Mybatis插件原理

1、概述

　　Mybatis允许使用插件拦截的方法调用包括：

• Excetor
• ParameterHandler
• ResultHandler
• StatementHandler

　　

　　上图是Mybatis框架的整体执行架构，Mybatis插件能够对四大对象接口进行拦截。

• Executor ： Mybatis执行器，是Mybatis调度核心，负责SQL语句的完成和查询缓存的维护
• StatementHandler ： 封装了JDBC Statement操作，负责对JDBC Statement的操作，如设置参数、将Statement结果集转换成list集合
• ParameterHandler ：负责将用户传递的参数转换成JDBC Statement所需要的参数
• ResultSetHandler ：负责将Java数据类型和JDBC数据类型之间的映射和转换

 2、插件实现原理

Executor代理对象

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;
}

StatementHandler代理对象

  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;
  }

ParameterHandler 

  public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
    return parameterHandler;
  }

ResultSetHandler

  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;
  }

可拦截类对象都是通过pluginAll来实现的

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

default Object plugin(Object target) {
  return Plugin.wrap(target, this);
}

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) {
    return Proxy.newProxyInstance(
        type.getClassLoader(),
        interfaces,
        new Plugin(target, interceptor, signatureMap));
  }
  return target;
}

1、遍历所有拦截器，调用拦截器的plugin方法生成代理对象

2、注意：生成的对象又重新赋值给了target，所以如果有多个拦截器，生成的代理对象会被另一个代理对象所代理，形成一个代理链条，执行的时候，依次执行所有拦截器的拦截逻辑代码。

3、Proxy.newProxyInstance(type.getClassLoader(),interfaces,new Plugin(target, interceptor,signatureMap)) 动态代理实现

3、拦截逻辑的执行

　　由于真正去执行Executor、ParameterHandler、ResultSetHandler和StatementHandler类中的方法的对象是代理对象，所以在执行方法时，首先调用的时Plugin类，如下：

　　

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
　　　//首先根据执行方法所属类获取拦截器中声明需要拦截的方法集合
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
　　　//判断当前方法需不需要执行拦截逻辑，需要的话，执行拦截逻辑方法（即Interceptor接口的intercept方法实现
      if (methods != null && methods.contains(method)) {
        return interceptor.intercept(new Invocation(target, method, args));
      }
　　　//不需要则直接执行原方法。
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }