How to implement thread-safe lazy initialization?

Question

What are some recommended approaches to achieving thread-safe lazy initialization? For instance,

// Not thread-safe
public Foo getI         


        

Answer 1

Try to defined the method which gets an instance as synchronized:

public synchronized Foo getInstance(){
   if(INSTANCE == null){
    INSTANCE = new Foo();
  }

  return INSTANCE;
 }

Or use a variable:

private static final String LOCK = "LOCK";
public synchronized Foo getInstance(){
  synchronized(LOCK){
     if(INSTANCE == null){
       INSTANCE = new Foo();
     }
  }
  return INSTANCE;
 }