...
@Transient
private boolean passwordConfirmValid;

public void setPasswordConfirmValid(boolean passwordConfirmValid) {
	this.passwordConfirmValid = passwordConfirmValid;
}

@AssertTrue(message = "Password do not match.")
public boolean isPasswordConfirmValid() {
	if (this.password == null) {
		passwordConfirmValid = this.password_again == null;
	} else {
		passwordConfirmValid = this.password_again.equals(this.password);
	}
	return passwordConfirmValid;
}
...