单件模式

public class {
    private static Singleton uniqueInstance;
    private (){
        
    }
    public static Singleton getInstance(){
        if(uniqueInstance == null){
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }
}

public class {
    private static Singleton uniqueInstance;
    private (){
        
    }
    
    //这种方式会使运行效率大幅下降
    public static synchronized Singleton getInstance(){
        if(uniqueInstance == null){
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }
}

public class {
    private static Singleton uniqueInstance = new Singleton();
    private Singleton(){   
    }
    public static Singleton getInstance(){
        return uniqueInstance;
    }
}

//jdk1.5 以后版本可用
public class Singleton{
    private volatile static Singleton uniqueInstance;
    private Singleton(){
        
    }
    public static Singleton getInstance(){
        if(uniqueInstance == null){
            synchronized (Singleton.class){
                if(uniqueInstance == null){
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;
    }
}