[TOC]
I. model definition
Appearance mode: appearance mode is to provide a unified interface to access a group of interfaces of subsystem. The facade pattern defines a high-level interface that makes subsystems easier to use. Appearance mode, also known as facade mode, is an object structural design mode.
Insert picture description here II. Model role
As you can see from the pattern definition, the appearance pattern should include the following roles:
- Frcade: appearance character
- SubSystem: SubSystem role
- Client: client role
Classic example:
public class Facade { private SubSystemA obj1 = new SubSystemA(); private SubSystemB obj2 = new SubSystemB(); private SubSystemC obj3 = new SubSystemC(); public void method() { obj1.method(); obj2.method(); obj3.method(); } }
3. Simple analysis of the model
The appearance mode provides convenience for the client class. The client class does not need to pay attention to the design of the subsystem. It is good to provide appearance class access directly
The appearance mode conforms to "Dimiter's law", introduces a single simple interface to call the client, thus reducing the coupling degree between the client and the subsystem
However, appearance pattern also has some disadvantages. Each design pattern has its disadvantages and advantages. It needs to be selected according to complex business scenarios. If an abstract appearance class is not referenced, the appearance class and client class code need to be adjusted once the business changes
Insert picture description here For some very complex business systems, sometimes multiple appearance classes can be designed for system decoupling
IV. simple example practice
JDBC database operation example, this example comes from Design mode A Book
import java.sql.*; public class JDBCFacade { private Connection conn=null; private Statement statement=null; public void open(String driver,String jdbcUrl,String userName,String userPwd) { try { Class.forName(driver).newInstance(); conn = DriverManager.getConnection(jdbcUrl,userName,userPwd); statement = conn.createStatement(); } catch (Exception e) { e.printStackTrace(); } } public int executeUpdate(String sql) { try { return statement.executeUpdate(sql); } catch (SQLException e) { e.printStackTrace(); return -1; } } public ResultSet executeQuery(String sql) { try { return statement.executeQuery(sql); } catch (SQLException e) { e.printStackTrace(); return null; } } public void close() { try { conn.close(); statement.close(); } catch (SQLException e) { e.printStackTrace(); } } }
V. mode application
Appearance mode is suitable for complex system and can be used for system decoupling. Here are some application scenarios of appearance mode
Session in Java EE framework uses appearance mode
Learning JSP's JDBC database operation is also often using appearance mode