Springboot项目中最核心也是入门级的注解就是@SpringBootApplication，这是由@EnableAutoConfiguration、@ComponentScan和@Configuration组成的一个组合注解。
接下来我们将分析其中的EnableAutoConfiguration注解的源码。
springboot的一大特性就是“约定大于配置”，EnableAutoConfiguration这个注解从字面上就可以理解是一个实现自动配置的注解，也是“约定”能够生效的一大保障。
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
/**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
Class<?>[] exclude() default {};
/**
* Exclude specific auto-configuration class names such that they will never be
* applied.
* @return the class names to exclude
* @since 1.3.0
*/
String[] excludeName() default {};
}
EnableAutoConfiguration有两大属性，exclude和excludeName都是用来指定不需要自动配置的配置类。
分析完EnableAutoConfiguration的属性，接下来和其它Enable类型的注解一下，我们进入@import注解指定的类，分析springboot是如何实现自动配置的。
AutoConfigurationImportSelector是一个ImportSelector类，selectImports的返回值就是其最后注入IOC容器的配置类。另外，AutoConfigurationImportSelector还用到了beanClassLoader、environment、resourceLoader，因此其还实现了BeanClassLoaderAware，EnviromentAware，ResourceLoaderAware接口。
@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
try {
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
.loadMetadata(this.beanClassLoader);
AnnotationAttributes attributes = getAttributes(annotationMetadata);
List<String> configurations = getCandidateConfigurations(annotationMetadata,
attributes);
configurations = removeDuplicates(configurations);
configurations = sort(configurations, autoConfigurationMetadata);
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
configurations = filter(configurations, autoConfigurationMetadata);
fireAutoConfigurationImportEvents(configurations, exclusions);
return StringUtils.toStringArray(configurations);
}
catch (IOException ex) {
throw new IllegalStateException(ex);
}
}
阅读代码，可以发现其步骤如下：
- 获取所有的候选配置
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
AnnotationAttributes attributes) {
List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
Assert.notEmpty(configurations,
"No auto configuration classes found in META-INF/spring.factories. If you "
+ "are using a custom packaging, make sure that file is correct.");
return configurations;
}
SpringFactoriesLoader的相关方法如下：
public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = cache.get(classLoader);
if (result != null)
return result;
try {
Enumeration<URL> urls = (classLoader != null ?
classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
result = new LinkedMultiValueMap<>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry<?, ?> entry : properties.entrySet()) {
List<String> factoryClassNames = Arrays.asList(
StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
result.addAll((String) entry.getKey(), factoryClassNames);
}
}
cache.put(classLoader, result);
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}
loadSpringFactories方法获取当前项目和所有jar包路径下的"META-INF/spring.factories"文件，其中就是约定的所有自动配置的类。
如spring-boot-autoconfigure-2.0.2.RELEASE.jar和spring-cloud-openfeign-core-2.0.0.M1.jar下的该文件：
- 去除重复项
protected final <T> List<T> removeDuplicates(List<T> list) {
return new ArrayList<>(new LinkedHashSet<>(list));
}
- 排序
如下所示，按order排序
private List<String> sort(List<String> configurations,
AutoConfigurationMetadata autoConfigurationMetadata) throws IOException {
configurations = new AutoConfigurationSorter(getMetadataReaderFactory(),
autoConfigurationMetadata).getInPriorityOrder(configurations);
return configurations;
}
public List<String> getInPriorityOrder(Collection<String> classNames) {
final AutoConfigurationClasses classes = new AutoConfigurationClasses(
this.metadataReaderFactory, this.autoConfigurationMetadata, classNames);
List<String> orderedClassNames = new ArrayList<>(classNames);
// Initially sort alphabetically
Collections.sort(orderedClassNames);
