代码文件目录：

 初始版本：

package Chapter5_SingletonPattern.origin;/*** @Author 竹心* @Date 2023/8/5**/public class Singleton {private static Singleton uniqueInstance;private Singleton(){}public static Singleton getInstance(){if(uniqueInstance == null){uniqueInstance = new Singleton();}return uniqueInstance;}//问题：多线程中会出现多个实例被创建的情况//other functions}

三种解决多线程问题的方法：

package Chapter5_SingletonPattern.Multithread;import Chapter5_SingletonPattern.origin.Singleton;/*** @Author 竹心* @Date 2023/8/5**/public class Singleton1 {private static Singleton1 uniqueInstance;private Singleton1(){}//使用synchronized迫使每个线程进入此方法之前必须轮到他//但是这样会导致性能开销大public static synchronized Singleton1 getInstance(){if(uniqueInstance == null){uniqueInstance = new Singleton1();}return uniqueInstance;}//other functions
}

package Chapter5_SingletonPattern.Multithread;/*** @Author 竹心* @Date 2023/8/5**/public class Singleton2 {//急切创建实例：在静态初始化器中就创建实例private static Singleton2 uniqueInstance = new Singleton2();private Singleton2(){}public static Singleton2 getInstance(){return uniqueInstance;}//other functions
}

package Chapter5_SingletonPattern.Multithread;import Chapter5_SingletonPattern.origin.Singleton;/*** @Author 竹心* @Date 2023/8/5**/public class Singleton3 {//用双重检查加锁减少使用同步，降低开销private volatile static Singleton3 uniqueInstance;//volatile保证uniqueInstance为单实例类的时候多个线程正确处理private Singleton3(){}public static Singleton3 getInstance(){if(uniqueInstance == null){synchronized (Singleton3.class){if(uniqueInstance == null){//进入同步区块后，再检查一次uniqueInstance = new Singleton3();}}}return uniqueInstance;}
}

Notes:

单件模式：确保一个类只有一个实例，并提供一个全局的访问点