java单例模式-懒加载

private static SingletonLazy instance = null;
private SingletonLazy(){};
public static  SingletonLazy getInstance(){
    if (null == instance){
        createInstance();
    }
    return instance;
}

private synchronized static SingletonLazy createInstance(){
    if (null == instance){
        instance = new SingletonLazy();
    }
    return instance;
}

private static SingletonLazyLock instance = null;
private SingletonLazyLock(){};

//加读写锁
private static ReadWriteLock rwl = new ReentrantReadWriteLock();

/**
 * 单例模式 懒加载并发
 * @return
 */
public static SingletonLazyLock getInstance(){
    rwl.readLock().lock();
    try {
        if (null == instance){
            rwl.readLock().unlock();
            rwl.writeLock().lock();
            if(null == instance){
                instance = new SingletonLazyLock();
            }
            rwl.writeLock().unlock();

        }
        rwl.readLock().lock();
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        rwl.readLock().unlock();
    }

    return instance;
}