java单例模式怎么实现

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Java 中单例模式的实现方法有以下几种：

1. 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;
}
}

1. 懒汉式（线程安全）：

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

1. 饿汉式：

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

1. 双重检查锁定（Double-Checked Locking）：

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;
}
}

1. 静态内部类：

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

以上是几种常见的单例模式实现方法，每种方法都有各自的优缺点，可以根据具体需求选择适合的实现方法。

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