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Built-in Endpoints Catalog

4 roadmaps6 questions

Actuator ships endpoints for health, info, metrics, environment, beans, mappings, config properties and conditions, discoverable from the index. Interviewers ask which ones you would ever expose publicly, and the answer is health and little else.

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questions

6

What is the Actuator discovery index at /actuator, and how do you find out which endpoints your running app actually exposes?

level: juniorimportance: must knowfreq 45%

answer

  1. GET /actuator = HAL index
  2. _links: id -> href
  3. templated = URI template
  4. only enabled + web-exposed appear
  5. health exposed by default

basics

~10 s

GET /actuator returns a JSON document with a _links section. Each entry maps an endpoint id (like health, metrics, beans) to its URL. It's the menu of endpoints currently exposed over the web.

solid answer

~40 s

Spring Boot Actuator serves a discovery index at the base path `/actuator`. It returns a HAL-style JSON document whose `_links` object lists every web-exposed endpoint by id together with its `href` and whether it is `templated`. Only endpoints that are both enabled and web-exposed appear, so it's the authoritative runtime view of what's reachable. The catalog of standard endpoints includes `health`, `info`, `metrics`, `env`, `beans`, `mappings`, `configprops`, `conditions`, `scheduledtasks`, and `caches`, among others. `health` is exposed by default; the rest must be opted into. The index itself is what tooling and humans use to navigate; each `href` is an absolute URL you can GET directly. Templated links (like `metrics/{requiredMetricName}`) show a URI template you fill in.

code

java · 15 lines
java
// Example response body of  GET http://localhost:8080/actuator
// (only endpoints that are enabled AND web-exposed are listed)
{
  "_links": {
    "self":    { "href": "http://localhost:8080/actuator", "templated": false },
    "health":  { "href": "http://localhost:8080/actuator/health", "templated": false },
    "health-path": {
      "href": "http://localhost:8080/actuator/health/{*path}", "templated": true
    },
    "metrics": { "href": "http://localhost:8080/actuator/metrics", "templated": false },
    "metrics-requiredMetricName": {
      "href": "http://localhost:8080/actuator/metrics/{requiredMetricName}", "templated": true
    }
  }
}

go deeper

for a junior

Know that GET /actuator lists exposed endpoints via _links, and that most endpoints are off by default.

for a middle

Explain templated links and that the index reflects live enable+expose state, not the full catalog.

for a senior

Contrast web (HAL index) vs JMX (MBeans) discovery, and note base-path relocation keeps ids stable.

for a principal

Discuss the index as an ops contract across environments and why prod typically exposes a minimal subset.

**Actuator** is Spring Boot's production-monitoring subsystem. It exposes a set of HTTP (or JMX) **endpoints** under a base path, `/actuator` by default. Each endpoint has a short **id** (`health`, `metrics`, etc.). **The discovery index.** A `GET /actuator` (with no id) returns the *discovery index* — a JSON document in **HAL** style (Hypertext Application Language). Its shape is: ```json { "_links": { "self": { "href": "http://localhost:8080/actuator", "templated": false }, "health": { "href": "http://localhost:8080/actuator/health", "templated": false }, "metrics": { "href": "http://localhost:8080/actuator/metrics", "templated": false }, "metrics-requiredMetricName": { "href": "http://localhost:8080/actuator/metrics/{requiredMetricName}", "templated": true } } } ``` Key points: - **`_links`** is a map of *relation name → link object*. The relation name is essentially the endpoint id (with a suffix when an endpoint has a path-variable variant). - **`href`** is the full URL to GET. - **`templated: true`** means the `href` contains a `{placeholder}` (a URI template, e.g. `metrics/{requiredMetricName}`) that you substitute before calling. - **`self`** points back at the index. **What shows up.** An endpoint appears in the index only if it is **enabled** *and* **exposed over the web**. Exposure and enablement are configured with `management.*` properties (that configuration belongs to the Spring Boot starter/config topic, not here) — but the *effect* you observe is: by default only `health` is web-exposed, so a fresh app's index is nearly empty. Once more endpoints are exposed, they each get a `_links` entry. This makes the index the single source of truth for 'what is actually reachable right now' — more reliable than guessing from docs, because it reflects the live configuration. **The standard endpoint catalog** (the ones this leaf covers): `health` (app/dependency health), `info` (arbitrary app info), `metrics` (Micrometer metric snapshots), `env` (Spring `Environment` property sources), `beans` (the bean graph), `mappings` (request mappings), `configprops` (`@ConfigurationProperties` bound values), `conditions` (auto-configuration condition report), `scheduledtasks` (`@Scheduled` inventory), and `caches` (Spring cache inventory). Others exist (`loggers`, `threaddump`, `heapdump`, `httpexchanges`, `prometheus`, `shutdown`, `flyway`/`liquibase`) but the ones above are the core catalog. **Gotchas.** - If you changed the base path (e.g. to `/manage`), the index moves too — the *ids* stay the same, only the prefix changes. - The index is not an authentication or authorization gate; it merely lists exposed endpoints. Anything listed is callable subject to your security config. - The `templated` links can't be GET-ed as-is; you must fill the placeholder. - Under JMX (rather than web), there is no HTTP index — endpoints appear as MBeans instead. **When to use.** During debugging or ops, hitting `/actuator` first tells you exactly which endpoints are live without reading config files, which is invaluable across environments where exposure differs (dev exposes many, prod exposes few).

