面试常见的几种单例模式
不支持多线程的单例模式:
懒汉式线程不安全:
public class Singleton {
private static Singleton instance;
private Singleton (){}
public static Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
支持多线程
synchronized意味着不高效
- 懒汉式:
效率很低
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
基于ClassLoader实现加载时就被创建出对象,推荐使用,但是同时也会浪费内存
public class Singleton {
private static Singleton instance = new Singleton();
private Singleton (){}
public static Singleton getInstance() {
return instance;
}
}
-
双重校验锁
在同步的情况下仍然能高效
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;
}
}