make the property resolver configurable

spring-context/src/main/java/org/springframework/context/support/PropertySourcesPlaceholderConfigurer.java

@@ -24,13 +24,7 @@
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.beans.factory.config.PlaceholderConfigurerSupport;
 import org.springframework.context.EnvironmentAware;
-import org.springframework.core.env.ConfigurablePropertyResolver;
-import org.springframework.core.env.Environment;
-import org.springframework.core.env.MutablePropertySources;
-import org.springframework.core.env.PropertiesPropertySource;
-import org.springframework.core.env.PropertySource;
-import org.springframework.core.env.PropertySources;
-import org.springframework.core.env.PropertySourcesPropertyResolver;
+import org.springframework.core.env.*;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.StringValueResolver;
@@ -154,10 +148,17 @@ public String getProperty(String key) {
 			}
 		}
 
-		processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));
+		processProperties(beanFactory, getPropertyResolver(this.propertySources));
 		this.appliedPropertySources = this.propertySources;
 	}
 
+	/**
+	 * Construct and provide a PropertyResolver from the given properties.
+	 */
+	public ConfigurablePropertyResolver getPropertyResolver(MutablePropertySources propertySources){
+		return new PropertySourcesPropertyResolver(propertySources);
+	}
+
 	/**
 	 * Visit each bean definition in the given bean factory and attempt to replace ${...} property
 	 * placeholders with values from the given properties.