在Java中实现线程安全的单例模式的常见方式

public class Singleton {
    private volatile static Singleton instance; // 使用 volatile 确保可见性

    private Singleton() {
        // 防止反射攻击
        if (instance != null) {
            throw new IllegalStateException("Singleton instance already created!");
        }
    }

    public static Singleton getInstance() {
        if (instance == null) { // 第一次检查
            synchronized (Singleton.class) { // 加锁
                if (instance == null) { // 第二次检查
                    instance = new Singleton(); // 实例化
                }
            }
        }
        return instance;
    }
}

public class Singleton {
    private Singleton() {}

    private static class SingletonHolder {
        private static final Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }
}

public enum Singleton {
    INSTANCE;

    public void someMethod() {
        // 实现单例的方法
    }
}

public class Singleton implements Serializable {
    private static final long serialVersionUID = 1L;

    private Singleton() {}

    private static class SingletonHolder {
        private static final Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }

    protected Object readResolve() {
        return getInstance();
    }
}