Using Beanshell Assertion to test a JSON value

I'm trying to test a JSON value that may be = null.

jp [ at ] gc - JSON Path Assertion doesn't appear to return anything or define the
variable if it encounters a null value, so I was trying to get it to work
like this:

- use jp [ at ] gc - JSON Path Extractor to try to extract the value to a variable.
- use Beanshell Assertion to try to test to see if the variable exists or
has a null value
- If the variable does not exist, or exists and has a null value, I know
that the JSON value was null.

I haven't written any scripts with Jmeter before, so I may be doing
something obviously wrong, but here is what I've tried:

try {
String blobvalue = vars.get("myValue");
log.info("myValue =" + myValue);

catch (e) {
log.info( "caught exception: "+e );
string myValue = null;

if (myValue.length() > 0 & myValue != 0 ){
Failure = true;
FailureMessage = "myValue was " + myValue + ".";

else{
Failure = false;

For this test, a null value actually passes the test.

The problem I'm running into is that the try/catch block doesn't work.
Instead, I see the following message in the log:

jmeter.assertions.BeanShellAssertion:
org.apache.jorphan.util.JMeterException: Error invoking bsh method: eval
Sourced file: inline evaluation of: ``import
org.apache.jmeter.util.JMeterUtils; try { //myValue = JMeterUtils.get . .
. '' : Attempt to resolve method: length() on undefined variable or class
name: myValue

Can anyone tell me what I'm doing wrong and/or how to get this test of a
null JSON value to work?

Thanks very much!