一、定義
橋接模式(Bridge Pattern)：将抽象部分與它的實作部分分離，使它們都可以獨立地變化。它是一種對象結構型模式，又稱為柄體(Handle and Body)模式或接口(Interface)模式。橋接模式主要目的是通過組合的方式建立兩個類之前的聯系，而不是繼承。但又類似于多重繼承方案，但是多重繼承方案往往違背了類得單一職責原則，其複用比較差，橋接模式是比多重繼承更好的替代方案。橋接模式的核心在解耦抽象和實作。
橋接模式主要包含如下幾個角色：
- Abstraction：抽象類，該類持有一個對實作角色的引用，抽象角色中的方法需要實作角色來實作。抽象角色一般為抽象類（構造函數規定子類要傳入一個實作對象）
- RefinedAbstraction：擴充抽象類，對Abstraction的方法進行完善和擴充
- Implementor：實作類接口，确定實作次元的基本操作，提供給Abstraction使用。該類一般為接口或抽象類
- ConcreteImplementor：Implementor具體實作類 。
二、橋接模式實際開發中應用場景
當一個類内部具備兩種或多種變化次元時，使用橋接模式可以解耦這些變化的次元，使高層代碼架構穩定
– JDBC驅動程式 – AWT中的Peer架構 – 銀行日志管理：
- 格式分類：記錄檔、交易日志、異常日志
- 距離分類：本地記錄日志、異地記錄日志
– 人力資源系統中的獎金計算子產品：
- 獎金分類：個人獎金、團體獎金、激勵獎金。
- 部門分類：人事部門、銷售部門、研發部門。
– OA系統中的消息處理：
業務類型：普通消息、加急消息、特急消息
示例：家電電器有很多種，也有很多品牌，電器和品牌存在關系
如圖所示，如果按照這樣的設計，每增加一種電器就需要對應的綁定品牌；另一種情況是，每增加一個品牌就需要在對應的電器下添加。這樣的話會導緻出現許多重複性的代碼，而且耦合度也很高。
如果添加新的品牌或者新的電器而不會改動先有的類，該怎麼設計呢？
根據橋接模式，代碼可以設計成下圖的流程
這個架構是不是看的很象抽象工廠的感覺，下面我們寫個橋接模式的通用代碼
ElectricAppliance接口：
public interface ElectricAppliance {
String description();
} AirConditioner類、WashingMachine類、WaterHeater類：
public class AirConditioner implements ElectricAppliance {
private final String name = "空調";
@Override
public String description() {
return name;
}
}
public class WashingMachine implements ElectricAppliance {
private final String name = "洗衣機";
@Override
public String description() {
return name;
}
}
public class WaterHeater implements ElectricAppliance {
private final String name = "熱水器";
@Override
public String description() {
return name;
}
} Brand抽象類
public abstract class Brand {
abstract String description();
} public class Gree extends Brand {
private final String name = "格力";
@Override
public String description() {
return name ;
}
} public class Haier extends Brand{
private final String name = "海爾";
@Override
public String description() {
return name ;
}
} public class Midea extends Brand{
private final String name = "美的";
@Override
public String description() {
return name ;
}
} 橋接
public class Abstraction {
private ElectricAppliance electricAppliance;
private Brand brand;
public void setElectricAppliance(ElectricAppliance electricAppliance) {
this.electricAppliance = electricAppliance;
}
public void setBrand(Brand brand) {
this.brand = brand;
}
} 對于這個案例有些人可能說還太抽象，更好我最近在給公司調研需求，我們相關方提出需求有短信通知和系統通知這兩個通知。但後期可能APP準備開發後，又會加上APP通知。因為我們公司管養的是泵站類業務，在處理消息時對于緊急情況和普通情況通知和處理的政策如果深入研究的話肯定是不一樣的，那這個需求分析後的場景就是一個很好的二個次元的，那麼很好的可以寫上橋接模式，當然抽象工廠模式也行
首先建立一個統一的消息發送接口：
//消息接口
public interface IMessage {
void send(String content,String name);
} //手機
public class PhoneMessage implements IMessage{
@Override
public void send(String content, String name) {
System.out.println("這是手機邏輯");
}
} //郵箱
public class EmailIMessage implements IMessage{
@Override
public void send(String content, String name) {
System.out.println("這是郵箱的邏輯");
}
} public abstract class Abstraction {
IMessage iMessage;
public Abstraction(IMessage iMessage){
this.iMessage=iMessage;
}
//發送消息
public void send(String content, String name){
this.iMessage.send(content,name);
}
} public class CommonAbstraction extends Abstraction{
public CommonAbstraction(IMessage iMessage) {
super(iMessage);
}
//發送消息
public void send(String content, String name){
this.iMessage.send(content,name);
System.out.println("普通消息的處理方式");
}
} public class UrgencyAbstraction extends Abstraction{
public UrgencyAbstraction(IMessage iMessage) {
super(iMessage);
}
//發送消息
public void send(String content, String name){
this.iMessage.send(content,name);
System.out.println("緊急消息的處理方式");
}
} public class Text {