// Then sort by order
orderedClassNames.sort((o1, o2) -> {
int i1 = classes.get(o1).getOrder();
int i2 = classes.get(o2).getOrder();
return Integer.compare(i1, i2);
});
// Then respect @AutoConfigureBefore @AutoConfigureAfter
orderedClassNames = sortByAnnotation(classes, orderedClassNames);
return orderedClassNames;
}
- exclusions项处理
如何对于项目中某些自动配置类，我们不希望其自动配置，name我们可以通过EnableAutoConfiguration的exclude或excludeName属性进行配置，或者也可以在配置文件里通过配置项“spring.autoconfigure.exclude”进行配置。
（1）找到不希望自动配置的配置类exclusions
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
protected Set<String> getExclusions(AnnotationMetadata metadata,
AnnotationAttributes attributes) {
Set<String> excluded = new LinkedHashSet<>();
excluded.addAll(asList(attributes, "exclude"));
excluded.addAll(Arrays.asList(attributes.getStringArray("excludeName")));
excluded.addAll(getExcludeAutoConfigurationsProperty());
return excluded;
}
private List<String> getExcludeAutoConfigurationsProperty() {
if (getEnvironment() instanceof ConfigurableEnvironment) {
Binder binder = Binder.get(getEnvironment());
return binder.bind(PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE, String[].class)
.map(Arrays::asList).orElse(Collections.emptyList());
}
String[] excludes = getEnvironment()
.getProperty(PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE, String[].class);
return (excludes == null ? Collections.emptyList() : Arrays.asList(excludes));
}
（2）对exclusions进行验证
exclusions必须是自动配置类，否则抛出异常。
checkExcludedClasses(configurations, exclusions);
private void checkExcludedClasses(List<String> configurations,
Set<String> exclusions) {
List<String> invalidExcludes = new ArrayList<>(exclusions.size());
for (String exclusion : exclusions) {
if (ClassUtils.isPresent(exclusion, getClass().getClassLoader())
&& !configurations.contains(exclusion)) {
invalidExcludes.add(exclusion);
}
}
if (!invalidExcludes.isEmpty()) {
handleInvalidExcludes(invalidExcludes);
}
}
/**
* Handle any invalid excludes that have been specified.
* @param invalidExcludes the list of invalid excludes (will always have at least one
* element)
*/
protected void handleInvalidExcludes(List<String> invalidExcludes) {
StringBuilder message = new StringBuilder();
for (String exclude : invalidExcludes) {
message.append("\t- ").append(exclude).append(String.format("%n"));
}
throw new IllegalStateException(String
.format("The following classes could not be excluded because they are"
+ " not auto-configuration classes:%n%s", message));
}
（3）移除exclusions
configurations.removeAll(exclusions);
- 根据项目中的AutoConfigurationImportFilter类进行过滤
在项目中找到所有AutoConfigurationImportFilter类进行过滤，对于自动配置类，只要其不满足任意一个filter的match方法，就将其进行过滤，不再自动配置。
最常见的filter类是OnClassCondition类，与ConditionalOnClass注解息息相关，判断当前类加载路径下是否有对应文件。
configurations = filter(configurations, autoConfigurationMetadata);
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);
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);
}
protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class,
this.beanClassLoader);
}
- 配置监听事件
fireAutoConfigurationImportEvents(configurations, exclusions);
private void fireAutoConfigurationImportEvents(List<String> configurations,
Set<String> exclusions) {
List<AutoConfigurationImportListener> listeners = getAutoConfigurationImportListeners();
if (!listeners.isEmpty()) {
AutoConfigurationImportEvent event = new AutoConfigurationImportEvent(this,
configurations, exclusions);
for (AutoConfigurationImportListener listener : listeners) {
invokeAwareMethods(listener);
listener.onAutoConfigurationImportEvent(event);
}
}
}
至此，自动配置的逻辑实现。
最后提一点，按照springboot的编码格式，springboot源码中所有形如XXXAutoConfiguration的配置类都是自动配置类，无需其他操作即可注入IOC容器。