Spring核心接口之InitializingBean

一、InitializingBean接口说明
InitializingBean接口为bean提供了属性初始化后的处理方法，它只包括afterPropertiesSet方法，凡是继承该接口的类，在bean的属性初始化后都会执行该方法。

package org.springframework.beans.factory;

/**
* Interface to be implemented by beans that need to react once all their
* properties have been set by a BeanFactory: for example, to perform custom
* initialization, or merely to check that all mandatory properties have been set.
*
* <p>An alternative to implementing InitializingBean is specifying a custom
* init-method, for example in an XML bean definition.
* For a list of all bean lifecycle methods, see the BeanFactory javadocs.
*
* @author Rod Johnson
* @see BeanNameAware
* @see BeanFactoryAware
* @see BeanFactory
* @see org.springframework.beans.factory.support.RootBeanDefinition#getInitMethodName
* @see org.springframework.context.ApplicationContextAware
*/
public interface InitializingBean {

/**
* Invoked by a BeanFactory after it has set all bean properties supplied
* (and satisfied BeanFactoryAware and ApplicationContextAware).
* <p>This method allows the bean instance to perform initialization only
* possible when all bean properties have been set and to throw an
* exception in the event of misconfiguration.
* @throws Exception in the event of misconfiguration (such
* as failure to set an essential property) or if initialization fails.
*/
void afterPropertiesSet() throws Exception;

}

从方法名afterPropertiesSet也可以清楚的理解该方法是在属性设置后才调用的。

二、源码分析接口应用
通过查看spring的加载bean的源码类(AbstractAutowireCapableBeanFactory)可以看到
protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd)
throws Throwable {
//判断该bean是否实现了实现了InitializingBean接口，如果实现了InitializingBean接口，则调用bean的afterPropertiesSet方法
boolean isInitializingBean = (bean instanceof InitializingBean);
if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
if (logger.isDebugEnabled()) {
logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
}
if (System.getSecurityManager() != null) {
try {
AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
public Object run() throws Exception {
//调用afterPropertiesSet
((InitializingBean) bean).afterPropertiesSet();
return null;
}
}, getAccessControlContext());
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
//调用afterPropertiesSet
((InitializingBean) bean).afterPropertiesSet();
}
}

if (mbd != null) { //判断是否指定了init-method方法，如果指定了init-method方法，则再调用制定的init-method
String initMethodName = mbd.getInitMethodName();
if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
!mbd.isExternallyManagedInitMethod(initMethodName)) {
//反射调用init-method方法
invokeCustomInitMethod(beanName, bean, mbd);
}
}
}
分析代码可以了解：
1：spring为bean提供了两种初始化bean的方式，实现InitializingBean接口，实现afterPropertiesSet方法，或者在配置文件中同过init-method指定，两种方式可以同时使用
2：实现InitializingBean接口是直接调用afterPropertiesSet方法，比通过反射调用init-method指定的方法效率相对来说要高点。但是init-method方式消除了对spring的依赖
3：如果调用afterPropertiesSet方法时出错，则不调用init-method指定的方法。

三、接口应用
  InitializingBean接口在spring框架中本身就很多应用，这就不多说了。我们在实际应用中如何使用该接口呢？

1、使用InitializingBean接口处理一个配置文件：
import java.io.File;
import java.io.FileInputStream;
import java.util.Properties;

import org.springframework.beans.factory.InitializingBean;

public class ConfigBean implements InitializingBean{

//微信公众号配置文件
private String configFile;

private String appid;

private String appsecret;

public String getConfigFile() {
return configFile;
}

public void setConfigFile(String configFile) {
this.configFile = configFile;
}

public void afterPropertiesSet() throws Exception {
if(configFile!=null){
File cf = new File(configFile);
if(cf.exists()){
Properties pro = new Properties();
pro.load(new FileInputStream(cf));
appid = pro.getProperty("wechat.appid");
appsecret = pro.getProperty("wechat.appsecret");
}
}
System.out.println(appid);
System.out.println(appsecret);
}
}
2、配置
spring配置文件：
<bean id="configBean" class="com.ConfigBean">
<property name="configFile" value="d:/wechat.properties"></property>
</bean>
wechat.properties配置文件
wechat.appid=wxappid
wechat.appsecret=wxappsecret
3、测试
     public static void main(String[] args) throws Exception {
        String config = Test.class.getPackage().getName().replace('.', '/') + "/bean.xml";
           ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(config);
           context.start();
        }


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