1、基本概念
装饰器模式( Decorator Pattern ) ,也称为包装模式( Wrapper Pattern )是指在不改变原有对象
的基础之上，将功能附加到对象上,扩展原有对象的功能), 属于结构型模式。
我们常用的各种io流就是一种装饰器模式，mybatis的catch也是。
生活中案例有很多如：煎饼果子，蛋糕，麻辣烫等
2、类图和角色
主要有角色：
抽象组件或接口：规定被装饰者的原有属性和行为
具体组件/基本组件/被装饰者：拥有最基本的属性和行为
抽象装饰器：规定装饰具体组件的属性和行为（看业务可不需要）
具体装饰器：扩展被装饰者的功能
3、案例
案例：蛋糕：蛋糕没有加任何东西的时候，价格是50，加了巧克力是+20，加了草莓是+30
类图：
接口ICake：
public interface ICake { String getName(); Integer getPrice(); }
被装饰者BaseCake:
public class BaseCake implements ICake { @Override public String getName() { return "蛋糕"; } @Override public Integer getPrice() { return 50; } }
具体装饰者1：ChocolateCakeDecorator
public class ChocolateCakeDecorator implements ICake { public ICake iCake; public ChocolateCakeDecorator(ICake iCake) { this.iCake = iCake; } @Override public String getName() { return (iCake.getName()+"+巧克力"); } @Override public Integer getPrice() { return (iCake.getPrice()+20); } }
具体装饰者2：StrawberryCakeDecorator
public class StrawberryCakeDecorator implements ICake { public ICake iCake; public StrawberryCakeDecorator(ICake iCake) { this.iCake = iCake; } @Override public String getName() { return iCake.getName()+"+草莓"; } @Override public Integer getPrice() { return iCake.getPrice()+30; } }
测试：
public class Test { public static void main(String[] args) { ICake iCake; iCake = new BaseCake(); iCake = new ChocolateCakeDecorator(iCake); iCake = new StrawberryCakeDecorator(iCake); System.out.printf("蛋糕: %s 价格:%d", iCake.getName(), iCake.getPrice()); } }
输出：
蛋糕: 蛋糕+加巧克力+草莓
价格:100
这里我们没有用到抽象的装饰器，如果加上抽象的装饰器
抽象的装饰器：AbstractCakeDecorator
public abstract class AbstractCakeDecorator implements ICake { public ICake iCake; public AbstractCakeDecorator(ICake iCake) { this.iCake = iCake; } public abstract void doOtherthing(); @Override public String getName() { return this.iCake.getName(); } @Override public Integer getPrice() { return this.iCake.getPrice(); } }
具体装饰器1:ChocolateCakeDecorator
public class ChocolateCakeDecorator extends AbstractCakeDecorator { public ChocolateCakeDecorator(ICake iCake) { super(iCake); } @Override public void doOtherthing() { } @Override public String getName() { return (super.getName()+"+巧克力"); } @Override public Integer getPrice() { return (super.getPrice()+20); } }
具体装饰器2:StrawberryCakeDecorator
public class StrawberryCakeDecorator extends AbstractCakeDecorator { public StrawberryCakeDecorator(ICake iCake) { super(iCake); } @Override public void doOtherthing() { } @Override public String getName() { return super.getName()+"+草莓"; } @Override public Integer getPrice() { return super.getPrice()+30; } }
其他都不变
4、优缺点
优点：
1、可以动态的扩展一个对象的功能
2、完全是遵循开闭原则，当需要扩展功能的时候，只需要加上一个装饰器即可
例如我们有这样的一个需求：
public class Test { private static Logger logger = LoggerFactory.getLogger(Test.class); public static void main(String[] args) { logger = new JsonLoggerDecorator(logger); logger.info("hhh"); } }
我们的日志打印是这样的：
[main] INFO com.springboot.designPattern.decorator.ex.Test - 哈哈哈
我们希望将日志信息转为json字符串，存入mongodb中，进行日志信息的处理
[main] INFO com.springboot.designPattern.decorator.ex.Test - {"date":"2020-07-07T16:05:14.292","message":"哈哈哈","class":"com.springboot.designPattern.decorator.ex.Test"}
那么就可以用到装饰器模式
导包：
<dependency>
<groupId>com.alibaba</groupId>
<artifactId>fastjson</artifactId>
<version>1.2.56</version>
</dependency>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-core</artifactId>
<version>2.12.1</version>
</dependency>
写个抽象的装饰器AbstractLoggerDecorator 实现Logger接口：
public abstract class AbstractLoggerDecorator implements Logger{ public Logger logger; public AbstractLoggerDecorator(Logger logger){ this.logger = logger; } @Override public String getName() { return null; } @Override public boolean isTraceEnabled() { return false; } @Override public void trace(String msg) { } @Override public void trace(String format, Object arg) { } @Override public void trace(String format, Object arg1, Object arg2) { } @Override public void trace(String format, Object... arguments) { } @Override public void trace(String msg, Throwable t) { } @Override public boolean isTraceEnabled(Marker marker) { return false; } @Override public void trace(Marker marker, String msg) { } @Override public void trace(Marker marker, String format, Object arg) { } @Override public void