Question

Here’s some code I saw once. Can you see what’s wrong with it?

[updated]

public class ResourceManager1 {     private final String mutex = '';     Object resource = null;      public Object getResource()     {         synchronized (mutex)         {             if (resource == null)             {                 resource = new Object();             }         }          return resource;     } }  public class ResourceManager2 {     private final String mutex = '';     Object resource = null;      public Object getResource()     {         synchronized (mutex)         {             if (resource == null)             {                 resource = new Object();             }         }          return resource;     } } 

Answer 1

Never synchronize on strings, particularly string literals which are interned. You’ve basically just got a single lock.

In general, never synchronize on any reference that might be visible outside your class (including ‘this’) unless the purpose of the external visibility is precisely for locking purposes. I usually use a private final variable created solely for the purpose of locking.