一、概述

在我们的日常开发中会有这样的场景，在容器启动完成之后去做一些事，例如：数据库初始化、缓存预热、启动后检查、任务调度等。 SpringBoot给我们提供了两个接口来帮助我们实现这种需求，分别是：ApplicationRunner和 CommandLineRunner。

这两个接口的主要区别是参数传递方式不同。ApplicationRunner接口的run()方法有一个ApplicationArguments类型的参数，CommandLineRunner接口的run()方法有一个String数组类型的参数。

二、源码

我们跟着启动类的run方法进去，来到SpringApplication的run方法中

public ConfigurableApplicationContext run(String... args) {
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    configureHeadlessProperty();
    SpringApplicationRunListeners listeners = getRunListeners(args);
    listeners.starting();
    try {
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                args);
        ConfigurableEnvironment environment = prepareEnvironment(listeners,
                applicationArguments);
        configureIgnoreBeanInfo(environment);
        Banner printedBanner = printBanner(environment);
        context = createApplicationContext();
        exceptionReporters = getSpringFactoriesInstances(
                SpringBootExceptionReporter.class,
                new Class[] { ConfigurableApplicationContext.class }, context);
        prepareContext(context, environment, listeners, applicationArguments,
                printedBanner);
        //刷新上下文
        refreshContext(context);
        afterRefresh(context, applicationArguments);
        stopWatch.stop();
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass)
                    .logStarted(getApplicationLog(), stopWatch);
        }
        listeners.started(context);
        //调用Runners
        callRunners(context, applicationArguments);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, listeners);
        throw new IllegalStateException(ex);
    }

    try {
        listeners.running(context);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, null);
        throw new IllegalStateException(ex);
    }
    return context;
}

第32行callRunners方法就是调用ApplicationRunner与CommandLineRunner的入口，可以看到是在容器启动完成之后调用的，我们进去看下

private void callRunners(ApplicationContext context, ApplicationArguments args) {
    List<Object> runners = new ArrayList<>();
    //添加所有ApplicationRunner接口的实例
    runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
    //添加所有CommandLineRunner接口的实例
    runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());
    //排序
    AnnotationAwareOrderComparator.sort(runners);
    for (Object runner : new LinkedHashSet<>(runners)) {
        if (runner instanceof ApplicationRunner) {
            //调用
            callRunner((ApplicationRunner) runner, args);
        }
        if (runner instanceof CommandLineRunner) {
            //调用
            callRunner((CommandLineRunner) runner, args);
        }
    }
}

callRunners方法主要就是获取ApplicationRunner与CommandLineRunner接口的实例，并按照Order排序后循环执行callRunner方法

三、示例

下面我们写两个例子看看

@Component
@Order(1)
public class MyCommandLineRunner implements CommandLineRunner {

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

@Component
@Order(2)
public class MyApplicationRunner implements ApplicationRunner {


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

启动后日志如下

MyCommandLineRunner……run

MyApplicationRunner……run