MyBatis插件开发

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：
Executor (update, query, flushStatements, commit, rollback,getTransaction, close, isClosed)
ParameterHandler (getParameterObject, setParameters)
ResultSetHandler (handleResultSets, handleOutputParameters)
StatementHandler (prepare, parameterize, batch, update, query)
常用的插件有分页插件com.github.pagehelper.PageInterceptor。下面自定义个打印sql的插件。
通过对 MyBatis org.apache.ibatis.executor.statement.StatementHandler 中的prepare 方法进行拦截即可。
prepare 方法签名如下：

Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException;

自定义类实现org.apache.ibatis.plugin.Interceptor接口，Interceptor接口代码如下：

public interface Interceptor {
  Object intercept(Invocation invocation) throws Throwable;
  Object plugin(Object target);
  void setProperties(Properties properties);
}

每一个拦截器都必须实现上面的三个方法，其中：
1) Object intercept(Invocation invocation)是实现拦截逻辑的地方，内部要通过invocation.proceed()调用下一个拦截器拦截目标方法。
2) Object plugin(Object target) 就是用当前这个拦截器生成对目标target的代理，实际是通过Plugin.wrap(target,this) 来完成的，把目标target和拦截器this传给了包装函数。
3) setProperties(Properties properties)用于设置额外的参数，参数配置在拦截器的Properties节点里。

注解里描述的是指定拦截方法的签名 [type,method,args] （即对哪种对象的哪种方法进行拦截），它在拦截前用于决断。
示例拦截器代码如下：

package com.cuisea.mybatis;

import org.apache.ibatis.executor.statement.StatementHandler;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.plugin.*;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.sql.Connection;
import java.util.Properties;

/**
 * Mybatis支持对Executor、StatementHandler、PameterHandler和ResultSetHandler 接口进行拦截，也就是说会对这4种对象进行代理
 * MyBatis插件 打印sql 
 * Created by cuisea on 2017/7/18.
 */
@Intercepts({ @Signature(type = StatementHandler.class, method = "prepare", args = { Connection.class, Integer.class}) })
public class SQLStatsInterceptor implements Interceptor{
    private final Logger logger = LoggerFactory.getLogger(this.getClass());

    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        StatementHandler statementHandler = (StatementHandler) invocation.getTarget();
        BoundSql boundSql = statementHandler.getBoundSql();
        String sql = boundSql.getSql();
        logger.info("mybatis intercept sql:{}", sql);
        return invocation.proceed();
    }

    @Override
    public Object plugin(Object o) {
        logger.info("mybatis intercept plugin "+o.getClass().getName());
        return Plugin.wrap(o, this);
    }

    @Override
    public void setProperties(Properties properties) {
        String dialect = properties.getProperty("dialect");
        logger.info("mybatis intercept dialect:{}", dialect);
    }
}

下面就要配置插件，有两种配置方式：
第一种：在MyBatis的配置文件mybatis-conf.xml的plugins节点下配置

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <plugins>
        <plugin interceptor="com.cuisea.mybatis.SQLStatsInterceptor">
            <property name="dialect" value="mysql"></property>
        </plugin>
    </plugins>
</configuration>

第二种：在spring配置文件spring-mybatis.xml中配置

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:tx="http://www.springframework.org/schema/tx"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans-4.0.xsd
       http://www.springframework.org/schema/context
       http://www.springframework.org/schema/context/spring-context-4.0.xsd
       http://www.springframework.org/schema/tx
       http://www.springframework.org/schema/tx/spring-tx-4.0.xsd
       http://www.springframework.org/schema/aop
       http://www.springframework.org/schema/aop/spring-aop-4.0.xsd">
    <!-- 扫描service包下所有使用注解的类型 -->
    <context:component-scan base-package="com.cuisea.service"/>

    <!-- 配置数据库相关参数properties的属性：${url} -->
    <context:property-placeholder location="classpath:jdbc.properties" ignore-resource-not-found="true"/>

    <!-- 数据库连接池c3p0-->
    <bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource">
        <property name="driverClass" value="${jdbc.driver}"/>
        <property name="jdbcUrl" value="${jdbc.url}"/>
        <property name="user" value="${jdbc.username}"/>
        <property name="password" value="${jdbc.password}"/>
        <property name="maxPoolSize" value="${c3p0.maxPoolSize}"/>
        <property name="minPoolSize" value="${c3p0.minPoolSize}"/>
        <property name="autoCommitOnClose" value="${c3p0.autoCommitOnClose}"/>
        <property name="checkoutTimeout" value="${c3p0.checkoutTimeout}"/>
        <property name="acquireRetryAttempts" value="${c3p0.acquireRetryAttempts}"/>
    </bean>

    <!-- 配置SqlSessionFactory对象 -->
    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <!-- 注入数据库连接池 -->
        <property name="dataSource" ref="dataSource"/>
        <property name="configLocation" value="classpath:mybatis-config.xml" />
        <!-- 扫描model包 使用别名,这样在mapper文件中可以省略包名；否则parameterType，resultType必须使用类的全限定名 -->
        <property name="typeAliasesPackage" value="com.cuisea.model"/>
        <!-- 扫描sql配置文件:mapper需要的xml文件 -->
        <property name="mapperLocations" value="classpath:mapper/*.xml"/>
        <property name="plugins">
            <array>
                <bean class="com.cuisea.mybatis.SQLStatsInterceptor">
                    <property name="properties">
                        <!-- config params as the following -->
                        <value>
                            dialect=mysql
                        </value>
                    </property>
                </bean>
            </array>
        </property>
    </bean>

    <!-- 配置扫描Dao接口包，动态实现Dao接口，注入到spring容器中 -->
    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
        <!-- 注入sqlSessionFactory -->
        <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"/>
        <!-- 给出需要扫描Dao接口包 -->
        <property name="basePackage" value="com.cuisea.dao"/>
    </bean>

    <!-- 配置事务管理器 -->
    <bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
        <!-- 注入数据库连接池 -->
        <property name="dataSource" ref="dataSource"/>
    </bean>

    <!-- 配置基于注解的声明式事务 -->
    <tx:annotation-driven transaction-manager="transactionManager"/>

</beans>

测试运行结果如下：

第一种配置方式通过org.apache.ibatis.builder.xml.XMLConfigBuilder解析xml文件将自定义plugin加载到org.apache.ibatis.session.Configuration实例的interceptorChain字段中

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);
        configuration.addInterceptor(interceptorInstance);
      }
    }
  }

第二种方式是在实例化org.mybatis.spring.SqlSessionFactoryBean的时候添加进去的，protected SqlSessionFactory buildSqlSessionFactory()的代码片段如下：

if (!isEmpty(this.plugins)) {
      for (Interceptor plugin : this.plugins) {
        configuration.addInterceptor(plugin);
        if (LOGGER.isDebugEnabled()) {
          LOGGER.debug("Registered plugin: '" + plugin + "'");
        }
      }
    }

参考资料：http://blog.youkuaiyun.com/top_code/article/details/55520948