trace(Marker marker, String format, Object arg1, Object arg2) { } @Override public void trace(Marker marker, String format, Object... argArray) { } @Override public void trace(Marker marker, String msg, Throwable t) { } @Override public boolean isDebugEnabled() { return false; } @Override public void debug(String msg) { } @Override public void debug(String format, Object arg) { } @Override public void debug(String format, Object arg1, Object arg2) { } @Override public void debug(String format, Object... arguments) { } @Override public void debug(String msg, Throwable t) { } @Override public boolean isDebugEnabled(Marker marker) { return false; } @Override public void debug(Marker marker, String msg) { } @Override public void debug(Marker marker, String format, Object arg) { } @Override public void debug(Marker marker, String format, Object arg1, Object arg2) { } @Override public void debug(Marker marker, String format, Object... arguments) { } @Override public void debug(Marker marker, String msg, Throwable t) { } @Override public boolean isInfoEnabled() { return false; } @Override public void info(String msg) { } @Override public void info(String format, Object arg) { } @Override public void info(String format, Object arg1, Object arg2) { } @Override public void info(String format, Object... arguments) { } @Override public void info(String msg, Throwable t) { } @Override public boolean isInfoEnabled(Marker marker) { return false; } @Override public void info(Marker marker, String msg) { } @Override public void info(Marker marker, String format, Object arg) { } @Override public void info(Marker marker, String format, Object arg1, Object arg2) { } @Override public void info(Marker marker, String format, Object... arguments) { } @Override public void info(Marker marker, String msg, Throwable t) { } @Override public boolean isWarnEnabled() { return false; } @Override public void warn(String msg) { } @Override public void warn(String format, Object arg) { } @Override public void warn(String format, Object... arguments) { } @Override public void warn(String format, Object arg1, Object arg2) { } @Override public void warn(String msg, Throwable t) { } @Override public boolean isWarnEnabled(Marker marker) { return false; } @Override public void warn(Marker marker, String msg) { } @Override public void warn(Marker marker, String format, Object arg) { } @Override public void warn(Marker marker, String format, Object arg1, Object arg2) { } @Override public void warn(Marker marker, String format, Object... arguments) { } @Override public void warn(Marker marker, String msg, Throwable t) { } @Override public boolean isErrorEnabled() { return false; } @Override public void error(String msg) { } @Override public void error(String format, Object arg) { } @Override public void error(String format, Object arg1, Object arg2) { } @Override public void error(String format, Object... arguments) { } @Override public void error(String msg, Throwable t) { } @Override public boolean isErrorEnabled(Marker marker) { return false; } @Override public void error(Marker marker, String msg) { } @Override public void error(Marker marker, String format, Object arg) { } @Override public void error(Marker marker, String format, Object arg1, Object arg2) { } @Override public void error(Marker marker, String format, Object... arguments) { } @Override public void error(Marker marker, String msg, Throwable t) { } }
具体装饰器：JsonLoggerDecorator
public class JsonLoggerDecorator extends AbstractLoggerDecorator { public JsonLoggerDecorator(Logger logger) { super(logger); } @Override public void info(String msg) { JSONObject jsonObject = new JSONObject(); jsonObject.put("message",msg); jsonObject.put("class",logger.getName()); jsonObject.put("date", LocalDateTime.now()); logger.info(jsonObject.toString()); } }
测试：
public class Test { private static Logger logger = LoggerFactory.getLogger(Test.class); public static void main(String[] args) { logger = new JsonLoggerDecorator(logger); logger.info("哈哈哈"); } }
打印结果：
[main] INFO com.springboot.designPattern.decorator.ex.Test - {"date":"2020-07-07T16:05:14.292","message":"哈哈哈","class":"com.springboot.designPattern.decorator.ex.Test"}