SpringBoot自动配置原理学习

介绍

构建Springboot项目时我们会创建一个启动类

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

1. 在之前使用Spring框架时,我们一般会创建web.xml和spring-context.xml等文件配置组件扫描、调度器、视图解析器等。

2. 而在SpringBoot中则简单了很多,这里就有自动配置发挥作用。如默认用的内嵌式容器是 Tomcat ,端口默认设置为 8080

正式进入流程解析

首先看@SpringBootApplication这个注解

@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) })
@ConfigurationPropertiesScan
public @interface SpringBootApplication {

}
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};
    String[] excludeName() default {};

}

@EnableAutoConfiguration的作用是开启自动配置

核心是@EnableAutoConfiguration注解

这就是Spring Boot自动配置实现的核心入口,重头戏是@Import注解,这个注解导入了AutoConfigurationImportSelector类,利用AutoConfigurationImportSelector(自动配置导入选择器)将所有符合自动装配条件的bean注入到IOC容器中

查看AutoConfigurationImportSelector类代码,重头戏是selectImports方法

selectImports何时被执行

Springboot应用启动过程中使用ConfigurationClassParser分析配置类时,发现注解中存在@Import(ImportSelector)的情况。

就会创建一个相应的ImportSelector对象, 并调用其方法 public String[] selectImports(AnnotationMetadata annotationMetadata)

selectImports()方法流程

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
        ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {

    // 省略。。

    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        // 1 判断是否开启自动配置,为false直接返回空字符数组
    
if (!isEnabled(annotationMetadata)) { return NO_IMPORTS; }
     // 2 loadMetadata()去加载Spring预先定义的自动配置的依赖信息,
下面会具体说明
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader .loadMetadata(this.beanClassLoader);
     // 3 该方法返回的就是配置项信息,下面会具体说明 AutoConfigurationEntry autoConfigurationEntry
= getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata); return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations()); } // 省略。。。 }

2. loadMetadata()去加载Spring预先定义的自动配置的依赖信息

 1 static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader, String path) {
 2         try {
 3             Enumeration<URL> urls = (classLoader != null) ? classLoader.getResources(path)
 4                     : ClassLoader.getSystemResources(path);
 5             Properties properties = new Properties();
 6             while (urls.hasMoreElements()) {
 7                 properties.putAll(PropertiesLoaderUtils.loadProperties(new UrlResource(urls.nextElement())));
 8             }
 9             return loadMetadata(properties);
10         }
11         catch (IOException ex) {
12             throw new IllegalArgumentException("Unable to load @ConditionalOnClass location [" + path + "]", ex);
13         }
14     }

path的地址是

protected static final String PATH = "META-INF/spring-autoconfigure-metadata.properties";

里面的自动配置文件(部分)

org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration.AutoConfigureBefore=org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration.ConditionalOnClass=javax.sql.DataSource,org.springframework.batch.core.launch.JobLauncher
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration.AutoConfigureAfter=org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration

  2.1 根据path获取文件的URL对象

  2.2 遍历URL节点,将spring-autoconfigure-metadata.properties中的依赖信息加载到properties对象中

  2.3 调用loadMetadata()方法把properties存储到PropertiesAutoConfigurationMetadata类中

3. 在这里面会调用一个方法getAutoConfigurationEntry(),该方法返回的就是配置项信息,进入这个方法

 1 protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
 2             AnnotationMetadata annotationMetadata) {
           // 3.1
3
if (!isEnabled(annotationMetadata)) { 4 return EMPTY_ENTRY; 5 } // 3.2
6
AnnotationAttributes attributes = getAttributes(annotationMetadata);       // 3.3
7
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes); // 3.4
8
configurations = removeDuplicates(configurations); // 3.5
9
Set<String> exclusions = getExclusions(annotationMetadata, attributes); 10 checkExcludedClasses(configurations, exclusions); 11 configurations.removeAll(exclusions); 12 configurations = filter(configurations, autoConfigurationMetadata);        // 3.6
13 fireAutoConfigurationImportEvents(configurations, exclusions);        // 3.7
14 return new AutoConfigurationEntry(configurations, exclusions); 15 }

  3.1 判断是否开启自动配置

  3.2 getAttributes()方法获取@SpringBootApplication(主要是为了获取@EnableAutoConfiguation)注解上的属性