public static void main(String[] args) {
IMessage iMessage=new PhoneMessage();
Abstraction abstraction=new CommonAbstraction(iMessage);
abstraction.send("正常開閘","張工");
}
} 上面場景很好的進行了兩個次元的解耦，後面在有兩個次元的場景時可以考慮用這兩種設計模式，上面的代碼其實還有優化空間，怎麼優化其實在前面設計模式也講過了
三、橋接模式在源碼中的應用
對于JDBC API用的人很多，其中有一個Driver類就是橋接對象。在使用的時候通過Class.forName()方法可以動态加載和個資料庫廠商實作的Driver類，具體用戶端應用代碼如下，以Mysql的實作為例
try {
//1.加載驅動
Class.forName("com.mysql.jdbc.Driver"); //反射機制加載驅動類
// 2.擷取連接配接Connection
//主機:端口号/資料庫名
Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "root");
// 3.得到執行sql語句的對象Statement
Statement stmt = conn.createStatement();
// 4.執行sql語句，并傳回結果
ResultSet rs = stmt.executeQuery("select *from table");
}catch (Exception e){
e.printStackTrace();
}
當執行Class.forName("com.mysql.jdbc.Driver");方法的時候， 執行com.mysql.jdbc.Driver這個類的靜态塊中的代碼，而靜态塊中的代碼隻是調用了一下DriverManager的registerDriver（）方法，然後将Driver對象注冊到DriverManager中
/*
* Copyright (c) 1996, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
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*
*
*
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*
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*/
package java.sql;
import java.util.Iterator;
import java.util.ServiceLoader;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.concurrent.CopyOnWriteArrayList;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
/**
* <P>The basic service for managing a set of JDBC drivers.<br>
* <B>NOTE:</B> The {@link javax.sql.DataSource} interface, new in the
* JDBC 2.0 API, provides another way to connect to a data source.
* The use of a <code>DataSource</code> object is the preferred means of
* connecting to a data source.
*
* <P>As part of its initialization, the <code>DriverManager</code> class will
* attempt to load the driver classes referenced in the "jdbc.drivers"
* system property. This allows a user to customize the JDBC Drivers
* used by their applications. For example in your
* ~/.hotjava/properties file you might specify:
* <pre>
* <CODE>jdbc.drivers=foo.bah.Driver:wombat.sql.Driver:bad.taste.ourDriver</CODE>
* </pre>
*<P> The <code>DriverManager</code> methods <code>getConnection</code> and
* <code>getDrivers</code> have been enhanced to support the Java Standard Edition
* <a href="../../../technotes/guides/jar/jar.html#Service%20Provider">Service Provider</a> mechanism. JDBC 4.0 Drivers must
* include the file <code>META-INF/services/java.sql.Driver</code>. This file contains the name of the JDBC drivers
* implementation of <code>java.sql.Driver</code>. For example, to load the <code>my.sql.Driver</code> class,
* the <code>META-INF/services/java.sql.Driver</code> file would contain the entry:
* <pre>
* <code>my.sql.Driver</code>
* </pre>
*
* <P>Applications no longer need to explicitly load JDBC drivers using <code>Class.forName()</code>. Existing programs
* which currently load JDBC drivers using <code>Class.forName()</code> will continue to work without
* modification.
*
* <P>When the method <code>getConnection</code> is called,
* the <code>DriverManager</code> will attempt to
* locate a suitable driver from amongst those loaded at
* initialization and those loaded explicitly using the same classloader
* as the current applet or application.
*
* <P>
* Starting with the Java 2 SDK, Standard Edition, version 1.3, a
* logging stream can be set only if the proper
* permission has been granted. Normally this will be done with
* the tool PolicyTool, which can be used to grant <code>permission
* java.sql.SQLPermission "setLog"</code>.
