设计模式-单例模式

说明
参考

饿汉式

/**
 * 饿汉式，类加载的时候就会初始化
 *
 *   问题：
 *   1. 通过反射可以破坏单例
 *   2. 通过反序列化破坏单例
 *   3. 通过Unsafe破坏单例，无法解决 
 *
 * @author: optimjie
 * @date: 2023-08-19 21:37
 */
public class Singleton1 {

    private Singleton1() {
        System.out.println("private Singleton1");
    }

    private static final Singleton1 INSTANCE = new Singleton1();

    public static Singleton1 getInstance() {
        return INSTANCE;
    }

}

/**
 * 饿汉式，解决上述问题
 *
 * @author: optimjie
 * @date: 2023-08-19 21:44
 */
class Singleton2 {

    private Singleton2() {
        if (INSTANCE != null) {  // 解决问题1
            throw new RuntimeException("单例对象不能重复创建");
        }
        System.out.println("private Singleton1");
    }

    private static final Singleton2 INSTANCE = new Singleton2();

    public static Singleton2 getInstance() {
        return INSTANCE;
    }

    public Object readResolve() {  // 解决问题2，反序列化时直接调用该方法
        return INSTANCE;
    }
}

懒汉式

/**
 * 懒汉式
 *
 *   问题：
 *   1. 线程安全问题
 *
 * @author: optimjie
 * @date: 2023-08-19 21:55
 */
public class Singleton3 {

    private Singleton3() {
        System.out.println("private Singleton3");
    }

    private static Singleton3 INSTANCE = null;

    public static Singleton3 getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new Singleton3();
        }
        return INSTANCE;
    }

}

/**
 * 懒汉式，解决上面问题
 *
 *   问题：
 *   1. 每一次调用getInstance时都会加锁，但实际上只有一开始还没有创建Singleton4时才会有可能出现问题，
 *      因此需要进一步优化
 *
 * @author: optimjie
 * @date: 2023-08-19 22:07
 */
class Singleton4 {

    private Singleton4() {
        System.out.println("private Singleton3");
    }

    private static Singleton4 INSTANCE = null;

    public static synchronized Singleton4 getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new Singleton4();
        }
        return INSTANCE;
    }

}

/**
 * 懒汉式，优化上面的问题，也称为双检锁优化，DCL
 *
 * @author: optimjie
 * @date: 2023-08-19 22:09
 */
class Singleton5 {

    private Singleton5() {
        System.out.println("private Singleton3");
    }

    private static volatile Singleton5 INSTANCE = null;  // 加volatile的原因，详情看volatile的那篇

    public static Singleton5 getInstance() {
        if (INSTANCE == null) {
            synchronized (Singleton5.class) {
                if (INSTANCE == null) {
                    INSTANCE = new Singleton5();
                }
            }
        }
        return INSTANCE;
    }

}