java单例模式实现代码怎么写

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在 Java 中实现单例模式有多种方式，以下是其中几种常用的实现方式：

饿汉式（线程安全）：

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）：

public class Singleton {private static Singleton instance;

    private Singleton() {}

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

懒汉式（线程安全，双重检查锁定）：

public class Singleton {private static volatile 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）：

public class Singleton {private Singleton() {}

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

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