单例模式

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单例模式：确保一个类只有一个实例，并提供一个全局访问点。

这里写图片描述
下面我们介绍几种常见的实现方式。

1. 饿汉式：线程安全

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

2. 懒汉式：非线程安全

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

3. 使用synchronized：线程安全

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

4. 双重检查加锁：线程安全

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

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