Cache key strategies in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify a cache key using Spring's @Cacheable annotation.

Spring Boot

@Cacheable(value = "users", key = "#[1]")
public User getUserById(Long id) {
    // method body
}

Drag options to blanks, or click blank then click option'

Avalue

BuserId

Ckey

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name that does not exist in the method signature.

Omitting the # prefix in the key expression.

The cache key is set to the method parameter id using #id in the SpEL expression.

2fill in blank

medium

Complete the code to create a cache key based on multiple method parameters.

Spring Boot

@Cacheable(value = "products", key = "#[1] + '-' + #[2]")
public Product getProduct(String category, Long productId) {
    // method body
}

Drag options to blanks, or click blank then click option'

Acategory

BproductId

Cid

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names that don't match the method signature.

Not concatenating parameters properly in the key expression.

The first part of the key uses the category parameter, combined with the productId parameter.

3fill in blank

hard

Fix the error in the cache key expression to correctly use the method parameter.

Spring Boot

@Cacheable(value = "orders", key = "#[1]")
public Order findOrder(Long orderId) {
    // method body
}

Drag options to blanks, or click blank then click option'

Aid

BorderId

Corder

DorderID

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name with incorrect case.

Using a parameter name that does not exist.

The method parameter is named orderId, so the key expression must use #orderId.

4fill in blank

hard

Fill both blanks to create a cache key using two parameters separated by an underscore.

Spring Boot

@Cacheable(value = "sessions", key = "#[1] + '_' + #[2]")
public Session getSession(String userId, String sessionId) {
    // method body
}

Drag options to blanks, or click blank then click option'

AuserId

BsessionId

Cid

Dtoken

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names.

Forgetting to add quotes around the underscore.

The cache key combines userId and sessionId separated by an underscore.

5fill in blank

hard

Fill all three blanks to create a cache key using a method parameter and a method call on it.

Spring Boot

@Cacheable(value = "employees", key = "#[1] + '-' + #[2].[3]()")
public Employee getEmployee(EmployeeId empId) {
    // method body
}

Drag options to blanks, or click blank then click option'

AempId

CtoString

DgetId

Attempts:

3 left

💡 Hint

Common Mistakes

Using different parameter names in blanks 1 and 2.

Using a method name that does not exist on the parameter.

The cache key uses the empId parameter and calls its toString() method.

Practice

1. What is the main purpose of a cache key in Spring Boot caching?

easy

A. To define the cache size limit

B. To store the actual data in the cache

C. To uniquely identify cached data for quick retrieval

D. To clear the cache automatically

Solution

1. Step 1: Understand cache key role

   A cache key uniquely identifies the cached data so the system can find it quickly without confusion.

2. Step 2: Differentiate from other cache functions

   Cache keys do not store data or control cache size; they only act as unique identifiers.

3. Final Answer:

   To uniquely identify cached data for quick retrieval -> Option C

4. Quick Check:

   Cache key = unique identifier [OK]

Hint: Cache keys uniquely identify data to find it fast [OK]

Common Mistakes:

• Thinking cache keys store data
• Confusing keys with cache size settings
• Assuming keys clear cache automatically

2. Which of the following is the correct way to specify a cache key using Spring Expression Language (SpEL) in the @Cacheable annotation?

easy

A. @Cacheable(key = "#user.id")

B. @Cacheable(key = user.id)

C. @Cacheable(key = "user.id")

D. @Cacheable(key = #user.id)

Solution

1. Step 1: Recognize SpEL syntax in @Cacheable

   SpEL expressions must be inside double quotes and start with # to reference method parameters.

2. Step 2: Check each option

   @Cacheable(key = "#user.id") correctly uses "#user.id" as a string with # for SpEL. @Cacheable(key = user.id) misses quotes, the option with "user.id" misses #, @Cacheable(key = #user.id) misses quotes.

3. Final Answer:

   @Cacheable(key = "#user.id") -> Option A

4. Quick Check:

   SpEL key syntax = quoted string with # [OK]

Hint: SpEL keys need quotes and # before variable [OK]

Common Mistakes:

• Omitting quotes around SpEL expression
• Forgetting # before parameter name
• Using plain variable without SpEL syntax

3. Given the method annotated as @Cacheable(value = "users", key = "#id"), what will be the cache key when calling getUser(42)?

medium

A. "id"

B. 42

C. "#id"

D. null

Solution

1. Step 1: Understand key expression #id

   The key uses SpEL #id which means the method parameter named id's value is used as the key.

2. Step 2: Substitute method call parameter

   Calling getUser(42) means id = 42, so the cache key is 42 (an integer).

3. Final Answer:

   42 -> Option B

4. Quick Check:

   Key = parameter value 42 [OK]

Hint: Cache key #param uses actual parameter value [OK]

Common Mistakes:

• Using the string '#id' instead of parameter value
• Confusing key with parameter name only
• Assuming null key if not specified

4. Identify the error in this cache key definition: @Cacheable(value = "items", key = "#item.id + #item.name")

medium

A. SpEL expressions cannot concatenate strings with + operator

B. Missing parentheses around the expression

C. The key must be a constant string, not an expression

D. Using + operator without converting to string may cause wrong key

Solution

1. Step 1: Analyze SpEL expression with + operator

   In SpEL, + can add numbers or concatenate strings. If item.id is numeric, adding to item.name (string) may cause type issues.

2. Step 2: Understand type conversion in keys

   Without explicit string conversion, adding number + string may produce unexpected keys or errors.

3. Final Answer:

   Using + operator without converting to string may cause wrong key -> Option D

4. Quick Check:

   Type mismatch in key expression = error [OK]

Hint: Convert all parts to string before concatenation in keys [OK]

Common Mistakes:

• Assuming + always concatenates strings
• Not converting numeric parts to string explicitly
• Thinking key must be constant string only

5. You want to cache results of a method that takes two parameters: userId (Long) and filter (String). Which cache key strategy ensures unique keys for each combination?

hard

A. Use key = "T(java.util.Objects).hash(#userId, #filter)"

B. Use key = "#userId + #filter" without conversion

C. Use key = "#userId" only

D. Use key = "#filter" only

Solution

1. Step 1: Understand need for unique keys per parameter combo

   Using only one parameter as key risks collisions when other parameter changes.

2. Step 2: Evaluate options for combining parameters

   Use key = "#userId + #filter" without explicit conversion risks type issues. Use key = "T(java.util.Objects).hash(#userId, #filter)" uses Java's Objects.hash to combine both safely. Using only one parameter causes collisions.

3. Final Answer:

   Use key = "T(java.util.Objects).hash(#userId, #filter)" -> Option A

4. Quick Check:

   Combine keys safely with Objects.hash [OK]

Hint: Use Objects.hash to combine multiple parameters safely [OK]

Common Mistakes:

• Concatenating keys without type safety
• Using only one parameter as key causing collisions
• Ignoring proper SpEL syntax for static method calls