring Cache abstraction will expire cached objects after a certain amount of time has elapsed. The default expiration time is two hours.
We can control the expiration time of cached objects by using the @Cacheable annotation's expire attribute:
@Cacheable(value = "users", expire = 60)
public List<User> getUsers() {
return userRepository.findAll();
}
In this example, we've specified that cached results should expire after 60 seconds.
We can also use the @CacheEvict annotation to explicitly expire an object:
@CacheEvict(value = "users", allEntries = true)
public void deleteUserById(Long id) {
userRepository.deleteById(id);
}
In this example, we've annotated the deleteUserById() method with the @CacheEvict annotation. The allEntries attribute tells Spring to expire all objects in the users cache.
The Spring Cache abstraction can be used to cache any type of object. However, it's important to be aware of the fact that the CacheManager will serialize and deserialize cached objects.
This can cause problems if the cached objects are not serializable. For example, consider the following User class:
public class User {
private Long id;
private String name;
private Date birthDate;
// Getters and setters...
}
The User class is not serializable because it contains a Date field. This means that we cannot cache User objects.
One way to work around this problem is to make the User class serializable:
public class User implements Serializable {
private static final long serialVersionUID = 1L;
private Long id;
private String name;
private Date birthDate;
// Getters and setters...
}
In this example, we've made the User class serializable by adding a serialVersionUID field. This field is required by the Serializable interface.
Another way to work around this problem is to use a Serializable wrapper class:
public class SerializableUser implements Serializable {
private static final long serialVersionUID = 1L;
private User user;
// Getters and setters...
}
In this example, we've created a SerializableUser class that wraps a User object. This allows us to cache the SerializableUser object without having to make the User class serializable.
In this article, we've looked at how to use the Flyweight pattern in Spring Boot development. We've start by looking at the problem that the pattern can solve. We've then looked at how to implement the pattern using the Spring Cache abstraction. Finally, we've looked at a few more advanced topics related to the Flyweight pattern.