一、前言
上一篇介绍了注解，也是为这一篇做铺垫，传统的都是通过配置文件来启动spring，那spring boot到底是做了什么能让我们快速开发昵？
二、启动原理
看下程序启动的入口，主要两处地方一是SpringBootApplication注解，另外就是run方法，首先我们看注解部分，上一篇我们也说过注解应该不难看懂，我们看下这个注解里面有什么神奇的东西；
@SpringBootApplication
public class DemoApplication { public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}
} @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 { /**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
@AliasFor(annotation = EnableAutoConfiguration.class)
Class<?>[] exclude() default {}; /**
* Exclude specific auto-configuration class names such that they will never be
* applied.
* @return the class names to exclude
* @since 1.3.0
*/
@AliasFor(annotation = EnableAutoConfiguration.class)
String[] excludeName() default {}; /**
* Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}
* for a type-safe alternative to String-based package names.
* @return base packages to scan
* @since 1.3.0
*/
@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
String[] scanBasePackages() default {}; /**
* Type-safe alternative to {@link #scanBasePackages} for specifying the packages to
* scan for annotated components. The package of each class specified will be scanned.
* <p>
* Consider creating a special no-op marker class or interface in each package that
* serves no purpose other than being referenced by this attribute.
* @return base packages to scan
* @since 1.3.0
*/
@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
Class<?>[] scanBasePackageClasses() default {}; }
看上面代码，除去元注解，主要有3个注解，
@ComponentScan
这个不需要我们多说太多，这个主要有2个作用，组件扫描和自动装配；
@SpringBootConfiguration
这个我们也不需要说太多，这个注解主要是继承@Configuration注解，这个我们就是为了加载配置文件用的；
@EnableAutoConfiguration
这个是我们的重点：
看图我们来走一下代码，这里有一个重点就是@Import注解，这个里面引入了AutoConfigurationImportSelector.class这个文件，所以我们就需要看下这里面有那些玩意，值得我们注意的，这个类里面代码有点多我将重点放到下一个代码片段中，让大家结构清晰一些；
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration { String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration"; /**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
Class<?>[] exclude() default {}; /**
* Exclude specific auto-configuration class names such that they will never be
* applied.
* @return the class names to exclude
* @since 1.3.0
*/
String[] excludeName() default {}; }
这是中间比较关键的代码，我们主要看下loadFactories方法，这个里面有个常量的配置，位置如下图所示，整段代码实现了把配置文件中的信息通过反射实例化成为@Configuration的配置文件，然后通过@Configuration最后汇总到容器当中；
protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class,
this.beanClassLoader);
}
public abstract class SpringFactoriesLoader {
/**
* The location to look for factories.
* <p>Can be present in multiple JAR files.
*/
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);
private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>();
/**
* Load and instantiate the factory implementations of the given type from
* {@value #FACTORIES_RESOURCE_LOCATION}, using the given class loader.
* <p>The returned factories are sorted through {@link AnnotationAwareOrderComparator}.
* <p>If a custom instantiation strategy is required, use {@link #loadFactoryNames}
* to obtain all registered factory names.
* @param factoryClass the interface or abstract class representing the factory
* @param classLoader the ClassLoader to use for loading (can be {@code null} to use the default)
* @see #loadFactoryNames
* @throws IllegalArgumentException if any factory implementation class cannot
* be loaded or if an error occurs while instantiating any factory
*/
public static <T> List<T> loadFactories(Class<T> factoryClass, @Nullable ClassLoader classLoader) {
Assert.notNull(factoryClass, "'factoryClass' must not be null");
ClassLoader classLoaderToUse = classLoader;
if (classLoaderToUse == null) {
classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
}
List<String> factoryNames = loadFactoryNames(factoryClass, classLoaderToUse);
if (logger.isTraceEnabled()) {
logger.trace("Loaded [" + factoryClass.getName() + "] names: " + factoryNames);
}
List<T> result = new ArrayList<>(factoryNames.size());
for (String factoryName : factoryNames) {
result.add(instantiateFactory(factoryName, factoryClass, classLoaderToUse));
}
AnnotationAwareOrderComparator.sort(result);
return result;
}
/**
* Load the fully qualified class names of factory implementations of the
* given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given
* class loader.
