java单列模式实现有几种请用代码实现

Java单例模式的实现一般有以下几种：

饿汉式（线程安全，但可能浪费资源）

public class Singleton {
    private static Singleton instance = new Singleton();
    private Singleton() {}
    public static Singleton getInstance() {
        return instance;
    }
}

懒汉式（线程不安全，需要加锁）

public class Singleton {
    private static Singleton instance;
    private Singleton() {}
    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

双重检查锁（线程安全，但可能存在双重检查锁失效问题）

public class Singleton {
    private volatile static Singleton instance;
    private Singleton() {}
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

静态内部类（线程安全，推荐使用）

public class Singleton {
    private Singleton() {}
    private static class SingletonHolder {
        private static final Singleton INSTANCE = new Singleton();
    }
    public static Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }
}