  3.3 加载META-INF/spring.factories文件, 这个文件配置了具有哪些自动配置类,文件内容如下 ,具体代码如下    

getCandidateConfigurations方法如下

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;
}

    3.3.1 getSpringFactoriesLoaderFactoryClass方法返回EnableAutoConfiguration.class

    3.3.2 SpringFactoriesLoader.loadFactoryNames会加载META-INF/spring.factories中EnableAutoConfiguration的值中定义的jar包,并将其封装到一个List中返回

  3.4 removeDuplicates() 用LinkedHashSet去重,在包装成ArrayList

  3.5 getExclusions()、checkExcludedClasses()、removeAll()、filter() 过滤注解中要排除的自动配置

  3.6 fireAutoConfigurationImportEvents() 将自动配置导入监听

  3.7 返回配置的AutoConfigurationEntry包装类

自动配置原理(简化版)

通过 @EnableAutoConfiguration 核心注解初始化,并扫描 ClassPath 目录中自动配置类。根据项目中需要添加的默认配置,如springMVC,就按一定规则获取默认配置并自动初始化所需要的 Bean。

1. 核心是@EnableAutoConfiguration
2. 里面会通过@Import注解导入(AutoConfigurationImportSelector.class)类
3. 调用importSelect方法,进行自动配置

  3.1 判断是否开启了自动配置
  3.2 getAutoConfigurationEntry方法会使用
    SpringFactoriesLoader.loadFactoryNames方法加载classpath中spring.factories文件中EnableAutoConfiguration的配置类
  3.3 经过去重,排除后会把这些配置导入监听

重点关注

@EnableAutoConfiguration

@Import

AutoConfigurationImportSelector类

selectImports方法

SpringFactoriesLoader.loadFactoryNames() 核心方法读取 ClassPath 目录下面的 META-INF/spring.factories 文件。

spring.factories 文件中存放springBoot自动配置类, 如Servlet、jpa

【示例】(JDBC)的DataSourceAutoConfiguration配置

在spring.factories中org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguratio

作用是:SpringBoot 自动配置DataSource

// 表示这是一个配置类
@Configuration(proxyBeanMethods = false)
// 判断当前项目classpath有没有这两个类 DataSource EmbeddedDatabaseType
@ConditionalOnClass({ DataSource.class, EmbeddedDatabaseType.class })
// 将配置文件中对应的值和 DataSourceProperties 绑定起来,并把DataSourceProperties的属性值放到ioc容器中
@EnableConfigurationProperties(DataSourceProperties.class)
// 导入数据源注册类、数据源初始化类
@Import({ DataSourcePoolMetadataProvidersConfiguration.class, DataSourceInitializationConfiguration.class })
public class DataSourceAutoConfiguration {
  // 省略。。
}

查看DataSourceProperties属性,提供的这些属性,就可以获取到在application.yml中提供的值来设置数据源了

@ConfigurationProperties(prefix = "spring.datasource")
public class DataSourceProperties implements BeanClassLoaderAware, InitializingBean {

    private ClassLoader classLoader;

    /**
     * Name of the datasource. Default to "testdb" when using an embedded database.
     */
    private String name;

    /**
     * Whether to generate a random datasource name.
     */
    private boolean generateUniqueName;

    /**
     * Fully qualified name of the connection pool implementation to use. By default, it
     * is auto-detected from the classpath.
     */
    private Class<? extends DataSource> type;

    /**
     * Fully qualified name of the JDBC driver. Auto-detected based on the URL by default.
     */
    private String driverClassName;

    /**
     * JDBC URL of the database.
     */
    private String url;

    /**
     * Login username of the database.
     */
    private String username;

    /**
     * Login password of the database.
     */
    private String password;

    /**
     * JNDI location of the datasource. Class, url, username & password are ignored when
     * set.
     */
    private String jndiName;

        。。。省略
}

 

总结

Spring Boot 内部提供了很多自动化配置的类, 配置类会更加classpath下是否又相关依赖类,来判断是否要配置字节,所以当我们在pom中引入了相关依赖,就可以自动配置了

 

======  【多学一点,for Better】======

 

posted @ 2019-10-26 14:59  Other+  阅读(1942)  评论(0编辑  收藏