组合模式在开源代码中的应用

Bertha 。 2023-03-04 15:25 21阅读 0赞

组合模式的作用：将对象组合成树状层次结构，使用户对单个对象和组合对象具有一致的访问性。

### **案例一** ###

JDK 的 AWT(Abstract Window Toolkit)，使用了组合模式。AWT 中包含了两种组件：容器组件和基本组件。

*  容器组件是 java.awt.Container 的子孙类
 *  基本组件是 java.awt.Component 的子孙类
 *  容器组件和基本组件都是 java.awt.Component 的子孙类

Container 类中包含了很多基本组件或容器，放在 ArrayList 中

public class Container extends Component {
        private java.util.List<Component> component = new ArrayList<>();
        
        //向容器类中添加基本组件或容器
        public Component add(Component comp) {
            addImpl(comp, null, -1);
            return comp;
        }
    }

基本组件与容器组件，构成了树状结构。由于都是 Component 的子孙类，对 Component 定义的方法，访问上具有一致性

### **案例二** ###

Mybatis 在处理 xml 动态 sql 中用到了组合模式。

*  抽象构建接口 SqlNode，定义了 apply 方法，根据传入参数构造 sql

public interface SqlNode {
      boolean apply(DynamicContext context);
    }

*  容器类 MixedSqlNode、ChooseSqlNode 实现 SqlNode 接口，List 存储  SqlNode

public class MixedSqlNode implements SqlNode {
      private List<SqlNode> contents;
    
      public MixedSqlNode(List<SqlNode> contents) {
        this.contents = contents;
      }
    
      @Override
      public boolean apply(DynamicContext context) {
        for (SqlNode sqlNode : contents) {
          sqlNode.apply(context);
        }
        return true;
      }
    }

public class ChooseSqlNode implements SqlNode {
      private SqlNode defaultSqlNode;
      private List<SqlNode> ifSqlNodes;
    
      public ChooseSqlNode(List<SqlNode> ifSqlNodes, SqlNode defaultSqlNode) {
        this.ifSqlNodes = ifSqlNodes;
        this.defaultSqlNode = defaultSqlNode;
      }
    
      @Override
      public boolean apply(DynamicContext context) {
        for (SqlNode sqlNode : ifSqlNodes) {
          if (sqlNode.apply(context)) {
            return true;
          }
        }
        if (defaultSqlNode != null) {
          defaultSqlNode.apply(context);
          return true;
        }
        return false;
      }
    }

*  其他叶子节点：TextSqlNode、StaticTextSqlNode、TrimSqlNode、SetSqlNode、WhereSqlNode ... 均是通过 apply 方法构建 SQL

SqlNode 的实现类通过容器类与叶子节点构造成树形结构，使用 apply 动态解析出完整的 SQL。

--------------------

### 【Java学习资源】整理推荐 ###

*  [组合模式在开源代码中的应用][Link 1]
 *  [享元模式在开源代码中的应用][Link 2]
 *  [外观模式在开源代码中的应用][Link 3]
 *  [装饰器模式在开源代码中的应用][Link 4]
 *  [桥接模式在开源代码中的应用][Link 5]
 *  [适配器模式在开源代码中的应用][Link 6]
 *  [代理模式在开源代码中的应用][Link 7]
 *  [原型模式在开源代码中的应用][Link 8]
 *  [建造者模式在开源代码中的应用][Link 9]
 *  [工厂模式在开源代码中的应用][Link 10]
 *  [单例模式在开源代码中的应用][Link 11]
 *  [编码规范][Link 12]
 *  [设计模式][Link 13]
 *  [重构][Link 14]
 *  [设计原则][Link 15]
 *  [面向对象到底是什么][Link 16]
 *  [代码质量有哪些评判标准？][Link 17]

