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i'm not sure if GRINDER is still best tool so far JAVA API performance testing tool. If anyone is using it, could you share your feedback? tx

I have used JMeter and Grinder and can tell you that grinder is awesome . JMeter is also great . Not sure why you will be testing a JAVA API.
Grinder is more like a programmers performance tester tool. Once you get a hang of it you will not lean away from it . The only complex part could be the recorded scripts that grinder generates .The initial recorded script can get you overwhelmed and you might think what have i picked up .
But actually it just a step by step script and easy to modify as per your needs.
Any performance testing will have only these steps .
Record - Generates the Initial Script of what you want to test.
Modify - Modify it as per you needs.For grinder you may require a basic knowledge of some scripting language. Many times this may not be required.
Test - Put the load you want to .Here you need to decide the number of threads plus agents .
My vote is for grinder specially for complicated scenarios.