SpringBoot 自动化配置，以quartz为例

总结来说：

SpringBoot的自动化配置是这样实现的，启动时会去加载jar包（一般是spring-boot-autoconfigure-2.1.1.RELEASE.jar）中如下路径的“META-INF/spring.factories”内容，如下：

org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.core.ReactorCoreAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,

然后以上述的全类名加载相关的bean，比如QuartzAutoConfiguration进行自动化配置，如下：

package org.springframework.boot.autoconfigure.quartz;

import java.util.Map;
import java.util.Properties;

import javax.sql.DataSource;

import org.quartz.Calendar;
import org.quartz.JobDetail;
import org.quartz.Scheduler;
import org.quartz.Trigger;

import org.springframework.beans.factory.ObjectProvider;
import org.springframework.boot.autoconfigure.AbstractDependsOnBeanFactoryPostProcessor;
import org.springframework.boot.autoconfigure.AutoConfigureAfter;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnSingleCandidate;
import org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration;
import org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.annotation.Order;
import org.springframework.core.io.ResourceLoader;
import org.springframework.scheduling.quartz.SchedulerFactoryBean;
import org.springframework.scheduling.quartz.SpringBeanJobFactory;
import org.springframework.transaction.PlatformTransactionManager;

/**
 * {@link EnableAutoConfiguration Auto-configuration} for Quartz Scheduler.
 *
 * @author Vedran Pavic
 * @author Stephane Nicoll
 * @since 2.0.0
 */
@Configuration
@ConditionalOnClass({ Scheduler.class, SchedulerFactoryBean.class,
        PlatformTransactionManager.class })
@EnableConfigurationProperties(QuartzProperties.class)
@AutoConfigureAfter({ DataSourceAutoConfiguration.class,
        HibernateJpaAutoConfiguration.class })
public class QuartzAutoConfiguration {

    private final QuartzProperties properties;

    private final ObjectProvider<SchedulerFactoryBeanCustomizer> customizers;

    private final JobDetail[] jobDetails;

    private final Map<String, Calendar> calendars;

    private final Trigger[] triggers;

    private final ApplicationContext applicationContext;

    public QuartzAutoConfiguration(QuartzProperties properties,
            ObjectProvider<SchedulerFactoryBeanCustomizer> customizers,
            ObjectProvider<JobDetail[]> jobDetails,
            ObjectProvider<Map<String, Calendar>> calendars,
            ObjectProvider<Trigger[]> triggers, ApplicationContext applicationContext) {
        this.properties = properties;
        this.customizers = customizers;
        this.jobDetails = jobDetails.getIfAvailable();
        this.calendars = calendars.getIfAvailable();
        this.triggers = triggers.getIfAvailable();
        this.applicationContext = applicationContext;
    }
    //最重要的一步是生成SchedulerFactoryBean，通过这个FactoryBean就可以生成Scheduler对象，达到自动化配置的效果
    @Bean
    @ConditionalOnMissingBean
    public SchedulerFactoryBean quartzScheduler() {
        SchedulerFactoryBean schedulerFactoryBean = new SchedulerFactoryBean();
        SpringBeanJobFactory jobFactory = new SpringBeanJobFactory();
        jobFactory.setApplicationContext(this.applicationContext);
        schedulerFactoryBean.setJobFactory(jobFactory);
        if (this.properties.getSchedulerName() != null) {
            schedulerFactoryBean.setSchedulerName(this.properties.getSchedulerName());
        }
        schedulerFactoryBean.setAutoStartup(this.properties.isAutoStartup());
        schedulerFactoryBean
                .setStartupDelay((int) this.properties.getStartupDelay().getSeconds());
        schedulerFactoryBean.setWaitForJobsToCompleteOnShutdown(
                this.properties.isWaitForJobsToCompleteOnShutdown());
        schedulerFactoryBean
                .setOverwriteExistingJobs(this.properties.isOverwriteExistingJobs());
        if (!this.properties.getProperties().isEmpty()) {
            schedulerFactoryBean
                    .setQuartzProperties(asProperties(this.properties.getProperties()));
        }
        if (this.jobDetails != null && this.jobDetails.length > 0) {
            schedulerFactoryBean.setJobDetails(this.jobDetails);
        }
        if (this.calendars != null && !this.calendars.isEmpty()) {
            schedulerFactoryBean.setCalendars(this.calendars);
        }
        if (this.triggers != null && this.triggers.length > 0) {
            schedulerFactoryBean.setTriggers(this.triggers);
        }
        customize(schedulerFactoryBean);
        return schedulerFactoryBean;
    }
        ......
}

那启动时是如何加载spring.factories文件中的类并生成对象的呢

这块每个版本的SpringBoot项目实现是不一样的，此处以2.1.1版本为例说明：

1、启动类的：

@SpringBootApplication

2、SpringootApplication的@EnableAutoConfiguration注解：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
        @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

3、@EnableAutoConfiguration中的@Import(AutoConfigurationImportSelector.class)注解：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

4、@Import注解的作用就是生成bean，而生成哪些bean就是根据他的参数AutoConfigurationImportSelector.class来决定。

5、AutoConfigurationImportSelector中定义了需要生成的bean的列表。