How do you ensure multiple threads can safely access a class field?

public class SomeClass {
    private int val;

    public synchronized int getVal() {
        return val;
    }

    private void setVal(int val) {
        this.val = val;
    }
}

int old = someThing.getVal();
 if (old == 1) {
    someThing.setVal(2);
 }

synchronized(someThing) {
    int old = someThing.getVal();
    if (old == 1) {
        someThing.setVal(2);
    }
}

public class ThreadSafeSomeClass {

    private final AtomicInteger value = new AtomicInteger(0);

    public void setValue(int x){
         value.set(x);
    }

    public int getValue(){
         return value.get();
    }

}

public class SomeClass {

  private Object mutex = new Object();
  private int    val   = -1; // TODO: Adjust initialization to a reasonable start
                             //       value
  public int getVal() {
    synchronized ( mutex ) {
      return val;
    }
  }

  private void setVal( int val ) {
    synchronized ( mutex ) {
      this.val = val;
    }
  }
}