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If the page uses Javascript to build up a URL or submit a form, you can use the Proxy Recording facility to create the necessary sampler. If this is not possible, then manual inspection of the code may be needed to determine what the Javascript is doing.

What about backward compatibility ?

Answer: JMeter 2.1 uses a new format. But you can set values to 2.0 to save to old format.

You should take a look at jmeter.properties in your bin directory.

You can uncomment those lines (remove # character) :

# Save test plans and test logs in 2.0 format
#file_format=2.0
# Just test plans (jmx)
#file_format.testplan=2.0
# Just test logs (jtl)
#file_format.testlog=2.0