Spring Boot啟動流程詳解
阿新 • • 發佈：2019-01-28
1 package com.microservice.framework; 2 3 import org.springframework.boot.SpringApplication; 4 import org.springframework.boot.autoconfigure.SpringBootApplication; 5 6 @SpringBootApplication 7 public class MySpringAplication { 8 9 public void run(String[] args) { 10 SpringApplication sa = newSpringApplication(MySpringAplication.class); 11 sa.run(args); 12 } 13 14 }
SpringBoot啟動過程：
1、構建SpringApplication物件
2、執行run()
一、構建SpringApplication物件
1 /** 2 * The application context will load beans from the specified sources 3 */ 4 public SpringApplication(Object... sources) {5 initialize(sources); 6 }
說明：
- 例項化該類的時候會載入bean到applicationContext中去
- 這裡的入參是MySpringApplication.class這樣一個Class<com.microservice.framework.MySpringApplication>物件
private final Set<Object> sources = new LinkedHashSet<Object>(); private boolean webEnvironment;private Class<?> mainApplicationClass; 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(); }
步驟：
- 將傳入的MySpringApplication.class物件放入Set集合
- 判斷是否是web環境
- 建立ApplicationInitializer列表
- 初始化ApplicationListener列表
- 初始化主類mainApplicationClass
1.1、將傳入的MySpringApplication.class物件放入Set集合
1.2、判斷是否是web環境：
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; }
說明：通過在classpath中檢視是否存在WEB_ENVIRONMENT_CLASSES這個陣列中所包含的所有類（實際上就是2個類），如果存在那麼當前程式即是一個Web應用程式，反之則不然。
1.3、建立ApplicationContextInitializer列表
1 private List<ApplicationContextInitializer<?>> initializers; 2 3 public void setInitializers( 4 Collection<? extends ApplicationContextInitializer<?>> initializers) { 5 this.initializers = new ArrayList<ApplicationContextInitializer<?>>(); 6 this.initializers.addAll(initializers); 7 } 8 9 private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type) { 10 return getSpringFactoriesInstances(type, new Class<?>[] {}); 11 } 12 13 private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type, 14 Class<?>[] parameterTypes, Object... args) { 15 ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); 16 17 // Use names and ensure unique to protect against duplicates 18 Set<String> names = new LinkedHashSet<String>( 19 SpringFactoriesLoader.loadFactoryNames(type, classLoader)); 20 List<T> instances = new ArrayList<T>(names.size()); 21 22 // Create instances from the names 23 for (String name : names) { 24 try { 25 Class<?> instanceClass = ClassUtils.forName(name, classLoader); 26 Assert.isAssignable(type, instanceClass); 27 Constructor<?> constructor = instanceClass.getConstructor(parameterTypes); 28 T instance = (T) constructor.newInstance(args); 29 instances.add(instance); 30 } 31 catch (Throwable ex) { 32 throw new IllegalArgumentException( 33 "Cannot instantiate " + type + " : " + name, ex); 34 } 35 } 36 37 AnnotationAwareOrderComparator.sort(instances); 38 return instances; 39 }
步驟：
- 呼叫SpringFactoriesLoader.loadFactoryNames(type, classLoader)來獲取所有Spring Factories的名字，（這裡是獲取了四個ApplicationContextInitializer實現類的全類名，見下邊）
- 為每一個Spring Factories根據讀取到的名字建立其物件。（這裡建立了4個物件）
- 將建立好的物件列表排序並返回。
其中，SpringFactoriesLoader.loadFactoryNames(type, classLoader)如下：
1 /** 2 * The location to look for factories. 3 * <p>Can be present in multiple JAR files. 4 */ 5 public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories"; 6 7 /** 8 * Load the fully qualified class names of factory implementations of the 9 * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given 10 * class loader. 11 */ 12 public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) { 13 String factoryClassName = factoryClass.getName(); 14 try { 15 Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) : 16 ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION)); 17 List<String> result = new ArrayList<String>(); 18 while (urls.hasMoreElements()) { 19 URL url = urls.nextElement(); 20 Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url)); 21 String factoryClassNames = properties.getProperty(factoryClassName); 22 result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames))); 23 } 24 return result; 25 } 26 catch (IOException ex) { 27 throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + 28 "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex); 29 } 30 }
META-INF/spring-factories
1 # Application Context Initializers 2 org.springframework.context.ApplicationContextInitializer=\ 3 org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\ 4 org.springframework.boot.context.ContextIdApplicationContextInitializer,\ 5 org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\ 6 org.springframework.boot.context.web.ServerPortInfoApplicationContextInitializer
說明：
- 從所有jar獲取所有的META-INF/spring-factories檔案。（這裡只有spring-boot-1.3.0.RELEASE.jar下有一個）
- 遍歷每一個spring-factories檔案，並獲取其下key為factoryClass.getName()（這裡是入參
org.springframework.context.ApplicationContextInitializer）的value（這裡有以上四個ApplicationContextInitializer實現類）
以上四個類的作用：
至此，設定ApplicationContextInitialize就完成了。
總結：整個setInitializers實際上就是初始化了SpringApplication的屬性List<ApplicationContextInitializer<?>> initializers為一個ArrayList列表，該列表中有四個例項：
- ConfigurationWarningsApplicationContextInitializer的例項
- ContextIdApplicationContextInitializer的例項
- DelegatingApplicationContextInitializer例項
- ServerPortInfoApplicationContextInitializer例項
1.4、初始化ApplicationListener列表
1 private List<ApplicationListener<?>> listeners; 2 3 /** 4 * Sets the {@link ApplicationListener}s that will be applied to the SpringApplication 5 * and registered with the {@link ApplicationContext}. 6 * @param listeners the listeners to set 7 */ 8 public void setListeners(Collection<? extends ApplicationListener<?>> listeners) { 9 this.listeners = new ArrayList<ApplicationListener<?>>(); 10 this.listeners.addAll(listeners); 11 }
META-INF/spring-factories
1 # Application Listeners 2 org.springframework.context.ApplicationListener=\ 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
以上八個listener的作用如下：
至此，整個setListeners方法結束，初始化了一個包含以上8個ApplicationListener例項的List集合。
1.5、初始化主類mainApplicationClass
1 private Class<?> mainApplicationClass; 2 3 private Class<?> deduceMainApplicationClass() { 4 try { 5 StackTraceElement[] stackTrace = new RuntimeException().getStackTrace(); 6 for (StackTraceElement stackTraceElement : stackTrace) { 7 if ("main".equals(stackTraceElement.getMethodName())) { 8 return Class.forName(stackTraceElement.getClassName()); 9 } 10 } 11 } 12 catch (ClassNotFoundException ex) { 13 // Swallow and continue 14 } 15 return null; 16 }
說明：獲取main()方法所在的主類Class物件，並賦值給SpringApplication的mainApplicationClass屬性。
至此，SpringApplication物件初始化完成了。
總結：整個SpringApplication初始化的過程，就是初始化了
- 一個包含入參MySpringApplication.class的sources的Set<Object>
- 一個當前環境是否是web環境的boolean webEnvironment
- 一個包含4個ApplicationContextInitializer例項的List
- 一個包含8個ApplicationListener例項的List
- 一個main方法所在的主類的Class物件。
注意：