首先我们先来看段代码
@SpringBootApplication
public class StartEurekaApplication
{
public static void main(String[] args)
{
SpringApplication.run(StartEurekaApplication.class, args);
}
}
我们点进@SpringBootApplication来看
@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 {
}
上面的元注解我们在这里不在做解释,相信大家在开发当中肯定知道,我们要来说@SpringBootConfiguration @EnableAutoConfiguration 这两个注解,到这里我们知道 SpringBootApplication注解里除了元注解,我们可以看到又是@SpringBootConfiguration,@EnableAutoConfiguration,@ComponentScan的组合注解,官网上也有详细说明,那我们现在把目光投向这三个注解。
首先我们先来看 @SpringBootConfiguration,那我们点进来看
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
}
我们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,那他是干嘛的呢,相信很多人都知道,它是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。
那我们继续来看下一个@EnableAutoConfiguration,这个注解官网说是 让Spring自动去进行一些配置,那我们点进来看
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
}
可以看到它是由 @AutoConfigurationPackage,@Import(EnableAutoConfigurationImportSelector.class)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。我们来看@Import(EnableAutoConfigurationImportSelector.class)这个是核心,之前我们说自动配置,那他到底帮我们配置了什么,怎么配置的?
就和@Import(EnableAutoConfigurationImportSelector.class)息息相关,程序中默认使用的类就自动帮我们找到。我们来看EnableAutoConfigurationImportSelector.class
public class EnableAutoConfigurationImportSelector
extends AutoConfigurationImportSelector {
@Override
protected boolean isEnabled(AnnotationMetadata metadata) {
if (getClass().equals(EnableAutoConfigurationImportSelector.class)) {
return getEnvironment().getProperty(
EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
true);
}
return true;
}
}
可以看到他继承了AutoConfigurationImportSelector我们继续来看AutoConfigurationImportSelector,这个类有一个方法
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
try {
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
.loadMetadata(this.beanClassLoader);
AnnotationAttributes attributes = getAttributes(annotationMetadata);
List<String> configurations = getCandidateConfigurations(annotationMetadata,
attributes);
configurations = removeDuplicates(configurations);
configurations = sort(configurations, autoConfigurationMetadata);
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
configurations = filter(configurations, autoConfigurationMetadata);
fireAutoConfigurationImportEvents(configurations, exclusions);
return configurations.toArray(new String[configurations.size()]);
}
catch (IOException ex) {
throw new IllegalStateException(ex);
}
}
这个类会帮你扫描那些类自动去添加到程序当中。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,我们点进去看一下
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
AnnotationAttributes attributes) {
List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
getSpringFactoriesLoaderFactoryClass(), 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;
}
这个类回去寻找的一个目录为META-INF/spring.factories,也就是说他帮你加载让你去使用也就是在这个META-INF/spring.factories目录装配的,他在哪里?
我们点进spring.factories来看
我们可以发现帮我们配置了很多类的全路径,比如你想整合activemq,或者说Servlet
可以看到他都已经帮我们引入了进来,我看随便拿几个来看
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\
比如我们经常用的security,可以看到已经帮你配置好,所以我们的EnableAutoConfiguration主要作用就是让你自动去配置,但并不是所有都是创建好的,是根据你程序去进行决定。那我们继续来看
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM,
classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM,
classes = AutoConfigurationExcludeFilter.class) })
这个注解大家应该都不陌生,扫描包,放入spring容器,那他在springboot当中做了什么策略呢?我们可以点跟烟去思考,帮我们做了一个排除策略,他在这里结合SpringBootConfiguration去使用,为什么是排除,因为不可能一上来全部加载,因为内存有限。
那么我们来总结下@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,那我们到底用不用?那我们继续来看一行代码
public static void main(String[] args)
{
SpringApplication.run(StartEurekaApplication.class, args);
}
那们来看下在执行run方法到底有没有用到哪些自动配置的东西,比如说内置的Tomcat,那我们来找找内置Tomcat,我们点进run
public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
return new SpringApplication(sources).run(args);
}
然后他调用又一个run方法,我们点进来看
public ConfigurableApplicationContext run(String... args) {
//计时器
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
FailureAnalyzers analyzers = null;
configureHeadlessProperty();
//监听器
SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.starting();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);
Banner printedBanner = printBanner(environment);
//准备上下文
context = createApplicationContext();
analyzers = new FailureAnalyzers(context);
//预刷新context
prepareContext(context, environment, listeners, applicationArguments,
printedBanner);
//刷新context
refreshContext(context);
//刷新之后的context
afterRefresh(context, applicationArguments);
listeners.finished(context, null);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}
return context;
}
catch (Throwable ex) {
handleRunFailure(context, listeners, analyzers, ex);
throw new IllegalStateException(ex);
}
}
那我们关注的就是 refreshContext(context); 刷新context,我们点进来看
private void refreshContext(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();
}
}
}
这点代码似曾相识啊 没错,就是一个spring的bean的加载过程我在,解析springIOC加载过程的时候介绍过这里面的方法,如果你看过Spring源码的话 ,应该知道这些方法都是做什么的。现在我们不关心其他的,我们来看一个方法叫做 onRefresh();方法
protected void onRefresh() throws BeansException {
// For subclasses: do nothing by default.
}
他在这里并没有实现,但是我们找他的其他实现,我们来找
我们既然要找Tomcat那就肯定跟web有关,我们可以看到有个ServletWebServerApplicationContext
@Override
protected void onRefresh() {
super.onRefresh();
try {
createWebServer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}
我们可以看到有一个createWebServer();方法他是创建web容器的,而Tomcat不就是web容器,那他是怎么创建的呢,我们继续看
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 != 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。因为run方法里面加载的东西很多,所以今天就浅谈到这里。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>
所以我们不防不定义一个stater来理解下,我们做一个需求,就是定制化不同的人跟大家说你们好,我们来看
<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-srater</artifactId>
<version>1.0-SNAPSHOT</version>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-autoconfigure</artifactId>
</dependency>
</dependencies>
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
public class GwServiceImpl implements GwService{
@Autowired
GwProperties properties;
@Override
public void Hello()
{
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