How to programmatically resolve property placeholder in Spring

问题

I currently work on a web application based on Spring 3.1.0.M1, annotations based, and I have a problem with resolving property placeholders in one specific place of my application.

Here is the story.

1) In my web application context (loaded by DispatcherServlet), i have

mvc-config.xml:

<!-- Handles HTTP GET requests for /resources/version/**  -->
<resources mapping="/${app.resources.path}/**" location="/static/" cache-period="31556926"/> 

...

<!-- Web properties -->
<context:property-placeholder location="
    classpath:app.properties
    "/>

2) Inside app.properties, there are 2 properties, among others:

app.properties:

# Properties provided (filtered) by Maven itself
app.version: 0.1-SNAPSHOT
...

# Static resources mapping
app.resources.path: resources/${app.version}

3) I have a JSP custom tag in my JSP 2.1 templates. This tag is responsible for full resource path construction depending on environment settings, app version, spring theme selection etc. Custom tag class extends spring:url implementation class, so it may be considered a usual url tag but with some additional knowledge about proper path.

My problem is that I cannot get ${app.resources.path} correctly resolved in my JSP custom tag implementation. JSP custom tags are managed by servlet container, not Spring, and therefore dont participate in DI. So I cannot just use usual @Value("${app.resources.path}") and get it resolved by Spring automatically.

All I have there is the web application context instance, so I have to resolve my property programmatically.

So far I tried:

ResourceTag.java:

// returns null
PropertyResolver resolver = getRequestContext().getWebApplicationContext().getBean(PropertyResolver.class);
resolver.getProperty("app.resources.path");


// returns null, its the same web context instance (as expected)
PropertyResolver resolver2 = WebApplicationContextUtils.getRequiredWebApplicationContext(pageContext.getServletContext()).getBean(PropertyResolver.class);
resolver2.getProperty("app.resources.path");


// throws NPE, resolver3 is null as StringValueResolver is not bound
StringValueResolver resolver3 = getRequestContext().getWebApplicationContext().getBean(StringValueResolver.class);
resolver3.resolveStringValue("app.resources.path");


// null, since context: property-placeholder does not register itself as PropertySource
Environment env = getRequestContext().getWebApplicationContext().getEnvironment();
env.getProperty("app.resources.path");

So now I'm kinda stuck with that. I know that the ability to resolve my placeholder is somewhere in the context, I just don't know the correct way to do it.
Any help or ideas to check are highly appreciated.

回答1:

I think rather than focusing on inner working of context place holder, you can simply define a new util:properties like this:

<util:properties id="appProperties" location="classpath:app.properties" />

and in your code, use it like this:

Properties props = appContext.getBean("appProperties", Properties.class);

OR like this wherever you can do DI:

@Value("#{appProperties['app.resources.path']}")

回答2:

Since Spring 3.0.3 there is EmbeddedValueResolverAware which will work same way as mentioned by another post which uses appContext.getBeanFactory().resolveEmbeddedValue("${prop}") call.

To solve the problem:

1. Make your class to implement EmbeddedValueResolverAware interface and you will get resolver injected for you

2. Then where you will be able to retrieve properties as demonstrated in a code snippet:

   String propertyValue = resolver.resolveStringValue("${your.property.name}");
   

Then your bean does not need to depend on ApplicationContext to retrieve properties you need.

回答3:

Since version 3.0, Spring keeps a list of String resolver in the beanFactory. You can use it like this:

String value = appContext.getBeanFactory().resolveEmbeddedValue("${prop}");

The javadoc states this method as for resolving embedded values such as annotation attributes so maybe we are circumventing its usage, but it works.

回答4:

One option is to add a PropertySource (here MapPropertySource to exemplify an in-memory configuration) to a ConfigurableEnvironment and ask it to resolve properties for you.

public class Foo {

    @Autowired
    private ConfigurableEnvironment env;

    @PostConstruct
    public void setup() {
        env.getPropertySources()
           .addFirst(new MapPropertySource("my-propertysource", 
               ImmutableMap.<String, Object>of("your.property.name", "the value")));
        env.resolvePlaceholders("your.property.name");
    }
}

Optionally annotate the Foo class with @Configuration to enjoy the power of programmatic configuration in favor of XML.

回答5:

There is one more possible solution: make tag classes @Configurable via AspectJ and enable either compile-time or load-time weaving. Then, I could use usual Spring @Value annotations in my custom tags. But, really, I don't want to set up weaving infrastructure just because of a couple of classes. Still searching for a way to resolve placeholder via ApplicationContext.

来源：https://stackoverflow.com/questions/5172392/how-to-programmatically-resolve-property-placeholder-in-spring