浅谈Spring @Order注解的使用

注解@Order或者接口Ordered的作用是定义Spring IOC容器中Bean的执行顺序的优先级，而不是定义Bean的加载顺序，Bean的加载顺序不受@Order或Ordered接口的影响；

1.@Order的注解源码解读

@Retention(RetentionPolicy.RUNTIME) @Target({ElementType.TYPE, ElementType.METHOD, ElementType.FIELD}) @Documented public @interface Order {  	/** 	 * 默认是最低优先级,值越小优先级越高 	 */ 	int value() default Ordered.LOWEST_PRECEDENCE;  }

• 注解可以作用在类(接口、枚举)、方法、字段声明（包括枚举常量）；
• 注解有一个int类型的参数，可以不传，默认是最低优先级；
• 通过常量类的值我们可以推测参数值越小优先级越高；

2.Ordered接口类

package org.springframework.core;  public interface Ordered {     int HIGHEST_PRECEDENCE = -2147483648;     int LOWEST_PRECEDENCE = 2147483647;      int getOrder(); }

3.创建BlackPersion、YellowPersion类，这两个类都实现CommandLineRunner

实现CommandLineRunner接口的类会在Spring IOC容器加载完毕后执行，适合预加载类及其它资源；也可以使用ApplicationRunner,使用方法及效果是一样的

package com.yaomy.common.order;  import org.springframework.boot.CommandLineRunner; import org.springframework.core.annotation.Order; import org.springframework.stereotype.Component;  /**  * @Description: Description  * @ProjectName: spring-parent  * @Version: 1.0  */ @Component @Order(1) public class BlackPersion implements CommandLineRunner {     @Override     public void run(String... args) throws Exception {         System.out.println("----BlackPersion----");     } }

package com.yaomy.common.order;  import org.springframework.boot.CommandLineRunner; import org.springframework.core.annotation.Order; import org.springframework.stereotype.Component;  /**  * @Description: Description  * @ProjectName: spring-parent  * @Version: 1.0  */ @Component @Order(0) public class YellowPersion implements CommandLineRunner {     @Override     public void run(String... args) throws Exception {         System.out.println("----YellowPersion----");     } }

4.启动应用程序打印出结果

----YellowPersion---- ----BlackPersion----

我们可以通过调整@Order的值来调整类执行顺序的优先级，即执行的先后；当然也可以将@Order注解更换为Ordered接口，效果是一样的

5.到这里可能会疑惑IOC容器是如何根据优先级值来先后执行程序的，那接下来看容器是如何加载component的

• 看如下的启动main方法

@SpringBootApplication public class CommonBootStrap {     public static void main(String[] args) {         SpringApplication.run(CommonBootStrap.class, args);     } }

这个不用过多的解释，进入run方法…

     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);             //这里是重点，调用具体的执行方法             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);         }     }    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());         //重点来了，按照定义的优先级顺序排序         AnnotationAwareOrderComparator.sort(runners);         Iterator var4 = (new LinkedHashSet(runners)).iterator();         //循环调用具体方法         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);             }         }      }      private void callRunner(ApplicationRunner runner, ApplicationArguments args) {         try {             //执行方法             runner.run(args);         } catch (Exception var4) {             throw new IllegalStateException("Failed to execute ApplicationRunner", var4);         }     }      private void callRunner(CommandLineRunner runner, ApplicationArguments args) {         try {             //执行方法             runner.run(args.getSourceArgs());         } catch (Exception var4) {             throw new IllegalStateException("Failed to execute CommandLineRunner", var4);         }     }

到这里优先级类的示例及其执行原理都分析完毕；不过还是要强调下@Order、Ordered不影响类的加载顺序而是影响Bean加载如IOC容器之后执行的顺序（优先级）；

个人理解是加载代码的底层要支持优先级执行程序，否则即使配置上Ordered、@Order也是不起任何作用的，
个人的力量总是很微小的，欢迎大家来讨论，一起努力成长！！

GitHub源码：https://github.com/mingyang66/spring-parent