有时根据项目要求需要在项目启动时去执行一些操作，比如资源的加载、系统IP的获取、 读取配置文件，数据库连接等，实现方法也有多种。

一、java自身加载方式实现

static静态代码块：

static关键字所修饰的内容在整个类执行前优先加载

构造方法：

在对象初始化时执行，执行顺序在静态代码块之后

@Component
public class Test {
    static{
        InetAddress ip = null;
        try {
            ip = InetAddress.getLocalHost();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
        System.out.println("IP Address1 : " + ip.getHostAddress());
    }
    public Test(){
        InetAddress ip = null;
        try {
            ip = InetAddress.getLocalHost();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
        System.out.println("IP Address2 : " + ip.getHostAddress());
    }
}

执行结果：

二、@PostConstruct注解

@PostConstruct是Java自带的注解，在方法上加该注解会在项目启动的时候执行该方法，在spring容器初始化的时候执行该方法，执行顺序在静态代码块和构造方法之后。

@Component
public class PostConstructTest {
    static{
        InetAddress ip = null;
        try {
            ip = InetAddress.getLocalHost();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
        System.out.println("IP Address1 : " + ip.getHostAddress());
    }
    public PostConstructTest(){
        InetAddress ip = null;
        try {
            ip = InetAddress.getLocalHost();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
        System.out.println("IP Address2 : " + ip.getHostAddress());
    }
    @PostConstruct
    public void init(){
        InetAddress ip = null;
        try {
            ip = InetAddress.getLocalHost();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
        System.out.println("IP Address3 : " + ip.getHostAddress());
    }
}

执行结果：

三、CommandLineRunner、ApplicationRunner接口

SpringBoot提供了两个启动加载接口分别是：CommandLineRunner和ApplicationRunner。

在applicationContext容器加载完成之后，会调用SpringApplication类的callRunners方法：该方法中会获取所有实现了ApplicationRunner和CommandLineRunner的接口bean，然后依次执行对应的run方法，并且是在同一个线程中执行。

CommandLineRunner

@Component
@Order(value = 1)
public class CommandLineRunnerTest1 implements CommandLineRunner{

    @Override
    public void run(String... args) throws Exception {
        System.out.println("CommandLineRunnerTest1");
    }
}
@Component
@Order(value = 2)
public class CommandLineRunnerTest2 implements CommandLineRunner{

    @Override
    public void run(String... args) throws Exception {
        System.out.println("CommandLineRunnerTest2");
    }
}

@Component
@Order(value = 1)
public class ApplicationRunnerTest1 implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) throws Exception {
        System.out.println("ApplicationRunnerTest1");
    }
}
@Component
@Order(value = 2)
public class ApplicationRunnerTest2 implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) throws Exception {
        System.out.println("ApplicationRunnerTest2");
    }
}

有多个类实现了CommandLineRunner、ApplicationRunner接口时，可使用@Order(1)注解来控制执行顺序，value值越小、越先执行@Order(1)>@Order(2)，ApplicationRunner执行顺序在CommandLineRunner之前

四、InitializingBean接口

InitializingBean接口为bean提供了初始化方法的方式，它只包括afterPropertiesSet方法，凡是继承该接口的类，在初始化bean的时候都会执行该方法 。

public class InitializingBeanTest implements InitializingBean {
    @Override
    public void afterPropertiesSet() throws Exception {
    System.out.println("InitializingBeanTest");
}

以上四种方式都可实现在项目启动后执行一次方法。执行顺序如下：
$$static静态方法>构造方法>@PostConstruct注解>InitializingBean>ApplicationRunner>CommandLineRunner$$
其中ApplicationRunner：@Order(1)>CommandLineRunner：@Order(1)>ApplicationRunner：@Order(2)>CommandLineRunner：@Order(2)

@Component
public class PostConstructTest {
    static {
        System.out.println("static静态方法");
    }

    public PostConstructTest() {
        System.out.println("构造方法");
    }

    @PostConstruct
    public void init() {
        System.out.println("@PostConstruct注解");
    }

    @Component
    @Order(value = 1)
    public class CommandLineRunnerTest1 implements CommandLineRunner {

        @Override
        public void run(String... args) throws Exception {
            System.out.println("CommandLineRunner：@Order(1)");
        }
    }

    @Component
    @Order(value = 2)
    public class CommandLineRunnerTest2 implements CommandLineRunner {

        @Override
        public void run(String... args) throws Exception {
            System.out.println("CommandLineRunner：@Order(2)");
        }
    }

    @Component
    @Order(value = 1)
    public class ApplicationRunnerTest1 implements ApplicationRunner {
        @Override
        public void run(ApplicationArguments args) throws Exception {
            System.out.println("ApplicationRunner：@Order(1)");
        }
    }

    @Component
    @Order(value = 2)
    public class ApplicationRunnerTest2 implements ApplicationRunner {
        @Override
        public void run(ApplicationArguments args) throws Exception {
            System.out.println("ApplicationRunner：@Order(2)");
        }
    }

    @Component
    public class InitializingBeanTest implements InitializingBean {

        @Override
        public void afterPropertiesSet() throws Exception {
            System.out.println("InitializingBean");
        }
    }
}