JMeter JTL Files

When Apache JMeter captures test results, the data is typically written to a log file known as a JTL file. Understanding the structure of these files is essential for effective performance test analysis.

JTL File Structure

The JTL file begins with a header line containing column names, followed by individual test point data on separate lines.

timeStamp,elapsed,label,responseCode,responseMessage,threadName,dataType,success,failureMessage,bytes,sentBytes,grpThreads,allThreads,Latency,IdleTime,Connect
1541088455861,603,0.0_Home Page,200,OK,Advertise Job 1-1,text,true,,2674,301,10,30,515,0,130

Understanding the Timestamp

The timeStamp field represents an Epoch timestamp (seconds since January 1, 1970). Apache JMeter adds three decimal places of precision to this value.

For example, the timestamp 1541088455861 corresponds to November 1, 2018 at 16:07 UTC.

Key Fields

Field	Description
timeStamp	Epoch time with milliseconds
elapsed	Response time in milliseconds
label	Sampler name
responseCode	HTTP response code
success	Pass/fail status
bytes	Response size
Latency	Time to first byte
Connect	Connection establishment time

Editing JTL Files

HTML test reports generated from JTL files include all line entries. When you need to exclude specific data sections, such as ramp-up or ramp-down periods, you can remove those rows if you know their timestamps.

A practical approach involves:

1. Calculate the epoch timestamp for the desired timeframe
2. Remove unwanted lines from the JTL file
3. Regenerate the HTML report

This preserves data integrity for analysis while focusing on the steady-state period.

Repairing Damaged Files

If a test fails or the Java process terminates unexpectedly, the JTL file may become corrupted. Since it’s a plain text file, repairs are straightforward.

Incomplete Final Line

The typical issue occurs when the final line isn’t completely written. When attempting to generate an HTML report, the system fails due to incorrect column counts.

Solution: Open the file in a text editor, remove the incomplete final line, and save.

Problematic String Values

Failure messages sometimes contain commas, which disrupts the comma-separated format and causes column mismatches during HTML report generation.

Solution:

1. Search the file for the problematic failure message
2. Remove the extra comma from the failure message
3. Save the file
4. Regenerate the report

Useful Tools

When working with epoch timestamps, online conversion tools can help:

• Unix Timestamp Converter

Further Reading

• JMeter Dashboard Report
• JMeter Backend Listeners