  • Why might /actuator/beans return 404 even though /actuator/health works?
    Because only `health` is web-exposed by default. `beans` is a valid endpoint but must be explicitly added to the web exposure set before it appears in the index and becomes reachable; otherwise you get 404.
  • What does `templated: true` mean for a link?
    The `href` contains a URI-template placeholder (e.g. `metrics/{requiredMetricName}`) that you must substitute with a real value before issuing the request; you cannot GET the templated URL verbatim.

saying these in an interview costs you the question

  • Thinking every standard endpoint is reachable by default (only health is web-exposed by default)
  • Believing the index performs authentication or hides secured endpoints
  • Trying to GET a templated href without filling the placeholder

context

resolving links…

open as a page

What do /actuator/health and /actuator/info return, and where does their content come from?

level: juniorimportance: must knowfreq 60%

basics

~20 s

/actuator/health reports whether the app and its dependencies are healthy, returning a status like UP or DOWN. /actuator/info returns arbitrary descriptive info (build version, git commit) contributed by the app; it is empty unless you configure contributors.

open as a page

Explain what /actuator/beans, /actuator/mappings, and /actuator/conditions each expose and when you'd reach for them.

level: middleimportance: should knowfreq 35%

basics

~20 s

/beans lists every Spring bean with its type, scope and dependencies. /mappings lists all request mappings (which URL/method routes to which handler). /conditions shows the auto-configuration report: which @Conditional auto-configs matched (were applied) and which didn't, with reasons.

open as a page

How does the /actuator/metrics endpoint work, and how do you drill into a specific metric and filter by its tags?

level: middleimportance: should knowfreq 50%

basics

~10 s

GET /actuator/metrics lists available metric names. GET /actuator/metrics/{name} (e.g. jvm.memory.used) returns that metric's current measurements plus its availableTags. Add ?tag=key:value to filter to a specific dimension.

open as a page

Walk through what /actuator/scheduledtasks, /actuator/caches, /actuator/env, and /actuator/configprops expose, and their notable operations and behaviors.

level: seniorimportance: should knowfreq 30%

basics

~20 s

/scheduledtasks lists all @Scheduled tasks grouped by trigger type (cron/fixedRate/fixedDelay/custom). /caches lists Spring caches by cache manager, and supports DELETE to clear them. /env shows the Environment's property sources and values. /configprops shows @ConfigurationProperties beans and their bound values. env and configprops mask sensitive values by default.

open as a page

Explain the composite structure behind /actuator/health: how the overall status is aggregated, how health groups and readiness/liveness probes fit in, and what edge cases you must handle.

level: principalimportance: should knowfreq 28%

basics

~20 s

The health endpoint composes many HealthIndicator/HealthContributor beans into a tree. A StatusAggregator reduces their statuses to one overall status (by default the worst wins, e.g. any DOWN → overall DOWN). Health groups (like readiness, liveness) expose subsets at /actuator/health/{group} for Kubernetes probes.

open as a page