what's purpose of local variable for a member variable?

Question

When I see some framework source code like dubbo, I often see code similar to the following:

public class Person {
   int age;
   String name;
   List<Person> persons = new ArrayList<Person>();

   public Person findPerson(int lowAge,int highAge){
         List<Person> localPersons = persons;
         for(Person p : localPersons){
              if( p.age >=lowAge && p.age <highAge){
                   return p;
              } 
         }
         return null;
   }

}

I just do not understand why not using the member variables persons directly. Local variable localPersons looks redundant to me.

Answer 1

It is possibly a micro-optimization, though I doubt that it would make any difference with a modern JVM. (If that was a worthwhile optimization, the JIT compiler's optimizer will probably do the equivalent transformation at the native code level.)

It is also possibly an (incorrect) attempt to make the code work if there are multiple threads. I say "incorrect" because the programmer is ignoring the requirements of the Java Memory Model. Since there are no clear happens before relationships between two threads accessing the persons variable or the list that it refers to, threads are liable to see stale data, leading to unpredictable behavior.

There could be other reasons for doing this that are not apparent in your example. (For example, if persons was declared as volatile and certain other preconditions were met, this could be thread-safe, and the supposedly redundant local variable could have a valid purpose.)