How to Build a Thread‑Safe Global Counter in JMeter with Groovy

Goal

Create a globally unique, incrementing variable in JMeter that can be accessed by all threads during a test run.

Approach

Use a Java‑level lock on JMeter's built‑in props object to guarantee that only one thread updates the counter at a time.

Test Plan

Create a simple Thread Group with a single HTTP Request (or any sampler).

Add a JSR223 PreProcessor to the request. The script will run before each sampler execution.

Groovy Script (Thread‑safe version)

props.put("MY", test() + "")
//log.info(props.get("MY") + "")

def test() {
    synchronized (props) {
        // Optional delay for demonstration: Thread.sleep(1000)
        int i = props.get("MY") as Integer
        log.info(i + "")
        return i + 1
    }
}

The props object lives in the JVM for the lifetime of the JMeter process, so any value stored in it persists across all threads until the test ends.

Initialize Counter

// Set an initial value before the test starts
props.put("MY", "999")

Execution Logs

Without locking (race condition)

2020-07-19 19:29:50,187 INFO o.a.j.t.JMeterThread: Thread started: Thread Group 1-3
2020-07-19 19:29:50,191 INFO o.a.j.t.JMeterThread: Thread started: Thread Group 1-4
... (additional log lines) ...
2020-07-19 19:29:50,241 INFO o.a.j.t.JMeterThread: Thread finished: Thread Group 1-3

With locking (strictly increasing sequence)

2020-07-19 19:28:40,988 INFO o.a.j.t.JMeterThread: Thread started: Thread Group 1-5
2020-07-19 19:28:41,015 INFO o.a.j.m.J.JSR223 变量: 999
2020-07-19 19:28:41,017 INFO o.a.j.t.JMeterThread: Thread is done: Thread Group 1-1
2020-07-19 19:28:41,031 INFO o.a.j.m.J.JSR223 变量: 1000
2020-07-19 19:28:41,041 INFO o.a.j.t.JMeterThread: Thread is done: Thread Group 1-2
2020-07-19 19:28:41,044 INFO o.a.j.m.J.JSR223 变量: 1001
2020-07-19 19:28:41,046 INFO o.a.j.m.J.JSR223 变量: 1002
2020-07-19 19:28:41,049 INFO o.a.j.m.J.JSR223 变量: 1003
... (additional log lines) ...

Conclusion

Synchronizing on the props object provides a simple, reliable way to maintain a global counter that is safe for concurrent access in JMeter. By setting an initial value before the test, the counter starts from a known state, and no external Java code or custom classes are required.