@Cacheable for read caching in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a method annotated with @Cacheable is called multiple times with the same argument?

Consider a Spring Boot service method annotated with @Cacheable. What is the behavior when this method is called multiple times with the same input?

Spring Boot

public class ProductService {

  @Cacheable("products")
  public Product getProductById(Long id) {
    simulateSlowService();
    return new Product(id, "Product " + id);
  }

  private void simulateSlowService() {
    try {
      Thread.sleep(3000L);
    } catch (InterruptedException e) {
      throw new IllegalStateException(e);
    }
  }
}

A The method executes only once per unique argument; subsequent calls return cached result instantly.

B The method executes fully every time, ignoring the cache.

C The method caches the result but still executes every time.

D The method throws an exception if called more than once with the same argument.

💡 Hint

Think about how caching avoids repeated work for the same input.

The @Cacheable annotation tells Spring to cache the method's result for each unique argument. On subsequent calls with the same argument, the cached value is returned immediately without executing the method body again.

📝 Syntax

intermediate

Which @Cacheable annotation syntax correctly caches a method result with a custom cache name and key?

Choose the correct @Cacheable annotation usage to cache a method result in cache named "users" using the method's first argument as the key.

Spring Boot

public User getUserById(Long id) { ... }

A @Cacheable(value = "users", key = "id")

B @Cacheable(cache = "users", key = "#id")

C @Cacheable(cacheNames = "users", key = "#id")

D @Cacheable(cacheNames = "users", key = "id")

💡 Hint

Remember the SpEL syntax for keys uses # before parameter names.

The correct syntax uses cacheNames or value for cache name and SpEL expression #id to refer to the method parameter named id.

🔧 Debug

advanced

Why does the @Cacheable annotation not cache the method result in this Spring service?

Given the following Spring service, why is the caching not working as expected?

Spring Boot

public class OrderService {

  @Cacheable("orders")
  public Order getOrder(Long id) {
    return fetchOrderFromDb(id);
  }

  public Order fetchOrderFromDb(Long id) {
    // Simulate DB call
    return new Order(id, "Order " + id);
  }

  public Order getCachedOrder(Long id) {
    return getOrder(id);
  }
}

A The fetchOrderFromDb method should be annotated with @Cacheable instead.

B The method getOrder is called internally by getCachedOrder, so Spring proxy does not intercept the call to apply caching.

C The cache name "orders" is invalid and causes caching to fail silently.

D The Order class must implement Serializable for caching to work.

💡 Hint

Think about how Spring AOP proxies work and method calls inside the same class.

Spring uses proxies to apply caching. Internal method calls within the same class bypass the proxy, so @Cacheable is not triggered when getOrder is called from getCachedOrder.

❓ state_output

advanced

What is the output when calling a @Cacheable method twice with different arguments?

Consider this Spring Boot service method:

  @Cacheable("items")
  public String getItemName(int id) {
    System.out.println("Fetching item " + id);
    return "Item" + id;
  }

What will be printed to the console when calling getItemName(1) and then getItemName(2)?

A

Fetching item 2
Fetching item 1

B

Fetching item 1
(no output for second call)

C (no output for both calls)

D

Fetching item 1
Fetching item 2

💡 Hint

Remember caching stores results per unique argument.

Each unique argument causes the method to execute once. So calling with 1 prints once, then calling with 2 prints once. Both calls print because arguments differ.

🧠 Conceptual

expert

Which statement about @Cacheable behavior is TRUE regarding null return values?

In Spring Boot, when a method annotated with @Cacheable returns null, what happens by default?

A Null values are never cached by default; the method executes every time for null results.

B Null values are cached by default, so subsequent calls return null from cache.

C Caching null values causes a runtime exception unless configured otherwise.

D Null values are cached only if the cache manager supports it explicitly.

💡 Hint

Think about why caching nulls might be problematic.

By default, Spring does not cache null return values to avoid caching absence of data. The method runs every time if it returns null.

Practice

1. What is the main purpose of using @Cacheable in a Spring Boot application?

easy

A. To store the result of a method call and reuse it for the same inputs to improve performance.

B. To delete data from the database automatically.

C. To make a method run asynchronously in a separate thread.

D. To validate user input before processing.

Solution

1. Step 1: Understand what @Cacheable does

   @Cacheable stores the output of a method so that if the method is called again with the same inputs, it returns the stored result instead of running the method again.

2. Step 2: Identify the main benefit

   This caching avoids repeated slow or expensive operations, improving performance.

3. Final Answer:

   To store the result of a method call and reuse it for the same inputs to improve performance. -> Option A

4. Quick Check:

   @Cacheable purpose = store and reuse results [OK]

Hint: Remember: @Cacheable speeds up repeated reads by caching results [OK]

Common Mistakes:

• Thinking @Cacheable deletes or modifies data
• Confusing caching with asynchronous execution
• Assuming it validates inputs automatically

2. Which of the following is the correct way to use @Cacheable on a method named getUserById with cache name "users"?

easy

A. @Cacheable(cacheNames = "users") public User getUserById(Long id) { ... }

B. @Cacheable(name = "users") public User getUserById(Long id) { ... }

C. @Cacheable(cache = "users") public User getUserById(Long id) { ... }

D. @Cacheable(cacheName = "users") public User getUserById(Long id) { ... }

Solution

1. Step 1: Recall the correct attribute name for cache name

   The correct attribute to specify cache names in @Cacheable is cacheNames.

