spring boot 源码解析-SpringApplication初始化

调皮仔

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:

1. @SpringBootApplication
   
2. public class SpringBootAnalysisApplication {
   
3. public static void main(String[] args) {
   
4. SpringApplication.run(SpringBootAnalysisApplication.class, args);
   
5. }
   
6. }

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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:
• 1. public static ConfigurableApplicationContext run(Object source, String... args) {
     
  2. return run(new Object[] { source }, args);
     
  3. }

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  接着调⽤如下代码:
• 1. public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
     
  2. return new SpringApplication(sources).run(args);
     
  3. }

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  可以发现 ⾸先初始化了SpringApplication,然后调⽤其实例⽅法:run.
  
  
  2. 在 SpringApplication 的构造器中,调⽤了 initialize ⽅法.
• 1. public SpringApplication(Object... sources) {
     
  2. initialize(sources);
     
  3. }

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  3. SpringApplication#initialize⽅法代码如下:
• 1. private void initialize(Object[] sources) {
     
  2. if (sources != null && sources.length > 0) {
     
  3. this.sources.addAll(Arrays.asList(sources));
     
  4. }
     
  5. this.webEnvironment = deduceWebEnvironment();
     
  6. setInitializers((Collection) getSpringFactoriesInstances(
     
  7. ApplicationContextInitializer.class));
     
  8. setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
     
  9. this.mainApplicationClass = deduceMainApplicationClass();
     
  10. }

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  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.
  
  
  4. deduceWebEnvironment代码如下:
• 1. private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",
     
  2. "org.springframework.web.context.ConfigurableWebApplicationContext" };
     
  3. private boolean deduceWebEnvironment() {
     
  4. for (String className : WEB_ENVIRONMENT_CLASSES) {
     
  5. if (!ClassUtils.isPresent(className, null)) {
     
  6. return false;
     
  7. }
     
  8. }
     
  9. return true;
     
  10. }

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  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.
  
  
  5. 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:
• 1. private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type) {
     
  2. return getSpringFactoriesInstances(type, new Class<?>[] {});
     
  3. }

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  转⽽调⽤如下代码:
• 1. private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type,
     
  2. Class<?>[] parameterTypes, Object... args) {
     
  3. ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
     
  4. // Use names and ensure unique to protect against duplicates
     
  5. // 使⽤Set保存names来避免重复元素
     
  6. Set<String> names = new LinkedHashSet<String>(
     
  7. SpringFactoriesLoader.loadFactoryNames(type, classLoader));
     
  8. // 根据names来进⾏实例化
     
  9. List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
     
  10. classLoader, args, names);
      
  11. // 对实例进⾏排序
      
  12. AnnotationAwareOrderComparator.sort(instances);
      
  13. return instances;
      
  14. }

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  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:
• 1. public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
     
  2. String factoryClassName = factoryClass.getName();
     
  3. try {
     
  4. Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURC
     
  5. E_LOCATION) :
     
  6. ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
     
  7. List<String> result = new ArrayList<String>();
     
  8. while (urls.hasMoreElements()) {
     
  9. URL url = urls.nextElement();
     
  10. Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
      
  11. String factoryClassNames = properties.getProperty(factoryClassName);
      
  12. result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassN
      
  13. ames)));
      
  14. }
      
  15. return result;
      
  16. }
      
  17. catch (IOException ex) {
      
  18. throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
      
  19. "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
      
  20. }
      
  21. }

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  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ropertiesLoaderUtils#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:
  1. 在spring-boot/META-INF/spring.factories中.org.springframework.context.ApplicationContextInitializer值如下:
• 1. org.springframework.context.ApplicationContextInitializer=\
     
  2. org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
     
  3. org.springframework.boot.context.ContextIdApplicationContextInitializer,\
     
  4. org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
     
  5. org.springframework.boot.context.embedded.ServerPortInfoApplicationContextInitializer

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  2. 在spring-boot-autoconfigure/src/main/resources/META-INF/spring.factories
  中.org.springframework.context.ApplicationContextInitializer值如下:
• 1. org.springframework.context.ApplicationContextInitializer=\
     
  2. org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
     
  3. org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

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  因此会加载6个.
  SpringApplication#createSpringFactoriesInstances⽅法如下:
• 1. private <T> List<T> createSpringFactoriesInstances(Class<T> type,
     
  2. Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args,
     
  3. Set<String> names) {
     
  4. List<T> instances = new ArrayList<T>(names.size());
     
  5. for (String name : names) {
     
  6. try {
     
  7. Class<?> instanceClass = ClassUtils.forName(name, classLoader);
     
  8. Assert.isAssignable(type, instanceClass);
     
  9. Constructor<?> constructor = instanceClass
     
  10. .getDeclaredConstructor(parameterTypes);
      
  11. T instance = (T) BeanUtils.instantiateClass(constructor, args);
      
  12. instances.add(instance);
      
  13. }
      
  14. catch (Throwable ex) {
      
  15. throw new IllegalArgumentException(
      
  16. "Cannot instantiate " + type + " : " + name, ex);
      
  17. }
      
  18. }
      
  19. return instances;
      
  20. }

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  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:
• 1. private static final String NAME_PATTERN = "${spring.application.name:${vcap.application.name:${s
     
  2. pring.config.name:application}}}";
     
  3. public ContextIdApplicationContextInitializer() {
     
  4. this(NAME_PATTERN);
     
  5. }
     
  6. public ContextIdApplicationContextInitializer(String name) {
     
  7. this.name = name;
     
  8. }

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  6. 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:
• 1. org.springframework.context.ApplicationListener=\
     
  2. org.springframework.boot.ClearCachesApplicationListener,\
     
  3. org.springframework.boot.builder.ParentContextCloserApplicationListener,\
     
  4. org.springframework.boot.context.FileEncodingApplicationListener,\
     
  5. org.springframework.boot.context.config.AnsiOutputApplicationListener,\
     
  6. org.springframework.boot.context.config.ConfigFileApplicationListener,\
     
  7. org.springframework.boot.context.config.DelegatingApplicationListener,\
     
  8. org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener,\
     
  9. org.springframework.boot.logging.ClasspathLoggingApplicationListener,\
     
  10. org.springframework.boot.logging.LoggingApplicationListener

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  这些类在构造器中都没有做任何事.
  7. 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:
• 1. private Class<?> deduceMainApplicationClass() {
     
  2. try {
     
  3. StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
     
  4. for (StackTraceElement stackTraceElement : stackTrace) {
     
  5. if ("main".equals(stackTraceElement.getMethodName())) {
     
  6. return Class.forName(stackTraceElement.getClassName());
     
  7. }
     
  8. }
     
  9. }
     
  10. catch (ClassNotFoundException ex) {
      
  11. // Swallow and continue
      
  12. }
      
  13. return null;
      
  14. }

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  流程图如下:
• 
  
  

参考内容：Spring Boot源码解析