spring boot 源码解析2-SpringApplication初始化

前⾔
我们⽣成⼀个spring boot 项⽬时,会⾃带⼀个启动类. 代码如下:

@SpringBootApplication public class SpringBootAnalysisApplication { public static void main(String[] args) { SpringApplication.run(SpringBootAnalysisApplication.class, args); } }

就是这么简单的代码,构成了spring boot的世界. 那么代码中只有⼀个@SpringBootApplication 注解 和 调⽤了SpringApplication#run
⽅法.那么我们先来解析SpringApplication的run⽅法.

解析
⾸先调⽤了org.springframework.boot.SpringApplication#run(Object, String...) ⽅法.代码如下:

public static ConfigurableApplicationContext run(Object source, String... args) { return run(new Object[] { source }, args); }

接着调⽤如下代码:

public static ConfigurableApplicationContext run(Object[] sources, String[] args) { return new SpringApplication(sources).run(args); }

可以发现 ⾸先初始化了SpringApplication,然后调⽤其实例⽅法:run.

1. 在 SpringApplication 的构造器中,调⽤了 initialize ⽅法.

public SpringApplication(Object... sources) { initialize(sources); }

1. SpringApplication#initialize⽅法代码如下:

private void initialize(Object[] sources) { if (sources != null && sources.length > 0) { this.sources.addAll(Arrays.asList(sources)); } this.webEnvironment = deduceWebEnvironment(); setInitializers((Collection) getSpringFactoriesInstances( ApplicationContextInitializer.class)); setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class)); this.mainApplicationClass = deduceMainApplicationClass(); }

可以看到做了如下5件事:

1. 如果sources⻓度⼤于0的话,加⼊到SpringApplication的sources中,该sources是⼀个LinkedHashSet.
2. 调⽤deduceWebEnvironment⽅法判断是否是web环境
3. 设置initializers.
4. 设置Listeners.
5. 设置mainApplicationClass.
6. deduceWebEnvironment代码如下:

private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet", "org.springframework.web.context.ConfigurableWebApplicationContext" }; private boolean deduceWebEnvironment() { for (String className : WEB_ENVIRONMENT_CLASSES) { if (!ClassUtils.isPresent(className, null)) { return false; } } return true; }

可以发现会调⽤ClassUtils类的isPresent⽅法，检查classpath中是否存在javax.servlet.Servlet类和
org.springframework.web.context.ConfigurableWebApplicationContext类,如果存在的话,返回true.否则返回false.

1. 在设置Initializers时⾸先调⽤getSpringFactoriesInstances⽅法加载ApplicationContextInitializer.然后直接赋值给initializers.代码如下:

private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type) { return getSpringFactoriesInstances(type, new Class<?>[] {}); }

转⽽调⽤如下代码:

private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) { ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); // Use names and ensure unique to protect against duplicates // 使⽤Set保存names来避免重复元素 Set<String> names = new LinkedHashSet<String>( SpringFactoriesLoader.loadFactoryNames(type, classLoader)); // 根据names来进⾏实例化 List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names); // 对实例进⾏排序 AnnotationAwareOrderComparator.sort(instances); return instances; }

该⽅法逻辑如下:

1. ⾸先获得ClassLoader.
2. 调⽤SpringFactoriesLoader#loadFactoryNames进⾏加载,然后放⼊到LinkedHashSet进⾏去重.
3. 调⽤createSpringFactoriesInstances进⾏初始化
4. 排序
   其中SpringFactoriesLoader#loadFactoryNames代码如下:

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) { String factoryClassName = factoryClass.getName(); try { Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURC E_LOCATION) : ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION)); List<String> result = new ArrayList<String>(); while (urls.hasMoreElements()) { URL url = urls.nextElement(); Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url)); String factoryClassNames = properties.getProperty(factoryClassName); result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassN ames))); } return result; } catch (IOException ex) { throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex); } }

逻辑如下:

