设计模式学习笔记（九）桥接模式及其应用

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

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：

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<DriverInfo> 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 Connection  extends 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：