15000 字详解 Spring Boot 注解

首先，先看 SpringBoot 的主配置类：

@SpringBootApplication
publicclassStartEurekaApplication
{publicstaticvoidmain(String[] args)
{SpringApplication.run(StartEurekaApplication.class, args);
}
}

点进 @SpringBootApplication 来看，发现 @SpringBootApplication 是一个组合注解。

@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) })
public @interface SpringBootApplication {
}

首先我们先来看 @SpringBootConfiguration：

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
}

可以看到这个注解除了元注解以外，就只有一个@Configuration，那也就是说这个注解相当于@Configuration，所以这两个注解作用是一样的，它让我们能够去注册一些额外的Bean，并且导入一些额外的配置。

那@Configuration还有一个作用就是把该类变成一个配置类，不需要额外的XML进行配置。所以@SpringBootConfiguration就相当于@Configuration。进入@Configuration，发现@Configuration核心是@Component，说明Spring的配置类也是Spring的一个组件。

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
@AliasFor(
annotation = Component.class)String value default "";
}

继续来看下一个@EnableAutoConfiguration,这个注解是开启自动配置的功能。

@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 {};
}

可以看到它是由 @AutoConfigurationPackage，@Import 这两个而组成的，我们先说@AutoConfigurationPackage，他是说：让包中的类以及子包中的类能够被自动扫描到spring容器中。

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

使用 @Import 来给 Spring 容器中导入一个组件 ，这里导入的是 Registrar.class。来看下这个Registrar：

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
Registrar {}publicvoidregisterBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
AutoConfigurationPackages.register(registry, (new AutoConfigurationPackages.PackageImport(metadata)).getPackageName);
}public Set<Object> determineImports(AnnotationMetadata metadata) {
return Collections.singleton(new AutoConfigurationPackages.PackageImport(metadata));
}}

就是通过以上这个方法获取扫描的包路径，可以 debug 查看具体的值：

那 metadata 是什么呢，可以看到是标注在 @SpringBootApplication 注解上的 DemosbApplication，也就是我们的主配置类Application：

其实就是将主配置类（即@SpringBootApplication标注的类）的所在包及子包里面所有组件扫描加载到Spring容器。因此我们要把DemoApplication放在项目的最高级中（最外层目录）。

看看注解@Import，@Import注解就是给Spring容器中导入一些组件，这里传入了一个组件的选择器
:AutoConfigurationImportSelector。

可以从图中看出
AutoConfigurationImportSelector 继承了 DeferredImportSelector 继承了 ImportSelector，ImportSelector有一个方法为：selectImports。将所有需要导入的组件以全类名的方式返回，这些组件就会被添加到容器中。

public String selectImports(AnnotationMetadata annotationMetadata) {
if (!this.isEnabled(annotationMetadata)) {
return NO_IMPORTS;
} else {
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry =this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata);
return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations);
}}

会给容器中导入非常多的自动配置类（xxxAutoConfiguration）；就是给容器中导入这个场景需要的所有组件，并配置好这些组件。

有了自动配置类，免去了我们手动编写配置注入功能组件等的工作。那是如何获取到这些配置类的呢，看看下面这个方法：

protected AutoConfigurationImportSelector.AutoConfigurationEntry 
getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, AnnotationMetadata annotationMetadata) {if (!this.isEnabled(annotationMetadata)) {
return EMPTY_ENTRY;
} else {
AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
configurations = this.removeDuplicates(configurations);
Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
this.checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);configurations = this.filter(configurations, autoConfigurationMetadata);
this.fireAutoConfigurationImportEvents(configurations, exclusions);
return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
}}

我们可以看到
getCandidateConfigurations这个方法，他的作用就是引入系统已经加载好的一些类，到底是那些类呢：

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

public static List<String> loadFactoryNames(Class<?> factoryClass, @able ClassLoader classLoader) {
String factoryClassName = factoryClass.getName;return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList);
}

