前言:本系列文章非本人原创,转自:http://tengj.top/2017/04/24/springboot0/
正文
我们开发任何一个Spring Boot项目,都会用到如下的启动类
1 @SpringBootApplication 2 public class Application { 3 public static void main(String[] args) { 4 SpringApplication.run(Application.class, args); 5 } 6 }
从上面代码可以看出,Annotation定义(@SpringBootApplication)和类定义(SpringApplication.run)最为耀眼,所以要揭开SpringBoot的神秘面纱,我们要从这两位开始就可以了。
SpringBootApplication背后的秘密
1 package org.springframework.boot.autoconfigure; 2 3 import java.lang.annotation.Annotation; 4 import java.lang.annotation.Documented; 5 import java.lang.annotation.Inherited; 6 import java.lang.annotation.Retention; 7 import java.lang.annotation.RetentionPolicy; 8 import java.lang.annotation.Target; 9 import org.springframework.boot.SpringBootConfiguration; 10 import org.springframework.context.annotation.ComponentScan; 11 import org.springframework.core.annotation.AliasFor; 12 13 @Target({java.lang.annotation.ElementType.TYPE}) 14 @Retention(RetentionPolicy.RUNTIME) 15 @Documented 16 @Inherited 17 @SpringBootConfiguration 18 @EnableAutoConfiguration 19 @ComponentScan(excludeFilters={@org.springframework.context.annotation.ComponentScan.Filter(type=org.springframework.context.annotation.FilterType.CUSTOM, classes={org.springframework.boot.context.TypeExcludeFilter.class}), @org.springframework.context.annotation.ComponentScan.Filter(type=org.springframework.context.annotation.FilterType.CUSTOM, classes={AutoConfigurationExcludeFilter.class})}) 20 public @interface SpringBootApplication 21 { 22 @AliasFor(annotation=EnableAutoConfiguration.class) 23 Class<?>[] exclude() default {}; 24 25 @AliasFor(annotation=EnableAutoConfiguration.class) 26 String[] excludeName() default {}; 27 28 @AliasFor(annotation=ComponentScan.class, attribute="basePackages") 29 String[] scanBasePackages() default {}; 30 31 @AliasFor(annotation=ComponentScan.class, attribute="basePackageClasses") 32 Class<?>[] scanBasePackageClasses() default {}; 33 } 34 35 /* Location: C:\Users\Administrator\.m2\repository\org\springframework\boot\spring-boot-autoconfigure\2.1.8.RELEASE\spring-boot-autoconfigure-2.1.8.RELEASE.jar 36 * Qualified Name: org.springframework.boot.autoconfigure.SpringBootApplication 37 * Java Class Version: 8 (52.0) 38 * JD-Core Version: 0.7.0.1 39 */
虽然定义使用了多个Annotation进行了原信息标注,但实际上重要的只有三个Annotation:
- @Configuration(@SpringBootConfiguration点开查看发现里面还是应用了@Configuration)
- @EnableAutoConfiguration
- @ComponentScan
所以,如果我们使用如下的SpringBoot启动类,整个SpringBoot应用依然可以与之前的启动类功能对等:
1 @Configuration 2 @EnableAutoConfiguration 3 @ComponentScan 4 public class Application { 5 public static void main(String[] args) { 6 SpringApplication.run(Application.class, args); 7 } 8 }
每次写这3个比较累,所以写一个@SpringBootApplication方便点。接下来分别介绍这3个Annotation。
@Configuration
这里的@Configuration对我们来说不陌生,它就是JavaConfig形式的Spring Ioc容器的配置类使用的那个@Configuration,SpringBoot社区推荐使用基于JavaConfig的配置形式,所以,这里的启动类标注了@Configuration之后,本身其实也是一个IoC容器的配置类。
举几个简单例子回顾下,XML跟config配置方式的区别:
1、表达形式层面
基于XML配置的方式是这样:
1 <?xml version="1.0" encoding="UTF-8"?> 2 <beans xmlns="http://www.springframework.org/schema/beans" 3 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 4 xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd" 5 default-lazy-init="true"> 6 <!--bean定义--> 7 </beans>
基于JavaConfig的配置方式是这样:
1 @Configuration 2 public class MockConfiguration{ 3 //bean定义 4 }
任何一个标注了@Configuration的Java类定义都是一个JavaConfig配置类。
2、注册bean定义层面
基于XML的配置形式是这样:
1 <bean id="mockService" class="..MockServiceImpl"> 2 ... 3 </bean>
基于JavaConfig的配置形式是这样的:
1 @Configuration 2 public class MockConfiguration{ 3 @Bean 4 public MockService mockService(){ 5 return new MockServiceImpl(); 6 } 7 }
任何一个标注了@Bean的方法,其返回值将作为一个bean定义注册到Spring的IoC容器,方法名将默认成该bean定义的id。
3、表达依赖注入关系层面
为了表达bean与bean之间的依赖关系,在XML形式中一般是这样:
1 <bean id="mockService" class="..MockServiceImpl"> 2 <propery name ="dependencyService" ref="dependencyService" /> 3 </bean> 4 5 <bean id="dependencyService" class="DependencyServiceImpl></bean>
基于JavaConfig的配置形式是这样的:
1 @Configuration 2 public class MockConfiguration{ 3 @Bean 4 public MockService mockService(){ 5 return new MockServiceImpl(dependencyService()); 6 } 7 8 @Bean 9 public DependencyService dependencyService(){ 10 return new DependencyServiceImpl(); 11 } 12 }
如果一个bean的定义依赖其他bean,则直接调用对应的JavaConfig类中依赖bean的创建方法就可以了。
@ComponentScan
@ComponentScan这个注解在Spring中很重要,它对应XML配置中的元素,@ComponentScan的功能其实就是自动扫描并加载符合条件的组件(比如@Component和@Repository等)或者bean定义,最终将这些bean定义加载到IoC容器中。
我们可以通过basePackages等属性来细粒度的定制@ComponentScan自动扫描的范围,如果不指定,则默认Spring框架实现会从声明@ComponentScan所在类的package进行扫描。
注:所以SpringBoot的启动类最好是放在root package下,因为默认不指定basePackages。
@EnableAutoConfiguration
个人感觉@EnableAutoConfiguration这个Annotation最为重要,所以放在最后来解读,大家是否还记得Spring框架提供的各种名字为@Enable开头的Annotation定义?比如@EnableScheduling、@EnableCaching、@EnableMBeanExport等,@EnableAutoConfiguration的理念和做事方式其实一脉相承,简单概括一下就是,借助@Import的支持,收集和注册特定场景相关的bean定义。
- @EnableScheduling是通过@Import将Spring调度框架相关的bean定义都加载到IoC容器。
- @EnableMBeanExport是通过@Import将JMX相关的bean定义加载到IoC容器。
而@EnableAutoConfiguration也是借助@Import的帮助,将所有符合自动配置条件的bean定义加载到IoC容器,仅此而已!
@EnableAutoConfiguration作为一个复合Annotation,其自身定义关键信息如下:
1 /* 2 * Copyright 2012-2019 the original author or authors. 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * https://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package org.springframework.boot.autoconfigure; 18 19 import java.lang.annotation.Documented; 20 import java.lang.annotation.ElementType; 21 import java.lang.annotation.Inherited; 22 import java.lang.annotation.Retention; 23 import java.lang.annotation.RetentionPolicy; 24 import java.lang.annotation.Target; 25 26 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean; 27 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass; 28 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean; 29 import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory; 30 import org.springframework.boot.web.servlet.server.ServletWebServerFactory; 31 import org.springframework.context.annotation.Conditional; 32 import org.springframework.context.annotation.Configuration; 33 import org.springframework.context.annotation.Import; 34 import org.springframework.core.io.support.SpringFactoriesLoader; 35 36 /** 37 * Enable auto-configuration of the Spring Application Context, attempting to guess and 38 * configure beans that you are likely to need. Auto-configuration classes are usually 39 * applied based on your classpath and what beans you have defined. For example, if you 40 * have {@code tomcat-embedded.jar} on your classpath you are likely to want a 41 * {@link TomcatServletWebServerFactory} (unless you have defined your own 42 * {@link ServletWebServerFactory} bean). 43 * <p> 44 * When using {@link SpringBootApplication}, the auto-configuration of the context is 45 * automatically enabled and adding this annotation has therefore no additional effect. 46 * <p> 47 * Auto-configuration tries to be as intelligent as possible and will back-away as you 48 * define more of your own configuration. You can always manually {@link #exclude()} any 49 * configuration that you never want to apply (use {@link #excludeName()} if you don't 50 * have access to them). You can also exclude them via the 51 * {@code spring.autoconfigure.exclude} property. Auto-configuration is always applied 52 * after user-defined beans have been registered. 53 * <p> 54 * The package of the class that is annotated with {@code @EnableAutoConfiguration}, 55 * usually via {@code @SpringBootApplication}, has specific significance and is often used 56 * as a 'default'. For example, it will be used when scanning for {@code @Entity} classes. 