SpringBoot自动化配置之三：深入SpringBoot:自定义EnableAutoConfiguration

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前言

上面几篇文章介绍了SpringFramework的一些原理，这里开始介绍一下SpringBoot，并通过自定义一些功能来介绍SpringBoot的原理。SpringBoot在SpringFramework的基础上集成了Web容器，日志等功能，可以快速的实现Web服务。
先看SpringBoot必要的依赖。

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-autoconfigure</artifactId>
    </dependency>

spring-boot 包含了 SpringBoot 的核心实现类。
spring-boot-autoconfigure 则是根据当前classpath的类来注入不同的实现类。

EnableAutoConfiguration

EnableAutoConfiguration源码：

package org.springframework.boot.autoconfigure;
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface 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 {};

}

观察EnableAutoConfiguration可以发现，这里Import了EnableAutoConfigurationImportSelector。
Import主要是配合Configuration来使用的，用来导出更多的Configuration类，ConfigurationClassPostProcessor会读取Import的内容来实现具体的逻辑。

EnableAutoConfigurationImportSelector分析：

EnableAutoConfigurationImportSelector实现了DeferredImportSelector接口,并实现了selectImports方法，用来导出指定配置Configuration类（在META-INF/spring.factories配置）。

@Order(Ordered.LOWEST_PRECEDENCE - 1)
public class EnableAutoConfigurationImportSelector implements DeferredImportSelector,
        BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware {

    private ConfigurableListableBeanFactory beanFactory;

    private Environment environment;

    private ClassLoader beanClassLoader;

    private ResourceLoader resourceLoader;

    @Override
    public String[] selectImports(AnnotationMetadata metadata) {
        try {
            AnnotationAttributes attributes = getAttributes(metadata);
            List<String> configurations = getCandidateConfigurations(metadata,
                    attributes);
            configurations = removeDuplicates(configurations);
            Set<String> exclusions = getExclusions(metadata, attributes);
            configurations.removeAll(exclusions);
            configurations = sort(configurations);
            recordWithConditionEvaluationReport(configurations, exclusions);
            return configurations.toArray(new String[configurations.size()]);
        }
        catch (IOException ex) {
            throw new IllegalStateException(ex);
        }
    }
...
}

导出的类是通过SpringFactoriesLoader.loadFactoryNames()读取了ClassPath下面的META-INF/spring.factories文件。

    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
            AnnotationAttributes attributes) {
        return SpringFactoriesLoader.loadFactoryNames(
                getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
    }

META-INF/spring.factories 文件，这个文件内容大致如下：

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
...
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration
...

可以看继续往里看源码：

package org.springframework.core.io.support;
...
public abstract class SpringFactoriesLoader {

    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
...
    public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        try {
            Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            List<String> result = new ArrayList<String>();
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
                String factoryClassNames = properties.getProperty(factoryClassName);
                result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
            }
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                    "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }
...
}

EmbeddedServletContainerAutoConfiguration 是实现web服务的主要的配置类，EmbeddedServletContainerAutoConfiguration这个类会根据当前存在的类的信息注入必要的EmbeddedServletContainerFactory类。

package org.springframework.boot.autoconfigure.web;

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration
@ConditionalOnWebApplication
@Import(EmbeddedServletContainerCustomizerBeanPostProcessorRegistrar.class)
public class EmbeddedServletContainerAutoConfiguration {

    /**
     * Nested configuration for if Tomcat is being used.
     */
    @Configuration
    @ConditionalOnClass({ Servlet.class, Tomcat.class })
    @ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
    public static class EmbeddedTomcat {

        @Bean
        public TomcatEmbeddedServletContainerFactory tomcatEmbeddedServletContainerFactory() {
            return new TomcatEmbeddedServletContainerFactory();
        }

