我们知道在Spring及SpringBoot里按条件创建Bean的核心是Condition接口与Conditional注解,其实在SpringBoot里还有一种AutoConfigure也可以来过滤配置，只不过使用这种技术，能够让SpringBoot更快速的启动，那么下面我们就来看一下具体怎么实现的。

autoconfigure Module

     SpringBoot使用一个Annotation的处理器来收集一些自动装配的条件，那么这些条件可以在META-INF/spring-autoconfigure-metadata.properties进行配置。SpringBoot会将收集好的@Configuration进行一次过滤进而剔除不满足条件的配置类。

演示示例

     在我们创建好的SpringBoot项目里添加一个AutoConfiguration:

package com.ys.zhshop.member.config;

import com.ys.zhshop.member.service.MemberRegisterService;
import org.springframework.context.annotation.Bean;

public class MemberAutoConfiguration {

    @Bean
    public MemberRegisterService registerService() {
        return new MemberRegisterService();
    }
}

     在MemberRegisterService里的构造函数输出一段内容看看Spring是否帮我们初始化
     紧接着在META-INF/spring.factories里配置对应的引导：

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.ys.zhshop.member.config.MemberAutoConfiguration

     随后我们需要在META-INF目录下创建一个spring-autoconfigure-metadata.properties 文件，内容如下：

com.ys.zhshop.member.config.MemberAutoConfiguration.ConditionalOnClass=java.lang.Strings

     格式：自动配置的类全名.条件=值

     在这里我们先指定一个类路径下不存在的Java类，启动后并没有相关信息的输出，那么把其值改成java.land.String，那么我们启动可以发现：

     在这里，我们可以在控制台看到构造函数输出的值，这就说明我们的Bean的的确确被创建了
     下面我贴出一个spring-cloud-netflix-core下的配置，主要来看看这些条件该怎么写，大家如果想使用可以参考人家的来配置：

#Mon Jun 18 19:13:37 UTC 2018
org.springframework.cloud.netflix.hystrix.HystrixAutoConfiguration.ConditionalOnClass=com.netflix.hystrix.Hystrix,org.springframework.boot.actuate.health.HealthIndicator
org.springframework.cloud.netflix.hystrix.HystrixAutoConfiguration=
org.springframework.cloud.netflix.hystrix.security.HystrixSecurityAutoConfiguration=
org.springframework.cloud.netflix.hystrix.HystrixAutoConfiguration.Configuration=
org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration=
org.springframework.cloud.netflix.hystrix.HystrixAutoConfiguration.AutoConfigureAfter=org.springframework.boot.actuate.autoconfigure.health.HealthIndicatorAutoConfiguration
org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration.Configuration=
org.springframework.cloud.netflix.hystrix.security.HystrixSecurityAutoConfiguration.Configuration=
org.springframework.cloud.netflix.hystrix.security.HystrixSecurityAutoConfiguration.ConditionalOnClass=com.netflix.hystrix.Hystrix,org.springframework.security.core.context.SecurityContext

     根据官网说法，使用这种配置方式可以有效的降低SpringBoot的启动时间，因为通过这种过滤方式能减少@Configuration类的数量，从而降低初始化Bean时的耗时,官网原话描述如下：

Spring Boot uses an annotation processor to collect the conditions on auto-configurations in a metadata file (META-INF/spring-autoconfigure-metadata.properties). If that file is present, it is used to eagerly filter auto-configurations that do not match, which will improve startup time.

源码追踪

     我们再次打开自动化配置的核心类AutoConfigurationImportSelector,看看它的selectImport方法:

     @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                .loadMetadata(this.beanClassLoader);//1
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        List<String> configurations = getCandidateConfigurations(annotationMetadata,
                attributes);//2
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = filter(configurations, autoConfigurationMetadata);//3
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return StringUtils.toStringArray(configurations);
    }

1. 在代码1处创建用于加载spring-autoconfigure-metadata.properties里的元数据，在这里我们可以看到其指定文件的位置：

protected static final String PATH = "META-INF/"
            + "spring-autoconfigure-metadata.properties";

