DoubleCheckedLocking

DoubleCheckedLocking.java

// Single-threaded version
public class DoubleCheckedLocking {
    private Helper helper;
    public Helper getHelper() {
        if (helper == null) {
            helper = new Helper();
        }
        return helper;
    }
 
    // other functions and members...
}

// Correct but possibly expensive multithreaded version
class DoubleCheckedLocking1 {
    private Helper helper;
    public synchronized Helper getHelper() {
        if (helper == null) {
            helper = new Helper();
        }
        return helper;
    }
 
    // other functions and members...
}

// Broken multithreaded version
// "Double-Checked Locking" idiom
class DoubleCheckedLocking2 {
    private Helper helper;
    public Helper getHelper() {
        if (helper == null) {
            synchronized(this) {
                if (helper == null) {
                    helper = new Helper();
                }
            }
        }
        return helper;
    }
 
    // other functions and members...
}

// Works with acquire/release semantics for volatile
// Broken under Java 1.4 and earlier semantics for volatile
class DoubleCheckedLocking3 {
    private volatile Helper helper;
    public Helper getHelper() {
        Helper result = helper;
        if (result == null) {
            synchronized(this) {
                result = helper;
                if (result == null) {
                    helper = result = new Helper();
                }
            }
        }
        return result;
    }
 
    // other functions and members...
}

// Semantics of final field in Java 5 can be employed to safely publish the helper object without using volatile
public class FinalWrapper<T> {
    public final T value;
    public FinalWrapper(T value) {
        this.value = value;
    }
}
 
public class DoubleCheckedLocking4 {
   private FinalWrapper<Helper> helperWrapper;
 
   public Helper getHelper() {
      FinalWrapper<Helper> wrapper = helperWrapper;
 
      if (wrapper == null) {
          synchronized(this) {
              if (helperWrapper == null) {
                  helperWrapper = new FinalWrapper<Helper>(new Helper());
              }
              wrapper = helperWrapper;
          }
      }
      return wrapper.value;
   }
}