Custom actuator endpoints in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this custom actuator endpoint?

Given the following Spring Boot custom actuator endpoint code, what will be the JSON output when accessing /actuator/customStatus?

Spring Boot

import org.springframework.boot.actuate.endpoint.annotation.Endpoint;
import org.springframework.boot.actuate.endpoint.annotation.ReadOperation;
import org.springframework.stereotype.Component;

@Component
@Endpoint(id = "customStatus")
public class CustomStatusEndpoint {

    @ReadOperation
    public Status status() {
        return new Status("UP", "All systems operational");
    }

    public record Status(String state, String message) {}
}

A {"status":"UP","info":"All systems operational"}

B {"state":"UP","message":"All systems operational"}

C {"state":"DOWN","message":"All systems operational"}

D {"state":"UP"}

💡 Hint

Look at the record fields and the returned object structure.

The endpoint returns a Status record with fields 'state' and 'message'. The JSON output matches these field names and values exactly.

📝 Syntax

intermediate

Which option correctly defines a write operation in a custom actuator endpoint?

You want to create a custom actuator endpoint with a write operation that accepts a String parameter. Which code snippet correctly defines the write operation method?

Spring Boot

import org.springframework.boot.actuate.endpoint.annotation.WriteOperation;
import org.springframework.boot.actuate.endpoint.annotation.Selector;

// Method to define:

A

@WriteOperation
public String updateStatus(@Selector String newStatus) { return "Updated to " + newStatus; }

B

@WriteOperation
public String updateStatus(String newStatus) { return "Updated to " + newStatus; }

C

@WriteOperation
public String updateStatus() { return "Updated"; }

D

@WriteOperation
public void updateStatus(String newStatus) { System.out.println(newStatus); }

💡 Hint

Write operations with parameters require @Selector annotation for each parameter.

In Spring Boot actuator custom endpoints, parameters for write operations must be annotated with @Selector to be bound from the request.

🔧 Debug

advanced

Why does this custom actuator endpoint fail to register?

Consider this custom endpoint class: @Component @Endpoint(id = "failEndpoint") public class FailEndpoint { public String status() { return "OK"; } } Why does Spring Boot fail to register this endpoint?

A The class must extend AbstractEndpoint to be recognized.

B The endpoint id 'failEndpoint' is invalid because it contains uppercase letters.

C The method 'status' lacks a @ReadOperation annotation.

D The class is missing @RestController annotation.

💡 Hint

Check if the method is properly annotated to expose it as an endpoint operation.

Spring Boot actuator requires methods to be annotated with @ReadOperation, @WriteOperation, or @DeleteOperation to expose them as endpoint operations.

❓ state_output

advanced

What is the state of the custom endpoint after calling the write operation twice?

Given this custom endpoint: @Component @Endpoint(id = "counter") public class CounterEndpoint { private int count = 0; @ReadOperation public int getCount() { return count; } @WriteOperation public void increment() { count++; } } If you call the write operation /actuator/counter twice, then call the read operation, what is the returned count?

A 2

B 1

C 0

D Cannot determine because endpoints are stateless

💡 Hint

Consider the instance variable and how many times increment is called.

The count starts at 0 and increments by 1 each time the write operation is called. After two calls, count is 2.

🧠 Conceptual

expert

Which statement about custom actuator endpoints is true?

Select the correct statement about custom actuator endpoints in Spring Boot.

A Custom actuator endpoints require extending AbstractEndpoint class to function.

B Custom actuator endpoints must be registered manually in application.properties to be exposed.

C Custom actuator endpoints cannot accept parameters in their operations.

D Custom actuator endpoints can expose multiple operations annotated with @ReadOperation, @WriteOperation, and @DeleteOperation within the same class.

💡 Hint

Think about the flexibility of operation types in one endpoint class.

A single custom actuator endpoint class can have multiple methods annotated with different operation annotations to expose various HTTP methods.

Practice

1. What annotation is used to define a custom actuator endpoint in Spring Boot?

easy

A. @Endpoint

B. @Controller

C. @Service

D. @RestController

Solution

1. Step 1: Understand the purpose of custom actuator endpoints

   Custom actuator endpoints allow you to add your own monitoring or info checks in Spring Boot.

2. Step 2: Identify the correct annotation for defining custom endpoints

   The @Endpoint annotation is specifically designed to mark a class as a custom actuator endpoint.

