JMeter - Using BSF Assertion to fail samples based on JSON response

I've tested some web applications whose unexpected errors or exceptions were handled by the application code and encoded in the response body as JSON.

A hypothetical response is shown below:

{
    data: null,
    message: "TimeoutException: Transaction rolled back after 3000 seconds.",
    success: false
}

The problem with this approach is that most of the times the HTTP response code is 200 (OK) which  jmeter interprets as a successful sample.
I'm not a big fan of this solution I'd rather have it returning 4XX or 5XX response codes, in which case jmeter would fail the sample.

In order to workaround this, I normally use a BSF Assertion containing javascript code similar to:

try {
   eval('var response = ' + prev.getResponseDataAsString());
   if (!response.success) {
      prev.setSuccessful(false);
      prev.setResponseMessage(response.message);
   }
} catch(e) {
      prev.setSuccessful(false);
      prev.setResponseMessage("Invalid response. Expected a valid JSON.");
}

First I try to evaluate the response string into a valid javascript object. If that succeeds I check the success flag and handle it properly, otherwise, in the catch block, I force the sample to fail due to a bad response format.
Note that in both failure scenarios I invoke the setSuccessful and the setResponseMessage methods to signal a sample failure and have better detailed error messages respectively.

And that's it!
Happy load testing!

JMeter - Changing sample label/name at runtime

When creating jmeter's test plans, sometimes its useful to change the samples names in order to have more accurate and meaningful reports. To achieve this I normally use one of these two approaches:

Using variables (placeholders) in the sampler name

If the variables you need to form the new sample name are already available just fill the the name's text field using the syntax ${variable name}.
The only problem with this approach is that the name shown in test plan tree (left panel) might end up being not so intuitive.

Dynamically setting it in BSF Post Processors

Just use the variable prev (which gives access to the previous SampleResult) and use the method setSampleLabel to set the new label.
In the example below I'm using the value of another variable (named url) previously put in the vars object: