Singleton - 单例模式

1. show you the code：

public class Singleton01 {
    private static Singleton01 instance;
    private Singleton01() {
    }
    // 线程不安全的单例实现
    public static Singleton01 getInstance(){
        if(instance == null)
            instance = new Singleton01();
        return instance;
    }
}

public class Singleton02 {
    private static Singleton02 instance;
    private Singleton02() {
    }
    //线程安全，但效率低
    public static synchronized Singleton02 getInstance(){
        if(instance == null)
            instance = new Singleton02();
        return instance;
    }
}

public class Singleton03 {
    private static Singleton03 instance = new Singleton03();
    private Singleton03() {
    }
    //线程安全，饿汉模式
    public static Singleton03 getInstance(){
        return instance;
    }
}

public class Singleton04 {
    private volatile static Singleton04 instance;//volatile 防止指令重排
    private Singleton04() {
    }
    //线程安全，双重检索模式
    public static Singleton04 getInstance() {
        if (instance == null) {
            synchronized (Singleton04.class) {
                if (instance == null) {
                    instance = new Singleton04();
                }
            }
        }
        return instance;
    }
}

// 内部静态类方式，线程安全
public class Singleton05 {
    private Singleton05() {
    }
    private static class SingletonHolder{
        private static final Singleton05 INSTANCE = new Singleton05();
    } 
    public static Singleton05 getInstance(){
        return SingletonHolder.INSTANCE;
    }
}

//枚举模式
public enum  SingletonEnum {
    INSTANCE;
}

2. 举例子

饿汉模式 懒汉模式

3. 左耳朵耗子的实现：https://coolshell.cn/articles/265.html
4. http://wuchong.me/blog/2014/08/28/how-to-correctly-write-singleton-pattern/