引言
前文说过，Spring Boot最大的优势在于自动配置，我们只需要引入一个个第三方提供的starter就可以快速集成第三方功能，既然starter这么方便，我们自己应该也可以写一个。
快速写一个starter
- 新建一个maven项目
<groupId>com.walm</groupId>
<artifactId>boot-test-starter</artifactId>
<version>1.0-SNAPSHOT</version>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-autoconfigure</artifactId>
<version>2.0.6.RELEASE</version>
</dependency>
</dependencies>
- 新建一个config类
@Configuration
public class HelloWorldConfig {
@Bean
public HelloWorld HelloWord() {
return new HelloWorld();
}
}
- 新建一个提供服务的service类
public class HelloWorld {
public void sayHello() {
System.out.println("hello world !!!!");
}
}
- 最重要的是一步，需要让spring能够扫描到我们的 config HelloWorldConfig 类
- 在resource 文件夹下新建一个 META-INF 文件夹
- 新建一个 spring.factories 文件
在spring.factories文件中配置我们的写的HelloWorldConfig类
org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.walm.boottest.HelloWorldConfig
这样一个简单的 springboot starter 就完成了，最后本地安装 install我们的starter jar包就可以在其他项目中引用了
- 引用starter，只需要在项目引用 该jar包即可
<dependency>
<groupId>com.walm</groupId>
<artifactId>boot-test-starter</artifactId>
<version>1.0-SNAPSHOT</version>
</dependency>
下面我们简单分析一下 stater的原理
starter 自动配置原理
首先自动配置的原理关键一定是在 spring.factories 文件是如何被加载到的这个点上去探究
需要从spring boot的启动过程一步一步的去看了
首先，我们启动一个spring-boot项目，肯定会有一个配置注解 SpringBootApplication 那么这个注解到底起什么作用呢？
- SpringBootApplication.class
@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.
* <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 {};
}
这里着重看 EnableAutoConfiguration 这个注解
@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 {};
}
可以看到 @Import(AutoConfigurationImportSelector.class)这个注解，这里就是我们自动配置的入口
注意，解析Import注解其实是通过 BeanFactoryPostProcessor 接口来扩展的
下面我们来看下AutoConfigurationImportSelector.class 类的实现，AutoConfigurationImportSelector 实现了ImportSelector接口
- ImportSelector
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);
}
在解析import注解的时候会调用指定ImportSelector 接口实现 的 selectImports方法。下面我们看下AutoConfigurationImportSelector的 selectImports方法实现
- selectImports
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
.loadMetadata(this.beanClassLoader);
AnnotationAttributes attributes = getAttributes(annotationMetadata);
// 获取 META-INF/spring.factories 文件中的配置信息
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);
// 触发事件
fireAutoConfigurationImportEvents(configurations, exclusions);
return StringUtils.toStringArray(configurations);
}
总结
写一个starter还是非常简单的，核心就是要让springboot自动扫描到我们的 配置类，核心逻辑其实就是通过扫描我们的自定义的META-INF/spring.factories文件来进行自动配置
Spring 系列
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