3. Final Answer:

   @Endpoint -> Option A

4. Quick Check:

   Custom actuator endpoint = @Endpoint [OK]

Hint: Custom actuator endpoints always use @Endpoint annotation [OK]

Common Mistakes:

• Confusing @Endpoint with @Controller
• Using @Service instead of @Endpoint
• Using @RestController which is for REST APIs

2. Which annotation should you use on a method inside a custom actuator endpoint to expose a read-only operation?

easy

A. @WriteOperation

B. @ReadOperation

C. @GetMapping

D. @PostMapping

Solution

1. Step 1: Recall method annotations for custom actuator endpoints

   Custom actuator endpoints use specific annotations to define operations: read or write.

2. Step 2: Match read-only operation annotation

   The @ReadOperation annotation marks a method as a read-only operation accessible via HTTP GET.

3. Final Answer:

   @ReadOperation -> Option B

4. Quick Check:

   Read-only method = @ReadOperation [OK]

Hint: Use @ReadOperation for read-only endpoint methods [OK]

Common Mistakes:

• Using @WriteOperation for read methods
• Confusing @GetMapping with actuator annotations
• Using @PostMapping which is for REST controllers

3. Given this custom endpoint class, what will be the HTTP response body when accessing the endpoint?

@Endpoint(id = "status")
public class StatusEndpoint {
  @ReadOperation
  public String getStatus() {
    return "UP";
  }
}

medium

A. "DOWN"

B. 404 Not Found

C. "UP"

D. 500 Internal Server Error

Solution

1. Step 1: Analyze the custom endpoint class

   The class is annotated with @Endpoint(id = "status") and has a method with @ReadOperation returning "UP".

2. Step 2: Understand the HTTP response from the read operation

   Accessing the actuator endpoint with id "status" triggers the getStatus() method, returning the string "UP" as the response body.

3. Final Answer:

   "UP" -> Option C

4. Quick Check:

   Read operation returns "UP" [OK]

Hint: ReadOperation returns method result as HTTP response body [OK]

Common Mistakes:

• Expecting 404 if endpoint is defined correctly
• Confusing return value with HTTP status code
• Assuming method returns JSON object instead of string

4. Identify the error in this custom actuator endpoint code:

@Endpoint(id = "metrics")
public class MetricsEndpoint {
  @ReadOperation
  public String metrics() {
    return 12345;
  }
}

medium

A. The @ReadOperation annotation is missing

B. The method name must be getMetrics

C. The @Endpoint annotation must be on a method, not class

D. The method declares String return type but returns an int

Solution

1. Step 1: Check method return type consistency

   The method metrics() returns an integer literal 12345 but declares return type String, causing a type mismatch.

2. Step 2: Verify annotations and method naming

   The @ReadOperation annotation is present, and @Endpoint is correctly on the class. Method name can be arbitrary.

3. Final Answer:

   The method declares String return type but returns an int -> Option D

4. Quick Check:

   Return type mismatch causes error [OK]

Hint: Method return type must match declared type [OK]

Common Mistakes:

• Thinking @Endpoint goes on methods
• Assuming method name must follow getter pattern
• Ignoring type mismatch errors

5. You want to create a custom actuator endpoint that accepts a POST request to update a configuration value. Which combination of annotations and method signature is correct? A)

@Endpoint(id = "config")
public class ConfigEndpoint {
  @WriteOperation
  public String updateConfig(String key, String value) {
    // update logic
    return "Updated";
  }
}

B)

@Endpoint(id = "config")
public class ConfigEndpoint {
  @ReadOperation
  public void updateConfig(String key, String value) {
    // update logic
  }
}

C)

@Endpoint(id = "config")
public class ConfigEndpoint {
  @WriteOperation
  public void updateConfig() {
    // update logic
  }
}

D)

@Endpoint(id = "config")
public class ConfigEndpoint {
  @WriteOperation
  public String updateConfig() {
    // update logic
    return "Updated";
  }
}

hard

A. Option A is correct because @WriteOperation supports parameters and returns a response

B. Option B is correct because @ReadOperation can update values

C. Option C is correct because @WriteOperation methods cannot have parameters

D. Option D is correct because @WriteOperation methods should not have parameters

Solution

1. Step 1: Understand @WriteOperation usage

   @WriteOperation marks methods that handle write operations, typically POST, and can accept parameters for input.

2. Step 2: Analyze method signatures and annotations

   The correct combination uses @WriteOperation with String key and String value parameters and returns a String response, which is valid for POST updates. Using @ReadOperation is wrong for update operations. The other snippets lack parameters needed to receive key and value.

3. Final Answer:

   Option A is correct because @WriteOperation supports parameters and returns a response -> Option A

4. Quick Check:

   @WriteOperation with parameters for POST updates [OK]

Hint: Use @WriteOperation with parameters for POST updates [OK]

Common Mistakes:

• Using @ReadOperation for write/update methods
• Omitting parameters needed for update
• Assuming @WriteOperation methods cannot return values