Mybatis-Plugin介绍及配置使用

MyBatis 允许你在已映射语句执行过程中的某一点进行拦截调用。默认情况下，MyBatis允许使用插件来拦截的方法调用包括：

1.Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed) 拦截执行器的方法，其中包含了query、update、commit、rollback方法
2.ParameterHandler (getParameterObject, setParameters) 拦截参数的处理；
3.ResultSetHandler (handleResultSets, handleOutputParameters) 拦截结果集的处理；
4.StatementHandler (prepare, parameterize, batch, update, query) 拦截Sql语法构建的处理

参考Mybatis 官方的一个例子：

1.在全局配置文件mybatis-config.xml 添加

<plugins>
    <plugin interceptor="org.format.mybatis.cache.interceptor.ExamplePlugin"></plugin>
</plugins>

2.写好拦截类代码

@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) {
    }
}

这个拦截器会拦截Executor接口的update方法

源码简要分析

首先从配置文件解析开始

public class XMLConfigBuilder extends BaseBuilder {
    //解析配置
  private void parseConfiguration(XNode root) {
    try {
     //省略部分代码
      pluginElement(root.evalNode("plugins"));
     
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }
    
   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);
        //调用InterceptorChain.addInterceptor
        configuration.addInterceptor(interceptorInstance);
      }
    }
  }
}

上面的代码主要是解析配置文件的plugin节点，根据配置的interceptor 属性实例化Interceptor 对象，然后添加到Configuration 对象中的InterceptorChain 属性中

public class InterceptorChain {


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

  public Object pluginAll(Object target) {
    //循环调用每个Interceptor.plugin方法
    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);
  }

}

这个就和我们上面实现的是一样的。定义了拦截器链，初始化配置文件的时候就把所有的拦截器添加到拦截器链中，下面来看一下什么时候把拦截器插入到需要拦截的接口中

public class Configuration {

    protected final InterceptorChain interceptorChain = new InterceptorChain();
    //创建参数处理器
  public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    //创建ParameterHandler
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    //插件在这里插入
    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
    return parameterHandler;
  }

  //创建结果集处理器
  public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
      ResultHandler resultHandler, BoundSql boundSql) {
    //创建DefaultResultSetHandler
    ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
    //插件在这里插入
    resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
    return resultSetHandler;
  }

  //创建语句处理器
  public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    //创建路由选择语句处理器
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    //插件在这里插入
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

  public Executor newExecutor(Transaction transaction) {
    return newExecutor(transaction, defaultExecutorType);
  }

  //产生执行器
  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    //这句再做一下保护,囧,防止粗心大意的人将defaultExecutorType设成null?
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    //然后就是简单的3个分支，产生3种执行器BatchExecutor/ReuseExecutor/SimpleExecutor
    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);
    }
    //如果要求缓存，生成另一种CachingExecutor(默认就是有缓存),装饰者模式,所以默认都是返回CachingExecutor
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    //此处调用插件,通过插件可以改变Executor行为
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }
}

从代码可以看出mybatis 在实例化Executor、ParameterHandler、ResultSetHandler、StatementHandler四大接口对象的时候调用interceptorChain.pluginAll() 方法插入进去的。其实就是循环执行拦截器链所有的拦截器的plugin() 方法，mybatis官方推荐的plugin方法是Plugin.wrap() 方法,这个类就是我们上面的TargetProxy类

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public class Plugin implements InvocationHandler {

    public static Object wrap(Object target, Interceptor interceptor) {
    //从拦截器的注解中获取拦截的类名和方法信息
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    //取得要改变行为的类(ParameterHandler|ResultSetHandler|StatementHandler|Executor)
    Class<?> type = target.getClass();
    //取得接口
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    //产生代理，是Interceptor注解的接口的实现类才会产生代理
    if (interfaces.length > 0) {
      return Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          new Plugin(target, interceptor, signatureMap));
    }
    return target;
  }
    
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      //获取需要拦截的方法
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
      //是Interceptor实现类注解的方法才会拦截处理
      if (methods != null && methods.contains(method)) {
        //调用Interceptor.intercept，也即插入了我们自己的逻辑
        return interceptor.intercept(new Invocation(target, method, args));
      }
      //最后还是执行原来逻辑
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }
    
    //取得签名Map,就是获取Interceptor实现类上面的注解，要拦截的是那个类（Executor，ParameterHandler，   ResultSetHandler，StatementHandler）的那个方法
  private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
    //取Intercepts注解，例子可参见ExamplePlugin.java
    Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
    // issue #251
    //必须得有Intercepts注解，没有报错
    if (interceptsAnnotation == null) {
      throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());      
    }
    //value是数组型，Signature的数组
    Signature[] sigs = interceptsAnnotation.value();
    //每个class里有多个Method需要被拦截,所以这么定义
    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;
  }
    
    //取得接口
  private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
    Set<Class<?>> interfaces = new HashSet<Class<?>>();
    while (type != null) {
      for (Class<?> c : type.getInterfaces()) {
        //拦截其他的无效
        if (signatureMap.containsKey(c)) {
          interfaces.add(c);
        }
      }
      type = type.getSuperclass();
    }
    return interfaces.toArray(new Class<?>[interfaces.size()]);
  }
}

总结

Mybatis 拦截器的使用是实现Interceptor接口

public interface Interceptor {

  //拦截
  Object intercept(Invocation invocation) throws Throwable;

  //插入
  Object plugin(Object target);

  //设置属性(扩展)
  void setProperties(Properties properties);

}

通过上面的分析可以知道，所有可能被拦截的处理类都会生成一个代理类,如果有N个拦截器，就会有N个代理，层层生成动态代理是比较耗性能的。而且虽然能指定插件拦截的位置，但这个是在执行方法时利用反射动态判断的，初始化的时候就是简单的把拦截器插入到了所有可以拦截的地方。所以尽量不要编写不必要的拦截器。