Java: Anonymous inner class cannot reference a variable of the non-final type of an outer class

When using a Java local inner class or an anonymous inner class, the local variable must be decorated with the final keyword if the class invokes a local variable of the method, or a compilation error will occur "cannot refer to a non-final variable * inside an Inner class defined in a different method ".
I do not know whether it is because of my programming style problem, this error I often encountered. And when I was learning Java, I didn't remember to say this question in my book. So this time take out Baidu a bit.
Simple Baidu, as if there is no detailed explanation, is probably involved in the JVM and the principle of compiling. Foundation is important. This question is left behind to think deeply.

Java: Anonymous inner class cannot reference a variable of the non-final type of an outer class