1.懒汉式 线程不安全,当有多个线程并行调用getInstance()的时候，就会创建多个实例。也就是说在多线程下不能正常工作。
public class Singleton{
private static Singleton instance;
private Singleton(){}
public static Singleton getInstance(){
if(instance == null){
instance = new Singleton();
}
return instance;
}
}
2.懒汉式 线程安全,虽然做到了线程安全，并且解决了多实例的问题，但是它并不高效。因为在任何时候只能有一个线程调用getInstance()方法。但是同步操作只需要在第一次调用时才被需要，即第一次创建单例实例对象时。这就引出了双重检验锁。
public class Singleton{
private static Singleton instance;
private Singleton(){}
public synchronized static Singleton getInstance(){
if(instance == null){
instance = new Singleton();
}
return instance;
}
}
3.双重检验锁
public static Singleton getSingleton() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance ;
}
>改进代码, instance 变量声明成 volatile
public class Singleton {
private volatile static Singleton instance; //声明成 volatile
private Singleton (){}
public static Singleton getSingleton() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}
4.饿汉式,在第一次加载类到内存中时就会初始化，所以创建实例本身是线程安全的。
public class Singleton {
//类加载的时候就初始化
private static final Singleton instance = new Singleton();
private Singleton(){}
public static Singleton getInstance() {
return instance;
}
}
5.静态内部类.
public class Singleton {
private static class SingletonHolder{
private static final INCTANCE = new Singleton();
}
private Singleton(){}
public static final Singleton getInstance(){
return SingletonHolder.INSTANCE;
}
}
6.枚举
public enum EasySingleton {
INSTANCE;
}