    }

    /**
     * Nested configuration if Jetty is being used.
     */
    @Configuration
    @ConditionalOnClass({ Servlet.class, Server.class, Loader.class,
            WebAppContext.class })
    @ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
    public static class EmbeddedJetty {

        @Bean
        public JettyEmbeddedServletContainerFactory jettyEmbeddedServletContainerFactory() {
            return new JettyEmbeddedServletContainerFactory();
        }

    }

    /**
     * Nested configuration if Undertow is being used.
     */
    @Configuration
    @ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })
    @ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
    public static class EmbeddedUndertow {

        @Bean
        public UndertowEmbeddedServletContainerFactory undertowEmbeddedServletContainerFactory() {
            return new UndertowEmbeddedServletContainerFactory();
        }

    }
//...
}

spring-boot-starter-tomcat引入了tomcat的依赖，所以EmbeddedServletContainerAutoConfiguration发现存在Tomcat.class就会注入TomcatEmbeddedServletContainerFactory来内置web容器。

示例1：

先看一个基本的例子，这个例子实现了一个简单的web服务。

引入必要的依赖，开启web服务需要引入对应的EmbedWeb容器。

<dependency>
     <groupId>org.springframework.boot</groupId>
     <artifactId>spring-boot-starter-tomcat</artifactId>
 </dependency>

程序入口，这里使用了 EnableAutoConfiguration 注解。

package com.dxz.autoconfig;

import java.util.HashMap;
import java.util.Map;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.context.annotation.Configuration;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

@Configuration
@EnableAutoConfiguration
public class Bootstrap {
    public static void main(String[] args) {
        SpringApplication springApplication = new SpringApplication(Bootstrap.class);
        springApplication.run(args);
    }

    @Controller
    public static class MyController {
        @RequestMapping
        @ResponseBody
        public Map index() {
            Map<String, String> map = new HashMap<String, String>();
            map.put("hello", "world");
            return map;
        }
    }
}

结果：

三、自定义EnableAutoConfiguration示例

1、自定义 EnableAutoConfiguration ，这里 Import 了 MyEnableAutoConfigurationImport 。

package com.dxz.autoconfig;

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.AutoConfigurationPackage;
import org.springframework.context.annotation.Import;

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(MyEnableAutoConfigurationImport.class)
public @interface MyEnableAutoConfiguration {
}

2、自定义 EnableAutoConfigurationImport ，注入了 ClassLoader ，并调用 SpringFactoriesLoader.loadFactoryNames() 方法，导出 Configuration 的类。

package com.dxz.autoconfig;

import java.util.List;

import org.springframework.beans.factory.BeanClassLoaderAware;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.context.annotation.DeferredImportSelector;
import org.springframework.core.io.support.SpringFactoriesLoader;
import org.springframework.core.type.AnnotationMetadata;

public class MyEnableAutoConfigurationImport implements DeferredImportSelector, BeanClassLoaderAware {
    private ClassLoader classLoader;

    public void setBeanClassLoader(ClassLoader classLoader) {
        this.classLoader = classLoader;
    }

    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        List<String> beanNames = SpringFactoriesLoader.loadFactoryNames(EnableAutoConfiguration.class, classLoader);
        return beanNames.toArray(new String[beanNames.size()]);
    }
}

3、入口类，这里使用了 MyEnableAutoConfiguration 注解。

package com.dxz.autoconfig;

import java.util.HashMap;
import java.util.Map;

import org.springframework.boot.SpringApplication;
import org.springframework.context.annotation.Configuration;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

@Configuration
@MyEnableAutoConfiguration
public class CustomizeEnableAutoConfigure {
    public static void main(String[] args) {
        SpringApplication application = new SpringApplication(CustomizeEnableAutoConfigure.class);
        application.run(args);
    }

    @Controller
    public static class MyController {
        @RequestMapping
        @ResponseBody
        public Map index() {
            Map<String, String> map = new HashMap<String, String>();
            map.put("hello", "world2");
            return map;
        }
    }
}

结果：