* @see Driver
* @see Connection
*/
public class DriverManager {
// List of registered JDBC drivers
private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();
private static volatile int loginTimeout = 0;
private static volatile java.io.PrintWriter logWriter = null;
private static volatile java.io.PrintStream logStream = null;
// Used in println() to synchronize logWriter
private final static Object logSync = new Object();
/* Prevent the DriverManager class from being instantiated. */
private DriverManager(){}
/**
* Load the initial JDBC drivers by checking the System property
* jdbc.properties and then use the {@code ServiceLoader} mechanism
*/
static {
loadInitialDrivers();
println("JDBC DriverManager initialized");
}
/**
* The <code>SQLPermission</code> constant that allows the
* setting of the logging stream.
* @since 1.3
*/
final static SQLPermission SET_LOG_PERMISSION =
new SQLPermission("setLog");
/**
* The {@code SQLPermission} constant that allows the
* un-register a registered JDBC driver.
* @since 1.8
*/
final static SQLPermission DEREGISTER_DRIVER_PERMISSION =
new SQLPermission("deregisterDriver");
//--------------------------JDBC 2.0-----------------------------
/**
* Retrieves the log writer.
*
* The <code>getLogWriter</code> and <code>setLogWriter</code>
* methods should be used instead
* of the <code>get/setlogStream</code> methods, which are deprecated.
* @return a <code>java.io.PrintWriter</code> object
* @see #setLogWriter
* @since 1.2
*/
public static java.io.PrintWriter getLogWriter() {
return logWriter;
}
/**
* Sets the logging/tracing <code>PrintWriter</code> object
* that is used by the <code>DriverManager</code> and all drivers.
* <P>
* There is a minor versioning problem created by the introduction
* of the method <code>setLogWriter</code>. The
* method <code>setLogWriter</code> cannot create a <code>PrintStream</code> object
* that will be returned by <code>getLogStream</code>---the Java platform does
* not provide a backward conversion. As a result, a new application
* that uses <code>setLogWriter</code> and also uses a JDBC 1.0 driver that uses
* <code>getLogStream</code> will likely not see debugging information written
* by that driver.
*<P>
* Starting with the Java 2 SDK, Standard Edition, version 1.3 release, this method checks
* to see that there is an <code>SQLPermission</code> object before setting
* the logging stream. If a <code>SecurityManager</code> exists and its
* <code>checkPermission</code> method denies setting the log writer, this
* method throws a <code>java.lang.SecurityException</code>.
*
* @param out the new logging/tracing <code>PrintStream</code> object;
* <code>null</code> to disable logging and tracing
* @throws SecurityException
* if a security manager exists and its
* <code>checkPermission</code> method denies
* setting the log writer
*
* @see SecurityManager#checkPermission
* @see #getLogWriter
* @since 1.2
*/
public static void setLogWriter(java.io.PrintWriter out) {
SecurityManager sec = System.getSecurityManager();
if (sec != null) {
sec.checkPermission(SET_LOG_PERMISSION);
}
logStream = null;
logWriter = out;
}
//---------------------------------------------------------------
/**
* Attempts to establish a connection to the given database URL.
* The <code>DriverManager</code> attempts to select an appropriate driver from
* the set of registered JDBC drivers.
*<p>
* <B>Note:</B> If a property is specified as part of the {@code url} and
* is also specified in the {@code Properties} object, it is
* implementation-defined as to which value will take precedence.
* For maximum portability, an application should only specify a
* property once.
*
* @param url a database url of the form
* <code> jdbc:<em>subprotocol</em>:<em>subname</em></code>
* @param info a list of arbitrary string tag/value pairs as
* connection arguments; normally at least a "user" and
* "password" property should be included
* @return a Connection to the URL
* @exception SQLException if a database access error occurs or the url is
* {@code null}
* @throws SQLTimeoutException when the driver has determined that the
* timeout value specified by the {@code setLoginTimeout} method
* has been exceeded and has at least tried to cancel the
* current database connection attempt
*/
@CallerSensitive
public static Connection getConnection(String url,
java.util.Properties info) throws SQLException {
return (getConnection(url, info, Reflection.getCallerClass()));
}
/**
* Attempts to establish a connection to the given database URL.
