Spring Boot自动配置原理

Spring Boot的自动配置注解是@EnableAutoConfiguration， 从上面的@Import的类可以找到下面自动加载自动配置的映射。

/**
     * Load the fully qualified class names of factory implementations of the
     * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given
     * class loader.
     * @param factoryClass the interface or abstract class representing the factory
     * @param classLoader the ClassLoader to use for loading resources; can be
     * {@code null} to use the default
     * @throws IllegalArgumentException if an error occurs while loading factory names
     * @see #loadFactories
     */
    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 {
            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);
        }
    }

这个方法会加载类路径及所有jar包下META-INF/spring.factories配置中映射的自动配置的类。

/**
     * The location to look for factories.
     * <p>Can be present in multiple JAR files.
     */
    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

查看Spring Boot自带的自动配置的包： spring-boot-autoconfigure-1.5.6.RELEASE.jar，打开其中的META-INF/spring.factories文件会找到自动配置的映射。

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,
...

我们看看数据源自动配置的实现注解

@Configuration
@ConditionalOnClass({ DataSource.class, EmbeddedDatabaseType.class })
@EnableConfigurationProperties(DataSourceProperties.class)
@Import({ Registrar.class, DataSourcePoolMetadataProvidersConfiguration.class })
public class DataSourceAutoConfiguration {
...

@Configuration,@ConditionalOnClass就是自动配置的核心，首先它得是一个配置文件，其次根据类路径下是否有这个类去自动配置。

创建一个自定义Starter

这个自定义 Starter 就实现一个根据属性的值是否配置Bean。

1、创建自动配置类

import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
@ConditionalOnProperty(prefix = "com.hzy.starter", name = "enabled", havingValue = "true")
public class TestServiceAutoConfiguration {

    @Bean
    public TestService testService() {
        return new TestService();
    }

}

这个自动配置类很简单，就是根据是否有 com.hzy.starter.enabled=true这个参数的值再配置一个Bean

TestService示例如下：

public class TestService {

    public String getServiceName() {
        return "com.hzy.starter";
    }

}

2、允许自动配置

创建 META-INF/spring.factories 文件，添加这个允许自动配置的类。

org.springframework.boot.autoconfigure.EnableAutoConfiguration=
com.hzy.springboot.starter.config.TestServiceAutoConfiguration

3、测试这个自定义Starter

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;


@SpringBootApplication
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class);
    }

    @Bean
    public CommandLineRunner commandLineRunner(TestService testService) {
        return (args) -> {
            System.out.println(testService.getServiceName());
        };
    }

}

4、开启配置

我们知道这个自定义 Starter 中需要有 com.hzy.starter.enabled=true 这个参数的值的，所以我们在 application.xml 配置文件中添加这个配置：

com.hzy.starter.enabled=true

5、运行测试

运行 Application 类的 main 方法，最后会输出结果：com.hzy.starter。