深入SpringBoot註解原理及使用,圖文並茂,萬字長文!
首先,先看 SpringBoot 的主配置類:
@SpringBootApplication
public class StartEurekaApplication
{
public static void main(String[] args)
{
SpringApplication.run(StartEurekaApplication.class, args);
}
}
點進 @SpringBootApplication 來看,發現 @SpringBootApplication 是一個組合註解。
@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 {
}
可以看到這個註解除了元註解以外,就只有一個 @Configuration,那也就是說這個註解相當於 @Configuration,所以這兩個註解作用是一樣的,它讓我們能夠去註冊一些額外的 Bean,並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 就相當於 @Configuration。進入 @Configuration,發現 @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 "";
}
繼續來看下一個 @EnableAutoConfiguration, 這個註解是開啓自動配置的功能。
@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 ,這裏導入的是 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 是什麼呢,可以看到是標註在 @SpringBootApplication 註解上的 DemosbApplication,也就是我們的主配置類 Application:
其實就是將主配置類(即 @SpringBootApplication 標註的類)的所在包及子包裏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)。
看看註解 @Import(AutoConfigurationImportSelector.class),@Import 註解就是給 Spring 容器中導入一些組件,這裏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 有一個方法爲: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,並配置好這些組件。
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,然後通過 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 在啓動的時候從類路徑下的 META-INF/spring.factories 中獲取 EnableAutoConfiguration 指定的值,將這些值作爲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。如下圖可以發現 Spring 常見的一些類已經自動導入。
接下來看 @ComponentScan 註解,@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) }),這個註解就是掃描包,然後放入 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,具體是否使用取決於我們的程序或者說配置。
接下來繼續看 run 方法:
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
來看下在執行 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);
//刷新之後的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。繼續來看一個方法叫做 onRefresh():
protected void onRefresh() throws BeansException {
// For subclasses: do nothing by default.
}
他在這裏並沒有直接實現,但是我們找他的具體實現:
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(); 方法他是創建 web 容器的,而 Tomcat 不就是 web 容器,那是如何創建的呢,我們繼續看:
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建的。
public interface ServletWebServerFactory {
WebServer getWebServer(ServletContextInitializer... initializers);
}
可以看到 它是一個接口,爲什麼會是接口。因爲我們不止是 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製 name 這個是具體實現。
@Component
@ConfigurationProperties(prefix = "spring.gwname")
public class GwProperties {
String ;
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 託管給spring
* @author zgw
* @return
*/
@Bean
@ConditionalOnMissingBean
public GwService gwService()
{
return new GwServiceImpl();
}
}
這個爲配置類,爲什麼這麼寫因爲,spring-boot 的 stater 都是這麼寫的,我們可以參照他仿寫 stater,以達到自動配置的目的,然後我們在通過 spring.factories 也來進行配置。
org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.gw.GwAutoConfiguration
然後這樣一個簡單的 stater 就完成了,然後可以進行 maven 的打包,在其他項目引入就可以使用
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來源:https://mp.weixin.qq.com/s/evk7UXoUaegXF3U9klr01Q