1. 获得factoryClassName,对于当前来说factoryClassName =org.springframework.context.ApplicationContextInitializer.
2. 通过传⼊的classLoader加载META-INF/spring.factories⽂件.
3. 通过调⽤PropertiesLoaderUtils#loadProperties将其转为Properties.
4. 获得factoryClassName对应的值进⾏返回.
   对于当前来说,由于我们只加⼊了spring-boot-starter-web的依赖,因此会加载如下的配置:
5. 在spring-boot/META-INF/spring.factories中.org.springframework.context.ApplicationContextInitializer值如下:

org.springframework.context.ApplicationContextInitializer=\ org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\ org.springframework.boot.context.ContextIdApplicationContextInitializer,\ org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\ org.springframework.boot.context.embedded.ServerPortInfoApplicationContextInitializer

1. 在spring-boot-autoconfigure/src/main/resources/META-INF/spring.factories
   中.org.springframework.context.ApplicationContextInitializer值如下:

org.springframework.context.ApplicationContextInitializer=\ org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\ org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

因此会加载6个.
SpringApplication#createSpringFactoriesInstances⽅法如下:

private <T> List<T> createSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args, Set<String> names) { List<T> instances = new ArrayList<T>(names.size()); for (String name : names) { try { Class<?> instanceClass = ClassUtils.forName(name, classLoader); Assert.isAssignable(type, instanceClass); Constructor<?> constructor = instanceClass .getDeclaredConstructor(parameterTypes); T instance = (T) BeanUtils.instantiateClass(constructor, args); instances.add(instance); } catch (Throwable ex) { throw new IllegalArgumentException( "Cannot instantiate " + type + " : " + name, ex); } } return instances; }

逻辑如下:遍历传⼊的names,也就是之前通过SpringFactoriesLoader加载的类名.通过遍历,依次调⽤其构造器进⾏初始化.加⼊到
instances.然后进⾏返回.
对于当前场景来说:
ConfigurationWarningsApplicationContextInitializer,DelegatingApplicationContextInitializer,ServerPortInfoApplicationContextInitializer
初始化没有做任何事.
ContextIdApplicationContextInitializer在初始化时.会获得spring boot的应⽤名.搜索路径如下:

1. spring.application.name
2. vcap.application.name
3. spring.config.name
4. 如果都没有配置的话,返回application.
   代码如下:

private static final String NAME_PATTERN = "${spring.application.name:${vcap.application.name:${s pring.config.name:application}}}"; public ContextIdApplicationContextInitializer() { this(NAME_PATTERN); } public ContextIdApplicationContextInitializer(String name) { this.name = name; }

1. 设置SpringApplication#setListeners时,还是同样的套路.调⽤getSpringFactoriesInstances加载META-INF/spring.factories中配置
   的org.springframework.context.ApplicationListener. 对于当前来说.加载的类如下:

org.springframework.context.ApplicationListener=\ org.springframework.boot.ClearCachesApplicationListener,\ org.springframework.boot.builder.ParentContextCloserApplicationListener,\ org.springframework.boot.context.FileEncodingApplicationListener,\ org.springframework.boot.context.config.AnsiOutputApplicationListener,\ org.springframework.boot.context.config.ConfigFileApplicationListener,\ org.springframework.boot.context.config.DelegatingApplicationListener,\ org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener,\ org.springframework.boot.logging.ClasspathLoggingApplicationListener,\ org.springframework.boot.logging.LoggingApplicationListener

这些类在构造器中都没有做任何事.

1. 调⽤SpringApplication#deduceMainApplicationClass⽅法.获得应⽤的启动类.该⽅法通过获取当前⽅法调⽤栈，找到main函数的
   类.代码如下:

private Class<?> deduceMainApplicationClass() { try { StackTraceElement[] stackTrace = new RuntimeException().getStackTrace(); for (StackTraceElement stackTraceElement : stackTrace) { if ("main".equals(stackTraceElement.getMethodName())) { return Class.forName(stackTraceElement.getClassName()); } } } catch (ClassNotFoundException ex) { // Swallow and continue } return null; }

流程图如下:

参考内容：Spring Boot源码解析