（其实直接在 main 方法里写也不是执行不了） 如果只是简单的一些语句，写在 main 中可能会方便一些
但如果需要调用 spring 容器中的对象可能会要吃瘪，因为 main 方法是 static 的，而获取 ioc 对象不能使用 static 直接获取（会报错） 当调用 @AutoWired 获得 ioc 容器中的对象时
@Autowiredprivate static TestService testService;
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Exception in thread "main" java.lang.NullPointerException
当调用 @Resource 获得 ioc 容器中的对象时
@Resourceprivate static TestService testService;
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Caused by: java.lang.IllegalStateException: @Resource annotation is not supported on static fields
两个函数接口
有两个函数接口类都可以实现
1.ApplicationRunner 接口
源码
package org.springframework.boot;
@FunctionalInterfacepublic interface ApplicationRunner { void run(ApplicationArguments args) throws Exception;}
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使用方法
使用 @SpringBootApplication 注释的 ApplicationMain 启动类实现 ApplicationRunner 接口，并实现 run 方法即可 在 main 方法将 spring 启动完成后会执行 run 方法中的程序
@SpringBootApplicationpublic class BootTestApplication implements ApplicationRunner{
public static void main(String[] args) { SpringApplication.run(BootTestApplication.class, args); }
@Override public void run(ApplicationArguments args) throws Exception { System.out.println("ApplicationRunner.run"); }}
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2.CommandLineRunner 接口
源码
package org.springframework.boot;
@FunctionalInterfacepublic interface CommandLineRunner { void run(String... args) throws Exception;}
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使用方法
其实 CommandLineRunner 和 ApplicationRunner 使用方法差不多
使用 @SpringBootApplication 注释的 ApplicationMain 启动类实现接口，并实现 run 方法即可 在 main 方法将 spring 启动完成后会执行 run 方法中的程序
@SpringBootApplicationpublic class BootTestApplication implements CommandLineRunner {
public static void main(String[] args) { SpringApplication.run(BootTestApplication.class, args); }
@Override public void run(String... args) throws Exception { System.out.println("CommandLineRunner.run"); }}
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为什么会被执行？两个接口谁先执行？为什么？
举个栗子
我们来看个例子 现在我们有一个 service 类
@Servicepublic class TestService { public void test(String name){ System.out.println(name); }}
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然后 main 启动类中同时实现 ApplicationRunner 和 CommandLineRunner 接口，并调用这个 service 的方法 看看是谁先被输出
@SpringBootApplicationpublic class BootTestApplication implements ApplicationRunner , CommandLineRunner {
@Autowired private TestService service;
public static void main(String[] args) { SpringApplication.run(BootTestApplication.class, args); }
@Override public void run(ApplicationArguments args) throws Exception { service.test("ApplicationRunner.run"); }
@Override public void run(String... args) throws Exception { service.test("CommandLineRunner.run"); }}
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结果
可以看到，无论运行几次，结果都是 ApplicationRunner 先执行，CommandLineRunner 后执行
ApplicationRunner.run CommandLineRunner.run
如图
简单源码分析
我们查看 SpringApplication.run(BootTestApplication.class, args) 一层一层被调用到
public ConfigurableApplicationContext run(String... args) { StopWatch stopWatch = new StopWatch(); stopWatch.start(); ConfigurableApplicationContext context = null; Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList(); this.configureHeadlessProperty(); SpringApplicationRunListeners listeners = this.getRunListeners(args); listeners.starting();
Collection exceptionReporters; try { ApplicationArguments applicationArguments = new DefaultApplicationArguments(args); ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments); this.configureIgnoreBeanInfo(environment); Banner printedBanner = this.printBanner(environment); context = this.createApplicationContext(); exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context); this.prepareContext(context, environment, listeners, applicationArguments, printedBanner); this.refreshContext(context); this.afterRefresh(context, applicationArguments); stopWatch.stop(); if (this.logStartupInfo) { (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch); }
listeners.started(context); this.callRunners(context, applicationArguments); } catch (Throwable var10) { this.handleRunFailure(context, var10, exceptionReporters, listeners); throw new IllegalStateException(var10); }
try { listeners.running(context); return context; } catch (Throwable var9) { this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners)null); throw new IllegalStateException(var9); } }
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其中有一条 this.callRunners(context, applicationArguments);就是回调方法了，我们点开可以看到
private void callRunners(ApplicationContext context, ApplicationArguments args) { List<Object> runners = new ArrayList(); runners.addAll(context.getBeansOfType(ApplicationRunner.class).values()); runners.addAll(context.getBeansOfType(CommandLineRunner.class).values()); AnnotationAwareOrderComparator.sort(runners); Iterator var4 = (new LinkedHashSet(runners)).iterator();
while(var4.hasNext()) { Object runner = var4.next(); if (runner instanceof ApplicationRunner) { this.callRunner((ApplicationRunner)runner, args); }
if (runner instanceof CommandLineRunner) { this.callRunner((CommandLineRunner)runner, args); } }}
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如果启动类是 ApplicationRunner 的实现类，那么会调用 runner.run(args); 也就是启动类中实现了 ApplicationRunner 的 run 方法
if(runner instanceof ApplicationRunner) { this.callRunner((ApplicationRunner)runner, args);}
private void callRunner(ApplicationRunner runner, ApplicationArguments args) { try { runner.run(args); } catch (Exception var4) { throw new IllegalStateException("Failed to execute ApplicationRunner", var4); }}
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如果启动类是 CommandLineRunner 的实现类，那么会调用 runner.run(args.getSourceArgs()); 也就是启动类中实现了 CommandLineRunner 的 run 方法
if (runner instanceof CommandLineRunner) { this.callRunner((CommandLineRunner)runner, args);}
private void callRunner(CommandLineRunner runner, ApplicationArguments args) { try { runner.run(args.getSourceArgs()); } catch (Exception var4) { throw new IllegalStateException("Failed to execute CommandLineRunner", var4); }}
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作者：ifyyf
链接：https://juejin.cn/post/7050361931210686472
来源：稀土掘金
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