Spring @Order注解的使用

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）；

個人理解是加載代碼的底層要支持優先級執行程序，否則即使配置上Ordered、@Order也是不起任何作用的。