spring boot 源码解析-SpringApplication初始化

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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:

2019101710071\_1.png

QQ截图20180515120216.png

参考视频教程:Spring Boot源码解析

小程序

作者:小红牛
链接:https://www.jianshu.com/p/7355972957ff
來源:简书
简书著作权归作者所有,任何形式的转载都请联系作者获得授权并注明出处。


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