Java中的单例设计模式

Singleton Pattern说, 只是“定义一个只有一个实例并提供对其全局访问点的类”。

换句话说, 一个类必须确保仅创建单个实例, 并且所有其他类都可以使用单个对象。

单例设计模式有两种形式

• 早期实例化：在加载时创建实例。
• 延迟实例化：在需要时创建实例。

Singleton设计模式的优势

• 因为没有在每个请求中创建对象, 所以节省了内存。只有单个实例一次又一次地被重用。

Singleton设计模式的用法

• 单例模式主要用于多线程和数据库应用程序。它用于日志记录, 缓存, 线程池, 配置设置等。

UML的Singleton设计模式

如何创建Singleton设计模式？

要创建单例类, 我们需要具有类的静态成员, 私有构造函数和静态工厂方法。

• 静态成员：由于静态, 它仅获取一次内存, 它包含Singleton类的实例。
• 私有构造函数：它将阻止从类外部实例化Singleton类。
• 静态工厂方法：此方法提供对Singleton对象的全局访问点, 并将实例返回给调用方。

了解Singleton模式的早期实例化

在这种情况下, 我们在声明静态数据成员时创建该类的实例, 因此该类的实例是在加载类时创建的。

让我们看一下使用早期实例化的单例设计模式的示例。

class A{
 private static A obj=new A();//Early, instance will be created at load time
 private A(){}
 
 public static A getA(){
  return obj;
 }

 public void doSomething(){
 //write your code
 }
}

了解单例模式的延迟实例化

在这种情况下, 我们将在同步方法或同步块中创建该类的实例, 因此将在需要时创建该类的实例。

让我们看一下使用延迟实例化的单例设计模式的简单示例。

class A{
 private static A obj;
 private A(){}
 
 public static A getA(){
   if (obj == null){
      synchronized(Singleton.class){
		if (obj == null){
			obj = new Singleton();//instance will be created at request time
		}
	}            
    }
  return obj;
 }

 public void doSomething(){
 //write your code
 }
}

类装载器在单例模式中的意义

如果单例类由两个类加载器加载, 则将创建两个单例类实例, 每个类加载器一个。

单例模式中序列化的意义

如果singleton类是Serializable, 则可以序列化singleton实例。序列化后, 你可以对其进行反序列化, 但不会返回单例对象。

若要解决此问题, 你需要重写强制执行单例的readResolve()方法。在对象反序列化之后立即调用它。它返回单例对象。

public class A implements Serializable {
		//your code of singleton
		protected Object readResolve() {
			return getA();
		}

	}

了解单例模式的真实示例

• 我们将创建一个JDBCSingleton类。该JDBCSingleton类包含其构造函数（私有）和自身的私有静态实例jdbc。
• JDBCSingleton类提供了一个静态方法来将其静态实例传递给外界。现在, JDBCSingletonDemo类将使用JDBCSingleton类来获取JDBCSingleton对象。

假设：你创建了一个表userdata, 该表在mysql数据库中具有uid, uname和upassword三个字段。数据库名称是ashwinirajput, 用户名是root, 密码是ashwini。

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

class JDBCSingleton {
     //Step 1
      // create a JDBCSingleton class.
     //static member holds only one instance of the JDBCSingleton class.
           
         private static JDBCSingleton jdbc;
         
     //JDBCSingleton prevents the instantiation from any other class.
       private JDBCSingleton() {  }
      
    //Now we are providing gloabal point of access.
         public static JDBCSingleton getInstance() {  
                                     if (jdbc==null)
                                   {
                                            jdbc=new  JDBCSingleton();
                                   }
                         return jdbc;
             }
          
   // to get the connection from methods like insert, view etc. 
          private static Connection getConnection()throws ClassNotFoundException, SQLException
          {
              
              Connection con=null;
              Class.forName("com.mysql.jdbc.Driver");
              con= DriverManager.getConnection("jdbc:mysql://localhost:3306/ashwanirajput", "root", "ashwani");
              return con;
              
          }
          
 //to insert the record into the database 
          public int insert(String name, String pass) throws SQLException
          {
              Connection c=null;
              
              PreparedStatement ps=null;
              
              int recordCounter=0;
              
              try {
                  
                      c=this.getConnection();
                      ps=c.prepareStatement("insert into userdata(uname, upassword)values(?, ?)");
                      ps.setString(1, name);
                      ps.setString(2, pass);
                      recordCounter=ps.executeUpdate();
                      
              } catch (Exception e) { e.printStackTrace(); } finally{
                    if (ps!=null){
                      ps.close();
                  }if(c!=null){
                      c.close();
                  } 
              }
             return recordCounter;
          }

