简单工厂(三)——JDK源码中的简单工厂

private static Calendar createCalendar(TimeZone zone,Locale aLocale)
            {
                CalendarProvider provider =
                    LocaleProviderAdapter.getAdapter(CalendarProvider.class, aLocale)
                                         .getCalendarProvider();
                if (provider != null) {
                    try {
                        return provider.getInstance(zone, aLocale);
                    } catch (IllegalArgumentException iae) {
                        // fall back to the default instantiation
                    }
                }

                Calendar cal = null;
            /*根据不同的地区来创建不同的日历对象，就好比日历这个工厂，生产着世界上各地区的日历，我需要这个地区日历，我只需要传参数告诉工厂即可，不需要知道日历制作过程和实例的过程*/
                if (aLocale.hasExtensions()) {
                    String caltype = aLocale.getUnicodeLocaleType("ca");
                    if (caltype != null) {
                        switch (caltype) {
                        case "buddhist":
                        cal = new BuddhistCalendar(zone, aLocale);
                            break;
                        case "japanese":
                            cal = new JapaneseImperialCalendar(zone, aLocale);
                            break;
                        case "gregory":
                            cal = new GregorianCalendar(zone, aLocale);
                            break;
                        }
                    }
                }
                if (cal == null) {
                    // If no known calendar type is explicitly specified,
                    // perform the traditional way to create a Calendar:
                    // create a BuddhistCalendar for th_TH locale,
                    // a JapaneseImperialCalendar for ja_JP_JP locale, or
                    // a GregorianCalendar for any other locales.
                    // NOTE: The language, country and variant strings are interned.
                    if (aLocale.getLanguage() == "th" && aLocale.getCountry() == "TH") {
                        cal = new BuddhistCalendar(zone, aLocale);
                    } else if (aLocale.getVariant() == "JP" && aLocale.getLanguage() == "ja"
                               && aLocale.getCountry() == "JP") {
                        cal = new JapaneseImperialCalendar(zone, aLocale);
                    } else {
                        cal = new GregorianCalendar(zone, aLocale);
                    }
                }
                return cal;
            }

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　Calendar类图

　　除了日历类还有JDBC，当我们需要MySQL数据库的驱动时，我们就传MySQL的参数，用Oracle的就传相应的参数。

在写JDBC的时候,JDK来实现的时候,

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

 

通过Class.forName把mysql的驱动加载进来,那如果写ORACLE的驱动呢,这里就变成对应的ORACLE的JDBC的jar包,ORACLE的driver类,然后调用DriverManager的getConnection方法,

      @CallerSensitive
      public static Connection getConnection(String url)
          throws SQLException {
          java.util.Properties info = new java.util.Properties();
          return (getConnection(url, info,Reflection.getCallerClass()));
     }

获取对应的数据库连接,JDBC的过程也是非常简单的,

      //  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");
    }

通过Class.forName这种方式,直接通过反射拿到对应的Video,只不过MSYQL这里面还是需要通过注册的,

        // 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());
            }

        }

因为这个可以看出来它是一个for循环,在遍历注册的一个驱动,
 
private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();
 
并且它是CopyOnWriteArrayList,里面是DriverInfo,初始化的时候他是一个空的,具体是什么时候完成注册的呢,

static {
    try {
        java.sql.DriverManager.registerDriver(new Driver());
    } catch (SQLException E) {
        throw new RuntimeException("Can't register driver!");
    }
}  

这个时候就会在registerDriver(new Driver())这个方法里面直接注册这个Driver,那里面的Driver自然就是MySQL的Driver,

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);

}

如果不存在就往里放

if(driver != null) {

registeredDrivers.addIfAbsent(new DriverInfo(driver, da));

}

此外，logback中也有简单工厂的影子。

public final class LoggerFactory

public static ILoggerFactory getILoggerFactory() {

}

最里面有一个getLogger方法,

public static Logger getLogger(Class clazz) {

return getLogger(clazz.getName());

}

public static Logger getLogger(String name) {

ILoggerFactory iLoggerFactory = getILoggerFactory();

return iLoggerFactory.getLogger(name);

}

这个还有个重载,一个是String name,还有一个是Class clazz,clazz是干嘛的,是clazz.getName(),

首先getLogger根据我们传来的name,从iLoggerFactory里面getLogger,先看一下

public interface ILoggerFactory   

  package org.slf4j;

  //抽象产品工厂

  public interface ILoggerFactory {

     //抽象工厂方法

      public Logger getLogger(String name);

 }

很明显ILoggerFactory它是一个接口,下面有一个方法,那这个呢是工厂方法,那在后面我们也会讲,这里先过去,

后面我们学习工厂方法的时候,再单独来说,然后通过iLoggerFactory.getLogger,因为它是一个接口,肯定有多个实现,

  LoggerContext   

  //具体工厂实现类

  public class LoggerContext extends ContextBase implements ILoggerFactory, LifeCycle {

  ...

      //具体工厂方法

      @Override

      public final Logger getLogger(final String name) {

       ...

      }

这里面我们看一下,传入的一个name,这里面要返回Logger,这里面对name进行了判断,很明显这个方法就是一个简单工厂方法,

根据传入的入参进行选择哪个Logger,那这个还是非常简单的,刚刚也说了,在我们的LoggerFactory里面,既存在了工厂方法,

又存在了简单工厂,所以设计模式在使用的时候,不一定要局限在使用一种,例如这里就是一个组合的使用,这个简单工厂比较简单,

在很多源码中也能够找到他的影子,在前面的JDK,Logback开源框架的,对于学习设计模式的讲解呢,我们在阅读源码的时候呢,

还可以以设计模式的角度,去聚焦源码,这样对我们理解源码也是有益处的 。