MyBatis源码中装饰器模式

MyBatis中关于Cache和CachingExecutor接口的实现类也使用了装饰者设计模式。Executor是MyBatis执行器，是MyBatis 调度的核心，负责SQL语句的生成和查询缓存的维护；CachingExecutor是一个Executor的装饰器，给一个Executor增加了缓存的功能。此时可以看做是对Executor类的一个增强，故使用装饰器模式是合适的。

2.1.1 Executor

首先我们看下Executor，打开MyBatis的源码org.apache.ibatis.session.Configuration

  1.   public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
  3.     executorType = executorType == null ? defaultExecutorType : executorType;
  4.     executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
  6.       Executor executor;
  8.       if (ExecutorType.BATCH == executorType) {
  9.       executor = new BatchExecutor(this, transaction);
  10.     } else if (ExecutorType.REUSE == executorType) {
  11.       executor = new ReuseExecutor(this, transaction);
  12.     } else {
  13.       executor = new SimpleExecutor(this, transaction);
  14.     }
  15.     //如果开启了二级缓存则装饰原先的Executor
  16.     if (cacheEnabled) {
  17.       executor = new CachingExecutor(executor);
  18.     }
  19.     executor = (Executor) interceptorChain.pluginAll(executor);
  20.     return executor;
  21.   }

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2.1.2 CachingExecutor (装饰器的具体实现对象)

  1. public class CachingExecutor implements Executor {
  2.     //持有组件对象
  3.   private Executor delegate;
  5.   private TransactionalCacheManager tcm = new TransactionalCacheManager();
  7.     //构造方法，传入组件对象
  8.   public CachingExecutor(Executor delegate) {
  9.     this.delegate = delegate;
  10.     delegate.setExecutorWrapper(this);
  11.   }
  13.   @Override
  14.   public int update(MappedStatement ms, Object parameterObject) throws SQLException {
  15.     //转发请求给组件对象，可以在转发前后执行一些附加动作
  16.     flushCacheIfRequired(ms);
  17.     return delegate.update(ms, parameterObject);
  18.   }
  19.   //...
  20.  }