java设计模式-单例模式

单例模式

1、饿汉式(静态常量)

@Slf4j
public class SingletonTest01 {
    public static void main(String[] args) {
        Singleton singleton = Singleton.getInstance();
        Singleton singleton2 = Singleton.getInstance();
        log.info("比对结果：{}",singleton==singleton2); //true
    }
}
/**
 * 饿汉式，(静态变量)
 */
class Singleton {
    //1、构造器私有化，外部new
    private Singleton() {
    }
    //本部内部类创建对象实例
    private final static Singleton instance = new Singleton();
    public static Singleton getInstance() {
        return instance;
    }
}

2、饿汉式(静态代码块)

也可以使用，但是稍微有点内存的浪费(并且线程是安全的)

@SuppressWarnings("All")
@Slf4j
public class SingletonTestStatic {
    public static void main(String[] args) {
        SingletonStatic singleton = SingletonStatic.getInstance();
        SingletonStatic singleton2 = SingletonStatic.getInstance();
        log.info("比对结果：{}",singleton == singleton2);
    }
}
/**
 * 饿汉式，(静态代码块)
 */
class SingletonStatic {
    /**
     * 1、构造器私有化，外部new
     */
    private SingletonStatic() {
        instance = new SingletonStatic();
    }
    //本部内部类创建对象实例
    private static SingletonStatic instance;
    public static SingletonStatic getInstance() {
        return instance;
    }
}

3、 懒汉式(线程不安全)

不推荐使用

@SuppressWarnings("All")
@Slf4j
public class SingletonHungryMan {
    public static void main(String[] args) {
        SingletonHungry singleton=SingletonHungry.getInstance();
        SingletonHungry singleton1=SingletonHungry.getInstance();
        log.info("比对结果：{}",singleton==singleton1);
        log.info("singleton:hasCode:{}",singleton.hashCode());
        log.info("singleton1:hasCode:{}",singleton1.hashCode());
    }
}
/**
 * 饿汉式，(静态代码块)
 */
class SingletonHungry {
    private static SingletonHungry instance;
    private SingletonHungry() {
    }
    //提供一个静态公有方法，当使用到该方法时候，才去创建instance
    //即懒汉式
    public static SingletonHungry getInstance() {
        if(null==instance){
            instance=new SingletonHungry();
        }
        return instance;
    }
}

4、懒汉式(线程安全，同步方法)

@SuppressWarnings("All")
@Slf4j
public class SingletonHungryManSync {
    public static void main(String[] args) {
        SingletonHungryManSync singleton= SingletonHungrySync.getInstance();
        SingletonHungryManSync singleton1= SingletonHungrySync.getInstance();
        log.info("比对结果：{}",singleton==singleton1);
        log.info("singleton:hasCode:{}",singleton.hashCode());
        log.info("singleton1:hasCode:{}",singleton1.hashCode());
    }
}
/**
 * 懒汉式，(静态代码块)
 */

class SingletonHungrySync {
    private static SingletonHungryManSync instance;
    private SingletonHungrySync() {
    }
    //提供一个静态的公有方法，同时加入同步处理代码synchronized， 解决线程安全问题
    public static synchronized SingletonHungryManSync getInstance() {
        if(null==instance){
            instance=new SingletonHungryManSync();
        }
        return instance;
    }
}

5、懒汉式(线程安全，同步代码块(线程不安全)

不推荐使用

@Slf4j
public class SingletonHungryManSyncCode {
    public static void main(String[] args) {
        SingletonHungrySyncCode singleton = SingletonHungrySyncCode.getInstance();
        SingletonHungrySyncCode singleton1 = SingletonHungrySyncCode.getInstance();
        log.info("比对结果：{}", singleton == singleton1);
        log.info("singleton :hasCode:{}", singleton.hashCode());
        log.info("singleton1:hasCode:{}", singleton1.hashCode());
    }
}
class SingletonHungrySyncCode {
    private static SingletonHungrySyncCode instance;
    private SingletonHungrySyncCode() {
    }
    public static  SingletonHungrySyncCode getInstance() {
        if (null == instance) {
            //进入if再加锁，创建多个实例，不安全(不推荐使用)
            synchronized (SingletonHungrySyncCode.class) {
                instance = new SingletonHungrySyncCode();
            }
        }
        return instance;
    }
}

6、双重检查

推荐使用

@Slf4j
public class SingletonDoubleCheckMain {
    public static void main(String[] args) {
        SingletonDoubleCheck singleton = SingletonDoubleCheck.getSingleton();
        SingletonDoubleCheck singleton2 = SingletonDoubleCheck.getSingleton();
        log.info("比对结果：{}", singleton == singleton2);
        log.info("singleton :hashCode：{}", singleton.hashCode());
        log.info("singleton2:hashCode：{}", singleton.hashCode());
    }
}
class SingletonDoubleCheck {
    private static volatile SingletonDoubleCheck singleton;
    private SingletonDoubleCheck() {
    }
    /**
     *  提供一个静态的公有方法，加入双重检查代码，解决线程安全问题，同时解决懒加载问题
     */
    public static synchronized SingletonDoubleCheck getSingleton() {
        if (null == singleton) {
            synchronized (SingletonDoubleCheck.class) {
                if (null == singleton) {
                    singleton = new SingletonDoubleCheck();
                }
            }
        }
        return singleton;
    }
}

7、静态内部类

推荐使用

@SuppressWarnings("All")
@Slf4j
public class SingletonStaticInnerMain {
    public static void main(String[] args) {
        SingletonStaticInner singleton = SingletonStaticInner.getInstance();
        SingletonStaticInner singleton2 = SingletonStaticInner.getInstance();
        log.info("静态内部类完成单例模式");
        log.info("比对结果：{}", singleton == singleton2);
        log.info("singleton :hashCode：{}", singleton.hashCode());
        log.info("singleton2:hashCode：{}", singleton.hashCode());
    }
}
class SingletonStaticInner {
    private SingletonStaticInner() {
    }
    /**
     * 写一个静态内部类，该类中有一个静态属性SingletonStaticInner
     */
    private static class SingletonInstance {
        private static final SingletonStaticInner INSTANCE = new SingletonStaticInner();
    }
    /**
     * 提供一个静态的公有方法，直接返回SingletonInstance.INSTANCE;
     */
    public static SingletonStaticInner getInstance() {
        return SingletonInstance.INSTANCE;
    }
}

8、枚举

@Slf4j
public class SingletonEnumMain {
    public static void main(String[] args) {
        SingletonEnum singleton = SingletonEnum.INSTANCE;
        SingletonEnum singleton2 = SingletonEnum.INSTANCE;

        log.info("比对结果：{}", singleton == singleton2);
        log.info("singleton :hashCode：{}", singleton.hashCode());
        log.info("singleton2:hashCode：{}", singleton.hashCode());
    }
}
enum SingletonEnum {
    //属性
    INSTANCE;
}

总结、以上推荐使用的是：
饿汉式，静态内部类，双重检查，枚举
单例使用注意事项：

1、单例模式保证了系统内存中该类只存在一个对象，节省了系统资源，对于一些需要平凡创建校会的对象，使用单例模式可以提高系统性能
2、想实例化一个单例类的时候，必须要记住使用相应的获取对象的方法，而不是使用new
3、单例模式适用的场景：需要频繁的进行创建和销毁对象，创建对象时消耗过多或者耗费资源过多(即：重量级对象)，
但又经常用到的对象、工具类对象、频繁访问数据库或文件的对象(比如数据源、session工厂等)