会从 META-INF/spring.factories 中获取资源，然后通过 Properties 加载资源：

private static Map<String, List<String>> loadSpringFactories(@able ClassLoader classLoader) {
MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
if (result != ) {
return result;
} else {
try {
Enumeration<URL> urls = classLoader !=? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
LinkedMultiValueMap result = new LinkedMultiValueMap;
while(urls.hasMoreElements) {
URL url = (URL)urls.nextElement;UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);Iterator var6 = properties.entrySet.iterator;while(var6.hasNext) {
Map.Entry<?, ?> entry = (Map.Entry)var6.next;
String factoryClassName = ((String)entry.getKey).trim;
String var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue);
int var10 = var9.length;for(int var11 = 0; var11 < var10; ++var11) {
String factoryName = var9[var11];
result.add(factoryClassName, factoryName.trim);}}}cache.put(classLoader, result);return result;
} catch (IOException var13) {
throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
}}}

可以知道SpringBoot在启动的时候从类路径下的META-INF/spring.factories中获取EnableAutoConfiguration指定的值，将这些值作为自动配置类导入到容器中，自动配置类就生效，帮我们进行自动配置工作。以前我们需要自己配置的东西，自动配置类都帮我们完成了。如下图可以发现Spring常见的一些类已经自动导入。

接下来看@ComponentScan注解，@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes =
AutoConfigurationExcludeFilter.class) })，这个注解就是扫描包，然后放入spring容器。

@ComponentScan(excludeFilters = {
@Filter(type = FilterType.CUSTOM,classes = {TypeExcludeFilter.class}),
@Filter(type = FilterType.CUSTOM,classes = {AutoConfigurationExcludeFilter.class})})
public @interface SpringBootApplication {}

总结下 @SpringbootApplication：就是说，他已经把很多东西准备好，具体是否使用取决于我们的程序或者说配置。

接下来继续看run方法：

publicstaticvoidmain(String[] args) {
SpringApplication.run(Application.class, args);
}

来看下在执行 run 方法到底有没有用到哪些自动配置的东西，我们点进 run：

public ConfigurableApplicationContext run(String... args) {
//计时器
StopWatch stopWatch = new StopWatch;stopWatch.start;ConfigurableApplicationContext context = ;Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList;this.configureHeadlessProperty;//监听器
SpringApplicationRunListeners listeners = this.getRunListeners(args);listeners.starting;Collection exceptionReporters;try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);this.configureIgnoreBeanInfo(environment);Banner printedBanner = this.printBanner(environment);//准备上下文
context = this.createApplicationContext;exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);
//预刷新context
this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
//刷新context
this.refreshContext(context);
//刷新之后的context
this.afterRefresh(context, applicationArguments);
stopWatch.stop;
if (this.logStartupInfo) {
(new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog, stopWatch);
}
listeners.started(context);
this.callRunners(context, applicationArguments);
} catch (Throwable var10) {
this.handleRunFailure(context, var10, exceptionReporters, listeners);
throw new IllegalStateException(var10);
}
try {
listeners.running(context);
return context;
} catch (Throwable var9) {
this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners));
throw new IllegalStateException(var9);
}
}

那我们关注的就是 refreshContext(context); 刷新 context，我们点进来看。

privatevoidrefreshContext(ConfigurableApplicationContext context) {
refresh(context);if (this.registerShutdownHook) {
try {
context.registerShutdownHook;}catch (AccessControlException ex) {
// Not allowed in some environments.
}
}
}

我们继续点进 refresh(context);

protected void refresh(ApplicationContext applicationContext) {
Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
((AbstractApplicationContext) applicationContext).refresh;}

他会调用 ((
AbstractApplicationContext) applicationContext).refresh;方法，我们点进来看：

public void refresh throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {// Prepare this context for refreshing.
prepareRefresh;// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory;// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);try {// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);// Initialize message source for this context.
initMessageSource;// Initialize event multicaster for this context.
initApplicationEventMulticaster;// Initialize other special beans in specific context subclasses.
onRefresh;// Check for listener beans and register them.
registerListeners;// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);// Last step: publish corresponding event.
finishRefresh;}catch (BeansException ex) {if (logger.isWarnEnabled) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}// Destroy already created singletons to avoid dangling resources.
destroyBeans;
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches;
}
}
}

