Unveiling the Inner Workings of @EnableAutoConfiguration in Spring Boot

Introduction

In our work we often use the @EnableAutoConfiguration annotation directly or indirectly.

Today we discuss the processing logic of @EnableAutoConfiguration.

Finding the Core Class

Annotations that start with @Enable usually have an @Import that points to the implementation class. For @EnableAutoConfiguration the @Import brings in AutoConfigurationImportSelector , which is the entry point.

The class hierarchy shows that AutoConfigurationImportSelector imports AutoConfigurationImportSelector.AutoConfigurationGroup and related classes.

The Aware series are used for resource injection, and Ordered handles sorting.

DeferredImportSelector runs after all @Configuration beans, allowing it to skip already loaded auto‑configuration classes.

process

private static class DeferredImportSelectorGrouping {<br/>    private final DeferredImportSelector.Group group;<br/>    public Iterable<Group.Entry> getImports() {<br/>        for (DeferredImportSelectorHolder deferredImport : this.deferredImports) {<br/>            this.group.process(deferredImport.getConfigurationClass().getMetadata(),<br/>                deferredImport.getImportSelector());<br/>        }<br/>        return this.group.selectImports();<br/>    }<br/>}

Key steps:

ImportSelector parsing is handled in ConfigurationClassParser#processImports, where DeferredImportSelector is queued.

DeferredImportSelector handling occurs in ConfigurationClassParser#parse via deferredImportSelectorHandler.process().

AutoConfigurationImportSelector.AutoConfigurationGroup.process

and selectImports are the core methods.

1. getAutoConfigurationMetadata()

public AutoConfigurationMetadata getAutoConfigurationMetadata() {<br/>    if (this.autoConfigurationMetadata == null) {<br/>        this.autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);<br/>    }<br/>    return this.autoConfigurationMetadata;<br/>}

This loads the metadata from META-INF/spring-autoconfigure-metadata.properties.

2. getAutoConfigurationEntry()

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata meta, AnnotationMetadata am) {<br/>    if (!isEnabled(am)) {<br/>        return EMPTY_ENTRY;<br/>    }<br/>    AnnotationAttributes attributes = getAttributes(am);<br/>    List<String> configurations = getCandidateConfigurations(am, attributes);<br/>    configurations = removeDuplicates(configurations);<br/>    Set<String> exclusions = getExclusions(am, attributes);<br/>    checkExcludedClasses(configurations, exclusions);<br/>    configurations.removeAll(exclusions);<br/>    configurations = filter(configurations, meta);<br/>    fireAutoConfigurationImportEvents(configurations, exclusions);<br/>    return new AutoConfigurationEntry(configurations, exclusions);<br/>}

It obtains annotation attributes, candidate configurations, removes duplicates, applies exclusions, filters with conditions, and fires import events.

getCandidateConfigurations

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {<br/>    List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());<br/>    Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories.");<br/>    return configurations;<br/>}

Loads factory names from META-INF/spring.factories.

filter

private List<String> filter(List<String> configurations, AutoConfigurationMetadata meta) {<br/>    String[] candidates = StringUtils.toStringArray(configurations);<br/>    boolean[] skip = new boolean[candidates.length];<br/>    boolean skipped = false;<br/>    for (AutoConfigurationImportFilter filter : getAutoConfigurationImportFilters()) {<br/>        invokeAwareMethods(filter);<br/>        boolean[] match = filter.match(candidates, meta);<br/>        for (int i = 0; i < match.length; i++) {<br/>            if (!match[i]) {<br/>                skip[i] = true;<br/>                candidates[i] = null;<br/>                skipped = true;<br/>            }<br/>        }<br/>    }<br/>    if (!skipped) {<br/>        return configurations;<br/>    }<br/>    List<String> result = new ArrayList<>(candidates.length);<br/>    for (int i = 0; i < candidates.length; i++) {<br/>        if (!skip[i]) {<br/>            result.add(candidates[i]);<br/>        }<br/>    }<br/>    return result;<br/>}

Implementations such as OnClassCondition, OnBeanCondition, and OnWebApplicationCondition decide which configurations to keep.

sortAutoConfigurations

private List<String> sortAutoConfigurations(Set<String> configurations, AutoConfigurationMetadata meta) {<br/>    return new AutoConfigurationSorter(getMetadataReaderFactory(), meta).getInPriorityOrder(configurations);<br/>}

Sorting follows alphabetical order, then @AutoConfigureOrder, then @AutoConfigureBefore / @AutoConfigureAfter.

Summary

The core processor of @EnableAutoConfiguration is AutoConfigurationImportSelector .

AutoConfigurationImportSelector implements DeferredImportSelector .

ImportSelector parsing occurs in ConfigurationClassParser#processImports, queuing the selector for later handling.

AutoConfigurationImportSelector.AutoConfigurationGroup.process

and selectImports are the focal methods. ConfigurationClassParser#doProcessConfigurationClass uses conditionEvaluator.shouldSkip(...) to apply various @Conditional annotations, enabling custom configurations to override auto‑configuration.