* @param factoryClass the interface or abstract class representing the factory
* @param classLoader the ClassLoader to use for loading resources; can be
* {@code null} to use the default
* @see #loadFactories
* @throws IllegalArgumentException if an error occurs while loading factory names
*/
public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = cache.get(classLoader);
if (result != null) {
return result;
}
try {
Enumeration<URL> urls = (classLoader != null ?
classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
result = new LinkedMultiValueMap<>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry<?, ?> entry : properties.entrySet()) {
List<String> factoryClassNames = Arrays.asList(
StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
result.addAll((String) entry.getKey(), factoryClassNames);
}
}
cache.put(classLoader, result);
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}
@SuppressWarnings("unchecked")
private static <T> T instantiateFactory(String instanceClassName, Class<T> factoryClass, ClassLoader classLoader) {
try {
Class<?> instanceClass = ClassUtils.forName(instanceClassName, classLoader);
if (!factoryClass.isAssignableFrom(instanceClass)) {
throw new IllegalArgumentException(
"Class [" + instanceClassName + "] is not assignable to [" + factoryClass.getName() + "]");
}
return (T) ReflectionUtils.accessibleConstructor(instanceClass).newInstance();
}
catch (Throwable ex) {
throw new IllegalArgumentException("Unable to instantiate factory class: " + factoryClass.getName(), ex);
}
}
}
基本上注解这块就是说完了，但是中间少说了几个比较重要的东西，这里要说下需要注意的2个问题，
1.exclude和excludeName这个两个主要时排除你不想加载的配置，用法很简答，不需要说他太多；
2.scanBasePackages和scanBasePackageClasses这个是为了指定运行目录，好多小伙伴做了项目分离以后，会读取不到Mappr等，可以考虑下是不是这个错误；
重点来了，上面说了加载什么东西，那这些东西啥时候被调用被触发，那我们看下我们重点run方法:
1.调用run方法之前，首先初始化SpringApplication对象实例,这个对象初始化的过程中也做了不少事情让我们来慢慢看起来，接上上面思路，继续完成我们的取经；
//初始化SpringApplication对象
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
//加载classpatch文件下面的配置文件
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
//判断是否是web运行环境
this.webApplicationType = deduceWebApplicationType();
//使用SpringFactoriesLoader在应用的classpath中查找并加载所有可用的ApplicationContextInitializer。
setInitializers((Collection) getSpringFactoriesInstances(
ApplicationContextInitializer.class));
//使用SpringFactoriesLoader在应用的classpath中查找并加载所有可用的ApplicationListener。
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
//获得当前执行main方法的类对象
this.mainApplicationClass = deduceMainApplicationClass();
}
ApplicationContextInitializer 接口是在spring容器刷新之前执行的一个回调函数，主要有2点作用:1.在上下文（ConfigurableApplicationContext）刷新（refresh）之前调用，2.通常被用作web应用，在一些程序设计在spring容器初始化使用。比如说注册一些配置或者激活一些配置文件针对（ConfigurableApplicationContext的getEnvironment()方法）。另外这个函数支持支持Order注解。并且代表着执行顺序。我在下面也写了一个简单的例子，同时这个也是支持在配置文件中配置的context.initializer.classes=后面加上回调函数的全限定名称;另外假设我们在当前项目中要引入别的jar,这个jar要在加载前做一些配置，这个时候我们项目下的resources下新建META-INF文件夹，文件夹下新建spring.factories文件，然后写上org.springframework.context.ApplicationContextInitializer=后面加上需要回调函数的全限定名称，这个是在主项目启动的时候就会优先加载了；
ApplicationListener接口是spring boot的监听器，有7种类型，我准备好了demo大家执行一下，我相信对下面run方法的运行就不是很迷惑了；
@Order(3)
public class TestApplicationContextInitializer implements ApplicationContextInitializer {
@Override
public void initialize(ConfigurableApplicationContext applicationContext) {
System.out.println(applicationContext.getBeanDefinitionCount()+applicationContext.getBeanDefinitionNames().toString());
}
} @Order(1)
public class TestApplicationContextInitializer2 implements ApplicationContextInitializer {
@Override
public void initialize(ConfigurableApplicationContext applicationContext) {
System.out.println(applicationContext.getDisplayName());
}
} @SpringBootApplication
