java单例模式（下）

五种实现方式：

//create instance with loading class.it's better to create instance the when you// will use.the instance should not be heavy and use possiblely.

在类加载到jvm初始化的时候创建对象，如果实例比较大，会影响性能。

class Singleton1{

private static final Singleton1 instance = new Singleton1();
private Singleton1(){

}
public static Singleton1 getInstance(){
return instance; 
}

}

//double check. 对Singleton2的class对象加锁。

class Singleton2{
private static Singleton2 instance;
private Singleton2(){

}
public static Singleton2 getInstance(){
if(instance == null){
synchronized(Singleton2.class){
if(instance == null){
instance = new Singleton2();
}
}
}
return instance;
}

}

//静态内部类使用，延迟实例化对象。在使用时创建而不是在类加载初始化创建。

class Singleton3{
private Singleton3(){
}
private static class Instance{
private static final Singleton3 instance = new Singleton3();
private Instance(){}
}
public static Singleton3 getInstance(){
return Instance.instance;
}

}

//can't clone and serializable.recommend use. 避免克隆和反序列化得到不同实例对象。

enum Singleton4{
INSTANCE;
private String name;
public void setName(String name){
this.name = name;
}
public String getName(){
return name;
}

}

//use in single thread 适用单线程

class Singleton5{
private static Singleton5 instance;
private Singleton5(){

}
public static Singleton5 getInstance(){
if(instance == null){
instance = new Singleton5();
}
return instance;
}

}

同步方法性能受到影响，适用于调用次数不多的情况。

class Singleton6{
private static Singleton6 instance;
private Singleton6(){

}
public static synchronized Singleton6 getInstance(){
if(instance == null){
instance = new Singleton6();
}
return instance;
}

}