《剑指offer》java语言优化版——实现Singleton模式

饿汉式
public class Singleton1 {
    private static Singleton1 instance = new Singleton1();
    private Singleton1(){
    }
    public Singleton1 getInstance(){
        return instance;
    }
}

懒汉式
public class Singleton2 {
    private static Singleton2 instance = null;//创建的时机
    private Singleton2(){
    }
    public Singleton2 getInstance(){
        if(instance == null){
            instance = new Singleton2();
        }
        return instance;
    }
}
Double-Check-Lock（DCL）
public class Singleton5 {
    private static Singleton5 instance = null;//创建的时机
    private Singleton5(){
    }
    public static Singleton5 getInstance(){
        if(instance==null){
            synchronized(Singleton5.class){
                if(null==instance){
                    instance=new Singleton5();
                }
            }
        }
        return instance;
    }
}
DCL+volatile
public class Singleton6 {
    private volatile static Singleton6 instance = null;//保证有序性
    private Singleton6(){
    }
    public static Singleton6 getInstance(){
        if(instance==null){
            synchronized(Singleton6.class){
                if(null==instance){
                    instance=new Singleton6();
                }
            }
        }
        return instance;
    }
}
Holder模式
public class Singleton7 {
    private volatile static Singleton7 instance = null;//保证有序性
    private Singleton7(){
    }
    private static class Holder{
        private static Singleton7 instance = new Singleton7();
    }
    public static Singleton7 getInstance(){
        return Holder.instance;
    }
}
枚举模式
public enum Singleton8 {
    uniqueInstance;// 定义一个枚举的元素，它就代表了Singleton的一个实例

    public void singletonOperation() {
        // 功能处理
        System.err.println("功能处理");
    }
}