一般来说，单例模式有五种写法：懒汉、饿汉、双重检验锁、静态内部类、枚举。宜信所说都是线程安全的实现。 就我个人而言，一般情况下直接使用饿汉式就好了，如果明确要求要懒加载（lazy initialization）会倾向于使用静态内部类，如果涉及到反序列化创建对象时会试着使用枚举的方式来实现单例。

懒汉式

public final class Singleton {    private static Singleton instance;    private Singleton() {        //    }    public static Singleton getInstance() {        synchronized (Singleton.class) {            if(instance == null) {                instance = new Singleton();            }        }        return instance;    }}

双重检验锁

public final class Singleton {    private volatile static Singleton instance; //用volatile修饰对象，禁止指令重排序优化    private Singleton() {        //    }    public static Singleton getInstance() {        if(instance == null) {            synchronized (Singleton.class) {                if(instance == null) {                    instance = new Singleton();                }            }        }        return instance;    }}

饿汉式

public final class Singleton {    //类加载时就初始化    private static final Singleton instance = new Singleton();    private Singleton() {        //    }    public static Singleton getInstance() {        return instance;    }}

静态内部类

public final class Singleton {    private Singleton() {        //    }    private static class SingletonHolder {        private static final Singleton INSTANCE = new Singleton();    }    public static Singleton getInstance() {        return SingletonHolder.INSTANCE;    }}

枚举

public enum EnumSingleton {    INSTANCE;    private EnumSingleton() {    }}