Sunday, September 17, 2017

JMeter: How to Verify JSON Response?

JSON (Javascript object notation) is a serialization format (in key-value pairs) of data structures .  For REST API, it is widely used for data transfer from server to client.  For example, a client sends an HTTP request with below header:

  • Accept: application/json

The server can respond with below sample JSON data:

    "result": [],
    "ccapiInfo": {
      "createdOn": "2017-09-07T15:25:29.000Z",
      "cachedOn": "2017-09-07T15:21:49.513Z",
      "origin": "cache",
      "canonicalLink": ""

with a response header of:
Content-Type: application/json

In this article, we will discuss how to achieve two tasks in Apache JMeter:

JSON Extractor / JSONPath

One of the advantages of XML is the availability of numerous tools to analyse, transform and selectively extract data out of XML documents. XPath is one of these powerful tools.  For JSON, we have a similar tool called JSONPath.

JSONPath is the XPath for JSON.  Since a JSON structure is normally anonymous, JSONPath assigns symbol $ as the root object.

Below is a side-by-side comparison of the JSONPath syntax elements with its XPath counterparts.[9]

/$the root object/element
.@the current object/element
/. or []child operator
..n/aparent operator
//..recursive descent. JSONPath borrows this syntax from E4X.
**wildcard. All objects/elements regardless their names.
@n/aattribute access. JSON structures don't have attributes.
[][]subscript operator. XPath uses it to iterate over element collections and for predicates. In Javascript and JSON it is the native array operator.
|[,]Union operator in XPath results in a combination of node sets. JSONPath allows alternate names or array indices as a set.
n/a[start:end:step]array slice operator borrowed from ES4.
[]?()applies a filter (script) expression.
n/a()script expression, using the underlying script engine.
()n/agrouping in Xpath

JSONPath expressions can use the dot–notation


or the bracket–notation


for input paths. For the internal or output paths, they will always be converted to the more general bracket–notation.  Below diagram shows the evaluation result using a JSONPath Online Evaluator with the input and JSONPath Expression as given in this article.

JSR223 Assertion

Assertion in JMeter help verify that your server under test returns the expected results. JMeter includes quite a few assertion elements for validating the sampler’s response, yet sometimes your validation decision might follow complex logic, and can’t be configured using the out-of-the-box JMeter assertions - scripting is then required.

If you need to write scripting assertion code to extend baseline JMeter functionality, JSR223, in combination with Groovy language is a good choice performance-wise—especially when its compilation caching is enabled.

Groovy Script

String jsonString =  vars.get("myCanonicalLink");
String userNameString = vars.get("user_name"); ("The canonicalLink is " + jsonString);

if ( jsonString != "" + 
      userNameString + "/api/v1/instance/Compute-" + userNameString + "/") 
AssertionResult.setFailureMessage("The canonicalLink is wrong");

However, every test element including assertion added to the test plan will increase the total CPU and memory requirements.  So, plan your use of assertions sparingly.

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