单例模式的定义：保证一个类仅有一个实例，并提供一个访问它的全局访问点！
使用一个私有构造器、一个私有静态变量和一个公有静态函数来实现。
私有构造函数保证了不能通过构造函数来创建对象实例，只能通过公有静态函数返回唯一的私有静态变量。
懒汉式
public class Singleton {
private static Singleton instance;
private Singleton (){}
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
饿汉式
public class Singleton {
private static Singleton instance = new Singleton();
private Singleton (){}
public static Singleton getInstance() {
return instance;
}
}
双重校验锁
public class Singleton {
private volatile static Singleton singleton;
private Singleton (){}
public static Singleton getSingleton() {
if (singleton == null) {
synchronized (Singleton.class) {
if (singleton == null) {
singleton = new Singleton();
}
}
}
return singleton;
}
}
枚举
public enum Singleton {
INSTANCE;
public void whateverMethod() {
}
}
静态内部类
public class Singleton {
private static class SingletonHolder {
private static final Singleton INSTANCE = new Singleton();
}
private Singleton (){}
public static final Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}