一、什么是启动加载器？
在项目启动的时候做一些初始化工作。
二、启动类加载器实践
2.1 实现 CommandLineRunner 接口
@Component
public class FirstCommandLineRunner implements CommandLineRunner {
@Override
public void run(String... args) throws Exception {
System.out.println("run FirstCommandLineRunner");
}
}
2.2 实现 ApplicationRunner 接口
@Component
public class FirstApplicationRunner implements ApplicationRunner {
@Override
public void run(ApplicationArguments args) throws Exception {
System.out.println("run FirstApplicationRunner");
}
}
启动项目，观察控制台输出：
run FirstApplicationRunner
run FirstCommandLineRunner
可以看到默认实现 ApplicationRunner 接口比 CommandLineRunner 优先级高。
2.3 使用 @Order 注解
我们试试看增加 @Order 注解之后会怎么样
@Component
@Order(1)
public class FirstCommandLineRunner implements CommandLineRunner {
......
}
@Component
@Order(2)
public class FirstApplicationRunner implements ApplicationRunner {
......
}
启动项目，观察控制台输出：
run FirstCommandLineRunner
run FirstApplicationRunner
发现 @Order 注解可以改变执行顺序。
三、启动类加载器原理
进入 SpringApplication.run 方法中，callRunners 方法就是入口：
public ConfigurableApplicationContext run(String... args) {
......
callRunners(context, applicationArguments);
......
}
查看 callRunners 方法实现：
private void callRunners(ApplicationContext context, ApplicationArguments args) {
List<Object> runners = new ArrayList<>();
// 添加接口实现类
runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());
// 进行排序
AnnotationAwareOrderComparator.sort(runners);
// 循环调用 callRunner 方法
for (Object runner : new LinkedHashSet<>(runners)) {
if (runner instanceof ApplicationRunner) {
callRunner((ApplicationRunner) runner, args);
}
if (runner instanceof CommandLineRunner) {
callRunner((CommandLineRunner) runner, args);
}
}
}
可以看到先添加的 ApplicationRunner 接口实现，然后添加 CommandLineRunner 接口实现，所以默认优先级以 ApplicationRunner 为先，后面有个排序实现，以 @Order 进行优先级排序
我们再来分别看下两个接口各自的 callRunner 的实现：
private void callRunner(ApplicationRunner runner, ApplicationArguments args) {
try {
(runner).run(args);
}
catch (Exception ex) {
throw new IllegalStateException("Failed to execute ApplicationRunner", ex);
}
}
private void callRunner(CommandLineRunner runner, ApplicationArguments args) {
try {
(runner).run(args.getSourceArgs());
}
catch (Exception ex) {
throw new IllegalStateException("Failed to execute CommandLineRunner", ex);
}
}