springboot自动装配原理

springboot自动装配原理

1.@SpringBootApplication

我们在使用springboot项目时，都会有一个在启动类上加一个@SpringBootApplication注解，下面看下这个注解源码

@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 {
    @AliasFor(annotation = EnableAutoConfiguration.class)
    Class<?>[] exclude() default {};
    @AliasFor(annotation = EnableAutoConfiguration.class)
    String[] excludeName() default {};
    @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
    String[] scanBasePackages() default {};
    @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
    Class<?>[] scanBasePackageClasses() default {};
    @AliasFor(annotation = Configuration.class)
    boolean proxyBeanMethods() default true;
}

这里包含了@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan这几个关键的注解。

2.@ComponentScan

由于@ComponentScan没有指定扫描包，因此它默认扫描的是与该类同级的类以及同级下的子包的所有类

3.@SpringBootConfiguration

@SpringBootConfiguration通过源码可以得知它是一个配置类，因为它加了@Configuration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {

    @AliasFor(annotation = Configuration.class)
    boolean proxyBeanMethods() default true;

}

4.@EnableAutoConfiguration

@EnableAutoConfiguration，一旦加上这个注解，将会开启自动装配功能，也就是说spring会在你的classpath下找到所有配置的bean进行装配，而且在装配bean的过程中，会根据条件来判断。

@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 {};

}

根据@Import(AutoConfigurationImportSelector.class)可以得知，springboot会加载AutoConfigurationImportSelector.class这个类，下面看下这个类

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
        ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {

    private static final AutoConfigurationEntry EMPTY_ENTRY = new AutoConfigurationEntry();

    private static final String[] NO_IMPORTS = {};

    private static final Log logger = LogFactory.getLog(AutoConfigurationImportSelector.class);

    private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude";

    private ConfigurableListableBeanFactory beanFactory;

    private Environment environment;

    private ClassLoader beanClassLoader;

    private ResourceLoader resourceLoader;

    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                .loadMetadata(this.beanClassLoader);
        AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
                annotationMetadata);
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }
    ...

AutoConfigurationImportSelector这个类实现了DeferredImportSelector接口，这个接口继承了ImportSelector，并重写了selectImports方法，这个方法首先会判断是否需要自动装配，然后会调用getAutoConfigurationEntry方法

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
            AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        }
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        // 从META-INF/spring.factories文件中拿到所有的配置
        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 new AutoConfigurationEntry(configurations, exclusions);
    }

调用getCandidateConfigurations方法通过SpringFactoriesLoader从META-INF/spring.factories文件中读取所有的配置，然后进行过滤得到相应的需要装配的类。最后让所有配置在META-INF/spring.factories下的AutoConfigurationImportListener执行AutoConfigurationImportEvent事件

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);
            }
        }
    }

    protected List<AutoConfigurationImportListener> getAutoConfigurationImportListeners() {
        return SpringFactoriesLoader.loadFactories(AutoConfigurationImportListener.class,
                this.beanClassLoader);
    }

总结：自动装配还是利用了SpringFactoriesLoader来加载META-INF/spring.factoires文件里所有配置的EnableAutoConfgruation，它会经过exclude和filter等操作，最终确定要装配的类