java MyBatis拦截器Inteceptor详细介绍

有许多java初学者对于MyBatis拦截器Inteceptor不是很了解，在这里我来为各位整理下篇关于java中MyBatis拦截器Inteceptor详解，

本文主要分析MyBatis的插件机制，实际就是Java动态代理实现的责任链模式实现。

根据官方文档。Mybatis只允许拦截以下方法，这个决定写拦截器注解签名参数。

 代码如下 

Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)
ParameterHandler (getParameterObject, setParameters)
ResultSetHandler (handleResultSets, handleOutputParameters)
StatementHandler (prepare, parameterize, batch, update, query)

拦截处理的源码如下，其中interceptorChain.pluginAll(..)即为织入自定义拦截器：

代码如下 

/* org.apache.ibatis.session.Configuration类中方法 */
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
  ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
  /* 拦截ParameterHandler*/
  parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
  return parameterHandler;
}
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);
  /* 拦截ResultSetHandler*/
  resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
  return resultSetHandler;
}
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);
   /* 拦截StatementHandler*/
  statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
  return statementHandler;
}
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);
 }
  /* 拦截Executor*/
 executor = (Executor) interceptorChain.pluginAll(executor);
 return executor;
}

实现一个自定义拦截器只需实现Interceptor接口即可，大致代码如下:

 代码如下 

/* 注解表明要拦截哪个接口的方法及其参数 */
@Intercepts({ @Signature(type = StatementHandler.class, method = "prepare", args = { Connection.class }) })
public class YourInterceptor implements Interceptor{
 public Object intercept(Invocation invocation) throws Throwable{
  doSomeThing();
  /* 注：此处实际上使用Invocation.proceed()方法完成interceptorChain链的遍历调用(即执行所有注册的Interceptor的intercept方法)，到最终被代理对象的原始方法调用 */
  return invocation.proceed();
 }
  /*生成成对目标target的代理，而@Intercepts的注解是在Plugin.wrap中用到*/
 @Override
 public Object plugin(Object target){
   /* 当目标类是StatementHandler类型时，才包装目标类，不做无意义的代理 */
  return (target instanceof StatementHandler)?Plugin.wrap(target, this):target;
 }
  /*用于设置自定义的拦截器配置参数*/
 @Override
 public void setProperties(Properties properties){
 }
}

其中，拦截调用的代码均在Plugin.wrap中：

代码如下

/* org.apache.ibatis.plugin.Plugin类 */
public class Plugin implements InvocationHandler {
 /* 省略代码... */
 public static Object wrap(Object target, Interceptor interceptor) {
  /* 此处即为获取Interceptor的注解签名 */
  Map, Set> signatureMap = getSignatureMap(interceptor);
  Class type = target.getClass();
  /* 获取拦截目标类相匹配的接口 */
  Class[] interfaces = getAllInterfaces(type, signatureMap);
  if (interfaces.length > 0) {
   /* 使用jdk动态代理 */
   return Proxy.newProxyInstance(type.getClassLoader(), interfaces, new Plugin(target, interceptor, signatureMap));
  }
  return target;
 }
 /* 拦截目标类的所有方法的执行都会变为在此执行 */
 @Override
 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  try {
   Set methods = signatureMap.get(method.getDeclaringClass());
   if (methods != null && methods.contains(method)) {
    /* 执行拦截器方法 */
    return interceptor.intercept(new Invocation(target, method, args));
   }
   return method.invoke(target, args);
  } catch (Exception e) {
   throw ExceptionUtil.unwrapThrowable(e);
  }
 }
 /* 省略代码... */
}

可以看到MyBatis的拦截器设计核心代码还是比较简单的，但是足够灵活。实际使用时注意，不做无意义的代理(Plugin.wrap)。