@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

a. pom依赖

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot</artifactId>
    <version>2.0.1.RELEASE</version>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-autoconfigure</artifactId>
    <version>2.0.1.RELEASE</version>
</dependency>

b. 主启动类

package com.rgh;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

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

c. 两个实体类

@Component
@Order(1)
public class BlackPersion implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("----BlackPersion----");
    }
}

@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）；