spring boot 源码解析2-SpringApplication初始化-阿里云开发者社区

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.

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

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

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:

如果sources⻓度⼤于0的话,加⼊到SpringApplication的sources中,该sources是⼀个LinkedHashSet.
调⽤deduceWebEnvironment⽅法判断是否是web环境
设置initializers.
设置Listeners.
设置mainApplicationClass.
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.

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:

⾸先获得ClassLoader.
调⽤SpringFactoriesLoader#loadFactoryNames进⾏加载,然后放⼊到LinkedHashSet进⾏去重.
调⽤createSpringFactoriesInstances进⾏初始化
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:

获得factoryClassName,对于当前来说factoryClassName =org.springframework.context.ApplicationContextInitializer.
通过传⼊的classLoader加载META-INF/spring.factories⽂件.
通过调⽤PropertiesLoaderUtils#loadProperties将其转为Properties.
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:
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

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:

spring.application.name
vcap.application.name
spring.config.name
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;
}

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.

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:
QQ_20180515120216

参考内容：Spring Boot源码解析