mybatis 源碼分析(三) 攔截器原理

mybatis 源碼分析(三) 插件原理

• mybatis 源碼分析(一) Xml解析,容器初始化
• mybatis 源碼分析(二) sql執行路徑分析
• mybatis 源碼分析(三) 攔截器原理
• mybatis 源碼分析(四) 自帶連接池

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在使用mybatis過程中 我們可能需要對sql 產生的構建的中間環節 進行一些特殊處理
(比如 更換主從庫連接 自定義分表操作 ….) 這個時候就需要使用到攔截器

Interceptor 官方文檔

/**
 * @author Clinton Begin
 */
public interface Interceptor {

  Object intercept(Invocation invocation) throws Throwable;

  Object plugin(Object target);

  void setProperties(Properties properties);

}

在第一章中 已經指明
plugins 插件配置 是 注入到 SqlSessionFactoryBean 實例中的

SqlSessionFactoryBean

public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> {

    protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
    /** 省略其他代碼 */
        if (!isEmpty(this.plugins)) {
      for (Interceptor plugin : this.plugins) {
        /** 配置類中添加插件實現 往下跟蹤會到 InterceptorChain*/
        configuration.addInterceptor(plugin);
        if (LOGGER.isDebugEnabled()) {
          LOGGER.debug("Registered plugin: '" + plugin + "'");
        }
      }
    }
 }
}

InterceptorChain

public class InterceptorChain {

  private final List<Interceptor> interceptors = new ArrayList<Interceptor>();

  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

  public void addInterceptor(Interceptor interceptor) {
    interceptors.add(interceptor);
  }

  public List<Interceptor> getInterceptors() {
    return Collections.unmodifiableList(interceptors);
  }
}

那麼攔截器是在什麼地方使用的呢？
第一章的 DefaultSqlSessionFactory 的 第21行代碼 創建executor

Configuration

public class Configuration {
  /** 省略其他代碼 */
  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);
    }
    /** 使用動態代理包裝了executor */
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }
}

由於 interceptorChain.pluginAll 實際上是循環調用interceptor的plugin方法
所以 我們根據官網提供的代碼 繼續分析
如果搜索下 就會發現 interceptor 在4個地方進行了代理攔截

ExamplePlugin

@Intercepts({@Signature(
  type= Executor.class,
  method = "update",
  args = {MappedStatement.class,Object.class})})
public class ExamplePlugin implements Interceptor {
  public Object intercept(Invocation invocation) throws Throwable {
    return invocation.proceed();
  }
  public Object plugin(Object target) {
    return Plugin.wrap(target, this);
  }
  public void setProperties(Properties properties) {
  }
}

Plugin

public class Plugin implements InvocationHandler {

  private Object target;
  private Interceptor interceptor;
  private Map<Class<?>, Set<Method>> signatureMap;

  private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {
    this.target = target;
    this.interceptor = interceptor;
    this.signatureMap = signatureMap;
  }

  public static Object wrap(Object target, Interceptor interceptor) {
    /**
     * 該方法是解析攔截器實現類上面的 Intercepts 註解
     * 標明executor中那些方法需要被攔截
     */
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    Class<?> type = target.getClass();
    /** 代理 org.apache.ibatis.executor.Executor 接口
      * Plugin 實現了動態代理接口 所以調用 Executor 任何方法
      * 都會進入到 invoke裏面
      */
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    if (interfaces.length > 0) {
      return Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          new Plugin(target, interceptor, signatureMap));
    }
    return target;
  }

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
      /** 判斷調用的方法是否在需要攔截的方法列表中 */
      if (methods != null && methods.contains(method)) {
      /** 執行攔截器方法 */
        return interceptor.intercept(new Invocation(target, method, args));
      }
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }

  private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
    //TO DO
    return signatureMap;
  }

  private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
    //TO DO
    return interfaces.toArray(new Class<?>[interfaces.size()]);
  }

}

看完之後是不是覺得 攔截器 就是不停的對 上一個 動態代理的Executor 在包裝一層動態代理類!!!