设计模式（装饰模式）

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;<E> List<E> 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 <E> List<E> 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 <E> List<E> 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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