java中单例模式

public class QuotationEngine {
    private static QuotationEngine instance;// 对象实例

    private QuotationEngine() {
    }

    public static QuotationEngine getInstance() {
        if (null == instance) {
            synchronized (QuotationEngine.class) {
                if (null == instance) {
                    instance = new QuotationEngine();
                }
            }
        }

        return instance;
    }
}