单例设计模式

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//不能保证线程安全的懒汉模式
public class Singleton{
private static Singleton instance;
//构造函数
private Singleton(){
}
public static Singleton getInstance(){
//为空的话构建一个
if(instance == null ){
   instance = new Singleton();
}
return instance;
}
}

//保证了线程安全的懒汉模式
public class Singleton{
//声明一个静态变量
private static Singleton instance;
//构造函数
private Singleton(){
}
//加锁的函数
public static synchronzied Singleton getInstance(){
//为空的话构建一个
if(instance == null ){
   instance = new Singleton();
}
return instance;
}
}

//饿汉模式
public class Singleton{
private static Singleton instance;
static{
instance = new Singleton();
}
private Singleton(){
}
public static Singleton getInstance(){
return instance;
}
}

//双重校验锁懒汉模式
public class Singleton{
private volatile static Singleton_volatile instance=null;

private Singleton(){
}

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

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