2. Step 2: Match the syntax with the method

   @Cacheable(cacheNames = "users") public User getUserById(Long id) { ... } uses @Cacheable(cacheNames = "users") which is correct syntax for Spring Boot caching.

3. Final Answer:

   @Cacheable(cacheNames = "users") public User getUserById(Long id) { ... } -> Option A

4. Quick Check:

   Correct attribute = cacheNames [OK]

Hint: Use cacheNames, not name or cache, in @Cacheable [OK]

Common Mistakes:

• Using wrong attribute like name or cache instead of cacheNames
• Missing parentheses or quotes around cache name
• Placing @Cacheable on class instead of method

3. Given the following code snippet, what will be printed when getData(5) is called twice?

@Cacheable(cacheNames = "dataCache")
public int getData(int id) {
    System.out.println("Fetching data for " + id);
    return id * 2;
}

medium

A. Fetching data for 5
Fetching data for 10

B. Fetching data for 5
Fetching data for 5

C. No output printed

D. Fetching data for 5

Solution

1. Step 1: Understand caching effect on method calls

   The first call to getData(5) runs the method and prints "Fetching data for 5". The result (10) is cached.

2. Step 2: Analyze the second call with same argument

   The second call with argument 5 returns cached result without running the method body, so no print happens.

3. Final Answer:

   Fetching data for 5 -> Option D

4. Quick Check:

   Cached call skips print = only one print [OK]

Hint: Cached calls skip method body, so no repeated prints [OK]

Common Mistakes:

• Assuming method runs every time and prints twice
• Thinking cache stores only values, not skips method
• Confusing argument values causing cache miss

4. Identify the error in the following code using @Cacheable:

@Cacheable(cacheNames = "items")
public Item getItem(Long id) {
    return itemRepository.findById(id).get();
}

// Usage
getItem(null);

medium

A. Calling getItem with null causes NullPointerException because cache key is null.

B. Cache name "items" is invalid and causes syntax error.

C. Missing @EnableCaching annotation causes caching to fail silently.

D. Method must be static to use @Cacheable.

Solution

1. Step 1: Check required configuration for caching

   @EnableCaching must be present on a @Configuration class to activate Spring's caching support. Without it, @Cacheable has no effect and caching fails silently.

2. Step 2: Rule out distractors

   Cache name "items" is valid. Null arguments are handled via NullValue.INSTANCE in the default key generator, no exception from cache key. Methods do not need to be static.

3. Final Answer:

   Missing @EnableCaching annotation causes caching to fail silently. -> Option C

4. Quick Check:

   @EnableCaching required for caching [OK]

Hint: Always add @EnableCaching to a @Configuration class [OK]

Common Mistakes:

• Assuming cache name "items" is invalid
• Thinking null causes NullPointerException due to cache key
• Believing method must be static for @Cacheable

5. You want to cache results of a method findProduct(String category, int id) but only cache when category is not null or empty. Which @Cacheable usage correctly implements this conditional caching?

hard

A. @Cacheable(cacheNames = "products", unless = "#category == null || #category == ''")
public Product findProduct(String category, int id) { ... }

B. @Cacheable(cacheNames = "products", condition = "#category != null && #category != ''")
public Product findProduct(String category, int id) { ... }

C. @Cacheable(cacheNames = "products", key = "#category + #id")
public Product findProduct(String category, int id) { ... }

D. @Cacheable(cacheNames = "products", condition = "#id > 0")
public Product findProduct(String category, int id) { ... }

Solution

1. Step 1: Understand conditional caching with condition

   The condition attribute controls when caching happens. It caches only if the condition is true.

2. Step 2: Analyze each option

   @Cacheable(cacheNames = "products", condition = "#category != null && #category != ''") public Product findProduct(String category, int id) { ... } caches only if category is not null and not empty, matching the requirement. @Cacheable(cacheNames = "products", unless = "#category == null || #category == ''") public Product findProduct(String category, int id) { ... } uses unless which caches unless the condition is true, so it inverts logic. @Cacheable(cacheNames = "products", key = "#category + #id") public Product findProduct(String category, int id) { ... } only sets key, no condition. @Cacheable(cacheNames = "products", condition = "#id > 0") public Product findProduct(String category, int id) { ... } caches based on id, not category.

3. Final Answer:

   @Cacheable(cacheNames = "products", condition = "#category != null && #category != ''") public Product findProduct(String category, int id) { ... } -> Option B

4. Quick Check:

   Use condition to cache only when category valid [OK]

Hint: Use condition attribute to control caching logic [OK]

Common Mistakes:

• Confusing condition and unless attributes
• Not checking for empty string in condition
• Using key attribute instead of condition for caching control