设计模式（装饰模式） -尊龙游戏旗舰厅官网

什么是装饰模式（decorator）：
　　装饰模式是为已有功能动态的添加更多功能的一种方式；在不必改变原类文件和使用继承的情况下，动态地扩展一个对象的功能。它是通过创建一个包装对象，也就是装饰来包裹真实的对象；

装饰模式由4种角色组成：
　　抽象构件（component）角色：给出一个抽象接口，以规范准备接收附加职责的对象；
　　具体构件（concrete component）角色：定义一个将要接收附加职责的类；
　　装饰（decorator）角色：持有一个构件（component）对象的实例，并实现一个与抽象构件接口一致的接口，从外类来扩展component类的功能，但对于component类来说，是无需知道decorato的存在的；
　　具体装饰（concrete decorator）角色：负责给构件对象添加上附加的职责；

装饰模式的优点：
　　1.把类中的装饰功能从类中搬移出去，这样可以简化原有的类；
　　2.有效的把类的核心职责和装饰功能分开，而且可以去除相关类中重复的装饰逻辑；
　　3.装饰模式与继承关系的目的都是要扩展对象的功能，但是装饰模式可以提供比继承更多的灵活性；
　　4.通过使用不同的具体装饰类以及这些装饰类的排列组合，设计师可以创造出很多不同行为的组合；

装饰模式的应用场景，以下引自百度百科：
　　1. 需要扩展一个类的功能，或给一个类添加附加职责；
　　2. 需要动态的给一个对象添加功能，这些功能可以再动态的撤销；
　　3. 需要增加由一些基本功能的排列组合而产生的非常大量的功能，从而使继承关系变的不现实；
　　4. 当不能采用生成子类的方法进行扩充时。
　　　　一种情况是，可能有大量独立的扩展，为支持每一种组合将产生大量的子类，使得子类数目呈爆炸性增长。
　　　　另一种情况可能是因为类定义被隐藏，或类定义不能用于生成子类；

模式简化
　　如果只有一个concrete component类而没有抽象的component接口时，可以让decorator继承concrete component；
　　如果只有一个concrete decorator类时，可以将decorator和concrete decorator合并；

uml类图

 

根据uml生成一个简单的demo：

interface component{void operation(); }class concreatecomponent implements component{@overridepublic void operation() {system.out.println("balabala....");} }abstract class decorator implements component{private component comp;public decorator(component comp){this.comp = comp;}@overridepublic void operation() {comp.operation();} }class concreatedecorator extends decorator{public concreatedecorator(component comp) {super(comp);}@overridepublic void operation() {system.out.println("begin...");super.operation();system.out.println("end...");} }public class testmain {public static void main(string[] args) {component component = new concreatecomponent();component = new concreatedecorator(component);component.operation();} }

 输出的结果为：

begin...
balabala....
end...

 

在mybatis的executor也采用了装饰模式，不过在使用方法上稍微有些区别，我们先看一下executor的实现图：

以下是我从mybatis中分离出来的一部分代码，标红部分可以重点关注一下：

package org.apache.ibatis.executor;import org.apache.ibatis.cache.cachekey; import org.apache.ibatis.mapping.boundsql; import org.apache.ibatis.mapping.mappedstatement; import org.apache.ibatis.session.resulthandler; import org.apache.ibatis.session.rowbounds; import org.apache.ibatis.transaction.transaction;import java.sql.sqlexception; import java.util.list;public interface executor {resulthandler no_result_handler = null; list query(mappedstatement ms, object parameter, rowbounds rowbounds, resulthandler resulthandler, cachekey cachekey, boundsql boundsql) throws sqlexception;void commit(boolean required) throws sqlexception;void rollback(boolean required) throws sqlexception;cachekey createcachekey(mappedstatement ms, object parameterobject, rowbounds rowbounds, boundsql boundsql);transaction gettransaction();void setexecutorwrapper(executor var1); }

