一、懒汉式单例
//懒汉式单例类.在第一次调用的时候实例化自己
public class Singleton {
private Singleton() {}
private static Singleton single=null;
//静态工厂方法
public static Singleton getInstance() {
if (single == null) {
single = new Singleton();
}
return single;
}
}
以上是线程不安全的 ，在多线程下可能出现多个Singleton 实例，同时多个线程执行getInstance()
要线程安全可以这样做：
1. 在getInstance方法上加同步
public static synchronized Singleton getInstance() {
if (single == null) {
single = new Singleton();
}
return single;
}
2. 双重检查锁定
public static Singleton getInstance() {
if (singleton == null) {
synchronized (Singleton.class) {
if (singleton == null) {
singleton = new Singleton();
}
}
}
return singleton;
}
3. 静态内部类
public class Singleton {
private static class LazyHolder {
private static final Singleton INSTANCE = new Singleton();
}
private Singleton (){}
public static final Singleton getInstance() {
return LazyHolder.INSTANCE;
}
}
二、饿汉式单例
//饿汉式单例类.在类初始化时，已经自行实例化
public class Singleton1 {
private Singleton1() {}
private static final Singleton1 single = new Singleton1();
//静态工厂方法
public static Singleton1 getInstance() {
return single;
}
}
类创建的同时创建了一个静态对象，以后不再改变，所以天生线程安全