spring的多个PropertyPlaceholderConfigurer实例装配的问题

1. 默认情况下，使用PropertyPlaceholderConfigurer多实例装配出现异常

    在项目中尝试 在不同的spring的配置文件中分别引入相应的properties文件，这样会在spring配置文件中配置多个PropertyPlaceholderConfigurer实例，但是这样使用的话就会出现key找不到的问题，异常信息如下：

“ Could not resolve placeholder 'key2”

信息: Destroying singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@38882d9a: defining beans [propertyConfigurer1,propertyConfigurer2,serviceA,serviceB,resourceServiceImpl]; root of factory hierarchy
Exception in thread "main" org.springframework.beans.factory.BeanDefinitionStoreException: Invalid bean definition with name 'serviceA' defined in class path resource [applicationContext.xml]: Could not resolve placeholder 'key2'
    at org.springframework.beans.factory.config.PropertyPlaceholderConfigurer.processProperties(PropertyPlaceholderConfigurer.java:287)
    at org.springframework.beans.factory.config.PropertyResourceConfigurer.postProcessBeanFactory(PropertyResourceConfigurer.java:75)
    at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:663)
    at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:638)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:407)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
    at org.simonme.srcstudy.spring3.demo.service.BeanAssemblyTest.main(BeanAssemblyTest.java:34)

配置形式如下（为了分析问题，配置形式做了简化，但是体现出了原本的意思）：

<bean id="propertyConfigurer1"
    class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
    <property name="locations">
        <list>
            <value>
                classpath:conf/test/test1.properties
            </value>
        </list>
    </property>
</bean>

<bean id="propertyConfigurer2"
    class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
    <property name="locations">
        <list>
            <value>
                classpath:conf/test/test2.properties
            </value>
        </list>
    </property>
</bean>

<bean id="serviceA" class="org.simonme.srcstudy.spring3.demo.service.impl.ServiceAImpl">
    <property name="field" value="field4AInstacne"/>
    <property name="key1" value="${key1}"/>
    <property name="key2" value="${key2}"/>
</bean>

Java的service代码如下：

public class ServiceAImpl implements ServiceA
{
    
    private String field;
    
    private String key1;
    
    private String key2;
    
    /**
     * 模拟返回测试数据
     * @return
     */
    @Override
    public String queryA()
    {
        System.out.println("key1:" + key1);
        System.out.println("key2:" + key2);
        return "Query A Result" + field;
    }

    public String getField()
    {
        return field;
    }

    public void setField(String field)
    {
        this.field = field;
    }

    public String getKey1()
    {
        return key1;
    }

    public void setKey1(String key1)
    {
        this.key1 = key1;
    }

    public String getKey2()
    {
        return key2;
    }

    public void setKey2(String key2)
    {
        this.key2 = key2;
    }
}

2. 如何装配能不出异常

如果仍然需要使用两个(或多个)PropertyPlaceholderConfigurer实例进行装配，怎样才能解决上面的异常？

<bean id="propertyConfigurer1"
    class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
    <property name="ignoreUnresolvablePlaceholders" value="true"/>
    <property name="locations">
        <list>
            <value>
                classpath:conf/test/test1.properties
            </value>
        </list>
    </property>
</bean>

<property name="ignoreUnresolvablePlaceholders" value="true"/>

加上上面的一行，表示可以忽略未解析到的占位符。这样就不会报错。如果对每个PropertyPlaceholderConfigurer实例都配置了这句话(忽略未解析占位符错误)，那什么时候检查未解析到的占位符呢？

3. 如何解决占位符未配置的检查问题？不完美

    对于未解析到的占位符，可以通过order属性来调整bean装配的优先级，然后在最后装配的PropertyPlaceholderConfigurer实例上面启用未解析到的占位符检查。

4.使PropertyPlaceholderConfigurer多实例装配出现异常对应的spring的代码在哪里

    分析上述问题涉及的spring代码主要在哪里？

在spring bean装配时，一个PropertyPlaceholderConfigurer就是一个后置处理器BeanFactoryPostProcessor。在装配完PropertyPlaceholderConfigurer之后，就会触发org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(Collection<? extends BeanFactoryPostProcessor>, ConfigurableListableBeanFactory)方法，代码如下：

/**
 * Invoke the given BeanFactoryPostProcessor beans.
 */
private void invokeBeanFactoryPostProcessors(
        Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {

    for (BeanFactoryPostProcessor postProcessor : postProcessors) {
        postProcessor.postProcessBeanFactory(beanFactory);
    }
}

在每调用完一个BeanFactoryPostProcessor之后，就会去解析所有的bean中引用properties的占位符，这时就会出现占位符不能解析的问题(不能解析的占位在后面的BeanFactoryPostProcessor中，也就是PropertyPlaceholderConfigurer实例)。