10000 字講清楚 Spring Boot 注解原理
@SpringBootApplication
public class StartEurekaApplication
{
public static void main(String[] args)
{
SpringApplication.run(StartEurekaApplication.class, args);
}
}
點進@SpringBootApplication來看，發(fā)現(xiàn)@SpringBootApplication是一個組合注解。
@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 {
}
首先我們先來看 @SpringBootConfiguration：
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
}
可以看到這個注解除了元注解以外，就只有一個@Configuration，那也就是說這個注解相當于@Configuration，所以這兩個注解作用是一樣的，它讓我們能夠去注冊一些額外的Bean，并且導入一些額外的配置。
那@Configuration還有一個作用就是把該類變成一個配置類，不需要額外的XML進行配置。所以@SpringBootConfiguration就相當于@Configuration。進入@Configuration，發(fā)現(xiàn)@Configuration核心是@Component，說明Spring的配置類也是Spring的一個組件。
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
@AliasFor(
annotation = Component.class
)
String value() default "";
}
繼續(xù)來看下一個@EnableAutoConfiguration,這個注解是開啟自動配置的功能。
@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(EnableAutoConfigurationImportSelector.class)這兩個而組成的，我們先說@AutoConfigurationPackage，他是說：讓包中的類以及子包中的類能夠被自動掃描到spring容器中。
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import({Registrar.class})
public @interface AutoConfigurationPackage {
}
使用@Import來給Spring容器中導入一個組件 ，這里導入的是Registrar.class。來看下這個Registrar：
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
Registrar() {
}
public void registerBeanDefinitions(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));
}
}
就是通過以上這個方法獲取掃描的包路徑，可以debug查看具體的值：
那metadata是什么呢，可以看到是標注在@SpringBootApplication注解上的DemosbApplication，也就是我們的主配置類Application：
其實就是將主配置類（即@SpringBootApplication標注的類）的所在包及子包里面所有組件掃描加載到Spring容器。因此我們要把DemoApplication放在項目的最高級中（最外層目錄）。
看看注解@Import(AutoConfigurationImportSelector.class)，@Import注解就是給Spring容器中導入一些組件，這里傳入了一個組件的選擇器:AutoConfigurationImportSelector。
可以從圖中看出AutoConfigurationImportSelector 繼承了 DeferredImportSelector 繼承了 ImportSelector，ImportSelector有一個方法為：selectImports。將所有需要導入的組件以全類名的方式返回，這些組件就會被添加到容器中。
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());
}
}
會給容器中導入非常多的自動配置類（xxxAutoConfiguration）；就是給容器中導入這個場景需要的所有組件，并配置好這些組件。
有了自動配置類，免去了我們手動編寫配置注入功能組件等的工作。那是如何獲取到這些配置類的呢，看看下面這個方法：
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);
}
}
我們可以看到getCandidateConfigurations()這個方法，他的作用就是引入系統(tǒng)已經加載好的一些類，到底是那些類呢：
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, @Nullable ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}
會從META-INF/spring.factories中獲取資源，然后通過Properties加載資源：
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
if (result != null) {
return result;
} else {
try {
Enumeration<URL> urls = classLoader !=
null ? 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在啟動的時候從類路徑下的META-INF/spring.factories中獲取EnableAutoConfiguration指定的值，將這些值作為自動配置類導入到容器中，自動配置類就生效，幫我們進行自動配置工作。以前我們需要自己配置的東西，自動配置類都幫我們完成了。如下圖可以發(fā)現(xiàn)Spring常見的一些類已經自動導入。
接下來看@ComponentScan注解，@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })，這個注解就是掃描包，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 {}
總結下@SpringbootApplication：就是說，他已經把很多東西準備好，具體是否使用取決于我們的程序或者說配置。
接下來繼續(xù)看run方法：
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
來看下在執(zhí)行run方法到底有沒有用到哪些自動配置的東西，我們點進run：
public ConfigurableApplicationContext run(String... args) {
//計時器
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();
this.configureHeadlessProperty();
//監(jiān)聽器
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);
//準備上下文
context = this.createApplicationContext();
exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);
//預刷新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)null);
throw new IllegalStateException(var9);
}
}
那我們關注的就是 refreshContext(context); 刷新context，我們點進來看。
private void refreshContext(ConfigurableApplicationContext context) {
refresh(context);
if (this.registerShutdownHook) {
try {
context.registerShutdownHook();
}
catch (AccessControlException ex) {
// Not allowed in some environments.
