桥接模式

 一、定义

桥接模式(Bridge Pattern)：将抽象部分与它的实现部分分离，使它们都可以独立地变化。它是一种对象结构型模式，又称为柄体(Handle and Body)模式或接口(Interface)模式。桥接模式主要目的是通过组合的方式建立两个类之前的联系，而不是继承。但又类似于多重继承方案，但是多重继承方案往往违背了类得单一职责原则，其复用比较差，桥接模式是比多重继承更好的替代方案。桥接模式的核心在解耦抽象和实现。

桥接模式主要包含如下几个角色：

• Abstraction：抽象类，该类持有一个对实现角色的引用，抽象角色中的方法需要实现角色来实现。抽象角色一般为抽象类（构造函数规定子类要传入一个实现对象）
• RefinedAbstraction：扩充抽象类，对Abstraction的方法进行完善和扩展
• Implementor：实现类接口，确定实现维度的基本操作，提供给Abstraction使用。该类一般为接口或抽象类
• ConcreteImplementor：Implementor具体实现类 。

二、桥接模式实际开发中应用场景

当一个类内部具备两种或多种变化维度时，使用桥接模式可以解耦这些变化的维度，使高层代码架构稳定

– JDBC驱动程序 – AWT中的Peer架构 – 银行日志管理：

• 格式分类：操作日志、交易日志、异常日志
• 距离分类：本地记录日志、异地记录日志

– 人力资源系统中的奖金计算模块：

• 奖金分类：个人奖金、团体奖金、激励奖金。
• 部门分类：人事部门、销售部门、研发部门。

– OA系统中的消息处理：

业务类型：普通消息、加急消息、特急消息

示例：家电电器有很多种，也有很多品牌，电器和品牌存在关系

如图所示，如果按照这样的设计，每增加一种电器就需要对应的绑定品牌；另一种情况是，每增加一个品牌就需要在对应的电器下添加。这样的话会导致出现许多重复性的代码，而且耦合度也很高。

如果添加新的品牌或者新的电器而不会改动先有的类，该怎么设计呢？

根据桥接模式，代码可以设计成下图的流程

 这个架构是不是看的很象抽象工厂的感觉，下面我们写个桥接模式的通用代码

ElectricAppliance接口：

public interface ElectricAppliance {
    String description();
}

AirConditioner类、WashingMachine类、WaterHeater类：

public class AirConditioner implements ElectricAppliance {

    private final String name = "空调";

    @Override
    public String description() {
        return name;
    }
}

public class WashingMachine implements ElectricAppliance {

    private final String name = "洗衣机";

    @Override
    public String description() {
        return name;
    }
}

public class WaterHeater implements ElectricAppliance {

    private final String name = "热水器";

    @Override
    public String description() {
        return name;
    }
}

Brand抽象类

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 = "海尔";


    @Override
    public String description() {
        return name  ;
    }
}

public class Midea extends Brand{

    private final  String name = "美的";



    @Override
    public String description() {
        return name ;
    }
}

桥接

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;
    }
}

对于这个案例有些人可能说还太抽象，更好我最近在给公司调研需求，我们相关方提出需求有短信通知和系统通知这两个通知。但后期可能APP准备开发后，又会加上APP通知。因为我们公司管养的是泵站类业务，在处理消息时对于紧急情况和普通情况通知和处理的策略如果深入研究的话肯定是不一样的，那这个需求分析后的场景就是一个很好的二个维度的，那么很好的可以写上桥接模式，当然抽象工厂模式也行

首先创建一个统一的消息发送接口：

//消息接口
public interface IMessage {
    void send(String content,String name);
}

//手机
public class PhoneMessage implements IMessage{
    @Override
    public void send(String content, String name) {
        System.out.println("这是手机逻辑");
    }
}

//邮箱
public class EmailIMessage implements IMessage{
    @Override
    public void send(String content, String name) {
        System.out.println("这是邮箱的逻辑");
    }
}

public abstract class Abstraction {
     IMessage iMessage;

    public Abstraction(IMessage iMessage){
        this.iMessage=iMessage;
    }

    //发送消息
    public void send(String content, String name){
        this.iMessage.send(content,name);
    }
}

public class CommonAbstraction extends Abstraction{
    public CommonAbstraction(IMessage iMessage) {
        super(iMessage);
    }
    //发送消息
    public void send(String content, String name){
        this.iMessage.send(content,name);
        System.out.println("普通消息的处理方式");
    }

}

public class UrgencyAbstraction extends Abstraction{
    public UrgencyAbstraction(IMessage iMessage) {
        super(iMessage);
    }
    //发送消息
    public void send(String content, String name){
        this.iMessage.send(content,name);
        System.out.println("紧急消息的处理方式");
    }

}

public class Text {
    public static void main(String[] args) {
        IMessage iMessage=new PhoneMessage();
        Abstraction abstraction=new CommonAbstraction(iMessage);
        abstraction.send("正常开闸","张工");


    }
}

上面场景很好的进行了两个维度的解耦，后面在有两个维度的场景时可以考虑用这两种设计模式，上面的代码其实还有优化空间，怎么优化其实在前面设计模式也讲过了

三、桥接模式在源码中的应用

对于JDBC API用的人很多，其中有一个Driver类就是桥接对象。在使用的时候通过Class.forName()方法可以动态加载和个数据库厂商实现的Driver类，具体客户端应用代码如下，以Mysql的实现为例

try {
            //1.加载驱动
            Class.forName("com.mysql.jdbc.Driver");  //反射机制加载驱动类
            // 2.获取连接Connection
            //主机:端口号/数据库名
            Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "root");
            // 3.得到执行sql语句的对象Statement
            Statement stmt = conn.createStatement();
            // 4.执行sql语句，并返回结果
            ResultSet rs = stmt.executeQuery("select *from table");
        }catch (Exception e){
            e.printStackTrace();
        }
    

 当执行Class.forName("com.mysql.jdbc.Driver");方法的时候， 执行com.mysql.jdbc.Driver这个类的静态块中的代码，而静态块中的代码只是调用了一下DriverManager的registerDriver（）方法，然后将Driver对象注册到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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 */

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;
    }
}

在注册之前，将传递过来的Driver对象，封装成一个DriverInfo对象。接着执行客户端代码的第二步，调用DriverManager的getConnection（）方法获取连接对象，在getConnection()中就又会调用各自厂商实现的Driver的connect()方法获得连接对象。这样避开了使用继承，为不同的数据库提供了相同的接口，JDBC API中DriverManager就是桥

 四、桥接模式的优缺点

优点：

1.分离抽象部分及其具体实现部分

2.提高了系统的扩展性

3.符合开闭原则及合成复用原则

缺点：

1.增加了系统的设计难度

2.需要正确地识别系统中两个独立变化的维度

git源码：https://github.com/ljx958720/design_patterns.git