文章目錄
引言
衆所周知,Spring Boot是通過各種場景啓動器starter來幫助我們自動引入相關Jar包依賴並對其版本進行控制的,Spring Boot完成此項功能的主要原理就是自動配置。那麼它是怎麼實現的呢?
環境準備
maven :Apache Maven 3.5.4
IDE :IntelliJ IDEA
Spring Boot:2.1.5.RELEASE 和 1.5.9.RELEASE
注意:對於Spring Boot 1.5.x與2.x在某些方面還是有一些不同,因此對兩個版本都進行分析。但是這兩個版本的自動配置原理是一樣的,只是具體實現不同而已。
項目結構:
pom:
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.frank.springboot_demo1</groupId>
<artifactId>springboot_demo1</artifactId>
<version>1.0-SNAPSHOT</version>
<!-- Inherit defaults from Spring Boot -->
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.1.5.RELEASE</version>
</parent>
<!-- Add typical dependencies for a web application -->
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
</dependency>
</dependencies>
<!-- Package as an executable jar -->
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>
</project>
Spring Boot 2.x 原理分析
@SpringBootApplication:
首先我們先看一下Spring Boot 主程序:
/**
* SpringBoot 主程序類,說明該項目是一個Spring Boot應用。
*/
@SpringBootApplication
public class SpringBootMainApplication {
public static void main(String[] args) {
SpringApplication.run(SpringBootMainApplication.class,args);
}
}
在這裏我們可以看到註解:
@SpringBootApplication:此註解是用於標註在Spring Boot主程序類上,表明該類是Spring Boot執行入口,同時也是表明該項目是一個Spring Boot項目的標誌之一。
點開此註解:
@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 {
我們可以看到該註解同時標註幾個註解,我們只對下面兩個註解進行分析:
@SpringBootConfiguration
@SpringBootConfiguration:Spring Boot配置類註解,表明該類是Spring Boot的配置類,將會把被該註解標註的類註冊到Spring IOC容器中。
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
通過其源碼我們可以知道,其本質上是利用了Spring的@Configuration註解。那麼我們來說一下@Configuration註解:
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
@Configuration:Spring配置類註解,相當於Spring.xml中的標籤,其標註着@Component註解,說明其本質上是就是Spring容器中的一個組件。
@EnableAutoConfiguration
@EnableAutoConfiguration:字面翻譯就是開啓自動配置功能註解。
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
在這裏我們可以看到兩個重要註解:
@AutoConfigurationPackage
@AutoConfigurationPackage:自動開啓包配置。
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {
我們可以發現該註解是通過@Import註解引入AutoConfigurationPackages.Registrar 類。
AutoConfigurationPackages.Registrar:
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
@Override
public void registerBeanDefinitions(AnnotationMetadata metadata,
BeanDefinitionRegistry registry) {
//new PackageImport(metadata).getPackageName()改參數查看發現是@SpringBootConfiguration標註類所在包。
register(registry, new PackageImport(metadata).getPackageName());
}
@Override
public Set<Object> determineImports(AnnotationMetadata metadata) {
return Collections.singleton(new PackageImport(metadata));
}
}
AutoConfigurationPackages.java:
public static void register(BeanDefinitionRegistry registry, String... packageNames) {
if (registry.containsBeanDefinition(BEAN)) {
BeanDefinition beanDefinition = registry.getBeanDefinition(BEAN);
ConstructorArgumentValues constructorArguments = beanDefinition
.getConstructorArgumentValues();
constructorArguments.addIndexedArgumentValue(0,
addBasePackages(constructorArguments, packageNames));
}
else {
GenericBeanDefinition beanDefinition = new GenericBeanDefinition();
beanDefinition.setBeanClass(BasePackages.class);
beanDefinition.getConstructorArgumentValues().addIndexedArgumentValue(0,
packageNames);
beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
registry.registerBeanDefinition(BEAN, beanDefinition);
}
}
通過源碼我們可以發現,register方法是自動掃描獲取@SpringBootApplication所標註類的所在包以及其子包中的所有組件掃描進Spring IOC容器中。
@Import(AutoConfigurationImportSelector.class)
AutoConfigurationImportSelector:自動配置選擇器,這就是我們自動配置的核心類,通過該類,Spring Boot確定將會把哪些組件加入到Spring容器中。其核心方法是selectImports。
AutoConfigurationImportSelector.java
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
.loadMetadata(this.beanClassLoader);
//獲取所有自動配置組件完整名稱。
AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
autoConfigurationMetadata, annotationMetadata);
return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}
//.......
protected AutoConfigurationEntry getAutoConfigurationEntry(
AutoConfigurationMetadata autoConfigurationMetadata,
AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return EMPTY_ENTRY;
}
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 new AutoConfigurationEntry(configurations, exclusions);
}
//........
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.java
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 {
// 初次進入此處獲取META-INF/spring.factories或META-INF/spring.factories重的所有要加載的類名並放入緩存中。
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()) {
String factoryClassName = ((String) entry.getKey()).trim();
for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
result.add(factoryClassName, factoryName.trim());
}
}
}
cache.put(classLoader, result);
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}
此處是與Spring Boot 1.5中不同,暫時按下不表,稍後再講。
通過查找我們找到了該文件:
再次我們截取部分,我們可以發現該文件中是很多的Spring Boot功能的完成包名並以逗號分隔。
# PropertySource Loaders
org.springframework.boot.env.PropertySourceLoader=\
org.springframework.boot.env.PropertiesPropertySourceLoader,\
org.springframework.boot.env.YamlPropertySourceLoader
# Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener
# Error Reporters
org.springframework.boot.SpringBootExceptionReporter=\
org.springframework.boot.diagnostics.FailureAnalyzers
# Application Context Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer
Spring Boot 1.5.x 分析
對於Spring Boot 1.5.x,其相比於2.x來說,其原理是相同的,都是加載META-INF/spring.factories或META-INF/spring.factories中的類,但是具體的實現有所差別。
EnableAutoConfigurationImportSelector.java
public class EnableAutoConfigurationImportSelector
extends AutoConfigurationImportSelector {
@Override
protected boolean isEnabled(AnnotationMetadata metadata) {
if (getClass().equals(EnableAutoConfigurationImportSelector.class)) {
return getEnvironment().getProperty(
EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
true);
}
return true;
}
AutoConfigurationImportSelector.java
@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);
}
}
SpringFactoriesLoader.java
public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
try {
Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
List<String> result = new ArrayList<String>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
String factoryClassNames = properties.getProperty(factoryClassName);
result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
}
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
"] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}
通過以上源碼我們可以發現:
1.相關方法的實現有所差別。
2.Spring Boot 2.x相較與1.5.x使用了緩存。
3.二者方法的調用與運行時期也有區別,有時間我會繼續補充。
總結
通過以上分析,我們可以知道Spring Boot主要是通過以上註解,自動加載META-INF/spring.factories或META-INF/spring.factories中羅列的相關類到Spring容器中。當然並不是這裏所有的類會被加載,其會通過一些相關注解進行過濾,我們以後再講。