桥接模式在开源代码中的应用

傷城~ 2023-03-01 13:40 29阅读 0赞

桥接模式的作用：将抽象与实现分离，使它们可以独立变化。它是用组合关系代替继承关系来实现，从而降低了抽象和实现这两个可变维度的耦合度。

**案例**

桥接模式比较经典的应用就是 JDBC 规范和不同数据库厂商驱动的实现

JDBC 中的驱动接口

public interface Driver {
    	Connection connect(String url, java.util.Properties info) throws SQLException;
    	boolean acceptsURL(String url) throws SQLException;
    	.
    	.
    	.
    }

JDBC 中的驱动管理器

public class DriverManager {
    	private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();
    	
    	private DriverManager(){}
    	
    	public static synchronized void registerDriver(java.sql.Driver driver, DriverAction da) throws SQLException {
            if(driver != null) {
                registeredDrivers.addIfAbsent(new DriverInfo(driver, da));
            } else {
                throw new NullPointerException();
            }
        }
    		
    	private static Connection getConnection( String url, java.util.Properties info, Class<?> caller) throws SQLException {
            ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
            synchronized(DriverManager.class) {
                if (callerCL == null) {
                    callerCL = Thread.currentThread().getContextClassLoader();
                }
            }
            if(url == null) {
                throw new SQLException("The url cannot be null", "08001");
            }
            SQLException reason = null;
            for(DriverInfo aDriver : registeredDrivers) {
                
                if(isDriverAllowed(aDriver.driver, callerCL)) {
                    try {
                        Connection con = aDriver.driver.connect(url, info);
                        if (con != null) {
                            return (con);
                        }
                    } catch (SQLException ex) {
                        if (reason == null) {
                            reason = ex;
                        }
                    }
                } else {
                }
            }
    		.
    		.
    		.
        }
    }

MySQL 的 Driver 继承的 NonRegisteringDriver 类

package com.mysql.jdbc;
    public class Driver extends NonRegisteringDriver implements java.sql.Driver {
        
    	//加载驱动的时候，自动注册到驱动管理器
        static {
            try {
                java.sql.DriverManager.registerDriver(new Driver());
            } catch (SQLException E) {
                throw new RuntimeException("Can't register driver!");
            }
        }
        
        public Driver() throws SQLException {
            
        }
    }

MySQL 的 NonRegisteringDriver 对 JDBC 中的 Driver 接口进行实现

public class NonRegisteringDriver implements java.sql.Driver {
    	private static final String ALLOWED_QUOTES = "\"'";
    
        private static final String REPLICATION_URL_PREFIX = "jdbc:mysql:replication://";
    
        private static final String URL_PREFIX = "jdbc:mysql://";
    	.
    	.
    	.
    	public java.sql.Connection connect(String url, Properties info) throws SQLException {
            .
    		.
    		.
            try {
                Connection newConn = com.mysql.jdbc.ConnectionImpl.getInstance(host(props), port(props), props, database(props), url);
    
                return newConn;
            } catch (SQLException sqlEx) {
                throw sqlEx;
            } catch (Exception ex) {
                SQLException sqlEx = SQLError.createSQLException(
                        Messages.getString("NonRegisteringDriver.17") + ex.toString() + Messages.getString("NonRegisteringDriver.18"),
                        SQLError.SQL_STATE_UNABLE_TO_CONNECT_TO_DATASOURCE, null);
                sqlEx.initCause(ex);
                throw sqlEx;
            }
        }
    }

示意图：

![format_png][]

JDBC 中 Driver 接口就是抽象部分，每个厂商有自己的实现，独立变化；JDBC 中 DriverManager 类通过组合的方式，持有注册进来的 Driver 对象，对应用系统提供数据库操作的 API，也可以根据需求独立变化。

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

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[format_png]: /images/20230208/841a594bd4da447e95ec7fca998c9c98.png
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