淺談Spring @Order注解的使用（轉）

注解@Order或者接口Ordered的作用是定義Spring IOC容器中Bean的執行順序的優先級，而不是定義Bean的加載順序，Bean的加載順序不受@Order或Ordered接口的影響；

1.@Order的注解源碼解讀

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.FIELD})
@Documented
public @interface Order {

    /**
     * 默認是最低優先級,值越小優先級越高
     */
    int value() default Ordered.LOWEST_PRECEDENCE;

}

• 注解可以作用在類(接口、枚舉)、方法、字段聲明（包括枚舉常量）；
• 注解有一個int類型的參數，可以不傳，默認是最低優先級；
• 通過常量類的值我們可以推測參數值越小優先級越高；

2.Ordered接口類

package org.springframework.core;

public interface Ordered {
    int HIGHEST_PRECEDENCE = -2147483648;
    int LOWEST_PRECEDENCE = 2147483647;

    int getOrder();
}

3.創建BlackPersion、YellowPersion類，這兩個類都實現CommandLineRunner
實現CommandLineRunner接口的類會在Spring IOC容器加載完畢后執行，適合預加載類及其它資源；也可以使用ApplicationRunner,使用方法及效果是一樣的。

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.啟動應用程序打印出結果

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

我們可以通過調整@Order的值來調整類執行順序的優先級，即執行的先后；當然也可以將@Order注解更換為Ordered接口，效果是一樣的

5.到這里可能會疑惑IOC容器是如何根據優先級值來先后執行程序的，那接下來看容器是如何加載component的

• 看如下的啟動main方法

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

  這個不用過多的解釋，進入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);
              //這里是重點，調用具體的執行方法
              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());
          //重點來了，按照定義的優先級順序排序
          AnnotationAwareOrderComparator.sort(runners);
          Iterator var4 = (new LinkedHashSet(runners)).iterator();
          //循環調用具體方法
          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 {
              //執行方法
              runner.run(args);
          } catch (Exception var4) {
              throw new IllegalStateException("Failed to execute ApplicationRunner", var4);
          }
      }
  
      private void callRunner(CommandLineRunner runner, ApplicationArguments args) {
          try {
              //執行方法
              runner.run(args.getSourceArgs());
          } catch (Exception var4) {
              throw new IllegalStateException("Failed to execute CommandLineRunner", var4);
          }
      }

  到這里優先級類的示例及其執行原理都分析完畢；不過還是要強調下@Order、Ordered不影響類的加載順序而是影響Bean加載如IOC容器之后執行的順序（優先級）；

• 原文地址：https://blog.csdn.net/yaomingyang/article/details/86649072