//to view the data from the database      
      public  void view(String name) throws SQLException
      {
                Connection con = null;
		PreparedStatement ps = null;
		ResultSet rs = null;
                
                try {
                    
                        con=this.getConnection();
                        ps=con.prepareStatement("select * from userdata where uname=?");
                        ps.setString(1, name);
                        rs=ps.executeQuery();
                        while (rs.next()) {
                                  System.out.println("Name= "+rs.getString(2)+"\t"+"Paasword= "+rs.getString(3));    
                       
                        }
              
          } catch (Exception e) { System.out.println(e);}
          finally{
                    if(rs!=null){
                        rs.close();
                    }if (ps!=null){
                      ps.close();
                  }if(con!=null){
                      con.close();
                  } 
                }
      }
      
     // to update the password for the given username
      public int update(String name, String password) throws SQLException  {
              Connection c=null;
              PreparedStatement ps=null;
              
              int recordCounter=0;
              try {
                      c=this.getConnection();
                      ps=c.prepareStatement(" update userdata set upassword=? where uname='"+name+"' ");
                      ps.setString(1, password);
                      recordCounter=ps.executeUpdate();
              } catch (Exception e) {  e.printStackTrace(); } finally{
                    
                  if (ps!=null){
                      ps.close();
                  }if(c!=null){
                      c.close();
                  } 
               }
             return recordCounter;
          }
          
// to delete the data from the database 
         public int delete(int userid) throws SQLException{
              Connection c=null;
              PreparedStatement ps=null;
              int recordCounter=0;
              try {
                       c=this.getConnection();
                      ps=c.prepareStatement(" delete from userdata where uid='"+userid+"' ");
                      recordCounter=ps.executeUpdate();
              } catch (Exception e) { e.printStackTrace(); } 
              finally{
              if (ps!=null){
                      ps.close();
             }if(c!=null){
                      c.close();
              } 
           }
             return recordCounter;
          }
 }// End of JDBCSingleton class

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
class JDBCSingletonDemo{
    static int count=1;
    static int  choice;
    public static void main(String[] args) throws IOException {
        
        JDBCSingleton jdbc= JDBCSingleton.getInstance();
        
        
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
   do{ 
        System.out.println("DATABASE OPERATIONS");
        System.out.println(" --------------------- ");
        System.out.println(" 1. Insertion ");
        System.out.println(" 2. View      ");
        System.out.println(" 3. Delete    ");
        System.out.println(" 4. Update    ");
        System.out.println(" 5. Exit      ");
        
        System.out.print("\n");
        System.out.print("Please enter the choice what you want to perform in the database: ");
        
        choice=Integer.parseInt(br.readLine());
        switch(choice) {
            
           case 1:{
                    System.out.print("Enter the username you want to insert data into the database: ");
                    String username=br.readLine();
                    System.out.print("Enter the password you want to insert data into the database: ");
                    String password=br.readLine();
                    
                    try {
                            int i= jdbc.insert(username, password);
                            if (i>0) {
                            System.out.println((count++) + " Data has been inserted successfully");
                            }else{
                                System.out.println("Data has not been inserted ");    
                            }
                        
                        } catch (Exception e) {
                          System.out.println(e);
                        }
                    
                     System.out.println("Press Enter key to continue...");
                     System.in.read();
                     
                   }//End of case 1
                   break;
            case 2:{
                    System.out.print("Enter the username : ");
                    String username=br.readLine();
             
                    try  {
                            jdbc.view(username);
                         } catch (Exception e) {
                          System.out.println(e);
                        }
                     System.out.println("Press Enter key to continue...");
                     System.in.read();
                     
                   }//End of case 2
                  break;
             case 3:{
                     System.out.print("Enter the userid, you want to delete: ");
                     int userid=Integer.parseInt(br.readLine());
           
                     try {
                            int i= jdbc.delete(userid);
                            if (i>0) {
                            System.out.println((count++) + " Data has been deleted successfully");
                            }else{
                                System.out.println("Data has not been deleted");    
                            }
                        
                         } catch (Exception e) {
                          System.out.println(e);
                         }
                     System.out.println("Press Enter key to continue...");
                     System.in.read();
                     
                    }//End of case 3
                   break;
             case 4:{
                    System.out.print("Enter the username, you want to update: ");
                    String username=br.readLine();
                    System.out.print("Enter the new password ");
                    String password=br.readLine();
                    
                    try {
                            int i= jdbc.update(username, password);
                            if (i>0) {
                            System.out.println((count++) + " Data has been updated successfully");
                            }
                        
                        } catch (Exception e) {
                          System.out.println(e);
                        }
                     System.out.println("Press Enter key to continue...");
                     System.in.read();
                    
                   }// end of case 4
                 break;
                 
             default:
                     return;
        }
        
       } while (choice!=4); 
   }
}

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