SpringBoot's automatic assembly is not only the basis of unpacking and ready to use, but also the premise of microservice. In fact, it is not so mysterious. I have written the most basic implementation before. You can refer to it This article . The main topic this time is to see how it is implemented. We can grasp the context of automatic assembly through source code.
1, Automatic assembly process analysis
1.1. About @ SpringBootApplication
When writing the SpringBoot project, @ SpringBootApplication is the most common annotation. We can take a look at the source code:
/*
* Copyright 2012-2017 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.boot.autoconfigure;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.context.TypeExcludeFilter;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.ComponentScan.Filter;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.FilterType;
import org.springframework.core.annotation.AliasFor;
/**
* Indicates a {@link Configuration configuration} class that declares one or more
* {@link Bean @Bean} methods and also triggers {@link EnableAutoConfiguration
* auto-configuration} and {@link ComponentScan component scanning}. This is a convenience
* annotation that is equivalent to declaring {@code @Configuration},
* {@code @EnableAutoConfiguration} and {@code @ComponentScan}.
*
* @author Phillip Webb
* @author Stephane Nicoll
* @since 1.2.0
*/
@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 {
/**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
@AliasFor(annotation = EnableAutoConfiguration.class, attribute = "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
*/
@AliasFor(annotation = EnableAutoConfiguration.class, attribute = "excludeName")
String[] excludeName() default {};
/**
* Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}
* for a type-safe alternative to String-based package names.
* @return base packages to scan
* @since 1.3.0
*/
@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
String[] scanBasePackages() default {};
/**
* Type-safe alternative to {@link #scanBasePackages} for specifying the packages to
* scan for annotated components. The package of each class specified will be scanned.
* <p>
* Consider creating a special no-op marker class or interface in each package that
* serves no purpose other than being referenced by this attribute.
* @return base packages to scan
* @since 1.3.0
*/
@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
Class<?>[] scanBasePackageClasses() default {};
}View Code
This includes @ SpringBootConfiguration, @ EnableAutoConfiguration, @ ComponentScan. Since @ ComponentScan does not specify a scanning package, it scans the classes of the same level or all classes under the same level package by default. In addition, @ SpringBootConfiguration is a @ Configuration:
This code is by Java Architect must see network-Architecture sorting
/*
* Copyright 2012-2016 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.boot;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Configuration;
/**
* Indicates that a class provides Spring Boot application
* {@link Configuration @Configuration}. Can be used as an alternative to the Spring's
* standard {@code @Configuration} annotation so that configuration can be found
* automatically (for example in tests).
* <p>
* Application should only ever include <em>one</em> {@code @SpringBootConfiguration} and
* most idiomatic Spring Boot applications will inherit it from
* {@code @SpringBootApplication}.
*
* @author Phillip Webb
* @since 1.4.0
*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
}View Code
From this, we can infer that @ SpringBootApplication is equivalent to @ Configuration @ComponentScan @EnableAutoConfiguration
1.2,@EnableAutoConfiguration
Once this annotation is added, the automatic assembly function will be enabled. To put it simply, Spring will try to find all configured beans under your classpath and then assemble them. Of course, when assembling a Bean, it will be initialized according to several (Conditional) customization rules. Let's take a look at its source code:
/*
* Copyright 2012-2017 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.boot.autoconfigure;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.context.embedded.EmbeddedServletContainerFactory;
import org.springframework.boot.context.embedded.tomcat.TomcatEmbeddedServletContainerFactory;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;
import org.springframework.core.io.support.SpringFactoriesLoader;
/**
* Enable auto-configuration of the Spring Application Context, attempting to guess and
* configure beans that you are likely to need. Auto-configuration classes are usually
* applied based on your classpath and what beans you have defined. For example, If you
* have {@code tomcat-embedded.jar} on your classpath you are likely to want a
* {@link TomcatEmbeddedServletContainerFactory} (unless you have defined your own
* {@link EmbeddedServletContainerFactory} bean).
