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：
|
1
2
3
4
5
6
7
|
package com.yk.test.executor.processorpublic class InstantiationTracingBeanPostProcessor implements ApplicationListener<ContextRefreshedEvent> {@Overridepublic 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：
|
1
2
|
<!-- 当Spring容器启动完成后执行下面的这个Bean --><bean class="com.yk.test.executor.processor.InstantiationTracingBeanPostProcessor"/> |
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
如下：
|
1
2
3
4
5
6
|
@Overridepublic void onApplicationEvent(ContextRefreshedEvent event) {if(event.getApplicationContext().getParent() == null){//root application context 没有parent，他就是老大.//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。}} |
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。
注解描述如下：
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
|
/* * 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 {} |