* The <code>DriverManager</code> attempts to select an appropriate driver from
* the set of registered JDBC drivers.
*<p>
* <B>Note:</B> If the {@code user} or {@code password} property are
* also specified as part of the {@code url}, it is
* implementation-defined as to which value will take precedence.
* For maximum portability, an application should only specify a
* property once.
*
* @param url a database url of the form
* <code>jdbc:<em>subprotocol</em>:<em>subname</em></code>
* @param user the database user on whose behalf the connection is being
* made
* @param password the user's password
* @return a connection to the URL
* @exception SQLException if a database access error occurs or the url is
* {@code null}
* @throws SQLTimeoutException when the driver has determined that the
* timeout value specified by the {@code setLoginTimeout} method
* has been exceeded and has at least tried to cancel the
* current database connection attempt
*/
@CallerSensitive
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()));
}
/**
* Attempts to establish a connection to the given database URL.
* The <code>DriverManager</code> attempts to select an appropriate driver from
* the set of registered JDBC drivers.
*
* @param url a database url of the form
* <code> jdbc:<em>subprotocol</em>:<em>subname</em></code>
* @return a connection to the URL
* @exception SQLException if a database access error occurs or the url is
* {@code null}
* @throws SQLTimeoutException when the driver has determined that the
* timeout value specified by the {@code setLoginTimeout} method
* has been exceeded and has at least tried to cancel the
* current database connection attempt
*/
@CallerSensitive
public static Connection getConnection(String url)
throws SQLException {
java.util.Properties info = new java.util.Properties();
return (getConnection(url, info, Reflection.getCallerClass()));
}
/**
* Attempts to locate a driver that understands the given URL.
* The <code>DriverManager</code> attempts to select an appropriate driver from
* the set of registered JDBC drivers.
*
* @param url a database URL of the form
* <code>jdbc:<em>subprotocol</em>:<em>subname</em></code>
* @return a <code>Driver</code> object representing a driver
* that can connect to the given URL
* @exception SQLException if a database access error occurs
*/
@CallerSensitive
public static Driver getDriver(String url)
throws SQLException {
println("DriverManager.getDriver(\"" + url + "\")");
Class<?> callerClass = Reflection.getCallerClass();
// Walk through the loaded registeredDrivers attempting to locate someone
// who understands the given URL.
for (DriverInfo aDriver : registeredDrivers) {
// If the caller does not have permission to load the driver then
// skip it.
if(isDriverAllowed(aDriver.driver, callerClass)) {
try {
if(aDriver.driver.acceptsURL(url)) {
// Success!
println("getDriver returning " + aDriver.driver.getClass().getName());
return (aDriver.driver);
}
} catch(SQLException sqe) {
// Drop through and try the next driver.
}
} else {
println(" skipping: " + aDriver.driver.getClass().getName());
}
}
println("getDriver: no suitable driver");
throw new SQLException("No suitable driver", "08001");
}
/**
* Registers the given driver with the {@code DriverManager}.
* A newly-loaded driver class should call
* the method {@code registerDriver} to make itself
* known to the {@code DriverManager}. If the driver is currently
* registered, no action is taken.
*
* @param driver the new JDBC Driver that is to be registered with the
* {@code DriverManager}
* @exception SQLException if a database access error occurs
* @exception NullPointerException if {@code driver} is null
*/
public static synchronized void registerDriver(java.sql.Driver driver)
throws SQLException {
registerDriver(driver, null);
}
/**
* Registers the given driver with the {@code DriverManager}.
* A newly-loaded driver class should call
* the method {@code registerDriver} to make itself
* known to the {@code DriverManager}. If the driver is currently
* registered, no action is taken.
*
* @param driver the new JDBC Driver that is to be registered with the
* {@code DriverManager}
* @param da the {@code DriverAction} implementation to be used when
* {@code DriverManager#deregisterDriver} is called
* @exception SQLException if a database access error occurs
* @exception NullPointerException if {@code driver} is null
* @since 1.8
*/
public static synchronized void registerDriver(java.sql.Driver driver,
DriverAction da)
throws SQLException {
/* Register the driver if it has not already been added to our list */
if(driver != null) {
registeredDrivers.addIfAbsent(new DriverInfo(driver, da));
} else {
// This is for compatibility with the original DriverManager
throw new NullPointerException();
}
println("registerDriver: " + driver);
}
/**
* Removes the specified driver from the {@code DriverManager}'s list of
* registered drivers.
