当spring 容器初始化完成后，执行某个方法

在做web项目开发中，尤其是企业级应用开发的时候，往往会在工程启动的时候做许多的前置检查。

比如检查是否使用了我们组禁止使用的Mysql的group_concat函数，如果使用了项目就不能启动，

并指出哪个文件的xml文件使用了这个函数。而在Spring的web项目中，我们可以介入Spring的启动过程。

我们希望在Spring容器将所有的Bean都初始化完成之后，做一些操作，这个时候我们就可以实现一个接口：

public class InstantiationTracingBeanPostProcessor implements ApplicationListener {
	@Override
	public void onApplicationEvent(ContextRefreshedEvent event) {
	//需要执行的逻辑代码，当spring容器初始化完成后就会执行该方法。
	}
}

同时在Spring的配置文件中，添加注入：

但是这个时候，会存在一个问题，在web 项目中（spring mvc），系统会存在两个容器，

一个是root application context ,

另一个就是我们自己的 projectName-servlet context（作为root application context的子容器）。

这种情况下，就会造成onApplicationEvent方法被执行两次。

为了避免上面提到的问题，我们可以只在root application context初始化完成后调用逻辑代码，

其他的容器的初始化完成，则不做任何处理，修改后代码

@Override
public void onApplicationEvent(ContextRefreshedEvent event) {
	//root application context 没有parent，他就是老大.
	if(event.getApplicationContext().getParent() == null){
	//需要执行的逻辑代码，当spring容器初始化完成后就会执行该方法。
	}
}

方法二、　　

其实更简单的方法是使用注解：`@PostConstruct`，

只需要在需要启动的时候执行的方法上标注这个注解就搞定了，注意，还要在该类上加上@Component注解，否则不能执行。

注解描述如下：

package javax.annotation;
 
import java.lang.annotation.*;
import static java.lang.annotation.ElementType.*;
import static java.lang.annotation.RetentionPolicy.*;
 
/**
 * The PostConstruct annotation is used on a method that needs to be executed
 * after dependency injection is done to perform any initialization. This
 * method MUST be invoked before the class is put into service. This
 * annotation MUST be supported on all classes that support dependency
 * injection. The method annotated with PostConstruct MUST be invoked even
 * if the class does not request any resources to be injected. Only one
 * method can be annotated with this annotation. The method on which the
 * PostConstruct annotation is applied MUST fulfill all of the following
 * criteria:
 * 

 * 

 * 
The method MUST NOT have any parameters except in the case of
 * interceptors in which case it takes an InvocationContext object as
 * defined by the Interceptors specification.
 * 
The method defined on an interceptor class MUST HAVE one of the
 * following signatures:
 * 

 * void (InvocationContext)
 * 

 * Object (InvocationContext) throws Exception
 * 

 * Note: A PostConstruct interceptor method must not throw application
 * exceptions, but it may be declared to throw checked exceptions including
 * the java.lang.Exception if the same interceptor method interposes on
 * business or timeout methods in addition to lifecycle events. If a
 * PostConstruct interceptor method returns a value, it is ignored by
 * the container.
 * 
 * 
The method defined on a non-interceptor class MUST HAVE the
 * following signature:
 * 

 * void ()
 * 
 * 
The method on which PostConstruct is applied MAY be public, protected,
 * package private or private.
 * 
The method MUST NOT be static except for the application client.
 * 
The method MAY be final.
 * 
If the method throws an unchecked exception the class MUST NOT be put into
 * service except in the case of EJBs where the EJB can handle exceptions and
 * even recover from them.
 * @since Common Annotations 1.0
 * @see javax.annotation.PreDestroy
 * @see javax.annotation.Resource
 */
@Documented
@Retention (RUNTIME)
@Target(METHOD)
public @interface PostConstruct {
}

方法三、
方法实现InitializingBean或者ServletContextAware