mybatis-issue

Summary:

Hello, 

Mybatis developers, I found you use distinct in class (`Jdbc3KeyGenerator.java`) to adjust whether the input params' count , 
but it will thorw a NullPointerException when we use the hashcode of id(is null before insert to database)  as the identifier of the object . 

In the early edition of mybatis(like 3.4.6), this  caused no exception, but after we upgrade the edition to mybatis 3.5.1, this exception occurs. 
Please fix it in the future!

Here is a demo to reproduce.

Thank you!

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The Usage of this test demo:

    maven compile -> run mybatis-issue/src/test/java/test/.java