Listeners facilitates viewers to view responses generated by samplers in the form of tables, graphs, trees or simple text in some log files and also give pictorial access to the data collected by JMeter about those test cases as a Sampler component of JMeter is executed.

To view the details of sampler requests and responses, add either "View Results Tree" or "View Results in table" Listeners to a test plan which displays basic HTML and XML representations of the response. To view the response time graphically, add Graph Results which plots the response times on a graph.

Different listeners will display the response information in different ways. However, they all write the same raw data to the output file - if one is specified. Every listener in JMeter has an option store data to the output file specified. There is also a Configuration button which can be used to specify which fields to write to the file, and whether to use CSV format or XML format.

Listeners can be added anywhere in the test, including directly under the test plan. They will collect data only from elements at or below their level.

For example, if we add a Listener to a Thread Group, they only collect information specific to that Thread Group. If you have 3 Thread Groups and a listener attached to each one, those three listeners will show three different sets of data.

Disable all the Listeners before running load Test.

Listeners should be disabled during load tests as listeners consume lots of resources that are needed by more important elements of your test. Generating load and a report at the same time is not a best practice. It is always recommend to run load tests using command-line mode. In order to analyse, we can open the .jtl results file with any listener once the test completes and perform its analysis.

Here is a list of available Listeners in JMeter:

• Aggregate Report
• Aggregate Graph
• Assertion Results
• BeanShell Listener
• Sample Result Save Configuration
• Graph Results
• Generate Summay Results
• View Results in Table
• View Results Tree
• Simple Data Writer
• Monitor Results
• Distribution Graph (alpha)
• Aggregate Graph
• Mailer Visualizer
• Summary Report

The default items to be saved can be defined in the jmeter.properties (or user.properties) file. The properties are used as the initial settings for the Listener Config pop-up. To change the default format, Navigate to in the /bin directory and find the line 'jmeter.save.saveservice.output_format=' in jmeter.properties file.

Source: JMeter