SpringBoot核心启动类的SpringApplication流程分析。上一篇用SpringBoot快速搭建并启动了一个WEB服务。有两个点与spring的项目启动有不同,这里我们分析SpringApplication的启动流程。更一篇才是说明关于@SpringBootApplication的SpringBoot注释做说明《SpringBoot-自动配置源码解析》,通过这两个点我们可以对SpringBoot的运行机制做一个比较细致的了解。
这一篇将从SpringApplication.run();开始:
/** * Static helper that can be used to run a {@link SpringApplication} from the * specified source using default settings. * @param source the source to load * @param args the application arguments (usually passed from a Java main method) * @return the running {@link ApplicationContext} */
public static ConfigurableApplicationContext run(Object source, String... args) {
return run(new Object[] { source }, args);
}
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进入重构run()方法
/** * Static helper that can be used to run a {@link SpringApplication} from the * specified sources using default settings and user supplied arguments. * @param sources the sources to load * @param args the application arguments (usually passed from a Java main method) * @return the running {@link ApplicationContext} */
public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
return new SpringApplication(sources).run(args);
}
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从这里可以看到首先创建了一个SpringApplication实例,然后在调用的其run()方法。首先我们先去创建实例这一流程:
/** * Create a new {@link SpringApplication} instance. The application context will load * beans from the specified sources (see {@link SpringApplication class-level} * documentation for details. The instance can be customized before calling * {@link #run(String...)}. * @param sources the bean sources * @see #run(Object, String[]) * @see #SpringApplication(ResourceLoader, Object...) */
public SpringApplication(Object... sources) {
initialize(sources);
}
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可以看到其调用了initialize()方法
@SuppressWarnings({ "unchecked", "rawtypes" })
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();
}
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从面的代码可以看到初始化过程做了以下几件事情
this.webEnvironment = deduceWebEnvironment();
这一个方法决定创建的是一个WEB应用还是一个SPRING的标准Standalone应用。如果入方法可以看到其是怎么判断的:
private boolean deduceWebEnvironment() {
for (String className : WEB_ENVIRONMENT_CLASSES) {
if (!ClassUtils.isPresent(className, null)) {
return false;
}
}
return true;
}
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可以看到是根据org.springframework.util.ClassUtils的静态方法去判断classpath里面是否有WEB_ENVIRONMENT_CLASSES包含的类,如果有都包含则返回true则表示启动一个WEB应用,否则返回false启动一个标准Spring的应用。然后通过代码:
private static final String[] WEB_ENVIRONMENT_CLASSES =
{ "javax.servlet.Servlet",
"org.springframework.web.context.ConfigurableWebApplicationContext" };
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可以看到是否启动一个WEB应用就是取决于classpath下是否有javax.servlet.Servlet和
org.springframework.web.context.ConfigurableWebApplicationContext。然后进入下一个阶段:
setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
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这个方法则是初始化classpath下的所有的可用的ApplicationContextInitializer
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
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这个方法则是初使化classpath下的所有的可用的ApplicationListener
this.mainApplicationClass = deduceMainApplicationClass();
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;
}
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最后找出main方法的全类名并返回其实例并设置到SpringApplication的this.mainApplicationClass完成初始化。然后调用SpringApplication实例的run方法来启动应用,代码如下:
/** * Run the Spring application, creating and refreshing a new * {@link ApplicationContext}. * @param args the application arguments (usually passed from a Java main method) * @return a running {@link ApplicationContext} */
public ConfigurableApplicationContext run(String... args) {
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
configureHeadlessProperty();
SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.started();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);
Banner printedBanner = printBanner(environment);
context = createApplicationContext();
prepareContext(context, environment, listeners, applicationArguments,
