Master Spring Boot Startup: From static blocks to @PostConstruct and Runner Interfaces

Java native startup loading methods

static code block

static block runs when the class is loaded.

Constructor

Runs during object initialization, after the static block.

Spring startup loading methods

@PostConstruct annotation

Method annotated with @PostConstruct executes after dependency injection initialization.

ApplicationRunner and CommandLineRunner

Spring Boot provides two interfaces to run code after the container has started. Both require implementing a run method; the difference lies in the method parameter: ApplicationRunner receives ApplicationArguments, while CommandLineRunner receives a String array.

What is ApplicationArguments

Provides access to the arguments that were used to run a SpringApplication.

It allows retrieving the arguments passed to SpringApplication.run(...) at runtime.

@Order annotation

When multiple classes implement CommandLineRunner or ApplicationRunner, adding @Order on the class sets the execution sequence.

Code test

TestPostConstruct

@Component
public class TestPostConstruct {
    static {
        System.out.println("static");
    }
    public TestPostConstruct() {
        System.out.println("constructer");
    }
    @PostConstruct
    public void init() {
        System.out.println("PostConstruct");
    }
}

TestApplicationRunner

@Component
@Order(1)
public class TestApplicationRunner implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments applicationArguments) throws Exception {
        System.out.println("order1:TestApplicationRunner");
    }
}

TestCommandLineRunner

@Component
@Order(2)
public class TestCommandLineRunner implements CommandLineRunner {
    @Override
    public void run(String... strings) throws Exception {
        System.out.println("order2:TestCommandLineRunner");
    }
}

Execution result

Summary

During Spring application startup, classes annotated with @Component are scanned, their static blocks execute first, followed by constructors, then @PostConstruct methods. After the container starts successfully, the run methods of CommandLineRunner and ApplicationRunner are invoked according to the order defined by @Order.

Therefore, the loading order is: static > constructor > @PostConstruct > CommandLineRunner & ApplicationRunner.