启动类
调用SpringApplication.run方法启动,启动类使用@SpringBootApplication注解
@SpringBootApplication
public class StringBootDemoApplication {
public static void main(String[] args) {
SpringApplication.run(StringBootDemoApplication.class, args);
}
}
@SpringBootApplication注解
@Target(ElementType.TYPE) // 注解的适用范围，其中TYPE用于描述类、接口（包括包注解类型）或enum声明
@Retention(RetentionPolicy.RUNTIME) // 注解的生命周期，保留到class文件中（三个生命周期中的运行时期）
@Documented // 表明这个注解应该被javadoc记录
@Inherited // 子类可以继承该注解
@SpringBootConfiguration // 继承了Configuration，表示当前是注解类
@EnableAutoConfiguration // 开启springboot的注解功能，springboot的四大神器之一，其借助@import的帮助
@ComponentScan(excludeFilters = { // 扫描路径设置
@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
...
}
@Configuration注解
不同于原来springmvc中用xml配置文件的形式，在springboot中我们大多用配置类来解决配置问题，能够被@Configuration注解加载的bean：
1. xml配置文件的形式配置bean
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd"
default-lazy-init="true">
<!--bean定义-->
</beans>
2. java configuration的配置形式配置bean
@Configuration
public class MockConfiguration{
//bean定义
}
任何一个标注了@Bean的方法，其返回值将作为一个bean定义注册到Spring的IoC容器，方法名将默认成该bean定义的id。
@ComponentScan注解
ComponentScan的功能其实就是自动扫描并加载符合条件的组件（比如@Component和@Repository等以及<context:component-scan>标签标注的xml文件里的bean）或者bean定义，将这些bean定义加载到IoC容器中
可以通过basePackages等属性来细粒度地定制@ComponentScan自动扫描的范围，如果不指定，则默认Spring框架实现会从声明@ComponentScan所在类的package进行扫描。
所以SpringBoot的启动类最好是放在root package下，因为默认可以不指定basePackages
/**
* Configures component scanning directives for use with @{@link Configuration} classes.
* Provides support parallel with Spring XML's {@code <context:component-scan>} element.
*
* <p>Either {@link #basePackageClasses} or {@link #basePackages} (or its alias
* {@link #value}) may be specified to define specific packages to scan. If specific
* packages are not defined, scanning will occur from the package of the
* class that declares this annotation.
*
* <p>Note that the {@code <context:component-scan>} element has an
* {@code annotation-config} attribute; however, this annotation does not. This is because
* in almost all cases when using {@code @ComponentScan}, default annotation config
* processing (e.g. processing {@code @Autowired} and friends) is assumed. Furthermore,
* when using {@link AnnotationConfigApplicationContext}, annotation config processors are
* always registered, meaning that any attempt to disable them at the
* {@code @ComponentScan} level would be ignored.
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Repeatable(ComponentScans.class)
public @interface ComponentScan {
@AliasFor("basePackages")
String[] value() default {};
@AliasFor("value")
String[] basePackages() default {};
Class<?>[] basePackageClasses() default {};
Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;
Class<? extends ScopeMetadataResolver> scopeResolver() default AnnotationScopeMetadataResolver.class;
ScopedProxyMode scopedProxy() default ScopedProxyMode.DEFAULT;
String resourcePattern() default ClassPathScanningCandidateComponentProvider.DEFAULT_RESOURCE_PATTERN;
boolean useDefaultFilters() default true;
Filter[] includeFilters() default {};
Filter[] excludeFilters() default {};
boolean lazyInit() default false;
@Retention(RetentionPolicy.RUNTIME)
@Target({})
@interface Filter {
FilterType type() default FilterType.ANNOTATION;
@AliasFor("classes")
Class<?>[] value() default {};
@AliasFor("value")
Class<?>[] classes() default {};
String[] pattern() default {};
}
}
@EnableAutoConfiguration
此注解顾名思义是可以自动配置，所以应该是springboot中最为重要的注解。注释信息里面对这个注解的描述比较详细：自动加载beans
在spring框架中就提供了各种以@Enable开头的注解，例如： @EnableScheduling、@EnableCaching、@EnableMBeanExport等； @EnableAutoConfiguration的理念和做事方式其实一脉相承简单概括一下就是，借助@Import的支持，收集和注册特定场景相关的bean定义。
- @EnableScheduling是通过@Import将Spring调度框架相关的bean定义都加载到IoC容器【定时任务、时间调度任务】
- @EnableMBeanExport是通过@Import将JMX相关的bean定义加载到IoC容器【监控JVM运行时状态】
- @EnableAutoConfiguration也是借助@Import的帮助，将所有符合自动配置条件的bean定义加载到IoC容器。
@EnableAutoConfiguration作为一个复合Annotation,其自身定义关键信息如下：
/**
* 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 TomcatServletWebServerFactory} (unless you have defined your own
* {@link ServletWebServerFactory} bean).
* <p>
* When using {@link SpringBootApplication @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 @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
* @since 1.0.0
* @see ConditionalOnBean
* @see ConditionalOnMissingBean
* @see ConditionalOnClass
* @see AutoConfigureAfter
* @see SpringBootApplication
*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
/**
* Environment property that can be used to override when auto-configuration is
* enabled.
