springboot

Web environment

下边这段话引用自官方文档

SpringApplication会尝试帮你创建正确的ApplicationContext，默认情况下会使用AnnotationConfigApplicationContext或者 AnnotationConfigEmbeddedWebApplicationContext，这取决于你开发的是否是web应用程序。

判断Web environment算法是非常简单的(基于某些类的存在与否)，如果有需要的话，你可以通过setWebEnvironment(boolean webEnvironment)覆盖这种行为。

1、 通常我们启动sping boot 应用都是用SpringApplication，比如：

@SpringBootApplicationpublic class Application {    public static void main(String[] args) {        SpringApplication.run(Application.class, args);    }}

2、 在SpringApplication构造方法中，最后一步调用了initialize方法

    public SpringApplication(Object... sources) {        this.bannerMode = Mode.CONSOLE;        this.logStartupInfo = true;        this.addCommandLineProperties = true;        this.headless = true;        this.registerShutdownHook = true;        this.additionalProfiles = new HashSet();        this.initialize(sources);    }

3、而在initialize方法中，可以看到 this.webEnvironment = this.deduceWebEnvironment(); 这行代码设置SpringApplication的属性webEnvironment，deduceWebEnvironment方法是推断是否是web应用的核心方法

private void initialize(Object[] sources) {        if(sources != null && sources.length > 0) {            this.sources.addAll(Arrays.asList(sources));        }        this.webEnvironment = this.deduceWebEnvironment();        this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class));        this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class));        this.mainApplicationClass = this.deduceMainApplicationClass();    }

4、 接下来我们到这个方法里deduceWebEnvironment看一下，确实如描述一样，算法非常简单，判断javax.servlet.Servlet, org.springframework.web.context.ConfigurableWebApplicationContext这两个类在classpath下是否存在。

private boolean deduceWebEnvironment() {    String[] var1 = WEB_ENVIRONMENT_CLASSES;    int var2 = var1.length;    for(int var3 = 0; var3 < var2; ++var3) {        String className = var1[var3];        if(!ClassUtils.isPresent(className, (ClassLoader)null)) {            return false;        }    }    return true;}WEB_ENVIRONMENT_CLASSES = new String[]{    "javax.servlet.Servlet",    "org.springframework.web.context.ConfigurableWebApplicationContext" };

5、 在SpringApplication的run方法中创建的ApplicationContext就是根据webEnvironment这个属性来判断。

public ConfigurableApplicationContext run(String... args) {       ...        try {            ...            context = this.createApplicationContext();            }            return context;           } catch (Throwable var9) {           ...        }    }protected ConfigurableApplicationContext createApplicationContext() {        Class contextClass = this.applicationContextClass;        if(contextClass == null) {            try {                contextClass = Class.forName(this.webEnvironment?"org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext":"org.springframework.context.annotation.AnnotationConfigApplicationContext");            } catch (ClassNotFoundException var3) {                throw new IllegalStateException("Unable create a default ApplicationContext, please specify an ApplicationContextClass", var3);            }        }        return (ConfigurableApplicationContext)BeanUtils.instantiate(contextClass);    }