Spring版本

5.2.5.RELEASE

参考

源码解读

PropertyOverrideConfigurer允许我们对 Spring 容器中配置的任何我们想处理的 bean 定义的 property 信息进行覆盖替换。它与PropertySourcesPlaceholderConfigurer的区别在于，PropertyOverrideConfigurer在于替换已经具体化的property信息，而PropertySourcesPlaceholderConfigurer做的事情是，property还只是一个占位符，是将占位符解析成具体的property信息。

Demo

1.1 Student

public class Student {

    private String id;

    private String name;

    private String desc;

   // 省略 getter、setter
}

1.2 spring.xml

<?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="propertyConfig" class="org.springframework.beans.factory.config.PropertyOverrideConfigurer">
        <property name="locations">
            <list>
                <value>classpath:application-dev.properties</value>
            </list>
        </property>
    </bean>
    <bean id="student" class="com.kungyu.custom.element.Student">
        <property name="name" value="name"/>
    </bean>
</beans>

1.3 application-dev.property

student.name=student-dev

1.4 测试

  public class Test {
    public static void main(String[] args) throws ClassNotFoundException {
        ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
        Student student = (Student) context.getBean("student");
        System.out.println(student.getName());
}

输出结果：

student-dev

可以看到，在spring.xml中配置的student的name属性原本的值name已经被替换成student-dev了。

那么，如果同时配置PropertySourcesPlaceholderConfigurer和PropertyOverrideConfigurer，哪个会生效呢？在测试之前，不妨猜想一下：

假如PropertySourcesPlaceholderConfigurer先生效，那么占位符首先会被解析成对应的配置值，之后再被PropertyOverrideConfigurer的值覆盖
如果PropertyOverrideConfigurer先生效，那么替换后，执行PropertySourcesPlaceholderConfigurer的时候可能查找不到对应的值，那么最后显示的也是PropertyOverrideConfigurer配置的值

俩者的结果是一样的，那么，动手实现一下。
添加以下俩个配置文件
application1.properties:

student.name=student-PropertySourcesPlaceholderConfigurer

application2.properties:

student.name=student-PropertyOverrideConfigurer

修改spring.xml如下:

<?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="propertySourcesPlaceholderConfigurer" class="org.springframework.context.support.PropertySourcesPlaceholderConfigurer">
        <property name="locations">
            <list>
                <value>classpath:application1.properties</value>
            </list>
        </property>
    </bean>
    <bean id="propertyOverrideConfigurer" class="org.springframework.beans.factory.config.PropertyOverrideConfigurer">
        <property name="locations">
            <list>
                <value>classpath:application2.properties</value>
            </list>
        </property>
    </bean>

    <bean id="student" class="com.kungyu.custom.element.Student">
        <property name="name" value="${student.name}"/>
    </bean>
</beans>

其余保持不变，执行结果如下：

student-PropertyOverrideConfigurer

可以看到和我们原来的猜想是一致的。

2. 源码解读

2.1 PropertyOverrideConfigurer#processProperties

    @Override
    protected void processProperties(ConfigurableListableBeanFactory beanFactory, Properties props)
            throws BeansException {

        for (Enumeration<?> names = props.propertyNames(); names.hasMoreElements();) {
            String key = (String) names.nextElement();
            try {
                processKey(beanFactory, key, props.getProperty(key));
            }
            catch (BeansException ex) {
                String msg = "Could not process key '" + key + "' in PropertyOverrideConfigurer";
                if (!this.ignoreInvalidKeys) {
                    throw new BeanInitializationException(msg, ex);
                }
                if (logger.isDebugEnabled()) {
                    logger.debug(msg, ex);
                }
            }
        }
    }

遍历属性，依次调用processKey执行属性覆盖逻辑

2.2 PropertyOverrideConfigurer#processKey

    protected void processKey(ConfigurableListableBeanFactory factory, String key, String value)
            throws BeansException {

        // 根据.进行切割，分别获取beanName和beanProperty，比如：配置项student.name切割成student和name
        int separatorIndex = key.indexOf(this.beanNameSeparator);
        if (separatorIndex == -1) {
            throw new BeanInitializationException("Invalid key '" + key +
                    "': expected 'beanName" + this.beanNameSeparator + "property'");
        }
        String beanName = key.substring(0, separatorIndex);
        String beanProperty = key.substring(separatorIndex + 1);
        this.beanNames.add(beanName);
        applyPropertyValue(factory, beanName, beanProperty, value);
        if (logger.isDebugEnabled()) {
            logger.debug("Property '" + key + "' set to value [" + value + "]");
        }
    }

将配置项根据.这个分割符进行切割，获得beanName和beanProperty，调用applyPropertyValue执行覆盖处理逻辑

2.3 PropertyOverrideConfigurer#applyPropertyValue

    protected void applyPropertyValue(
            ConfigurableListableBeanFactory factory, String beanName, String property, String value) {

        BeanDefinition bd = factory.getBeanDefinition(beanName);
        BeanDefinition bdToUse = bd;
        while (bd != null) {
            bdToUse = bd;
            bd = bd.getOriginatingBeanDefinition();
        }
        PropertyValue pv = new PropertyValue(property, value);
        pv.setOptional(this.ignoreInvalidKeys);
        // 进行覆盖
        bdToUse.getPropertyValues().addPropertyValue(pv);
    }

构造一个PropertyValue对象pv，调用addPropertyValue将pv加入到propertyValues当中

2.4 MutablePropertyValues#addPropertyValue

    public MutablePropertyValues addPropertyValue(PropertyValue pv) {
        for (int i = 0; i < this.propertyValueList.size(); i++) {
            PropertyValue currentPv = this.propertyValueList.get(i);
            if (currentPv.getName().equals(pv.getName())) {
                // 如果有需要，进行合并
                pv = mergeIfRequired(pv, currentPv);
                // 覆盖
                setPropertyValueAt(pv, i);
                return this;
            }
        }
        // 查找不到，直接新增
        this.propertyValueList.add(pv);
        return this;
    }

如果查找得到对应的属性，进行合并（如果有需要）和覆盖，否则，直接新增该属性

3. 总结

PropertyOverrideConfigurer很好理解，源码也很简单，功能就是覆盖bean已经定义好的属性值。

Spring源码解析（十八）-PropertyOverrideConfigurer