* <p>
* If a {@code null} value is specified for the driver to be removed, then no
* action is taken.
* <p>
* If a security manager exists and its {@code checkPermission} denies
* permission, then a {@code SecurityException} will be thrown.
* <p>
* If the specified driver is not found in the list of registered drivers,
* then no action is taken. If the driver was found, it will be removed
* from the list of registered drivers.
* <p>
* If a {@code DriverAction} instance was specified when the JDBC driver was
* registered, its deregister method will be called
* prior to the driver being removed from the list of registered drivers.
*
* @param driver the JDBC Driver to remove
* @exception SQLException if a database access error occurs
* @throws SecurityException if a security manager exists and its
* {@code checkPermission} method denies permission to deregister a driver.
*
* @see SecurityManager#checkPermission
*/
@CallerSensitive
public static synchronized void deregisterDriver(Driver driver)
throws SQLException {
if (driver == null) {
return;
}
SecurityManager sec = System.getSecurityManager();
if (sec != null) {
sec.checkPermission(DEREGISTER_DRIVER_PERMISSION);
}
println("DriverManager.deregisterDriver: " + driver);
DriverInfo aDriver = new DriverInfo(driver, null);
if(registeredDrivers.contains(aDriver)) {
if (isDriverAllowed(driver, Reflection.getCallerClass())) {
DriverInfo di = registeredDrivers.get(registeredDrivers.indexOf(aDriver));
// If a DriverAction was specified, Call it to notify the
// driver that it has been deregistered
if(di.action() != null) {
di.action().deregister();
}
registeredDrivers.remove(aDriver);
} else {
// If the caller does not have permission to load the driver then
// throw a SecurityException.
throw new SecurityException();
}
} else {
println(" couldn't find driver to unload");
}
}
/**
* Retrieves an Enumeration with all of the currently loaded JDBC drivers
* to which the current caller has access.
*
* <P><B>Note:</B> The classname of a driver can be found using
* <CODE>d.getClass().getName()</CODE>
*
* @return the list of JDBC Drivers loaded by the caller's class loader
*/
@CallerSensitive
public static java.util.Enumeration<Driver> getDrivers() {
java.util.Vector<Driver> result = new java.util.Vector<>();
Class<?> callerClass = Reflection.getCallerClass();
// Walk through the loaded registeredDrivers.
for(DriverInfo aDriver : registeredDrivers) {
// If the caller does not have permission to load the driver then
// skip it.
if(isDriverAllowed(aDriver.driver, callerClass)) {
result.addElement(aDriver.driver);
} else {
println(" skipping: " + aDriver.getClass().getName());
}
}
return (result.elements());
}
/**
* Sets the maximum time in seconds that a driver will wait
* while attempting to connect to a database once the driver has
* been identified.
*
* @param seconds the login time limit in seconds; zero means there is no limit
* @see #getLoginTimeout
*/
public static void setLoginTimeout(int seconds) {
loginTimeout = seconds;
}
/**
* Gets the maximum time in seconds that a driver can wait
* when attempting to log in to a database.
*
* @return the driver login time limit in seconds
* @see #setLoginTimeout
*/
public static int getLoginTimeout() {
return (loginTimeout);
}
/**
* Sets the logging/tracing PrintStream that is used
* by the <code>DriverManager</code>
* and all drivers.
*<P>
* In the Java 2 SDK, Standard Edition, version 1.3 release, this method checks
* to see that there is an <code>SQLPermission</code> object before setting
* the logging stream. If a <code>SecurityManager</code> exists and its
* <code>checkPermission</code> method denies setting the log writer, this
* method throws a <code>java.lang.SecurityException</code>.
*
* @param out the new logging/tracing PrintStream; to disable, set to <code>null</code>
* @deprecated Use {@code setLogWriter}
* @throws SecurityException if a security manager exists and its
* <code>checkPermission</code> method denies setting the log stream
*
* @see SecurityManager#checkPermission
* @see #getLogStream
*/
@Deprecated
public static void setLogStream(java.io.PrintStream out) {
SecurityManager sec = System.getSecurityManager();
if (sec != null) {
sec.checkPermission(SET_LOG_PERMISSION);
}
logStream = out;
if ( out != null )
logWriter = new java.io.PrintWriter(out);
else
logWriter = null;
}
/**
* Retrieves the logging/tracing PrintStream that is used by the <code>DriverManager</code>
* and all drivers.
