Cache key strategies in Spring Boot

Introduction

Cache keys help store and find data quickly in a cache. Good keys make your app faster and avoid mistakes.

When you want to speed up repeated data fetching in your Spring Boot app.

When you need to avoid recalculating expensive results multiple times.

When you want to share cached data safely between different users or requests.

When you want to control how cached data is stored and retrieved.

When you want to avoid cache conflicts or wrong data being returned.

Syntax

Spring Boot

@Cacheable(value = "cacheName", key = "#keyExpression")

Use SpEL (Spring Expression Language) inside the key attribute to build custom keys.

If you don't specify a key, Spring uses all method parameters by default.

Examples

Cache uses the book's ISBN as the key.

Spring Boot

@Cacheable(value = "books", key = "#isbn")
public Book findBook(String isbn) { ... }

Uses the user's id property as the cache key.

Spring Boot

@Cacheable(value = "users", key = "#user.id")
public User getUser(User user) { ... }

Combines method name and id to create a unique key.

Spring Boot

@Cacheable(value = "products", key = "#root.methodName + '_' + #id")
public Product getProduct(Long id) { ... }

No key specified, so all parameters (orderId) are used as the key.

Spring Boot

@Cacheable(value = "orders")
public Order getOrder(Long orderId) { ... }

Sample Program

This service method caches books by their ISBN. The first call waits 1 second, later calls return instantly from cache.

Spring Boot

import org.springframework.cache.annotation.Cacheable;
import org.springframework.stereotype.Service;

@Service
public class BookService {

    @Cacheable(value = "books", key = "#isbn")
    public String findBook(String isbn) {
        // Simulate slow service
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
        return "Book with ISBN: " + isbn;
    }
}

Important Notes

Always choose keys that uniquely identify the cached data to avoid wrong cache hits.

Use simple keys like IDs or combine multiple parameters if needed.

Remember that complex keys can slow down cache lookup.

Summary

Cache keys help find cached data fast and correctly.

Use Spring Expression Language (SpEL) to customize keys.

Pick keys that uniquely identify the data you want to cache.

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