spring源码分析初始化bean的遗留问题

作者: 会上树的程序猿 | 来源:发表于2019-08-05 23:15 被阅读0次

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我们在spring初始化bean的过程中在最后遗留了一个问题,本篇来说,问题是在#InitializingBean(...)中首先是对bean的判断,可以通过#afterPropertiesSet()方法来处理,具体详情请看spring容器之创建bean的终结篇,当然还有我们在配置文件中配置的init-method方法等

InitializingBean

该接口位于org.springframework.beans.factory包下,只有一个afterPropertiesSet()方法,其主要的作用是对bean自定义的实现过程,属性设置的检查等操作.

public interface InitializingBean {

/**
 * Invoked by the containing {@code BeanFactory} after it has set all bean properties
 * and satisfied {@link BeanFactoryAware}, {@code ApplicationContextAware} etc.
 * <p>This method allows the bean instance to perform validation of its overall
 * configuration and final initialization when all bean properties have been set.
 * @throws Exception in the event of misconfiguration (such as failure to set an
 * essential property) or if initialization fails for any other reason
 */
void afterPropertiesSet() throws Exception;

大概的可以了解到afterPropertiesSet()是在实例化时进行对bean的属性的检查,前提是我们此刻的bean是已经完成了Aware和BeanPostProcesser的设置过程,接下来我们简单的写个Demo来了解下该方法的作用:

case

定义一个类实现InitializingBean接口

public class InitializingBeanCase implements InitializingBean {

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

private String name;

public void afterPropertiesSet() throws Exception {

    System.out.println("实例化bean开始了....");
    this.name = "9527";

}

代码简单,我们直接给name属性赋值,将该类交给spring容器来创建:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="initializingBeanCase"  class="com.sgcc.InitializingBean.InitializingBeanCase">
    <property name="name" value="9528"/>
    </bean>
</beans>

在配置文件中我们是想让spring容器帮我们创建name为9528的bean

测试代码:

public class App {

public static void main(String[] args) {
    ClassPathResource resource = new ClassPathResource("InitializingBeanCaseConfig.xml");
    DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
    XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory);
    reader.loadBeanDefinitions(resource);
    InitializingBeanCase initializingBeanCase = (InitializingBeanCase) factory.getBean("initializingBeanCase");

    System.out.println("name:"+initializingBeanCase.getName());
}

}

运行结果:

image.png

看到结果的时候我也蒙了,奥明白了,该方法原来是spring给我们提供了一种在初始化时,动态修改bean的属性的途径,使得我们更加随心操作bean的一种手段.

初始过程

既然该方法是在初始化时来操作bean的属性的,我们来看一下,初始化过程#invokeInitMethods(...)的过程,该方法调用的前提是判断我们的bean是否是实现了InitializingBean接口,来看代码:

protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
        throws Throwable {
    //首先是检查是否是InitializingBean,如果是需要调用afterPropertiesSet
    boolean isInitializingBean = (bean instanceof InitializingBean);
    if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
        if (logger.isTraceEnabled()) {
            logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
        }
        //在系统安全管理器的环境下
        if (System.getSecurityManager() != null) {
            try {
                AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
                    //1.初始化属性
                    ((InitializingBean) bean).afterPropertiesSet();
                    return null;
                }, getAccessControlContext());
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            //同上
            ((InitializingBean) bean).afterPropertiesSet();
        }
    }

    if (mbd != null && bean.getClass() != NullBean.class) {
        //判断是否指定了 init-method()
        //如果指定了 init-method()，则再调用指定的init-method
        String initMethodName = mbd.getInitMethodName();
        if (StringUtils.hasLength(initMethodName) &&
                !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
                !mbd.isExternallyManagedInitMethod(initMethodName)) {
            //2.激活自定义方法的入口
             // 利用反射机制执行
            invokeCustomInitMethod(beanName, bean, mbd);
        }
    }
}

简单的来看下流程:

首先检查bean是否是实现了InitializingBean接口,如果实现了则调用afterPropertiesSet方法
同时也检查是否指定了init-method,如果指定了,则通过反射机制调用指定的init-method

init-method是spring提供的另外一种方法,我们来看

init-method

还记得我们在spring容器之Bean标签的解析文章中说过关于init-method标签,该标签在这里体现了它的作用,主要是在bean初始化的过程中调用指定的init-method方法来替代InitializingBean接口,我们通过案例来看:

init-method案例

我们自定义一个类,其中自定义一个方法:

public class InitMethodCase {

private String name;

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}
//自定义方法setInitMethod
public void setInitMethod(){

    System.out.println("调用了init的初始化过程......");
    this.name = "9527";
}

来看配置文件:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

<bean id="initMethodCase" class="com.sgcc.initMethod.InitMethodCase" init-method="setInitMethod">
    <property name="name" value="9529"/>
</bean>
</beans>

测试代码:

public class App {

public static void main(String[] args) {
    ClassPathResource resource = new ClassPathResource("InitMethodCaseConfig.xml");
    DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
    XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory);
    reader.loadBeanDefinitions(resource);
    InitMethodCase initMethodCase = (InitMethodCase) factory.getBean("initMethodCase");
    System.out.println("name="+initMethodCase.getName());

}