57 * It is generally recommended that you place {@code @EnableAutoConfiguration} (if you're 58 * not using {@code @SpringBootApplication}) in a root package so that all sub-packages 59 * and classes can be searched. 60 * <p> 61 * Auto-configuration classes are regular Spring {@link Configuration} beans. They are 62 * located using the {@link SpringFactoriesLoader} mechanism (keyed against this class). 63 * Generally auto-configuration beans are {@link Conditional @Conditional} beans (most 64 * often using {@link ConditionalOnClass @ConditionalOnClass} and 65 * {@link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations). 66 * 67 * @author Phillip Webb 68 * @author Stephane Nicoll 69 * @since 1.0.0 70 * @see ConditionalOnBean 71 * @see ConditionalOnMissingBean 72 * @see ConditionalOnClass 73 * @see AutoConfigureAfter 74 * @see SpringBootApplication 75 */ 76 @Target(ElementType.TYPE) 77 @Retention(RetentionPolicy.RUNTIME) 78 @Documented 79 @Inherited 80 @AutoConfigurationPackage 81 @Import(AutoConfigurationImportSelector.class) 82 public @interface EnableAutoConfiguration { 83 84 String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration"; 85 86 /** 87 * Exclude specific auto-configuration classes such that they will never be applied. 88 * @return the classes to exclude 89 */ 90 Class<?>[] exclude() default {}; 91 92 /** 93 * Exclude specific auto-configuration class names such that they will never be 94 * applied. 95 * @return the class names to exclude 96 * @since 1.3.0 97 */ 98 String[] excludeName() default {}; 99 100 }
其中,最关键的要属@Import(EnableAutoConfigurationImportSelector.class),借助EnableAutoConfigurationImportSelector,@EnableAutoConfiguration可以帮助SpringBoot应用将所有符合条件的@Configuration配置都加载到当前SpringBoot创建并使用的IoC容器。
借助于Spring框架原有的一个工具类:SpringFactoriesLoader的支持,@EnableAutoConfiguration可以智能的自动配置功效才得以大功告成!
自动配置幕后英雄:SpringFactoriesLoader详解
SpringFactoriesLoader属于Spring框架私有的一种扩展方案,其主要功能就是从指定的配置文件META-INF/spring.factories加载配置。
1 /* 2 * Copyright 2002-2018 the original author or authors. 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * https://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package org.springframework.core.io.support; 18 19 import java.io.IOException; 20 import java.net.URL; 21 import java.util.ArrayList; 22 import java.util.Collections; 23 import java.util.Enumeration; 24 import java.util.List; 25 import java.util.Map; 26 import java.util.Properties; 27 28 import org.apache.commons.logging.Log; 29 import org.apache.commons.logging.LogFactory; 30 31 import org.springframework.core.annotation.AnnotationAwareOrderComparator; 32 import org.springframework.core.io.UrlResource; 33 import org.springframework.lang.Nullable; 34 import org.springframework.util.Assert; 35 import org.springframework.util.ClassUtils; 36 import org.springframework.util.ConcurrentReferenceHashMap; 37 import org.springframework.util.LinkedMultiValueMap; 38 import org.springframework.util.MultiValueMap; 39 import org.springframework.util.ReflectionUtils; 40 import org.springframework.util.StringUtils; 41 42 /** 43 * General purpose factory loading mechanism for internal use within the framework. 44 * 45 * <p>{@code SpringFactoriesLoader} {@linkplain #loadFactories loads} and instantiates 46 * factories of a given type from {@value #FACTORIES_RESOURCE_LOCATION} files which 47 * may be present in multiple JAR files in the classpath. The {@code spring.factories} 48 * file must be in {@link Properties} format, where the key is the fully qualified 49 * name of the interface or abstract class, and the value is a comma-separated list of 50 * implementation class names. For example: 51 * 52 * <pre class="code">example.MyService=example.MyServiceImpl1,example.MyServiceImpl2</pre> 53 * 54 * where {@code example.MyService} is the name of the interface, and {@code MyServiceImpl1} 55 * and {@code MyServiceImpl2} are two implementations. 