设计模式学习笔记（九）桥接模式及其应用设计模式学习笔记（九）桥接

桥接（Bridge）模式是指将抽象部分与实现部分相分离，使它们都可以独立的发生变化。
一、桥接模式介绍 【设计模式学习笔记（九）桥接模式及其应用】我们知道，抽象部分一般与实现部分连接有两种方式：继承和实现。那么如何将其解耦分离，桥接模式提供一种方式，也就是将强关联转为弱关联，将继承转换为组合关系。如下图所示，取消两者的继承关系，改用组合关系：

文章图片

1.1 桥接模式的结构
我们可以看看桥接模式是怎么解耦，利用组合连接抽象和实现部分，如下所示：

其结构中包含如下角色：

Abstraction：抽象化角色，定义抽象类，包含一个对实现化对象的引用（组合）
RefinedAbstraction：扩展抽象化角色，实现抽象化角色的子类，由此通过组合关系调用实现化角色中的业务方法
Implementor：实现化角色的接口，供扩展抽象化角色调用
ImplementorA、ImplementorB：实现化角色的具体实现

1.2 桥接模式的实现
我们可以根据上面的UML图实现对应的代码：

//客户端类 public class Client { public static void main(String[] args) { Implementor imple = new ImplementorA(); Abstraction abs = new RefinedAbstraction(imple); abs.Operation(); } } //实现化角色 interface Implementor { public void OperationImpl(); } //具体的实现化角色 class ImplementorA implements Implementor { public void OperationImpl() { System.out.println("我是具体实现化角色A"); } } class ImplementorB implements Implementor { public void OperationImpl() { System.out.println("我是具体实现化角色B"); } } //抽象化角色 abstract class Abstraction { protected Implementor imple; protected Abstraction(Implementor imple) { this.imple = imple; }public abstract void Operation(); } //扩展抽象化角色 class RefinedAbstraction extends Abstraction { protected RefinedAbstraction(Implementor imple) { super(imple); }public void Operation() { System.out.println("扩展抽象化角色被访问"); imple.OperationImpl(); } }

实现结果：

扩展抽象化角色被访问我是具体实现化角色A

二、桥接模式的应用场景 2.1 JDBC 驱动器
JDBC为所有的关系型数据库提供一个通用的标准，这就是一个桥接模式的典型应用。我们先回顾一下JDBC的使用，用JDBC连接MySQL数据库主要分为这样几步：

//1.加载MySQL驱动注入到DriverManager Class.forName("com.mysql.cj.jdbc.Driver"); //2.提供JDBC连接的URL、用户名和密码 String url = "jdbc:mysql://localhost:3306/test_db?"; String username = "root"; String password = "root"; //3.创建数据库的连接 Connection connection = DriverManager.getConnection(url, username, password); //4.创建statement实例 Statement statement = connection.createStatement(); //5.执行SQL语句 String query = "select * from test"; //查询语句，也可以换成CRUD的其他语句 ResultSet resultSet = statement.executeQuery(query); //6.关闭连接对象 connection.close();

我们一步步来看，先看步骤1：

Class.forName("com.mysql.cj.jdbc.Driver");

查看对应的 com.mysql.cj.jdbc.Driver路径下的源码：

package com.mysql.cj.jdbc; import java.sql.DriverManager; import java.sql.SQLException; public class Driver extends NonRegisteringDriver implements java.sql.Driver { public Driver() throws SQLException { }static { try { DriverManager.registerDriver(new Driver()); } catch (SQLException var1) { throw new RuntimeException("Can't register driver!"); } } }

是通过静态方法调用registerDriver()方法来将MySQL驱动注入到DriverManager，registerDriver()方法具体如下：

public static synchronized void registerDriver(java.sql.Driver driver) throws SQLException { //直接调用下面的同名静态方法 registerDriver(driver, null); }public static synchronized void registerDriver(java.sql.Driver driver,DriverAction da)throws SQLException { /* registeredDrivers是一个list,用DriverInfo实例封装Driver */ if(driver != null) { registeredDrivers.addIfAbsent(new DriverInfo(driver, da)); } else { // This is for compatibility with the original DriverManager throw new NullPointerException(); } println("registerDriver: " + driver); }

registeredDrivers静态变量其实是一个list：

public class DriverManager { // List of registered JDBC drivers private final static CopyOnWriteArrayList registeredDrivers = new CopyOnWriteArrayList<>(); //... }

而DriverInfo类中封装了java.sql.Driver接口:

class DriverInfo {final Driver driver; DriverAction da; DriverInfo(Driver driver, DriverAction action) { this.driver = driver; da = action; } //... }

再看步骤2、3，重点是步骤3

Connection connection = DriverManager.getConnection(url, username, password);

Connection接口是和特定数据库的连接会话，不同的数据库的连接会话都不相同：

public interface Connectionextends Wrapper, AutoCloseable {Statement createStatement() throws SQLException; //... }

是通过DriverManager中的getConnection方法，从registeredDrivers进行选择对应数据库驱动下的连接实例：

public static Connection getConnection(String url,String user, String password) throws SQLException { java.util.Properties info = new java.util.Properties(); if (user != null) { info.put("user", user); } if (password != null) { info.put("password", password); }return (getConnection(url, info, Reflection.getCallerClass())); } // 实际上调用的是下面的静态方法getConnection //Worker method called by the public getConnection() methods. private static Connection getConnection( String url, java.util.Properties info, Class caller) throws SQLException { /* * When callerCl is null, we should check the application's * (which is invoking this class indirectly) * classloader, so that the JDBC driver class outside rt.jar * can be loaded from here. */ ClassLoader callerCL = caller != null ? caller.getClassLoader() : null; synchronized(DriverManager.class) { // synchronize loading of the correct classloader. if (callerCL == null) { callerCL = Thread.currentThread().getContextClassLoader(); } }if(url == null) { throw new SQLException("The url cannot be null", "08001"); }println("DriverManager.getConnection(\"" + url + "\")"); // Walk through the loaded registeredDrivers attempting to make a connection. // Remember the first exception that gets raised so we can reraise it. SQLException reason = null; for(DriverInfo aDriver : registeredDrivers) { // If the caller does not have permission to load the driver then // skip it. if(isDriverAllowed(aDriver.driver, callerCL)) { try { println("trying " + aDriver.driver.getClass().getName()); Connection con = aDriver.driver.connect(url, info); if (con != null) { // Success! println("getConnection returning " + aDriver.driver.getClass().getName()); return (con); } } catch (SQLException ex) { if (reason == null) { reason = ex; } }} else { println("skipping: " + aDriver.getClass().getName()); } }// if we got here nobody could connect. if (reason != null){ println("getConnection failed: " + reason); throw reason; }println("getConnection: no suitable driver found for "+ url); throw new SQLException("No suitable driver found for "+ url, "08001"); }

在Connection接口的具体实现部分，MySQL的连接是通过两层实现完成抽象部分的实现：

public class ConnectionImpl implements JdbcConnection, SessionEventListener, Serializable { private static final long serialVersionUID = 4009476458425101761L; private static final SQLPermission SET_NETWORK_TIMEOUT_PERM = new SQLPermission("setNetworkTimeout"); //... } public interface JdbcConnection extends Connection, MysqlConnection, TransactionEventHandler { JdbcPropertySet getPropertySet(); void changeUser(String var1, String var2) throws SQLException; //... }

综上我们可以画出对应的类图：