门面模式详解

门面模式

门面模式的应用场景

门面模式的通用写法

public class SubSystemA{ 
    public void doA(){ 
    System.out.println("doing A stuff"); } } public class SubSystemB{ 
    public void doB(){ 
    System.out.println("doing B stuff"); } } public class SubSystemC{ 
    public void doC(){ 
    System.out.println("doing C stuff"); } } 

然后，创建外观角色Facade类：

public class Facade{ 
    private SubSystemA a = new SubSystemA(); private SubSystemB b = new SubSystemB(); private SubSystemC c = new SubSystemC(); //对外接口 public void doA(){ 
    this.a.doA(); } //对外接口 public void doB(){ 
    this.b.doB(); } //对外接口 public void doC(){ 
    this.c.doC(); } } 

来看客户端代码：

public static void main(String[] args){ 
    Facade facede = new Facade(); facade.doA(); facade.doB(); facade.doC(); } 

门面模式业务场景实例

@Data public class GiftInfo{ 
    private String name; } 

然后，编写各个子系统的业务逻辑代码，分别创建积分系统QualifyService类：

public class QualifyService{ 
    public boolean isAvailable(GiftInfo giftInfo){ 
    System.out.println("校验"+giftInfo.getName()+"积分资格通过，库存通过"); } } 

支付系统PaymentService类：

public class PaymentService{ 
    public boolean pay(GiftInfo pointsGift){ 
    //扣减积分 System.out.println("支付"+pointsGift.getName()+"积分成功"); return true; } } 

物流系统ShippingService类：

public class shippingService{ 
    //发货 System.out.println(giftInfo.getName()+"进入物流系统"); String shippingOrderNo = "666"; return shippingOrderNo; } 

然后创建外观角色GiftFacadeService类，对外只开放一个月换礼物的exchange()方法，在exchange()方法内部整合3个子系统的所有功能。

public class giftFacadeService{ 
    private QualifyService qualifyService = new QualifyService(); private PaymentService paymentService = new PaymentService(); private ShippingService shippingService = new ShippingService(); public void exchange(GiftInfo giftInfo){ 
    if(qualifyService.isAvailable(giftInfo)){ 
    if(paymentService.pay(giftInfo)){ 
    String shippingNo = shippingService.delivery(giftInfo); System.out.println("物流系统下单成功，物流单号是：" + shippingNo); } } } } 

最后，来看客户端代码：

public class Test { 
    public static void main(String[] args) { 
    FacadeService facadeService = new FacadeService(); GiftInfo giftInfo = new GiftInfo("《Spring 5核心原理》"); facadeService.exchange(giftInfo); } } 

门面模式在源码中的应用

 下面我们来门面模式在源码中的应用，先来看Spring JDBC模块下的JdbcUtils类，它封装了和JDBC相关的所有操作，它一个代码片段： 

public abstract class JdbcUtils { 
    public static final int TYPE_UNKNOWN = -; private static final Log logger = LogFactory.getLog(JdbcUtils.class); private static final Map<Integer, String> typeNames = new HashMap(); public JdbcUtils() { 
    } public static void closeConnection(@Nullable Connection con) { 
    if (con != null) { 
    try { 
    con.close(); } catch (SQLException var2) { 
    logger.debug("Could not close JDBC Connection", var2); } catch (Throwable var3) { 
    logger.debug("Unexpected exception on closing JDBC Connection", var3); } } } public static void closeStatement(@Nullable Statement stmt) { 
    if (stmt != null) { 
    try { 
    stmt.close(); } catch (SQLException var2) { 
    logger.trace("Could not close JDBC Statement", var2); } catch (Throwable var3) { 
    logger.trace("Unexpected exception on closing JDBC Statement", var3); } } } public static void closeResultSet(@Nullable ResultSet rs) { 
    if (rs != null) { 
    try { 
    rs.close(); } catch (SQLException var2) { 
    logger.trace("Could not close JDBC ResultSet", var2); } catch (Throwable var3) { 
    logger.trace("Unexpected exception on closing JDBC ResultSet", var3); } } } ... } 

再来看一个MyBatis中的Configuration类。它其中有很多new开头的方法，来看一下源代码：

public MetaObject newMetaObject(Object object) { 
    return MetaObject.forObject(object, this.objectFactory, this.objectWrapperFactory, this.reflectorFactory); } public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) { 
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql); parameterHandler = (ParameterHandler)this.interceptorChain.pluginAll(parameterHandler); return parameterHandler; } public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler, ResultHandler resultHandler, BoundSql boundSql) { 
    ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds); ResultSetHandler resultSetHandler = (ResultSetHandler)this.interceptorChain.pluginAll(resultSetHandler); return resultSetHandler; } public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { 
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql); StatementHandler statementHandler = (StatementHandler)this.interceptorChain.pluginAll(statementHandler); return statementHandler; } public Executor newExecutor(Transaction transaction) { 
    return this.newExecutor(transaction, this.defaultExecutorType); } ... 

上面的这些方法都是对JDBC中关键组件操作的封装。另外地在Tomcat的源码中也有体现，也非常的有意思。举个例子RequestFacade类，来看源码：

public class RequestFacade implements HttpServletRequest { 
    ... public String getContentType() { 
    if (this.request == null) { 
    throw new IllegalStateException(sm.getString("requestFacade.nullRequest")); } else { 
    return this.request.getContentType(); } } public ServletInputStream getInputStream() throws IOException { 
    if (this.request == null) { 
    throw new IllegalStateException(sm.getString("requestFacade.nullRequest")); } else { 
    return this.request.getInputStream(); } } public String getParameter(String name) { 
    if (this.request == null) { 
    throw new IllegalStateException(sm.getString("requestFacade.nullRequest")); } else { 
    return Globals.IS_SECURITY_ENABLED ? (String)AccessController.doPrivileged(new RequestFacade.GetParameterPrivilegedAction(name)) : this.request.getParameter(name); } } public Enumeration<String> getParameterNames() { 
    if (this.request == null) { 
    throw new IllegalStateException(sm.getString("requestFacade.nullRequest")); } else { 
    return Globals.IS_SECURITY_ENABLED ? (Enumeration)AccessController.doPrivileged(new RequestFacade.GetParameterNamesPrivilegedAction()) : this.request.getParameterNames(); } } ... } 

我们看名字就知道他用了门面模式。它封装了非常多的request的操作，也整合了很多servlet-api以外的一些内容，给用户提供了很大的便捷。同样，Tomcat对Response和Session当也封装了ResponseFacade和StandardSessionFacade类，感兴趣的小伙伴可以去深入了解一下。

门面模式的优缺点