}
}
}
我們繼續(xù)點進refresh(context);
protected void refresh(ApplicationContext applicationContext) {
Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
((AbstractApplicationContext) applicationContext).refresh();
}
他會調用 ((AbstractApplicationContext) applicationContext).refresh();方法，我們點進來看：
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，就是一個spring的bean的加載過程。繼續(xù)來看一個方法叫做 onRefresh()：
protected void onRefresh() throws BeansException {
// For subclasses: do nothing by default.
}
他在這里并沒有直接實現(xiàn)，但是我們找他的具體實現(xiàn)：
比如Tomcat跟web有關，我們可以看到有個ServletWebServerApplicationContext：
@Override
protected void onRefresh() {
super.onRefresh();
try {
createWebServer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}
可以看到有一個createWebServer();方法他是創(chuàng)建web容器的，而Tomcat不就是web容器，那是如何創(chuàng)建的呢，我們繼續(xù)看：
private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
ServletWebServerFactory factory = getWebServerFactory();
this.webServer = factory.getWebServer(getSelfInitializer());
}
else if (servletContext != null) {
try {
getSelfInitializer().onStartup(servletContext);
}
catch (ServletException ex) {
throw new ApplicationContextException("Cannot initialize servlet context",
ex);
}
}
initPropertySources();
}
factory.getWebServer(getSelfInitializer());他是通過工廠的方式創(chuàng)建的。
public interface ServletWebServerFactory {
WebServer getWebServer(ServletContextInitializer... initializers);
}
可以看到 它是一個接口，為什么會是接口。因為我們不止是Tomcat一種web容器。
我們看到還有Jetty，那我們來看TomcatServletWebServerFactory：
@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
Tomcat tomcat = new Tomcat();
File baseDir = (this.baseDirectory != null) ? 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);
}
那這塊代碼，就是我們要尋找的內置Tomcat，在這個過程當中，我們可以看到創(chuàng)建Tomcat的一個流程。
如果不明白的話， 我們在用另一種方式來理解下，大家要應該都知道stater舉點例子。
<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>
首先自定義一個stater。
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.1.4.RELEASE</version>
<relativePath/>
</parent>
<groupId>com.zgw</groupId>
<artifactId>gw-spring-boot-starter</artifactId>
<version>1.0-SNAPSHOT</version>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-autoconfigure</artifactId>
</dependency>
</dependencies>
我們先來看maven配置寫入版本號，如果自定義一個stater的話必須依賴spring-boot-autoconfigure這個包,我們先看下項目目錄。
public class GwServiceImpl implements GwService{
@Autowired
GwProperties properties;
@Override
public void Hello()
{
String name=properties.getName();
System.out.println(name+"說:你們好啊");
}
}
我們做的就是通過配置文件來定制name這個是具體實現(xiàn)。
@Component
@ConfigurationProperties(prefix = "spring.gwname")
public class GwProperties {
String name="zgw";
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
這個類可以通過@ConfigurationProperties讀取配置文件。
@Configuration
@ConditionalOnClass(GwService.class) //掃描類
@EnableConfigurationProperties(GwProperties.class) //讓配置類生效
public class GwAutoConfiguration {
/**
* 功能描述 托管給spring
* @author zgw
* @return
*/
@Bean
@ConditionalOnMissingBean
public GwService gwService()
{
return new GwServiceImpl();
}
}
這個為配置類，為什么這么寫因為，spring-boot的stater都是這么寫的，我們可以參照他仿寫stater，以達到自動配置的目的，然后我們在通過spring.factories也來進行配置。
org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.gw.GwAutoConfiguration
鏈接：cnblogs.com/cmt/p/14553189.html
PS：如果覺得我的分享不錯，歡迎大家隨手點贊、在看。
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