2.在代码2处主要通过SpringFactoriesLoader加载spring.factories里的EnableAutoConfiguration
3.根据AutoConfigurationMetaData进行一次过滤：

private List<String> filter(List<String> configurations,
            AutoConfigurationMetadata autoConfigurationMetadata) {
        long startTime = System.nanoTime();
        String[] candidates = StringUtils.toStringArray(configurations);
        boolean[] skip = new boolean[candidates.length];
        boolean skipped = false;
        for (AutoConfigurationImportFilter filter : getAutoConfigurationImportFilters()) {
            invokeAwareMethods(filter);
            boolean[] match = filter.match(candidates, autoConfigurationMetadata);//代码1
            for (int i = 0; i < match.length; i++) {
                if (!match[i]) {
                    skip[i] = true;
                    skipped = true;
                }
            }
        }
        if (!skipped) {
            return configurations;
        }
        List<String> result = new ArrayList<>(candidates.length);
        for (int i = 0; i < candidates.length; i++) {
            if (!skip[i]) {
                result.add(candidates[i]);
            }
        }
        if (logger.isTraceEnabled()) {
            int numberFiltered = configurations.size() - result.size();
            logger.trace("Filtered " + numberFiltered + " auto configuration class in "
                    + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime)
                    + " ms");
        }
        return new ArrayList<>(result);
    }

我们在这里主要关注代码1。OnClassCondition是AutoConfigurationImportFilter接口的实现类，我这里贴一下与主题相关的代码：

@Order(Ordered.HIGHEST_PRECEDENCE)
class OnClassCondition extends SpringBootCondition
        implements AutoConfigurationImportFilter, BeanFactoryAware, BeanClassLoaderAware {


@Override
    public boolean[] match(String[] autoConfigurationClasses,
            AutoConfigurationMetadata autoConfigurationMetadata) {
        ConditionEvaluationReport report = getConditionEvaluationReport();
        ConditionOutcome[] outcomes = getOutcomes(autoConfigurationClasses,
                autoConfigurationMetadata);
        boolean[] match = new boolean[outcomes.length];
        for (int i = 0; i < outcomes.length; i++) {
            match[i] = (outcomes[i] == null || outcomes[i].isMatch());
            if (!match[i] && outcomes[i] != null) {
                logOutcome(autoConfigurationClasses[i], outcomes[i]);
                if (report != null) {
                    report.recordConditionEvaluation(autoConfigurationClasses[i], this,
                            outcomes[i]);
                }
            }
        }
        return match;
    }
//.....

private ConditionOutcome[] getOutcomes(String[] autoConfigurationClasses,
                int start, int end, AutoConfigurationMetadata autoConfigurationMetadata) {
            ConditionOutcome[] outcomes = new ConditionOutcome[end - start];
            for (int i = start; i < end; i++) {
                String autoConfigurationClass = autoConfigurationClasses[i];
                Set<String> candidates = autoConfigurationMetadata
                        .getSet(autoConfigurationClass, "ConditionalOnClass");
                if (candidates != null) {
                    outcomes[i - start] = getOutcome(candidates);
                }
            }
            return outcomes;
        }
}

上述代码虽然多，但最终还是要调用AutoConfigurationMetadata 的getSet方法，我们继续追踪一下这个接口的实现类，它是位于AutoConfigurationMetadataLoader的内部类：

    /**
     * {@link AutoConfigurationMetadata} implementation backed by a properties file.
     */
    private static class PropertiesAutoConfigurationMetadata
            implements AutoConfigurationMetadata {
    
      //....省略其他代码

      @Override
        public Set<String> getSet(String className, String key) {
            return getSet(className, key, null);
        }

        @Override
        public Set<String> getSet(String className, String key,
                Set<String> defaultValue) {
            String value = get(className, key);
            return (value != null ? StringUtils.commaDelimitedListToSet(value)
                    : defaultValue);
        }

        @Override
        public String get(String className, String key) {
            return get(className, key, null);
        }

        @Override
        public String get(String className, String key, String defaultValue) {
            String value = this.properties.getProperty(className + "." + key);
            return (value != null ? value : defaultValue);
        }
  }

最后我们可以发现其get方法还是通过java.util.Properties的getProperty方法来获取的值