printedBanner);
refreshContext(context);
afterRefresh(context, applicationArguments);
listeners.finished(context, null);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}
return context;
}
catch (Throwable ex) {
handleRunFailure(context, listeners, ex);
throw new IllegalStateException(ex);
}
}
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由于过程比较长,则把详细说明放入下一篇博客进行说明,这里只做简要的过程说明run()方法的代码中:蓝色部分为加载SpringApplicationRunListener对整个容器的初始化过程进行监听,这里先不做解释,然后先观察剩下的几行的代码:
ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
Banner printedBanner = printBanner(environment);
context = createApplicationContext();
prepareContext(context, environment, listeners, applicationArguments, printedBanner);
refreshContext(context);
afterRefresh(context, applicationArguments);
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首先是获取启动时传入参数args并初始化为ApplicationArguments对象
SpringApplication.run(Application.class, args);取这里传入值。
然后配置SpringBoot应用的环境:
ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
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下面的则打印标志这个方法不说明,因为没有什么实质性作用,反应到控制台则是以下的效果如果确实想玩玩修改一下标志,那可以在项目的classpath下新建一个banner.txt文件,把想打印到控制台的数据放到文件中即可。比如:
src/main/resources/banner.txt中加入以下内容:
_ _ _ _ __ __ _ _ _____ _ _ ___
| | | | | | | | \ \ / / | | | | | ____| | | | | / |
| | | | | | | | \ \/ / | | | | | |__ | | | | / /| |
| | | | | | | | } { | | | | | __| _ | | | | / / | |
| |___ | | | |_| | / /\ \ | |_| | | |___ | |_| | | | / / | |
|_____| |_| \_____/ /_/ \_\ \_____/ |_____| \_____/ |_| /_/ |_|
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那么启动的时候就可以看到些标识。
然后下面代码就是比较核心的:
context = createApplicationContext();
prepareContext(context, environment, listeners, applicationArguments, printedBanner);
refreshContext(context);
afterRefresh(context, applicationArguments);
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首先是createApplicationContext()方法:
/** * Strategy method used to create the {@link ApplicationContext}. By default this * method will respect any explicitly set application context or application context * class before falling back to a suitable default. * @return the application context (not yet refreshed) * @see #setApplicationContextClass(Class) */
protected ConfigurableApplicationContext createApplicationContext() {
Class<?> contextClass = this.applicationContextClass;
if (contextClass == null) {
try {
contextClass = Class.forName(this.webEnvironment
? DEFAULT_WEB_CONTEXT_CLASS : DEFAULT_CONTEXT_CLASS);
}
catch (ClassNotFoundException ex) {
throw new IllegalStateException(
"Unable create a default ApplicationContext, "
+ "please specify an ApplicationContextClass",
ex);
}
}
return (ConfigurableApplicationContext) BeanUtils.instantiate(contextClass);
}
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可以看出根据这前初始化过程初始化的this.webEnvironment来决定初始化一个什么容器。如果classpath下是否有javax.servlet.Servlet和
org.springframework.web.context.ConfigurableWebApplicationContext类,
则使用DEFAULT_WEB_CONTEXT_CLASS初始化容器,如果不存在则用DEFAULT_CONTEXT_CLASS初始化容器。
public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
+ "annotation.AnnotationConfigApplicationContext";
/** * The class name of application context that will be used by default for web * environments. */
public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework."
+ "boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext";
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以上是代码指定了容器的类名,最后通过Spring的工具类初始化容器类bean
BeanUtils.instantiate(contextClass);
完成容器的创建工作。然后执行以下的几个步骤完成整个容器的创建与启动以及bean的注入功能。
prepareContext(context, environment, listeners, applicationArguments, printedBanner);
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以下这一句代码是实现spring-boot-starter-*的自动化配置的关键。包括spring.factories的加载以及自动化配置类的加载以及注入容器的功能可以参考《SpringBoot-自动配置源码解析》
refreshContext(context);
afterRefresh(context, applicationArguments);
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至此通过SpringBoot启动的容器已经构造完成。这里忽略了启动流程中的收集各种Listener,创建Environment及Environment的初始化的因为这些地方都是SpringBoot提供的各种扩展点,后面的博客会详细的说明各个扩展点的用处以及扩展的方式。