--------------------

### 【Java面试题与答案】整理推荐 ###

*  [基础与语法][Link 18]
 *  [集合][Link 19]
 *  [网络编程][Link 20]
 *  [并发编程][Link 21]
 *  [Web][]
 *  [安全][Link 22]
 *  [设计模式][Link 23]
 *  [框架][Link 24]
 *  [算法与数据结构][Link 25]
 *  [异常][Link 26]
 *  [文件解析与生成][Link 27]
 *  [Linux][]
 *  [MySQL][]
 *  [Oracle][]
 *  [Redis][]
 *  [Dubbo][]

[Link 1]: https://www.javanav.com/val/12f35b774b514c47a219bd860148f179.html
[Link 2]: https://www.javanav.com/val/72ac6d1ddd2d46b2a5312ac9b3050986.html
[Link 3]: https://www.javanav.com/val/f4a27593b46b46bc89d77d9f92c0f50c.html
[Link 4]: https://www.javanav.com/val/40716af16cda47ffae5fcf4090e69e14.html
[Link 5]: https://www.javanav.com/val/75bfae90731d40c781926a0dbec9edca.html
[Link 6]: https://www.javanav.com/val/f35536fdc771448f8d70574d830b7616.html
[Link 7]: https://www.javanav.com/val/24468130d26e460187c9280b90146649.html
[Link 8]: https://www.javanav.com/val/ffd6d3d1ceed414daafeb3ce10794c60.html
[Link 9]: https://www.javanav.com/val/e8afb788fdc54712aaf981c705796b46.html
[Link 10]: https://www.javanav.com/val/6f367ed6ee894adb830987f11536141c.html
[Link 11]: https://www.javanav.com/val/838b98837f574aa6a09c840921984707.html
[Link 12]: https://www.javanav.com/val/2b9f8790e8564bc0a2cb36357a66502f.html
[Link 13]: https://www.javanav.com/val/90b430ff0cbc4ef997e64cf7a0e1e745.html
[Link 14]: https://www.javanav.com/val/d9095cb3712040c0bee8054732321d6d.html
[Link 15]: https://www.javanav.com/val/9916617e210c4e0eae8649874a2cedd5.html
[Link 16]: https://www.javanav.com/val/5a834b46cfd945f7be9c4ea652306fd4.html
[Link 17]: https://www.javanav.com/val/0cea6c7c082043ab83ad87a9c5a48d9d.html
[Link 18]: https://www.javanav.com/interview/93b0069472fd479393006c0e73043fc4.html
[Link 19]: https://www.javanav.com/interview/4147d95e693945e7a50345fd33ec817e.html
[Link 20]: https://www.javanav.com/interview/68a57ffc78de4133bb890bdd502ac418.html
[Link 21]: https://www.javanav.com/interview/36a24fa3258e45c89873f474ce914912.html
[Web]: https://www.javanav.com/interview/4e27cb63adb440c6825a159d35cf4010.html
[Link 22]: https://www.javanav.com/interview/03affb2ce3f04fda8853f714e5764906.html
[Link 23]: https://www.javanav.com/interview/07a32a9ff17a4006a53550ce658cafed.html
[Link 24]: https://www.javanav.com/interview/5c466456e9b54ab0a749b5948e51f1d3.html
[Link 25]: https://www.javanav.com/interview/943f917041ee4f05add24539f5b861b2.html
[Link 26]: https://www.javanav.com/interview/d3f058cfc05d4c6dadd821b622e84d36.html
[Link 27]: https://www.javanav.com/interview/67a50996c28f418e83b109ea42fc3eff.html
[Linux]: https://www.javanav.com/interview/00cccb81729540d1923bd0a0ef49533a.html
[MySQL]: https://www.javanav.com/interview/d311e78502a845f1a4bb307bd6aec2b3.html
[Oracle]: https://www.javanav.com/interview/5f29e9677edf4a16be3dce30d257c9ab.html
[Redis]: https://www.javanav.com/interview/066c62b3af4d4827b2243a822c3084a9.html
[Dubbo]: https://www.javanav.com/interview/9a3e7a7740324757abaab953f7ebc455.html