错误解法一：当处于多线程环境时，会出现竞争。

public class Singleton1     {    private Singleton1()    {    }    int a =5;    private static Singleton1 singleton1 = null;    public static Singleton1 getSingleton1(){        if(singleton1 == null){            singleton1 = new Singleton1();        }        return singleton1;    }}

 

错误解法二：虽然加了锁，解决了多线程下的竞争。但是在创建后的每次访问都加锁，影响效率。

public class Singleton2 {    private Singleton2()    {    }    int a =5;    private static Singleton2 singleton2 = null;    public static Singleton2 getSingleton2(){        synchronized(Singleton2.class)        {            if(singleton2 == null){                singleton2 = new Singleton2();            }        }        return singleton2;    }}

 

可行的解法三：在锁前面再加一个if判断将是一种可行的解法。

public class Singleton3 {    private Singleton3()    {    }    int a =5;    private static Singleton3 Singleton3 = null;    public Singleton3 getSingleton3(){        if(Singleton3 == null){            synchronized(Singleton3.class)            {                if(Singleton3 == null){                    Singleton3 = new Singleton3();                }            }        }        return Singleton3;    }}

 

 推荐的解法4：利用静态构造函数

public class Singleton4 {    private Singleton4() {    }    private static Singleton4 Singleton4 = new Singleton4();    public Singleton4 getSingleton4() {        return Singleton4;    }}

 推荐的解法5：实现按需创建实例，涉及语言特性

public class Singleton5 {    Singleton5() {    }    public Singleton5 getSingleton5() {        return Nested.instance;    }        static class Nested{        Nested(){        }        static Singleton5 instance = new Singleton5();    }}