public class DemoApplication { public static void main(String[] args) {
// SpringApplication.run(DemoApplication.class, args);
SpringApplication springApplication=new SpringApplication(DemoApplication.class);
springApplication.addListeners((ApplicationListener<ApplicationStartingEvent>) event->{
System.out.println("Starting");
});
springApplication.addListeners((ApplicationListener<ApplicationStartedEvent>) event->{
System.out.println("Started");
});
springApplication.addListeners((ApplicationListener<ApplicationFailedEvent>) event->{
System.out.println("Failed");
});
springApplication.addListeners((ApplicationListener<ApplicationPreparedEvent>) event->{
System.out.println("Prepared");
}); springApplication.addListeners((ApplicationListener<SpringApplicationEvent>) event->{
System.out.println("SpringApplication");
}); springApplication.addListeners((ApplicationListener<ApplicationEnvironmentPreparedEvent>) event->{
System.out.println("EnvironmentPrepare");
}); springApplication.addListeners((ApplicationListener<ApplicationReadyEvent>) event->{
System.out.println("Ready");
});
springApplication.addInitializers(new TestApplicationContextInitializer());
springApplication.addInitializers(new TestApplicationContextInitializer2()); springApplication.run(args);
}
}
2.实例化完成开始执行run方法，这个里面流程比较多，我们先来看一个继承关系，然后结合上面ApplicationListener的demo我相信大家已经对其广播实现已经有了一个了解，这里我还是提一下通过SpringApplicationRunListener在ApplicationContext初始化过程中各个时点发布各种广播事件，并由ApplicationListener负责接收广播事件。接下来我们看下启动流程:
public ConfigurableApplicationContext run(String... args) {
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
//收集异常
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
//设置Headless模式为全局
configureHeadlessProperty();
//加载所有classpath下面的META-INF/spring.factories SpringApplicationRunListener(不同的时间点发送事件通知）
SpringApplicationRunListeners listeners = getRunListeners(args);
//spring boot启动初始化开始
listeners.starting();
try {
//装配参数和环境
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);
configureIgnoreBeanInfo(environment);
//打印Banner
Banner printedBanner = printBanner(environment);
//创建ApplicationContext()
context = createApplicationContext();
//返回异常
exceptionReporters = getSpringFactoriesInstances(
SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
//装配Context
prepareContext(context, environment, listeners, applicationArguments,
printedBanner);
//执行context的refresh方法，并且调用context的registerShutdownHook方法（这一步执行完成之后，spring容器加载完成）
refreshContext(context);
//回调，获取容器中所有的ApplicationRunner、CommandLineRunner接口
afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}
//容器初始化完成
listeners.started(context);
//遍历所有注册的ApplicationRunner和CommandLineRunner，并执行其run()方法。
//该过程可以理解为是SpringBoot完成ApplicationContext初始化前的最后一步工作，
callRunners(context, applicationArguments);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, listeners);
throw new IllegalStateException(ex);
}
try {
//容器开始被调用
listeners.running(context);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, null);
throw new IllegalStateException(ex);
}
return context;
}
写了这么多我忘记放入执行结果了这里补进去:
三、总结
要是想在spring boot初始化的时候搞点事情的化，那么有3种方法：
1.创建ApplicationContextInitializer的实现类
2.创建ApplicationListener的实现类
3.创建ApplicationRunner和CommandLineRunner的实现类
上面2种已经有了demo，我再来写一个第3种的demo;
@Order(2)
@Component
public class CommandLineRunnerDemo implements CommandLineRunner {
@Override
public void run(String... args) throws Exception {
System.out.println("CommandLineRunnerDemo");
}
} @Order(1)
@Component
public class ApplicationRunnerDemo implements ApplicationRunner {
@Override
public void run(ApplicationArguments args) throws Exception {
System.out.println("ApplicationRunner");
}
}
知道启动的流程又懂了扩展，我们接下来开始spring cloud吧。
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