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<class<?>, set<method>> 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<method> 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)。