Jmeter Groovy replacing JSON file - groovy

Question

In Jmeter JSR223Preprocessor with Groovy I load a generic JSON file xyz looking like this:
{
"name": "dummy",
"block1": {
"var1": 1,
"var2": {
"value": 1,
"unit": "Miles",
},
"var3": {
"value": 3,
"unit": "Seconds",
},
"myList": [{"Id": 0}]
}
I like to come up with an elegant way to replace the var2 "Value" with a configurable value sayconfVal. This works:
String path = vars.get("basePath")+"xyz.json" ;
xyz = new File(path).getText("UTF-8");
xyz = xyz.replaceAll ('"value": 1', '"value": ${confVal}');
However I am not comfortable with this because it is vulnerable with spaces, and moreover I have another Value on var3 and someone could accidentally change 1 to 3. So I like to index to that child var2.Value then get Value.
Thank you

Answer 1

Add JSR223 PreProcessor as a child of the HTTP Request which body you need to amend
Put the following code into "Script" area:
def xyz = new File(vars.get("basePath")+"xyz.json")
def request = new groovy.json.JsonSlurper().parse(xyz)
request.block1.var2.value=vars.get('confVal') as int
xyz.newWriter().withWriter { writer ->
writer << new groovy.json.JsonBuilder(request).toPrettyString()
}
That's it, the value in the file should be replaced with what you have in the ${confVal} variable in the runtime.
More information:
Groovy: Parsing and producing JSON
Apache Groovy - Why and How You Should Use It