由此可知，就是一个 spring 的 bean 的加载过程。继续来看一个方法叫做 onRefresh：

protected void onRefresh throws BeansException {
// For subclasses: do nothing by default.
}

他在这里并没有直接实现，但是我们找他的具体实现：

比如 Tomcat 跟 web 有关，我们可以看到有个
ServletWebServerApplicationContext：

@Override
protectedvoidonRefresh {
super.onRefresh;
try {
createWebServer;}catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}}

可以看到有一个 createWebServer;方法他是创建web容器的，而Tomcat不就是web容器，那是如何创建的呢，我们继续看：

private void createWebServer {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext;if (webServer == && servletContext == ) {
ServletWebServerFactory factory = getWebServerFactory;this.webServer = factory.getWebServer(getSelfInitializer);
}else if (servletContext != ) {
try {
getSelfInitializer.onStartup(servletContext);}catch (ServletException ex) {
throw new ApplicationContextException("Cannot initialize servlet context",
ex);}}initPropertySources;}

factory.getWebServer(getSelfInitializer);他是通过工厂的方式创建的。

public interface ServletWebServerFactory {
WebServer getWebServer(ServletContextInitializer... initializers);
}

可以看到 它是一个接口，为什么会是接口。因为我们不止是Tomcat一种web容器。

我们看到还有Jetty，那我们来看
TomcatServletWebServerFactory：

@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
Tomcat tomcat = new Tomcat;File baseDir = (this.baseDirectory != ) ? this.baseDirectory
: createTempDir("tomcat");
tomcat.setBaseDir(baseDir.getAbsolutePath);Connector connector = new Connector(this.protocol);
tomcat.getService.addConnector(connector);customizeConnector(connector);tomcat.setConnector(connector);tomcat.getHost.setAutoDeploy(false);
configureEngine(tomcat.getEngine);for (Connector additionalConnector : this.additionalTomcatConnectors) {
tomcat.getService.addConnector(additionalConnector);}prepareContext(tomcat.getHost, initializers);return getTomcatWebServer(tomcat);
}

那这块代码，就是我们要寻找的内置Tomcat，在这个过程当中，我们可以看到创建Tomcat的一个流程。

如果不明白的话， 我们在用另一种方式来理解下，大家要应该都知道stater举点例子。

<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-redis</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-freemarker</artifactId>
</dependency>

首先自定义一个stater。

<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-redis</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-freemarker</artifactId>
</dependency>

我们先来看 maven 配置写入版本号，如果自定义一个 stater 的话必须依赖 spring-boot-autoconfigure 这个包,我们先看下项目目录。

publicclassGwServiceImplimplementsGwService{
@Autowired
GwProperties properties;
@Override
publicvoidHello
{String name=properties.getName;System.out.println(name+"说:你们好啊");
}}

我们做的就是通过配置文件来定制name这个是具体实现。

@Component
@ConfigurationProperties(prefix = "spring.gwname")
public class GwProperties {
String name="zgw";
public String getName {
return name;
}public void setName(String name) {
this.name = name;
}}

这个类可以通过 @ConfigurationProperties 读取配置文件。

@Configuration
@ConditionalOnClass(GwService.class) //扫描类
@EnableConfigurationProperties(GwProperties.class) //让配置类生效
public class GwAutoConfiguration {
/**
* 功能描述 托管给spring
* @author zgw
* @return
*/
@Bean
@ConditionalOnMissingBean
public GwService gwService
{
return new GwServiceImpl;
}
}

这个为配置类，为什么这么写因为，spring-boot的stater都是这么写的，我们可以参照他仿写stater，以达到自动配置的目的，然后我们在通过spring.factories也来进行配置。

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.gw.GwAutoConfiguration

然后这样一个简单的stater就完成了，然后可以进行maven的打包，在其他项目引入就可以使用。

来源：
cnblogs.com/jing99/p/11504113.html