java中单例模式

java中单例模式

饿汉式单例

public class Single{

private static Single mInstance = new Single();

private Single(){}

public static Single getInstance(){

return mInstance;

}

}

优点是线程安全，缺点时没有办法实现懒加载。

懒汉式+同步方法单例

public class Single{

private static Signle mInstance;

private Single(){}

public static synchronized Single getInstance(){

if(mInstance == null){

mInstance = new Single();

}

return mInstance;

}

}

优点是线程安全，懒加载

缺点是效率不高

volatile+Double Check方式

public class Single{

private volatile static Single mInstance;

private Single(){}

private static Single getInstance(){

if(mInstance == null){

synchronized(Single.class){

if(mInstance == null){

mInstance = new Single();

}

}

}

return mInstance;

}

}

优点是线程安全，效率还可以

Holder模式

public class Single{

private Single(){}

private static class Holder{

private static Single instance = new Single();

}

public static Single getInstance(){

return Holder.instance;

}

}

优点是线程安全，懒加载，效率高。

枚举方式

public enum Single{

//单例变量，也是枚举变量

INSTANCE;

public static getInstance(){

return INSTANCE;

}

}

优点，线程安全

缺点，没有懒加载

枚举+Holder

public class Single{

private Single(){}

//这个枚举类型的Holder主要是为了确保单例

private enum EnumHolder{

INSTANCE;

//单例的枚举型中包含一个Single类型的变量

private Single instance;

EnumHolder(){

//在构造函数中延时初始化

this.instance = new Single();

}

//提供方法返回单例

private Single getInstance(){

return instance;

}

}

//通过枚举类型返回单例实例

public static Single getInstance(){

return Instance();

}

}