Allow PropertySourcesPlaceholderConfigurer subclass to customize PropertyResolver

[1zg12]

Background

Prior to Spring 5.2, when we need to customize the PropertiesLoader, we can subclass PropertyPlaceholderConfigurer, and override the resolvePlaceholder method.

Now with PropertySourcesPlaceholderConfigurer deprecated PropertyPlaceholderConfigurer, the resolvePlaceholder method has been moved out of the PlaceHolderConfigurer and into a separate PropertyResolver.

Somehow the PropertyResolver is hardcoded [*]:

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

Line 157 in 01bea34

processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));

Due to the hardcode, the only way left for developers to override the resolvePlaceholder now becomes to duplicate the whole postProcessBeanFactory method and then provide custom PropertyResolver.

Changes

Instead of hardcoding a PropertyResolver as existing and make it very difficult to customize, this PR will point to a factory method to provide the PropertyResolver as needed.

I believe there is no side effect on this. It only adds the flexibility for customization which has been lost.

Note [*]

Even though PropertyPlaceholderConfigurer was also using a hardcoded class,

spring-framework/spring-beans/src/main/java/org/springframework/beans/factory/config/PropertyPlaceholderConfigurer.java

Line 210 in 01bea34

StringValueResolver valueResolver = new PlaceholderResolvingStringValueResolver(props);

It's a private inner class, which even doesn't allow override. However, after several redirects, it eventually points to the resolvePlaceholder of the current class implementation

spring-framework/spring-beans/src/main/java/org/springframework/beans/factory/config/PropertyPlaceholderConfigurer.java

Line 250 in 01bea34

return PropertyPlaceholderConfigurer.this.resolvePlaceholder(placeholderName,

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[1zg12]

hi @rstoyanchev , can you review this ?

[1zg12]

Bump up this, as we are still having issue to overwrite this setting in the newer version of spring. Thanks.

[snicoll]

@1zg12 thanks very much for making your first contribution to the Spring Framework.