* <p>
* When using {@link SpringBootApplication}, the auto-configuration of the context is
* automatically enabled and adding this annotation has therefore no additional effect.
* <p>
* Auto-configuration tries to be as intelligent as possible and will back-away as you
* define more of your own configuration. You can always manually {@link #exclude()} any
* configuration that you never want to apply (use {@link #excludeName()} if you don't
* have access to them). You can also exclude them via the
* {@code spring.autoconfigure.exclude} property. Auto-configuration is always applied
* after user-defined beans have been registered.
* <p>
* The package of the class that is annotated with {@code @EnableAutoConfiguration},
* usually via {@code @SpringBootApplication}, has specific significance and is often used
* as a 'default'. For example, it will be used when scanning for {@code @Entity} classes.
* It is generally recommended that you place {@code @EnableAutoConfiguration} (if you're
* not using {@code @SpringBootApplication}) in a root package so that all sub-packages
* and classes can be searched.
* <p>
* Auto-configuration classes are regular Spring {@link Configuration} beans. They are
* located using the {@link SpringFactoriesLoader} mechanism (keyed against this class).
* Generally auto-configuration beans are {@link Conditional @Conditional} beans (most
* often using {@link ConditionalOnClass @ConditionalOnClass} and
* {@link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations).
*
* @author Phillip Webb
* @author Stephane Nicoll
* @see ConditionalOnBean
* @see ConditionalOnMissingBean
* @see ConditionalOnClass
* @see AutoConfigureAfter
* @see SpringBootApplication
*/
@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.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 {};
}View Code
Although it instructs us to look at the enable AutoConfigurationImportSelector according to the instructions in the document comments. But this class is in springboot1 Version 5. X is outdated, so let's take a look at its parent class AutoConfigurationImportSelector:
This code is by Java Architect must see network-Architecture sorting
/*
* Copyright 2012-2017 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.boot.autoconfigure;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.TimeUnit;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.Aware;
import org.springframework.beans.factory.BeanClassLoaderAware;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.boot.bind.RelaxedPropertyResolver;
import org.springframework.context.EnvironmentAware;
import org.springframework.context.ResourceLoaderAware;
import org.springframework.context.annotation.DeferredImportSelector;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.AnnotationAttributes;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.Environment;
import org.springframework.core.io.ResourceLoader;
import org.springframework.core.io.support.SpringFactoriesLoader;
import org.springframework.core.type.AnnotationMetadata;
import org.springframework.core.type.classreading.CachingMetadataReaderFactory;
import org.springframework.core.type.classreading.MetadataReaderFactory;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.StringUtils;
/**
* {@link DeferredImportSelector} to handle {@link EnableAutoConfiguration
* auto-configuration}. This class can also be subclassed if a custom variant of
* {@link EnableAutoConfiguration @EnableAutoConfiguration}. is needed.
*
* @author Phillip Webb
* @author Andy Wilkinson
* @author Stephane Nicoll
* @author Madhura Bhave
* @since 1.3.0
* @see EnableAutoConfiguration
*/
public class AutoConfigurationImportSelector
implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
BeanFactoryAware, EnvironmentAware, Ordered {
private static final String[] NO_IMPORTS = {};
private static final Log logger = LogFactory
.getLog(AutoConfigurationImportSelector.class);
private ConfigurableListableBeanFactory beanFactory;
private Environment environment;
private ClassLoader beanClassLoader;
private ResourceLoader resourceLoader;
@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 configurations.toArray(new String[configurations.size()]);
}
catch (IOException ex) {
throw new IllegalStateException(ex);
}
}
protected boolean isEnabled(AnnotationMetadata metadata) {
return true;
}
/**
* Return the appropriate {@link AnnotationAttributes} from the
* {@link AnnotationMetadata}. By default this method will return attributes for
* {@link #getAnnotationClass()}.
