org.springframework.beans.factory.config.PropertyPlaceholderConfigurer。由这个类别，您可以将一些组态设定，移出至.properties档案中，如此的安排可以让XML定义档负责系统相关设定，而.properties档可 以作为客户根据需求，自定义一些相关的参数。实际上，PropertyPlaceholderConfigurer起的作用就是将占位符指向的数据库配置 信息放在bean中定义的工具。

（1）使用以下方式读取单个本地文件到xml中：

                
                       
       
        file:/opt/config/config1.properties
                   
        
     

（2）使用以下方式读取多个本地文件到xml中：

        
            
       　　　　 
                  
                       
        
         file:/opt/config/config1.properties
         　　　　　　　　　 
        
         file:/opt/config/config2.properties
                    
               
              
          
         
         
         
     

注意读取单个配置文件，使用location，多个使用locations

上面是介绍org.springframework.beans.factory.config.PropertyPlaceholderConfigurer工具类的具体使用，下面浅析Spring框架下PropertyPlaceholderConfigurer类

package org.springframework.beans.factory.config;import java.util.Properties;import java.util.Set;import org.springframework.beans.BeansException;import org.springframework.core.Constants;import org.springframework.util.PropertyPlaceholderHelper;import org.springframework.util.PropertyPlaceholderHelper.PlaceholderResolver;import org.springframework.util.StringValueResolver;public class PropertyPlaceholderConfigurer extends PlaceholderConfigurerSupport {    /** 是否覆盖proprties文件 */    public static final int SYSTEM_PROPERTIES_MODE_NEVER = 0;    /**     * 默认值，不存在时覆盖     */    public static final int SYSTEM_PROPERTIES_MODE_FALLBACK = 1;    /**覆盖proprties文件*/    public static final int SYSTEM_PROPERTIES_MODE_OVERRIDE = 2;    private static final Constants constants = new Constants(PropertyPlaceholderConfigurer.class);    private int systemPropertiesMode = SYSTEM_PROPERTIES_MODE_FALLBACK;    private boolean searchSystemEnvironment = true;    public void setSystemPropertiesModeName(String constantName) throws IllegalArgumentException {        this.systemPropertiesMode = constants.asNumber(constantName).intValue();    }       public void setSystemPropertiesMode(int systemPropertiesMode) {        this.systemPropertiesMode = systemPropertiesMode;    }    public void setSearchSystemEnvironment(boolean searchSystemEnvironment) {        this.searchSystemEnvironment = searchSystemEnvironment;    }       protected String resolvePlaceholder(String placeholder, Properties props, int systemPropertiesMode) {        String propVal = null;        if (systemPropertiesMode == SYSTEM_PROPERTIES_MODE_OVERRIDE) {            propVal = resolveSystemProperty(placeholder);        }        if (propVal == null) {            propVal = resolvePlaceholder(placeholder, props);        }        if (propVal == null && systemPropertiesMode == SYSTEM_PROPERTIES_MODE_FALLBACK) {            propVal = resolveSystemProperty(placeholder);        }        return propVal;    }      protected String resolvePlaceholder(String placeholder, Properties props) {        return props.getProperty(placeholder);    }      protected String resolveSystemProperty(String key) {        try {            String value = System.getProperty(key);            if (value == null && this.searchSystemEnvironment) {                value = System.getenv(key);            }            return value;        }        catch (Throwable ex) {            if (logger.isDebugEnabled()) {                logger.debug("Could not access system property '" + key + "': " + ex);            }            return null;        }    }    @Override    protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess, Properties props)            throws BeansException {        StringValueResolver valueResolver = new PlaceholderResolvingStringValueResolver(props);        this.doProcessProperties(beanFactoryToProcess, valueResolver);    }       @Deprecated    protected String parseStringValue(String strVal, Properties props, Set
      visitedPlaceholders) {        PropertyPlaceholderHelper helper = new PropertyPlaceholderHelper(                placeholderPrefix, placeholderSuffix, valueSeparator, ignoreUnresolvablePlaceholders);        PlaceholderResolver resolver = new PropertyPlaceholderConfigurerResolver(props);        return helper.replacePlaceholders(strVal, resolver);    }    private class PlaceholderResolvingStringValueResolver implements StringValueResolver {        private final PropertyPlaceholderHelper helper;        private final PlaceholderResolver resolver;        public PlaceholderResolvingStringValueResolver(Properties props) {            this.helper = new PropertyPlaceholderHelper(                    placeholderPrefix, placeholderSuffix, valueSeparator, ignoreUnresolvablePlaceholders);            this.resolver = new PropertyPlaceholderConfigurerResolver(props);        }        public String resolveStringValue(String strVal) throws BeansException {            String value = this.helper.replacePlaceholders(strVal, this.resolver);            return (value.equals(nullValue) ? null : value);        }    }    private class PropertyPlaceholderConfigurerResolver implements PlaceholderResolver {        private final Properties props;        private PropertyPlaceholderConfigurerResolver(Properties props) {            this.props = props;        }        public String resolvePlaceholder(String placeholderName) {            return PropertyPlaceholderConfigurer.this.resolvePlaceholder(placeholderName, props, systemPropertiesMode);        }    }}

 

首先看下PropertyPlaceholderConfigurer类的结构

1、注入会初始化property的location属性

 

public void setLocation(Resource location) {    this.locations = new Resource[] {location};}public void setLocations(Resource[] locations) {    this.locations = locations;}

 

2、SystemPropertiesModeName：SystemPropertiesModeName属性用来指定系统变量（System.getProperty）是否覆盖proprties文件。默认是在配置文件里找不到时使用。三个选项分别是：

 

　　SYSTEM_PROPERTIES_MODE_NEVER（不覆盖）

　　SYSTEM_PROPERTIES_MODE_FALLBACK（默认值，不存在时覆盖）

　　SYSTEM_PROPERTIES_MODE_OVERRIDE（覆盖）

　　从源码中可以看出searchSystemEnvironment属性的默认值为true，即允许环境变量覆盖properties中的值。