15.1 Introducing Actuator

In a machine, an actuator is a component that’s responsible for controlling and moving a mechanism. In a Spring Boot application, the Spring Boot Actuator plays that same role, enabling us to see inside of a running application and, to some degree, control how the application behaves.

Using endpoints exposed by Actuator, we can ask things about the internal state of a running Spring Boot application, such as the following:

• What configuration properties are available in the application environment?
• What are the logging levels of various packages in the application?
• How much memory is being consumed by the application?
• How many times has a given HTTP endpoint been requested?
• What is the health of the application and any external services it coordinates with?

To enable Actuator in a Spring Boot application, you simply need to add Actuator’s starter dependency to your build. In any Spring Boot application Maven pom.xml file, the following <dependency> entry does the trick:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

Once the Actuator starter is part of the project build, the application will be equipped with several out-of-the-box Actuator endpoints, including those described in table 15.1.

Table 15.1：Actuator endpoints for peeking inside and manipulating the state of a running Spring Boot application

HTTP method	Path	Description	是否默认启用
GET	/auditevents	Produces a report of any audit events that have been fired	否
GET	/beans	Describes all the beans in the Spring application context	否
GET	/conditions	Produces a report of autoconfiguration conditions that either passed or failed, leading to the beans created in the application context	否
GET	/configprops	Describes all configuration properties along with the current values	否
GET, POST, DELETE	/env	Produces a report of all property sources and their properties available to the Spring application	否
GET	/env/	Describes the value of a single environment property	否
GET	/health	Returns the aggregate health of the application and (possibly) the health of external dependent applications	是
GET	/heapdump	Downloads a heap dump	否
GET	/httptrace	Produces a trace of the most recent 100 requests	否
GET	/info	Returns any developer-defined information about the application	是
GET	/loggers	Produces a list of packages in the application along with their configured and effective logging levels	否
GET，POST	/loggers/	Returns the configured and effective logging level of a given logger; the effective logging level can be set with a POST request	否
GET	/mappings	Produces a report of all HTTP mappings and their corresponding handler methods	否
GET	/metrics	Returns a list of all metrics categories	否
GET	/metrics/	Returns a multidimensional set of values for a given metric	否
GET	/scheduledtasks	Lists all scheduled tasks	否。
GET	/threaddump	Returns a report of all application threads	否

In addition to HTTP endpoints, all of the Actuator endpoints in table 15.1, with the lone exception of /heapdump, are also exposed as JMX MBeans. We’ll look at the JMX side of Actuator in chapter 17.