单例模式
保证每个类只有一个实例,并提供一个全局访问点
第一步 构造方法私有化
第二步 公有化静态方法获取的实例
懒汉式
public class Bank{ private Bank(){}
private static Bank bank = null;
public static Bank getInstances(){ if(bank == null){
bank = new Bank();
}
return bank;
} }
构造方法在私有化之后在main方法中是不能创建对象的,也就不能用对象点方法
调用getInstances,所以getInstances方法要加static直接用Bank.getInstances.
在静态的方法中是不能有实例变量的,所以bank要加static
对于懒汉式有时候会造成线程安全问题,最好加synchronnized
public class Bank{ private Bank(){}
private static Bank bank = null;
public static synchronized Bank getInstances(){ if(bank == null){
bank = new Bank();
}
return bank;
} }
double check版本
public class Bank{ private Bank(){}
private static Bank bank = null;
public static Bank getInstances(){ if(bank == null){
synchronized(this){
if(bank == null){
bank = new Bank();
}
}
}
return bank;
} }
饿汉式
public class Bank{ private Bank(){}
private static Bank bank = new Bank();
public static Bank getInstances(){ return bank;
} }
内部类方式
public class SingleTon {
private SingleTon() {
} private static class SingleTonHolder {
private static SingleTon instance = new SingleTon();
} public static SingleTon getInstance() {
return SingleTonHolder.instance;
} }
采用内部类方式维护单例,可以做到懒加载又可以做到不采用关键字synchronized,避免双重锁机制判断