Facade 模式（外观模式）

最新推荐文章于 2024-09-03 07:00:00 发布

转载最新推荐文章于 2024-09-03 07:00:00 发布 · 130 阅读

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原文链接：https://yq.aliyun.com/articles/396408

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#数据库

本文介绍了Facade模式的概念及其在软件设计中的应用。通过一个具体的例子——抵押贷款申请系统，展示了如何使用Facade模式来简化复杂的子系统接口，提高代码的可读性和可维护性。

为子系统中的一组接口提供一个一致的界面。Facade模式定义了一个高层接口，这个接口使得这一系统更容易使用。

Facade（MortgageApplication）

知道哪些子系统类负责处理请求。

将客户的请求代理给适当的子系统对象。

Subsystem classes((Bank, Credit, Loan)

实现子系统的功能。

处理有facade对象指派的任务。

没有facade的任何相关信息；即没有引用facade。

代码

package design.pattern.facade;

/**

* Subsystem ClassA

* @author tyrone

public class Bank {

// Methods

/** @modelguid {0204A2F3-F161-4786-B0AE-34388CFBD12B} */

public boolean SufficientSavings( Customer c )

{

System.out.println('Check bank for '+ c.getName() );

return true;

}

package design.pattern.facade;

/**

* Subsystem ClassB

* @author tyrone

public class Credit {

// Methods

public boolean GoodCredit( int amount, Customer c )

{

System.out.println( 'Check credit for '+ c.getName() );

return true;

}

package design.pattern.facade;

/**

* Subsystem ClassC

* @author tyrone

public class Loan {

// Methods

public boolean GoodLoan( Customer c )

{

System.out.println( 'Check loan for '+ c.getName());

return true;

}

package design.pattern.facade;

/**

* @author tyrone

public class Customer {

private String name;

public Customer(String name){

this.name=name;

}

public void setName(String name){

this.name=name;

}

public String getName(){

return this.name;

}

package design.pattern.facade;

/**

* Facade

* @author tyrone

public class MortgageApplication {

// Fields

int amount;

private Bank bank = new Bank();

private Loan loan = new Loan();

private Credit credit = new Credit();

// Constructors

public MortgageApplication( int amount )

{

this.amount = amount;

}

// Methods

public boolean IsEligible( Customer c )

{

// Check creditworthyness of applicant

if( !bank.SufficientSavings( c ) )

return false;

if( !loan.GoodLoan( c ) )

return false;

if( !credit.GoodCredit( amount, c ))

return false;

return true;

}

package design.pattern.facade;

/**

* <summary>

* Facade Client

* </summary>

* @author tyrone

public class FacadeApp {

/** @modelguid {67B55D21-888A-422E-B88E-1FBC34D3AA5C} */

public static void main(String[] args)

{

// Create Facade

MortgageApplication mortgage =

new MortgageApplication( 125000 );

// Call subsystem through Facade

mortgage.IsEligible(

new Customer( 'Gabrielle McKinsey' ) );

}

Output

Check bank for Gabrielle McKinsey
Check loan for Gabrielle McKinsey
Check credit for Gabrielle McKinsey

-----------------------------------------------------------------------------------------------------------------------------------------------------

Facade模式的定义: 为子系统中的一组接口提供一个一致的界面.

Facade一个典型应用就是数据库JDBC的应用,如下例对数据库的操作:

public class DBCompare {

　　Connection conn = null;
PreparedStatement prep = null;
ResultSet rset = null;
try {
Class.forName( "<driver>" ).newInstance();
conn = DriverManager.getConnection( "<database>" );

String sql = "SELECT * FROM <table> WHERE <column name> = ?";
prep = conn.prepareStatement( sql );
prep.setString( 1, "<column value>" );
rset = prep.executeQuery();
if( rset.next() ) {
System.out.println( rset.getString( "<column name" ) );
}
} catch( SException e ) {
e.printStackTrace();
} finally {
rset.close();
prep.close();
conn.close();
}
}

上例是Jsp中最通常的对数据库操作办法.

在应用中,经常需要对数据库操作,每次都写上述一段代码肯定比较麻烦,需要将其中不变的部分提炼出来,做成一个接口,这就引入了facade外观对象.如果以后我们更换Class.forName中的<driver>也非常方便,比如从Mysql数据库换到Oracle数据库,只要更换facade接口中的driver就可以.

我们做成了一个Facade接口,使用该接口,上例中的程序就可以更改如下:

public class DBCompare {

String sql = "SELECT * FROM <table> WHERE <column name> = ?";　　

try {
Mysql msql=new mysql(sql);
prep.setString( 1, "<column value>" );
rset = prep.executeQuery();
if( rset.next() ) {
System.out.println( rset.getString( "<column name" ) );
}
} catch( SException e ) {
e.printStackTrace();
} finally {
mysql.close();
mysql=null;
}
}