Mybatis 攔截器實現原理

標簽（空格分隔）： mybatis

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攔截器概述

• 像springmvc一樣，mybatis也提供了攔截器實現，只是他們攔截的對象不同。
• mybatis給Executor、StatementHandler、ResultSetHandler、ParameterHandler提供了攔截器功能，
  • Executor提供了增刪改查的接口.
  • StatementHandler負責處理Mybatis與JDBC之間Statement的交互.
  • ResultSetHandler負責處理Statement執行后產生的結果集，生成結果列表.
  • ParameterHandler是Mybatis實現Sql入參設置的對象。
• 攔截器采用了責任鏈模式，把請求發送者和請求處理者分開，各司其職。

立即開始

<plugins>
    <plugin interceptor="com.tamguo.demo.interceptor.StatementInterceptor"></plugin>
</plugins>

@Intercepts({@Signature(type = StatementHandler.class, method = "prepare", args = {Connection.class, Integer.class})})
public class StatementInterceptor implements Interceptor {

    public Object intercept(Invocation invocation) throws Throwable {
        System.out.println(" hello Statement interceptor !");
        return invocation.proceed();
    }

    public Object plugin(Object target) {
        Object wrap = Plugin.wrap(target, this);
        return wrap;
    }

    public void setProperties(Properties properties) {

    }
}

開始一個Statement攔截器，在mybatis-config.xml加入plugin配置,編寫一個StatementInterceptor類。這樣就已經完成了一個攔截器的編寫，該類會鏈接StatementHandler的prepare方法。

MyBatis 加載intercetor過程

pluginElement() 是Confiration中，解析XML並把interceptorInstance加入到interceptorChain。

   private void pluginElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        String interceptor = child.getStringAttribute("interceptor");
        Properties properties = child.getChildrenAsProperties();
        Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
        interceptorInstance.setProperties(properties);
        configuration.addInterceptor(interceptorInstance);
      }
    }
  }

到這里Confiration對象已經持有了一個InterceptorChain屬性， InterceptorChain中只有一個intercetor的list集合。

sqlSessionFactory.openSession(),會創建Executor、StatementHandler、ResultSetHandler、ParameterHandler對象。

  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(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

executor = (Executor) interceptorChain.pluginAll(executor); 注意這一行代碼，executor在這里已經被修改成一個代理對象。