Mybatis的核心实现Plugin、Interceptor

本章讲Interceptor其实在分享怎样的设计才是具备良好的拓展性的程序，注重内功修为的童靴可能更感兴趣。Mybatis中的插件允许你针对核心组件接口Executor 、StatementHandler、ParameterHandler、ResultSetHandler中任何一个方法进行拦截调用。而每个Interceptor（拦截的接口）其实是通过JDK的动态代理技术生成的代理类，每当执行这4种接口中的方法时，就会进入拦截方法（具体就是InvocationHandler的invoke()方法）。

关于Mybatis的系统架构及Executor、ParameterHandler、ResultSetHandler、StatementHandler可参阅之前的相关的分享。

责任链模式

责任链模式是一种对象行为模式，插件用的是它，很多对象由每一个对象对其下家的引用而连接起来形成一条链，请求在这个链上传递，直到链上的某一个对象决定处理此请求。

用法

用法一点也不复杂：只需实现 Interceptor 接口，并指定想要拦截的方法签名即可，更多用法可参阅官方教程

除了用插件来修改 MyBatis 核心行为之外，还可以通过完全覆盖配置类来达到目的。只需继承后覆盖其中的每个方法，再把它传递到 SqlSessionFactoryBuilder.build(myConfig) 方法即可。再次重申，这可能会严重影响 MyBatis 的行为，务请慎之又慎。

<!-- mybatis-config.xml -->
<plugins>
  <plugin interceptor="org.mybatis.example.ExamplePlugin">
    <property name="someProperty" value="100"/>
  </plugin>
</plugins>

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

源码

在上面的源码中Plugin.wrap()，它就是当前拦截器（ExamplePlugin）的代理类。而InterceptorChain.pluginAll()则定义了一个组件接口（比如：ParameterHandler）执行的责任链。

public interface Interceptor {
  //拦截器具体实现
  Object intercept(Invocation invocation) throws Throwable;
  //拦截器的代理类
  Object plugin(Object target);
  //添加属性
  void setProperties(Properties properties);
}
//源码Plugin.wrap其实是一个代理
public class Plugin implements InvocationHandler {
  private Object target;
  private Interceptor interceptor;//拦截器
  private Map<Class<?>, Set<Method>> signatureMap;
  //为拦截器生成代理类 
  public static Object wrap(Object target, Interceptor interceptor) {
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    Class<?> type = target.getClass();
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    if (interfaces.length > 0) {//目标列的接口与拦截器匹配，则生成代理类
      return Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          new Plugin(target, interceptor, signatureMap));
    }
    return target;//原生类返回
  }
  //代理类执行入口
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
      if (methods != null && methods.contains(method)) {//执行插件类的拦截规则
        return interceptor.intercept(new Invocation(target, method, args));
      }
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }
  //处理拦截器类上的各种注解
  private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
    Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
    // issue #251
    if (interceptsAnnotation == null) {
      throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());      
    }
    Signature[] sigs = interceptsAnnotation.value();
    Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
    for (Signature sig : sigs) {
      Set<Method> methods = signatureMap.get(sig.type());
      if (methods == null) {
        methods = new HashSet<Method>();
        signatureMap.put(sig.type(), methods);
      }
      try {
        Method method = sig.type().getMethod(sig.method(), sig.args());
        methods.add(method);
      } catch (NoSuchMethodException e) {
        throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
      }
    }
    return signatureMap;
  }
}
//拦截器的请求
public class Invocation {
  private Object target;
  private Method method;
  private Object[] args;
  //调用原先真实方法(非代理)
  public Object proceed() throws InvocationTargetException, IllegalAccessException {
    return method.invoke(target, args);
  }
}

拦截器链执行过程

拦截器代理类对象->拦截器->目标方法，即：InvocationHandler.invoke()->Interceptor.intercept()->Invocation.proceed()。

也就是说其实在执行每个Executor、ParameterHandler、ResultSetHandler、StatementHandler都会构建一个拦截器链，如果针对每个配置了plugin，一个sql执行下来会陆续经过四个拦截器链。

//拦截器管理类
public class InterceptorChain {
  private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
  //构建责任链
  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {//存在拦截器则构建
      //有多个同类型的拦截器则构成链，将代理类作为target再成代理类
      target = interceptor.plugin(target);
    }
    return target;//可以没有拦截器
  }
  public void addInterceptor(Interceptor interceptor) {
    interceptors.add(interceptor);
  }
  public List<Interceptor> getInterceptors() {
    return Collections.unmodifiableList(interceptors);
  }
}
//重要的配置类
public class Configuration {
  protected final InterceptorChain interceptorChain = new InterceptorChain();
  //有匹配的拦截器则构建责任链，没有则不生成
  public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
    return parameterHandler;
  }
  //同newParameterHandler
  public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
      ResultHandler resultHandler, BoundSql boundSql) {
    ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
    resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
    return resultSetHandler;
  }
  //同newParameterHandler
  public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

  public Executor newExecutor(Transaction transaction) {
    return newExecutor(transaction, defaultExecutorType);
  }
  //同newParameterHandler
  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;
  }
}
//负责解析Mybatis 全局配置文件
public class XMLConfigBuilder extends BaseBuilder {
  private void pluginElement(XNode parent) throws Exception {//解析interceptor
    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);
      }
    }
  }
}
public class SimpleExecutor extends BaseExecutor {
  @Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      //生成责任链
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);//可拦截query()
    } finally {
      closeStatement(stmt);
    }
  }
  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());//可拦截prepare()
    handler.parameterize(stmt);//可拦截parameterize()
    return stmt;
  }
}

mybatis用拦截器的设计非常巧妙的利用了jdk的动态代理，其实复杂度并不高。