在做web項目開發中，尤其是企業級應用開發的時候，往往會在工程啟動的時候做許多的前置檢查。
比如檢查是否使用了我們組禁止使用的Mysql的group_concat函數，如果使用了項目就不能啟動，並指出哪個文件的xml文件使用了這個函數。
而在Spring的web項目中，我們可以介入Spring的啟動過程。我們希望在Spring容器將所有的Bean都初始化完成之后，做一些操作，這個時候我們就可以實現一個接口：
package com.yk.test.executor.processor
public class InstantiationTracingBeanPostProcessor implements ApplicationListener<ContextRefreshedEvent> {
@Override
public void onApplicationEvent(ContextRefreshedEvent event) {
//需要執行的邏輯代碼，當spring容器初始化完成后就會執行該方法。
}
}
同時在Spring的配置文件中，添加注入：
<!-- 當Spring容器啟動完成后執行下面的這個Bean --> <bean class="com.yk.test.executor.processor.InstantiationTracingBeanPostProcessor"/>
但是這個時候，會存在一個問題，在web 項目中（spring mvc），系統會存在兩個容器，一個是root application context ,另一個就是我們自己的 projectName-servlet context（作為root application context的子容器）。
這種情況下，就會造成onApplicationEvent方法被執行兩次。為了避免上面提到的問題，我們可以只在root application context初始化完成后調用邏輯代碼，其他的容器的初始化完成，則不做任何處理，修改后代碼
如下：
@Override
public void onApplicationEvent(ContextRefreshedEvent event) {
if(event.getApplicationContext().getParent() == null){//root application context 沒有parent，他就是老大.
//需要執行的邏輯代碼，當spring容器初始化完成后就會執行該方法。
}
}
其實更簡單的方法是使用注解：`@PostConstruct`，只需要在需要啟動的時候執行的方法上標注這個注解就搞定了。
注解描述如下：
/*
* Copyright (c) 2005, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
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:
* <p>
* <ul>
* <li>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.</li>
* <li>The method defined on an interceptor class MUST HAVE one of the
* following signatures:
* <p>
* void <METHOD>(InvocationContext)
* <p>
* Object <METHOD>(InvocationContext) throws Exception
* <p>
* <i>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.</i>
* </li>
* <li>The method defined on a non-interceptor class MUST HAVE the
* following signature:
* <p>
* void <METHOD>()
* </li>
* <li>The method on which PostConstruct is applied MAY be public, protected,
* package private or private.</li>
* <li>The method MUST NOT be static except for the application client.</li>
* <li>The method MAY be final.</li>
* <li>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.</li></ul>
* @since Common Annotations 1.0
* @see javax.annotation.PreDestroy
* @see javax.annotation.Resource
*/
@Documented
@Retention (RUNTIME)
@Target(METHOD)
public @interface PostConstruct {
}