java - What "inconsistent synchronization" means? -

this java 1.6 class:

public class foo {   private arraylist<string> names;   public void scan() {     if (names == null) {       synchronized (this) {         this.names = new arraylist<string>();         // fill array data       }     }   } } 

findbugs says:

inconsistent synchronization of com.xxx.foo.names; locked 40% of time 

what mean , i'm doing wrong? i'm trying avoid problems when 2 or more clients call foo.scan() @ same time.

it's beacuse synchronizing when set names variable , not when read it. between read , write thread execute , you'd create 2 arraylists , fill them data, first 1 created gc'ed.

you need put synchronized block around read , write or add synchronized modifier method.

public class foo {   private arraylist<string> names;     public void scan() {       synchronized (this)         if (names == null) {            this.names = new arraylist<string>();            // fill array data          }        }      }   }