Complete the given apex method for validating password requirements. The password must be an alphanumeric value with a minimum length of 8 characters.

Instructions:

1. Accept a string parameter as input and return a boolean value.

2. If the length of the input string is 8 characters long and is alphanumeric, return true.

3. If the criteria from step 2 are not met, then return false.

Example:

String input = 'Pa$sw@rd123#';
Boolean result = passwordManager.isValidPassword(input);
// result will be true

Note: String is a primitive data type in apex. Apex provides a set of built in functions to operate on strings. See the official documentation for apex string methods and utilize them to solve this scenario.