*
* @return the logging/tracing PrintStream; if disabled, is <code>null</code>
* @deprecated Use {@code getLogWriter}
* @see #setLogStream
*/
@Deprecated
public static java.io.PrintStream getLogStream() {
return logStream;
}
/**
* Prints a message to the current JDBC log stream.
*
* @param message a log or tracing message
*/
public static void println(String message) {
synchronized (logSync) {
if (logWriter != null) {
logWriter.println(message);
// automatic flushing is never enabled, so we must do it ourselves
logWriter.flush();
}
}
}
//------------------------------------------------------------------------
// Indicates whether the class object that would be created if the code calling
// DriverManager is accessible.
private static boolean isDriverAllowed(Driver driver, Class<?> caller) {
ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
return isDriverAllowed(driver, callerCL);
}
private static boolean isDriverAllowed(Driver driver, ClassLoader classLoader) {
boolean result = false;
if(driver != null) {
Class<?> aClass = null;
try {
aClass = Class.forName(driver.getClass().getName(), true, classLoader);
} catch (Exception ex) {
result = false;
}
result = ( aClass == driver.getClass() ) ? true : false;
}
return result;
}
private static void loadInitialDrivers() {
String drivers;
try {
drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
public String run() {
return System.getProperty("jdbc.drivers");
}
});
} catch (Exception ex) {
drivers = null;
}
// If the driver is packaged as a Service Provider, load it.
// Get all the drivers through the classloader
// exposed as a java.sql.Driver.class service.
// ServiceLoader.load() replaces the sun.misc.Providers()
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
Iterator<Driver> driversIterator = loadedDrivers.iterator();
/* Load these drivers, so that they can be instantiated.
* It may be the case that the driver class may not be there
* i.e. there may be a packaged driver with the service class
* as implementation of java.sql.Driver but the actual class
* may be missing. In that case a java.util.ServiceConfigurationError
* will be thrown at runtime by the VM trying to locate
* and load the service.
*
* Adding a try catch block to catch those runtime errors
* if driver not available in classpath but it's
* packaged as service and that service is there in classpath.
*/
try{
while(driversIterator.hasNext()) {
driversIterator.next();
}
} catch(Throwable t) {
// Do nothing
}
return null;
}
});
println("DriverManager.initialize: jdbc.drivers = " + drivers);
if (drivers == null || drivers.equals("")) {
return;
}
String[] driversList = drivers.split(":");
println("number of Drivers:" + driversList.length);
for (String aDriver : driversList) {
try {
println("DriverManager.Initialize: loading " + aDriver);
Class.forName(aDriver, true,
ClassLoader.getSystemClassLoader());
} catch (Exception ex) {
println("DriverManager.Initialize: load failed: " + ex);
}
}
}
// 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");
}
}
/*
* Wrapper class for registered Drivers in order to not expose Driver.equals()
* to avoid the capture of the Driver it being compared to as it might not
* normally have access.
*/
class DriverInfo {
final Driver driver;
DriverAction da;
DriverInfo(Driver driver, DriverAction action) {
this.driver = driver;
da = action;
}
@Override
public boolean equals(Object other) {
return (other instanceof DriverInfo)
&& this.driver == ((DriverInfo) other).driver;
}
@Override
public int hashCode() {
return driver.hashCode();
}
@Override
public String toString() {
return ("driver[className=" + driver + "]");
}
DriverAction action() {
return da;
}
} 在注冊之前，将傳遞過來的Driver對象，封裝成一個DriverInfo對象。接着執行用戶端代碼的第二步，調用DriverManager的getConnection（）方法擷取連接配接對象，在getConnection()中就又會調用各自廠商實作的Driver的connect()方法獲得連接配接對象。這樣避開了使用繼承，為不同的資料庫提供了相同的接口，JDBC API中DriverManager就是橋
四、橋接模式的優缺點
優點：
1.分離抽象部分及其具體實作部分
2.提高了系統的擴充性
3.符合開閉原則及合成複用原則
缺點：
1.增加了系統的設計難度
2.需要正确地識别系統中兩個獨立變化的次元
git源碼：https://gitee.com/TongHuaShuShuoWoDeJieJu/design_pattern.git
這短短的一生我們最終都會失去，不妨大膽一點，愛一個人，攀一座山，追一個夢