Using Groovy for variable expansion in Java properties

Question

I frequently use standard Java property files for configuring my Groovy applications. One feature I have been missing is the ability to use variables as part of the property value so they can be expand dynamically during use. I thought I could provide this functionality using the following design:

Use a special format to annotate the properties that should be expanded. I have chosen to enclose such templates in double exclamation marks (!!). These property values are essentially a template to be expanded with the local variables
Before using the properties in the application, use the groovy 'evaluate' method to expand application variables in the template
Re-assign the original property key to the new value before use

So, if I have a property file config.properties with properties like:

version=2.3
local_lib=!!${env['GROOVY_HOME']}/${configProps.getProperty('version')}/lib!!

The local_lib property will be expanded from the GROOVY_HOME environment variable and the version property value.

In my application, I have coded this as follows:

//Load the environment variables and configuration file
env=System.getenv()
configFile=new File('config.properties')
configProps= new Properties()
configProps.load(configFile.newDataInputStream())

//Replace configuration property values with their expanded equivalent
configProps.each{
  //if a property value is a template we evaluate it
  if (it.value.startsWith('!!')){
    valTemplate=it.value.replace('!!','"')
    it.value=evaluate(valTemplate)
  }
}

 //then we use the expanded property values

This seems to work. When I do

println configProps

I see that the value is expanded and not null

However, the getProperty method for the expanded property returns null.

assert configProps.getProperty('local_lib')=='C:\\DEVTOOLS\\groovy-2.4.7/2.3/lib'
   |           |                       |
   |           null                    false
   [local_lib:C:\DEVTOOLS\groovy-2.4.7/2.3/lib, version:2.3]

What is causing this discrepancy? I would have expected to return the value shown in the property map.

BalRog BalRog · Accepted Answer · 2016-08-19T16:14:19

Your local_lib value looks like a String, but it isn't. It is a GString, only lazily coerced to String as needed (like when printing out the configProps map value).

Thus, a little known effect of Properties.getProperty() takes effect here. When the actual map value is not a String, Properties.getProperty() returns null.

So, in order to get the desired behavior, you need to coerce the GString to String before you store the value in the property map. Like so:

it.value=evaluate(valTemplate).toString()

or

it.value=evaluate(valTemplate) as String

Then you should see the desired results downstream.

Using Groovy for variable expansion in Java properties

1 Answers