 

package org.apache.ibatis.executor;import org.apache.ibatis.cache.cachekey; import org.apache.ibatis.logging.log; import org.apache.ibatis.logging.jdbc.connectionlogger; import org.apache.ibatis.mapping.boundsql; import org.apache.ibatis.mapping.mappedstatement; import org.apache.ibatis.session.configuration; import org.apache.ibatis.session.resulthandler; import org.apache.ibatis.session.rowbounds; import org.apache.ibatis.transaction.transaction;import java.sql.connection; import java.sql.sqlexception; import java.util.list;public abstract class baseexecutor implements executor {protected transaction transaction;protected executor wrapper;protected configuration configuration;protected int querystack;protected baseexecutor(configuration configuration, transaction transaction) {this.transaction = transaction;this.configuration = configuration;this.wrapper = this;}@overridepublic list query(mappedstatement ms, object parameter, rowbounds rowbounds, resulthandler resulthandler, cachekey cachekey, boundsql boundsql) throws sqlexception {return null;}@overridepublic void commit(boolean required) throws sqlexception {}@overridepublic void rollback(boolean required) throws sqlexception {}@overridepublic cachekey createcachekey(mappedstatement ms, object parameterobject, rowbounds rowbounds, boundsql boundsql) {return null;}@overridepublic transaction gettransaction() {return null;}@overridepublic void setexecutorwrapper(executor var1) {this.wrapper = wrapper;}protected connection getconnection(log statementlog) throws sqlexception {connection connection = transaction.getconnection();if (statementlog.isdebugenabled()) {return connectionlogger.newinstance(connection, statementlog, querystack);} else {return connection;}}}

 

package org.apache.ibatis.executor;import org.apache.ibatis.executor.statement.statementhandler; import org.apache.ibatis.logging.log; import org.apache.ibatis.session.configuration; import org.apache.ibatis.transaction.transaction;import java.sql.connection; import java.sql.sqlexception; import java.sql.statement;public class simpleexecutor extends baseexecutor {protected simpleexecutor(configuration configuration, transaction transaction) {super(configuration, transaction);}private statement preparestatement(statementhandler handler, log statementlog) throws sqlexception {statement stmt;connection connection = getconnection(statementlog);stmt = handler.prepare(connection, transaction.gettimeout());handler.parameterize(stmt);return stmt;}}

 

package org.apache.ibatis.executor;import org.apache.ibatis.cache.cachekey; import org.apache.ibatis.cache.transactionalcachemanager; import org.apache.ibatis.mapping.boundsql; import org.apache.ibatis.mapping.mappedstatement; import org.apache.ibatis.session.resulthandler; import org.apache.ibatis.session.rowbounds; import org.apache.ibatis.transaction.transaction;import java.sql.sqlexception; import java.util.list;public class cachingexecutor implements executor{private final executor delegate;private final transactionalcachemanager tcm = new transactionalcachemanager();public cachingexecutor(executor delegate) {this.delegate = delegate;delegate.setexecutorwrapper(this);}@overridepublic list query(mappedstatement ms, object parameter, rowbounds rowbounds, resulthandler resulthandler, cachekey cachekey, boundsql boundsql) throws sqlexception {return null;}@overridepublic void commit(boolean required) throws sqlexception {}@overridepublic void rollback(boolean required) throws sqlexception {}@overridepublic cachekey createcachekey(mappedstatement ms, object parameterobject, rowbounds rowbounds, boundsql boundsql) {return null;}@overridepublic transaction gettransaction() {return null;}@overridepublic void setexecutorwrapper(executor var1) {throw new unsupportedoperationexception("this method should not be called");} }

 

package org.apache.ibatis.session;import org.apache.ibatis.executor.*; import org.apache.ibatis.plugin.interceptorchain; import org.apache.ibatis.transaction.transaction;public class configuration {private configuration conf;public configuration(configuration conf){this.conf = conf;}protected boolean cacheenabled = true;protected executortype defaultexecutortype = executortype.simple;protected final interceptorchain interceptorchain = new interceptorchain();public executor newexecutor(transaction transaction, executortype executortype) {executortype = executortype == null ? defaultexecutortype : executortype;executortype = executortype == null ? executortype.simple : executortype;executor executor;if (executortype.batch == executortype) {executor = new batchexecutor(this, transaction);} else if (executortype.reuse == executortype) {executor = new reuseexecutor(this, transaction);} else {executor = new simpleexecutor(conf, transaction);}if (cacheenabled) {executor = new cachingexecutor(executor);}executor = (executor) interceptorchain.pluginall(executor);return executor;} }

 

转载于:https://www.cnblogs.com/jiangyaxiong1990/p/9236764.html

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