* @param metadata the annotation metadata
* @return annotation attributes
*/
protected AnnotationAttributes getAttributes(AnnotationMetadata metadata) {
String name = getAnnotationClass().getName();
AnnotationAttributes attributes = AnnotationAttributes
.fromMap(metadata.getAnnotationAttributes(name, true));
Assert.notNull(attributes,
"No auto-configuration attributes found. Is " + metadata.getClassName()
+ " annotated with " + ClassUtils.getShortName(name) + "?");
return attributes;
}
/**
* Return the source annotation class used by the selector.
* @return the annotation class
*/
protected Class<?> getAnnotationClass() {
return EnableAutoConfiguration.class;
}
/**
* Return the auto-configuration class names that should be considered. By default
* this method will load candidates using {@link SpringFactoriesLoader} with
* {@link #getSpringFactoriesLoaderFactoryClass()}.
* @param metadata the source metadata
* @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
* attributes}
* @return a list of candidate configurations
*/
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;
}
/**
* Return the class used by {@link SpringFactoriesLoader} to load configuration
* candidates.
* @return the factory class
*/
protected Class<?> getSpringFactoriesLoaderFactoryClass() {
return EnableAutoConfiguration.class;
}
private void checkExcludedClasses(List<String> configurations,
Set<String> exclusions) {
List<String> invalidExcludes = new ArrayList<String>(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));
}
/**
* Return any exclusions that limit the candidate configurations.
* @param metadata the source metadata
* @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
* attributes}
* @return exclusions or an empty set
*/
protected Set<String> getExclusions(AnnotationMetadata metadata,
AnnotationAttributes attributes) {
Set<String> excluded = new LinkedHashSet<String>();
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) {
RelaxedPropertyResolver resolver = new RelaxedPropertyResolver(
this.environment, "spring.autoconfigure.");
Map<String, Object> properties = resolver.getSubProperties("exclude");
if (properties.isEmpty()) {
return Collections.emptyList();
}
List<String> excludes = new ArrayList<String>();
for (Map.Entry<String, Object> entry : properties.entrySet()) {
String name = entry.getKey();
Object value = entry.getValue();
if (name.isEmpty() || name.startsWith("[") && value != null) {
excludes.addAll(new HashSet<String>(Arrays.asList(StringUtils
.tokenizeToStringArray(String.valueOf(value), ","))));
}
}
return excludes;
}
RelaxedPropertyResolver resolver = new RelaxedPropertyResolver(getEnvironment(),
"spring.autoconfigure.");
String[] exclude = resolver.getProperty("exclude", String[].class);
return (Arrays.asList(exclude == null ? new String[0] : exclude));
}
private List<String> sort(List<String> configurations,
AutoConfigurationMetadata autoConfigurationMetadata) throws IOException {
configurations = new AutoConfigurationSorter(getMetadataReaderFactory(),
autoConfigurationMetadata).getInPriorityOrder(configurations);
return configurations;
}
private List<String> filter(List<String> configurations,
AutoConfigurationMetadata autoConfigurationMetadata) {
long startTime = System.nanoTime();
String[] candidates = configurations.toArray(new String[configurations.size()]);
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<String>(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<String>(result);
}
protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class,
this.beanClassLoader);
}
private MetadataReaderFactory getMetadataReaderFactory() {
try {
return getBeanFactory().getBean(
SharedMetadataReaderFactoryContextInitializer.BEAN_NAME,
MetadataReaderFactory.class);
}
catch (NoSuchBeanDefinitionException ex) {
return new CachingMetadataReaderFactory(this.resourceLoader);
}
}
protected final <T> List<T> removeDuplicates(List<T> list) {
return new ArrayList<T>(new LinkedHashSet<T>(list));
}
protected final List<String> asList(AnnotationAttributes attributes, String name) {
String[] value = attributes.getStringArray(name);
return Arrays.asList(value == null ? new String[0] : value);
}