56 * 57 * @author Arjen Poutsma 58 * @author Juergen Hoeller 59 * @author Sam Brannen 60 * @since 3.2 61 */ 62 public final class SpringFactoriesLoader { 63 64 /** 65 * The location to look for factories. 66 * <p>Can be present in multiple JAR files. 67 */ 68 public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories"; 69 70 71 private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class); 72 73 private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>(); 74 75 76 private SpringFactoriesLoader() { 77 } 78 79 80 /** 81 * Load and instantiate the factory implementations of the given type from 82 * {@value #FACTORIES_RESOURCE_LOCATION}, using the given class loader. 83 * <p>The returned factories are sorted through {@link AnnotationAwareOrderComparator}. 84 * <p>If a custom instantiation strategy is required, use {@link #loadFactoryNames} 85 * to obtain all registered factory names. 86 * @param factoryClass the interface or abstract class representing the factory 87 * @param classLoader the ClassLoader to use for loading (can be {@code null} to use the default) 88 * @throws IllegalArgumentException if any factory implementation class cannot 89 * be loaded or if an error occurs while instantiating any factory 90 * @see #loadFactoryNames 91 */ 92 public static <T> List<T> loadFactories(Class<T> factoryClass, @Nullable ClassLoader classLoader) { 93 Assert.notNull(factoryClass, "'factoryClass' must not be null"); 94 ClassLoader classLoaderToUse = classLoader; 95 if (classLoaderToUse == null) { 96 classLoaderToUse = SpringFactoriesLoader.class.getClassLoader(); 97 } 98 List<String> factoryNames = loadFactoryNames(factoryClass, classLoaderToUse); 99 if (logger.isTraceEnabled()) { 100 logger.trace("Loaded [" + factoryClass.getName() + "] names: " + factoryNames); 101 } 102 List<T> result = new ArrayList<>(factoryNames.size()); 103 for (String factoryName : factoryNames) { 104 result.add(instantiateFactory(factoryName, factoryClass, classLoaderToUse)); 105 } 106 AnnotationAwareOrderComparator.sort(result); 107 return result; 108 } 109 110 /** 111 * Load the fully qualified class names of factory implementations of the 112 * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given 113 * class loader. 114 * @param factoryClass the interface or abstract class representing the factory 115 * @param classLoader the ClassLoader to use for loading resources; can be 116 * {@code null} to use the default 117 * @throws IllegalArgumentException if an error occurs while loading factory names 118 * @see #loadFactories 119 */ 120 public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) { 121 String factoryClassName = factoryClass.getName(); 122 return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList()); 123 } 124 125 private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) { 126 MultiValueMap<String, String> result = cache.get(classLoader); 127 if (result != null) { 128 return result; 129 } 130 131 try { 132 Enumeration<URL> urls = (classLoader != null ? 133 classLoader.getResources(FACTORIES_RESOURCE_LOCATION) : 134 ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION)); 135 result = new LinkedMultiValueMap<>(); 136 while (urls.hasMoreElements()) { 137 URL url = urls.nextElement(); 138 UrlResource resource = new UrlResource(url); 139 Properties properties = PropertiesLoaderUtils.loadProperties(resource); 140 for (Map.Entry<?, ?