```java
class ServiceLocator {
private static ServiceLocator instance = null;
private ServiceLocator() {}
public static ServiceLocator getInstance() {
if (instance == null) {
synchronized(ServiceLocator.class) {
instance = new ServiceLocator();
}
}
return instance;
}
public Engine getEngine() {
return new Engine();
}
}
class Car {
private Engine engine = ServiceLocator.getInstance().getEngine();
public void start() {
engine.start();
}
}
class MyApp {
public static void main(String[] args) {
Car car = new Car();
car.start();
}
}
其中下面这部分是什么意思?
if (instance == null) {
synchronized(ServiceLocator.class) {
instance = new ServiceLocator();
}
}