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);
}
private void invokeAwareMethods(Object instance) {
if (instance instanceof Aware) {
if (instance instanceof BeanClassLoaderAware) {
((BeanClassLoaderAware) instance)
.setBeanClassLoader(this.beanClassLoader);
}
if (instance instanceof BeanFactoryAware) {
((BeanFactoryAware) instance).setBeanFactory(this.beanFactory);
}
if (instance instanceof EnvironmentAware) {
((EnvironmentAware) instance).setEnvironment(this.environment);
}
if (instance instanceof ResourceLoaderAware) {
((ResourceLoaderAware) instance).setResourceLoader(this.resourceLoader);
}
}
}
@Override
public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
Assert.isInstanceOf(ConfigurableListableBeanFactory.class, beanFactory);
this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;
}
protected final ConfigurableListableBeanFactory getBeanFactory() {
return this.beanFactory;
}
@Override
public void setBeanClassLoader(ClassLoader classLoader) {
this.beanClassLoader = classLoader;
}
protected ClassLoader getBeanClassLoader() {
return this.beanClassLoader;
}
@Override
public void setEnvironment(Environment environment) {
this.environment = environment;
}
protected final Environment getEnvironment() {
return this.environment;
}
@Override
public void setResourceLoader(ResourceLoader resourceLoader) {
this.resourceLoader = resourceLoader;
}
protected final ResourceLoader getResourceLoader() {
return this.resourceLoader;
}
@Override
public int getOrder() {
return Ordered.LOWEST_PRECEDENCE - 1;
}
}View Code
First, this class implements the DeferredImportSelector interface, which inherits ImportSelector:
/*
* Copyright 2002-2013 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.context.annotation;
import org.springframework.core.type.AnnotationMetadata;
/**
* Interface to be implemented by types that determine which @{@link Configuration}
* class(es) should be imported based on a given selection criteria, usually one or more
* annotation attributes.
*
* <p>An {@link ImportSelector} may implement any of the following
* {@link org.springframework.beans.factory.Aware Aware} interfaces, and their respective
* methods will be called prior to {@link #selectImports}:
* <ul>
* <li>{@link org.springframework.context.EnvironmentAware EnvironmentAware}</li>
* <li>{@link org.springframework.beans.factory.BeanFactoryAware BeanFactoryAware}</li>
* <li>{@link org.springframework.beans.factory.BeanClassLoaderAware BeanClassLoaderAware}</li>
* <li>{@link org.springframework.context.ResourceLoaderAware ResourceLoaderAware}</li>
* </ul>
*
* <p>ImportSelectors are usually processed in the same way as regular {@code @Import}
* annotations, however, it is also possible to defer selection of imports until all
* {@code @Configuration} classes have been processed (see {@link DeferredImportSelector}
* for details).
*
* @author Chris Beams
* @since 3.1
* @see DeferredImportSelector
* @see Import
* @see ImportBeanDefinitionRegistrar
* @see Configuration
*/
public interface ImportSelector {
/**
* Select and return the names of which class(es) should be imported based on
* the {@link AnnotationMetadata} of the importing @{@link Configuration} class.
*/
String[] selectImports(AnnotationMetadata importingClassMetadata);
}View Code
This interface is mainly used to import Configuration items of @ Configuration, while DeferredImportSelector is used to postpone the import. It will not be executed until all @ Configuration items are processed.
Let's look back at the selectImport method of AutoConfigurationImportSelector:
@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 configurations.toArray(new String[configurations.size()]);
}
catch (IOException ex) {
throw new IllegalStateException(ex);
}
}At first, the method will judge whether to perform automatic assembly, and then start from meta-inf / spring-autoconfigure-metadata Properties read metadata and metadata related properties, and then call getCandidateConfigurations method:
/**
* Return the auto-configuration class names that should be considered. By default
* this method will load candidates using {@link SpringFactoriesLoader} with
* {@link #getSpringFactoriesLoaderFactoryClass()}.
