spring-autoconfigure-metadata.properties配置文件作用之前一直不清楚SpringBoot的jar包中的spring-autoconfigure-metadata.properties的作用，今天查了一下官方文档，发现是有介绍的。记录一下。
autoconfigure模块包含了库启动必须的所有信息。也可以包含配置key的定义（如@ConfigurationProperties）以及任何用于定制组件如何初始化的回调接口。
最好标识对库的依赖为可选，这样你就可以更加容易引入
autoconfigure模块。如果你这样做了（指标识对库的依赖为可选），如果库没有提供，那么SpringBoot会使用默认的方式。
SpringBoot使用注解处理器从元数据文件（metadata file）中收集自动配置条件（META-INF/spring-autoconfigure-metadata.properties）。如果文件存在，则用它来过滤不匹配的自动配置，这样可以提高启动时间。建议添加如下包括自动配置的依赖。
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-autoconfigure-processor</artifactId>
<optional>true</optional>
</dependency>
SpringBoot中关于spring-autoconfigure-metadata.properties文件作用的描述就上面一点点，看了和没看一个样。看来只能通过查看源代码的方式看它的作用了
其实关于这个文件，在@SpringBootApplication注解中就有看到，只不过当时没有注意而已。这里接着@SpringBootApplication注解中AutoConfigurationImportSelector类那里来写。
AutoConfigurationImportSelector类中selectImports方法如下：
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
.loadMetadata(this.beanClassLoader);
AnnotationAttributes attributes = getAttributes(annotationMetadata);
List<String> configurations = getCandidateConfigurations(annotationMetadata,
attributes);
configurations = removeDuplicates(configurations);
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
configurations = filter(configurations, autoConfigurationMetadata);
fireAutoConfigurationImportEvents(configurations, exclusions);
return StringUtils.toStringArray(configurations);
}
其中会调用AutoConfigurationMetadataLoader类的loadMetadata方法
loadMetadata方法如下：
protected static final String PATH = “META-INF/“
+ “spring-autoconfigure-metadata.properties”;
public static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader) {
return loadMetadata(classLoader, PATH);
}
static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader, String path) {
try {
Enumeration<URL> urls = (classLoader != null) ? classLoader.getResources(path)
: ClassLoader.getSystemResources(path);
Properties properties = new Properties();
while (urls.hasMoreElements()) {
properties.putAll(PropertiesLoaderUtils
.loadProperties(new UrlResource(urls.nextElement())));
}
return loadMetadata(properties);
}
catch (IOException ex) {
throw new IllegalArgumentException(
“Unable to load @ConditionalOnClass location [“ + path + “]”, ex);
}
}
static AutoConfigurationMetadata loadMetadata(Properties properties) {
return new PropertiesAutoConfigurationMetadata(properties);
}
loadMetadata会加载所有jar包中的META-INF/spring-autoconfigure-metadata.properties文件，然后返回PropertiesAutoConfigurationMetadata类的实例。
回到selectImports方法中，通过调用getCandidateConfigurations方法获取所有META-INF/spring.factories文件中key为org.springframework.boot.autoconfigure.EnableAutoConfiguration的值，将配置了exclude和excludeName的类排除掉。然后调用filter方法
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);
...
}
...
}
getAutoConfigurationImportFilters方法返回的是META-INF/spring.factories文件中key为org.springframework.boot.autoconfigure.AutoConfigurationImportFilter的值。
AutoConfigurationImportFilter的实现类只有一个，org.springframework.boot.autoconfigure.condition.OnClassCondition,对应的match方法及相关方法如下：
public boolean[] match(String[] autoConfigurationClasses,
AutoConfigurationMetadata autoConfigurationMetadata) {
ConditionEvaluationReport report = getConditionEvaluationReport();
ConditionOutcome[] outcomes = getOutcomes(autoConfigurationClasses,
autoConfigurationMetadata);
...
}
private ConditionOutcome[] getOutcomes(String[] autoConfigurationClasses,
AutoConfigurationMetadata autoConfigurationMetadata) {
// Split the work and perform half in a background thread. Using a single
// additional thread seems to offer the best performance. More threads make
// things worse
int split = autoConfigurationClasses.length / 2;
OutcomesResolver firstHalfResolver = createOutcomesResolver(
autoConfigurationClasses, 0, split, autoConfigurationMetadata);
OutcomesResolver secondHalfResolver = new StandardOutcomesResolver(
autoConfigurationClasses, split, autoConfigurationClasses.length,
autoConfigurationMetadata, this.beanClassLoader);
ConditionOutcome[] secondHalf = secondHalfResolver.resolveOutcomes();
ConditionOutcome[] firstHalf = firstHalfResolver.resolveOutcomes();
ConditionOutcome[] outcomes = new ConditionOutcome[autoConfigurationClasses.length];
System.arraycopy(firstHalf, 0, outcomes, 0, firstHalf.length);
System.arraycopy(secondHalf, 0, outcomes, split, secondHalf.length);
return outcomes;
}
org.springframework.boot.autoconfigure.condition.OnClassCondition.StandardOutcomesResolver的resolveOutcomes方法
public ConditionOutcome[] resolveOutcomes() {
return getOutcomes(this.autoConfigurationClasses, this.start, this.end,this.autoConfigurationMetadata);
}
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;
}
整体读下来，发现其实获取的是META-INF/spring-autoconfigure-metadata.properties文件中配置了类名.ConditionalOnClass对应的值。
打开spring-boot-actuator-autoconfigure-2.0.9.RELEASE.jar中的/META-INF/spring-autoconfigure-metadata.properties文件，随便找一个类的配置，比如RedisHealthIndicatorAutoConfiguration，发现这里的配置和class文件中注解其实是一一对应的。 所以感觉spring-autoconfigure-metadata.properties应该是工具自动生成出来的。但现在还不知道这个猜测对不对。后续知道了再补充吧