单例4种实现

「爱情、让人受尽委屈。」 2022-03-09 05:30 150阅读 0赞

## 懒汉式 ##

public class Singleton {
        private static Singleton singleton;
        private Singleton() {}  //此类不能被实例化
        public static synchronized Singleton getInstance() {
            if (singleton == null) {
                singleton = new Singleton();
            }
            return singleton;
        }
    }

优点：延迟加载  
缺点：加锁，影响性能

懒汉式dcl

public class Singleton {
        private static volatile Singleton singleton;
        private Singleton() {}  //此类不能被实例化
        public static Singleton getInstance() {
            if (singleton == null) {
                synchronized (Singleton.class) {
                    singleton = new Singleton();
                }
            }
            return singleton;
        }
    }

用synchronized和volatile加锁，影响性能

## 饿汉式 ##

public class Singleton {
        private static Singleton singleton = new Singleton();
        private Singleton() {}  //此类不能被实例化
        public static Singleton getInstance() {
            return singleton;
        }
    }

类加载时，就要初始化静态变量，对象初始化比较复杂，会导致程序初始化缓慢

## 静态内部类 ##

public class Singleton {
        private Singleton() {}  //此类不能被实例化
        private static class SingletonHolder {
            private static final Singleton SINGLETON = new Singleton();
        }
        public static Singleton getInstance() {
            return SingletonHolder.SINGLETON;
        }
    }

类加载时，不会加载内部类，做到了延时加载

## 静态公共实例变量，直接外部访问 ##

class Singleton {
    				private Singleton() {}
    			
    				public static final Singleton s = new Singleton();//final是最终的意思,被final修饰的变量不可以被更改,public可以被外部直接访问
    			}
    }

类加载时，就要初始化静态变量，对象初始化比较复杂，会导致程序初始化缓慢