> entry : properties.entrySet()) { 141 String factoryClassName = ((String) entry.getKey()).trim(); 142 for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) { 143 result.add(factoryClassName, factoryName.trim()); 144 } 145 } 146 } 147 cache.put(classLoader, result); 148 return result; 149 } 150 catch (IOException ex) { 151 throw new IllegalArgumentException("Unable to load factories from location [" + 152 FACTORIES_RESOURCE_LOCATION + "]", ex); 153 } 154 } 155 156 @SuppressWarnings("unchecked") 157 private static <T> T instantiateFactory(String instanceClassName, Class<T> factoryClass, ClassLoader classLoader) { 158 try { 159 Class<?> instanceClass = ClassUtils.forName(instanceClassName, classLoader); 160 if (!factoryClass.isAssignableFrom(instanceClass)) { 161 throw new IllegalArgumentException( 162 "Class [" + instanceClassName + "] is not assignable to [" + factoryClass.getName() + "]"); 163 } 164 return (T) ReflectionUtils.accessibleConstructor(instanceClass).newInstance(); 165 } 166 catch (Throwable ex) { 167 throw new IllegalArgumentException("Unable to instantiate factory class: " + factoryClass.getName(), ex); 168 } 169 } 170 171 }
配合@EnableAutoConfiguration使用的话,它更多是提供一种配置查找的功能支持,即根据@EnableAutoConfiguration的完整类名org.springframework.boot.autoconfigure.EnableAutoConfiguration作为查找的Key,获取对应的一组@Configuration类
上图就是从SpringBoot的spring-boot-autoconfigure-2.1.8.RELEASE.jar依赖包中的META-INF/spring.factories配置文件中摘录的一段内容,可以很好地说明问题。
所以,@EnableAutoConfiguration自动配置的魔法骑士就变成了:从classpath中搜寻所有的META-INF/spring.factories配置文件,并将其中org.springframework.boot.autoconfigure.EnableutoConfiguration对应的配置项通过反射(Java Refletion)实例化为对应的标注了@Configuration的JavaConfig形式的IoC容器配置类,然后汇总为一个并加载到IoC容器。
深入探索SpringApplication执行流程
SpringApplication的run方法的实现是我们本次旅程的主要线路,该方法的主要流程大体可以归纳如下:
1) 如果我们使用的是SpringApplication的静态run方法,那么,这个方法里面首先要创建一个SpringApplication对象实例,然后调用这个创建好的SpringApplication的实例方法。在SpringApplication实例初始化的时候,它会提前做几件事情:
- 根据classpath里面是否存在某个特征类(org.springframework.web.context.ConfigurableWebApplicationContext)来决定是否应该创建一个为Web应用使用的ApplicationContext类型。
- 使用SpringFactoriesLoader在应用的classpath中查找并加载所有可用的ApplicationContextInitializer。
- 使用SpringFactoriesLoader在应用的classpath中查找并加载所有可用的ApplicationListener。
- 推断并设置main方法的定义类。
2) SpringApplication实例初始化完成并且完成设置后,就开始执行run方法的逻辑了,方法执行伊始,首先遍历执行所有通过SpringFactoriesLoader可以查找到并加载的SpringApplicationRunListener。调用它们的started()方法,告诉这些SpringApplicationRunListener,“嘿,SpringBoot应用要开始执行咯!”。
3) 创建并配置当前Spring Boot应用将要使用的Environment(包括配置要使用的PropertySource以及Profile)。
4) 遍历调用所有SpringApplicationRunListener的environmentPrepared()的方法,告诉他们:“当前SpringBoot应用使用的Environment准备好了咯!”。
5) 如果SpringApplication的showBanner属性被设置为true,则打印banner。
6) 根据用户是否明确设置了applicationContextClass类型以及初始化阶段的推断结果,决定该为当前SpringBoot应用创建什么类型的ApplicationContext并创建完成,然后根据条件决定是否添加ShutdownHook,决定是否使用自定义的BeanNameGenerator,决定是否使用自定义的ResourceLoader,当然,最重要的,将之前准备好的Environment设置给创建好的ApplicationContext使用。
7) ApplicationContext创建好之后,SpringApplication会再次借助SpringFactoriesLoader,查找并加载classpath中所有可用的ApplicationContextInitializer,然后遍历调用这些ApplicationContextInitializer的initialize(applicationContext)方法来对已经创建好的ApplicationContext进行进一步的处理。
8) 遍历调用所有SpringApplicationRunListener的contextPrepared()方法。
9) 最核心的一步,将之前通过@EnableAutoConfiguration获取的所有配置以及其他形式的IoC容器配置加载到已经准备完毕的ApplicationContext。
10) 遍历调用所有SpringApplicationRunListener的contextLoaded()方法。
11) 调用ApplicationContext的refresh()方法,完成IoC容器可用的最后一道工序。
12) 查找当前ApplicationContext中是否注册有CommandLineRunner,如果有,则遍历执行它们。
13) 正常情况下,遍历执行SpringApplicationRunListener的finished()方法。(如果整个过程出现异常,则依然调用所有SpringApplicationRunListener的finished()方法,只不过这种情况下会将异常信息一并传入处理)
去除事件通知点后,整个流程如下:
总结
到此,SpringBoot的核心组件完成了基本的解析,综合来看,大部分都是Spring框架背后的一些概念和实践方式,SpringBoot只是在这些概念和实践上对特定的场景事先进行了固化和升华,而也恰恰是这些固化让我们开发基于Sping框架的应用更加方便高效。
参考
参考了《SpringBoot揭秘快速构建为服务体系》这本书的第三章,感兴趣的可以查阅。