* @param metadata the source metadata
* @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
* attributes}
* @return a list of candidate configurations
*/
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;
}
/**
* Return the class used by {@link SpringFactoriesLoader} to load configuration
* candidates.
* @return the factory class
*/
protected Class<?> getSpringFactoriesLoaderFactoryClass() {
return EnableAutoConfiguration.class;
}Here we meet our old acquaintance -- spring factoryiesloader, which can read meta-inf / spring The configuration of EnableAutoConfiguration under factories is followed by exclusion and filtering to obtain the classes to be assembled. Finally, let all configurations in meta-inf / spring The AutoConfigurationImportListener under factories executes the AutoConfigurationImportEvent event event. The code is as follows:
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);
}2, When to perform automatic assembly
In the previous link, we just need to finally determine which classes need to be assembled, and when to deal with these automatically assembled classes during SpringBoot? Let's briefly analyze:
2.1. refresh method of AbstractApplicationContext:
This method is commonplace. Please pay attention to this method:
// Invoke factory processors registered as beans in the context. invokeBeanFactoryPostProcessors(beanFactory);
Here we are dealing with beanfactory postprocessor. Let's take a look at the interface BeanDefinitionRegistryPostProcessor:
/*
* Copyright 2002-2010 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.beans.factory.support;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
/**
* Extension to the standard {@link BeanFactoryPostProcessor} SPI, allowing for
* the registration of further bean definitions <i>before</i> regular
* BeanFactoryPostProcessor detection kicks in. In particular,
* BeanDefinitionRegistryPostProcessor may register further bean definitions
* which in turn define BeanFactoryPostProcessor instances.
*
* @author Juergen Hoeller
* @since 3.0.1
* @see org.springframework.context.annotation.ConfigurationClassPostProcessor
*/
public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {
/**
* Modify the application context's internal bean definition registry after its
* standard initialization. All regular bean definitions will have been loaded,
* but no beans will have been instantiated yet. This allows for adding further
* bean definitions before the next post-processing phase kicks in.
* @param registry the bean definition registry used by the application context
* @throws org.springframework.beans.BeansException in case of errors
*/
void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException;
}View Code
This interface inherits beanfactoryprocessor.
2.2. ConfigurationClassPostProcessor class
This class mainly processes the @ Configuration annotation. It implements BeanDefinitionRegistryPostProcessor, and indirectly implements BeanFactoryPostProcessor. The key codes are as follows:
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
int factoryId = System.identityHashCode(beanFactory);
if (this.factoriesPostProcessed.contains(factoryId)) {
throw new IllegalStateException(
"postProcessBeanFactory already called on this post-processor against " + beanFactory);
}
this.factoriesPostProcessed.add(factoryId);
if (!this.registriesPostProcessed.contains(factoryId)) {
// BeanDefinitionRegistryPostProcessor hook apparently not supported...
// Simply call processConfigurationClasses lazily at this point then.
processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);
}
enhanceConfigurationClasses(beanFactory);
beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
}
/**
* Build and validate a configuration model based on the registry of
* {@link Configuration} classes.
