饿汉式：
在类加载的时候就初始化创建单例对象，线程安全，但存在的问题就是无论使用与否都创建对象，造成内存浪费
代码实现：
/*饿汉式*/
public class HungrySingleton {private HungrySingleton(){}private static HungrySingleton instance = new HungrySingleton();public static HungrySingleton getInstance(){return instance;}public static void main(String[] args) {HungrySingleton obj1 = HungrySingleton.getInstance();HungrySingleton obj2 = HungrySingleton.getInstance();System.out.println(obj1 == obj2);}
}
懒汉式：
在外部调用时才会去加载，线程不安全，可以通过线程加锁来保证安全，但效率较低
代码实现：
/*** 懒汉式*/
public class LazySingleton {private LazySingleton(){}private static LazySingleton instance;public static LazySingleton getInstance(){if (instance == null){instance = new LazySingleton();}return instance;}public static void main(String[] args) {LazySingleton obj1 = LazySingleton.getInstance();LazySingleton obj2 = LazySingleton.getInstance();System.out.println(obj1 == obj2);}
}
双重校验锁：
使用volatile以及多重检查，来减小锁范围，提升效率。
/*** 双重校验锁*/
public class DoubleCheckSingleton {private DoubleCheckSingleton(){}private volatile static DoubleCheckSingleton instance;public static DoubleCheckSingleton getInstance(){synchronized (DoubleCheckSingleton.class){if (instance == null){instance =new DoubleCheckSingleton();}}return instance;}public static void main(String[] args) {DoubleCheckSingleton obj1 = DoubleCheckSingleton.getInstance();DoubleCheckSingleton obj2 = DoubleCheckSingleton.getInstance();System.out.println(obj1 == obj2);}
}
静态内部类：
解决饿汉式（内存浪费）和懒汉式（线程不安全）的问题。
代码实现：
/*** 静态内部类*/
public class StaticSingleton {private StaticSingleton(){}public static StaticSingleton getInstance(){return StaticClass.instance;}private static class StaticClass{private static final StaticSingleton instance = new StaticSingleton();}public static void main(String[] args) {StaticSingleton obj1 = StaticSingleton.getInstance();StaticSingleton obj2 = StaticSingleton.getInstance();System.out.println(obj1 == obj2);}}