单例模式160905

/**
 * 单例模式:保证只有一个实例 private Singleton(){};
 * 饿汉式:先创建 private static final Singleton INSTANCE = new Singleton(); 用的时候调用 public static Singleton getInstance(){return INSTANCE;}
 * 懒汉式:用的时候创建实例。
 *         synchronized在方法内,则主要double-check;
 *         同步在方法上 public static synchronized Singleton getInstance(){ if(instanceLazy==null) instanceLazy=new Singleton();return instanceLazy;}
 * 静态代码块:
 *         static{ instanceStaticBlock = new Singleton();}
 * 静态内部类:
 *         private static class SingletonHolder{ public static final INSTANCE_STATIC_CLASS=new Singleton();}
 * 枚举:
 *         public enum SingletonEnum{INSTANCE;}
 */
public class Singleton implements Serializable{
    private Singleton(){};  // 私有化构造方法,外部无法创建本类 
    // 饿汉式
    private static final Singleton INSTANCE = new Singleton();
    public static Singleton getInstanceEager(){ return INSTANCE; }
    // 懒汉式
    private static volatile Singleton instanceLazy = null;
    public static Singleton getInstanceLazy1(){
        if(instanceLazy == null){
            synchronized(Singleton.class){
                if(instanceLazy == null){
                    instanceLazy = new Singleton();
                }
            }
        }
        return instanceLazy;
    }
    public static synchronized Singleton getInstanceLazy2(){
        if(instanceLazy == null){
            instanceLazy = new Singleton();
        }
        return instanceLazy;
    }
    // 静态代码块
    private static Singleton instanceStaticBlock = null;
    static{
        instanceStaticBlock = new Singleton();
    }
    public static Singleton getInstanceStaticBlock(){ return instanceStaticBlock; }
    // 静态内部类
    private static class SingletonHolder{ public static final Singleton INSTANCE = new Singleton(); }
    public static Singleton getInstanceStaticClass(){ return SingletonHolder.INSTANCE; }
    
    // 砖家建议:功能完善,性能更佳,不存在序列化等问题的单例 就是 静态内部类 + 如下的代码
    private static final long serialVersionUID = 1L;
    protected Object readResolve(){ return getInstanceStaticClass(); }
}
//枚举
enum SingletonEnum{
    INSTANCE;
}