*/
public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
//..... Omit some codes
// Parse each @Configuration class
ConfigurationClassParser parser = new ConfigurationClassParser(
this.metadataReaderFactory, this.problemReporter, this.environment,
this.resourceLoader, this.componentScanBeanNameGenerator, registry);
Set<BeanDefinitionHolder> candidates = new LinkedHashSet<BeanDefinitionHolder>(configCandidates);
Set<ConfigurationClass> alreadyParsed = new HashSet<ConfigurationClass>(configCandidates.size());
do {
parser.parse(candidates);
parser.validate();
Set<ConfigurationClass> configClasses = new LinkedHashSet<ConfigurationClass>(parser.getConfigurationClasses());
configClasses.removeAll(alreadyParsed);
// Read the model and create bean definitions based on its content
if (this.reader == null) {
this.reader = new ConfigurationClassBeanDefinitionReader(
registry, this.sourceExtractor, this.resourceLoader, this.environment,
this.importBeanNameGenerator, parser.getImportRegistry());
}
this.reader.loadBeanDefinitions(configClasses);
alreadyParsed.addAll(configClasses);
candidates.clear();
if (registry.getBeanDefinitionCount() > candidateNames.length) {
String[] newCandidateNames = registry.getBeanDefinitionNames();
Set<String> oldCandidateNames = new HashSet<String>(Arrays.asList(candidateNames));
Set<String> alreadyParsedClasses = new HashSet<String>();
for (ConfigurationClass configurationClass : alreadyParsed) {
alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
}
for (String candidateName : newCandidateNames) {
if (!oldCandidateNames.contains(candidateName)) {
BeanDefinition bd = registry.getBeanDefinition(candidateName);
if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&
!alreadyParsedClasses.contains(bd.getBeanClassName())) {
candidates.add(new BeanDefinitionHolder(bd, candidateName));
}
}
}
candidateNames = newCandidateNames;
}
}
while (!candidates.isEmpty());
// .... Omit some codes
} In fact, the comments here are very clear. We can clearly see that the key class for parsing each @ ConfigurationClass is ConfigurationClassParser. Let's continue to take a look at the parse method of this class:
public void parse(Set<BeanDefinitionHolder> configCandidates) {
this.deferredImportSelectors = new LinkedList<DeferredImportSelectorHolder>();
for (BeanDefinitionHolder holder : configCandidates) {
BeanDefinition bd = holder.getBeanDefinition();
try {
if (bd instanceof AnnotatedBeanDefinition) {
parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
}
else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) {
parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
}
else {
parse(bd.getBeanClassName(), holder.getBeanName());
}
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(
"Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
}
}
processDeferredImportSelectors();
}Here, please pay attention to the processDeferredImportSelectors method in the last sentence. DeferredImportSelector will be processed here, so we will be combined with AutoConfigurationSelectImporter:
private void processDeferredImportSelectors() {
List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
this.deferredImportSelectors = null;
Collections.sort(deferredImports, DEFERRED_IMPORT_COMPARATOR);
for (DeferredImportSelectorHolder deferredImport : deferredImports) {
ConfigurationClass configClass = deferredImport.getConfigurationClass();
try {
String[] imports = deferredImport.getImportSelector().selectImports(configClass.getMetadata());
processImports(configClass, asSourceClass(configClass), asSourceClasses(imports), false);
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(
"Failed to process import candidates for configuration class [" +
configClass.getMetadata().getClassName() + "]", ex);
}
}
}Please pay attention to this Code: String [] imports = deferredImport getImportSelector(). selectImports(configClass.getMetadata()); Here, the type of deferredImport is DeferredImportSelectorHolder:
private static class DeferredImportSelectorHolder {
private final ConfigurationClass configurationClass;
private final DeferredImportSelector importSelector;
public DeferredImportSelectorHolder(ConfigurationClass configClass, DeferredImportSelector selector) {
this.configurationClass = configClass;
this.importSelector = selector;
}
public ConfigurationClass getConfigurationClass() {
return this.configurationClass;
}
public DeferredImportSelector getImportSelector() {
return this.importSelector;
}
}A reference to DeferredImportSelector is held in this inner class, and all operations of automatic assembly will be performed so far
3, Summary
1) automatic assembly or spring factoriesloader is used to load meta-inf / spring All configured enableautoconflation in the factoires file will go through operations such as exclude and filter to finally determine the class to be assembled
2) the core of @ Configuration processing is ConfigurationClassPostProcessor, which implements beanfactoryprocessor. Therefore, when AbstractApplicationContext executes invokebeanfactoryprocessors (beanfactory) method in refresh method, automatic assembly will be executed