@SpringBootApplication 注解

一时失言乱红尘 2022-04-12 04:08 299阅读 0赞

### **@SpringBootApplication **查看源码 ###

/*
 * 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)
 	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)
 	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.
 	 * 
 	 * 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 {};
 
 }

### 重 点的注解有3个：  ###

- @SpringBootConfiguration
    - @EnableAutoConfiguration
    - @ComponentScan

### @SpringBootConfiguration ###

通过这段我们可以看出，在这个注解上面，又有一个@Configuration注解。通过上面的注释阅读我们知道：
    这个注解的作用就是声明当前类是一个配置类，然后Spring会自动扫描到添加了@Configuration的类，
    并且读取其中的配置信息。而@SpringBootConfiguration是来声明当前类是SpringBoot应用的配置类，
    项目中只能有一个。所以一般我们无需自己添加。

### @EnableAutoConfiguration ###

第二级的注解@EnableAutoConfiguration，告诉SpringBoot基于你所添加的依赖，
    去“猜测”你想要如何配置Spring。比如我们引入了spring-boot-starter-web，
    而这个启动器中帮我们添加了tomcat、SpringMVC的依赖。
    此时自动配置就知道你是要开发一个web应用，所以就帮你完成了web及SpringMVC的默认配置了！

/*
 * 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.web.embedded.tomcat.TomcatServletWebServerFactory;
 import org.springframework.boot.web.servlet.server.ServletWebServerFactory;
 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 TomcatServletWebServerFactory} (unless you have defined your own
 * {@link ServletWebServerFactory} bean).
 * 
 * When using {@link SpringBootApplication}, the auto-configuration of the context is
 * automatically enabled and adding this annotation has therefore no additional effect.
 * 
 * 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.
 * 
 * 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.
 * 
 * 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
 */
 @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 {};
 
 }

### @ComponentScan ###

配置组件扫描的指令。提供了类似与<context:component-scan>标签的作用
 
 通过basePackageClasses或者basePackages属性来指定要扫描的包。如果没有指定这些属性，
 那么将从声明这个注解的类所在的包开始，扫描包及子包

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