*/
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 {};
}
这个注解上还使用了两个注解：
- @AutoConfigurationPackage
- @Import(AutoConfigurationImportSelector.class)
@AutoConfigurationPackage和@Import(AutoConfigurationImportSelector.class)都使用了@Import注解
@AutoConfigurationPackage
源代码：
/**
* Registers packages with {@link AutoConfigurationPackages}. When no {@link #basePackages
* base packages} or {@link #basePackageClasses base package classes} are specified, the
* package of the annotated class is registered.
*
* @author Phillip Webb
* @since 1.3.0
* @see AutoConfigurationPackages
*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {
/**
* Base packages that should be registered with {@link AutoConfigurationPackages}.
* <p>
* Use {@link #basePackageClasses} for a type-safe alternative to String-based package
* names.
* @return the back package names
* @since 2.3.0
*/
String[] basePackages() default {};
/**
* Type-safe alternative to {@link #basePackages} for specifying the packages to be
* registered with {@link AutoConfigurationPackages}.
* <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 the base package classes
* @since 2.3.0
*/
Class<?>[] basePackageClasses() default {};
}
通过这个注解的源代码可以看出，这个注解又使用了@Import(AutoConfigurationPackages.Registrar.class)注解，打开AutoConfigurationPackages.Registrar
/**
* {@link ImportBeanDefinitionRegistrar} to store the base package from the importing
* configuration.
*/
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
//注册org.springframework.boot.autoconfigure.AutoConfigurationPackages的BeanDefinition
//实际上就是注册当前启动类的根package
@Override
public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
register(registry, new PackageImports(metadata).getPackageNames().toArray(new String[0]));
}
@Override
public Set<Object> determineImports(AnnotationMetadata metadata) {
return Collections.singleton(new PackageImports(metadata));
}
}
Import(AutoConfigurationImportSelector.class)
1）AutoConfigurationImportSelector类
AutoConfigurationImportSelector 实现了 DeferredImportSelector 从 ImportSelector继承的方法：selectImports。
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@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);
return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}
protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return EMPTY_ENTRY;
}
AnnotationAttributes attributes = getAttributes(annotationMetadata);
//加载各个组件jar下的"META-INF/spring.factories"
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
configurations = removeDuplicates(configurations);
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
configurations = getConfigurationClassFilter().filter(configurations);
fireAutoConfigurationImportEvents(configurations, exclusions);
return new AutoConfigurationEntry(configurations, exclusions);
}
Return the auto-configuration class names that should be considered选择要进行加载的全路径类名，默认加载spring.factories文件下org.springframework.boot.autoconfigure.EnableAutoConfiguration=\下面的所有类，META-INF/spring.factories长这样：
/**
* 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) {
//就是默认即将要选择加载的全类名集合
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;
}
代码中的configurations其实就是这些全类名集合，可以在这里打上断点以调试模式启动，可以看到里面的内容就是spring.factories里面的内容
该方法在springboot启动流程——bean实例化前被执行，返回要实例化的类信息列表；
实际上，除了org.springframework.boot.autoconfigure.EnableAutoConfiguration，spring.factories里面还定义了很多其他bean。
所有的这些bean都是通过Spring工厂加载器SpringFactoriesLoader进行加载。
**2)Spring工厂加载器SpringFactoriesLoader
它更多是提供一种配置查找的功能支持，即根据@EnableAutoConfiguration的完整类名org.springframework.boot.autoconfigure.EnableAutoConfiguration作为查找的Key,获取对应的一组@Configuration类
通过下面的这个方法来获取要再加载的类:
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
ClassLoader classLoaderToUse = classLoader;
if (classLoaderToUse == null) {
classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
}
String factoryTypeName = factoryType.getName();
//他的key就是我们的注解类名，如“EnableAutoConfiguration”，通过key从map中取出我们的目标类名集合
return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
}
//以Map<String, List<String>>形式返回spring.factories文件信息，内容如下面截图
private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
Map<String, List<String>> result = cache.get(classLoader);
if (result != null) {
return result;
}
result = new HashMap<>();
try {
Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry<?, ?> entry : properties.entrySet()) {
String factoryTypeName = ((String) entry.getKey()).trim();
String[] factoryImplementationNames =
StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
for (String factoryImplementationName : factoryImplementationNames) {
result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
.add(factoryImplementationName.trim());
}
}
}
// Replace all lists with unmodifiable lists containing unique elements
result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
.collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
cache.put(classLoader, result);
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
return result;
}
如果获取到类信息，spring可以通过类加载器将类加载到jvm中，现在我们已经通过spring-boot的starter依赖方式依赖了我们需要的组件，那么这些组件的类信息在select方法中就可以被获取到。
所以，@EnableAutoConfiguration自动配置的魔法其实就变成了：
从classpath中搜寻所有的META-INF/spring.factories配置文件，并将其中org.springframework.boot.autoconfigure.EnableAutoConfiguration对应的配置项通过反射（Java Refletion）实例化为对应的标注了@Configuration的JavaConfig形式的IoC容器配置类，然后汇总为一个并加载到IoC容器。