MyBatis允许我们在其执行过程中对特定的一些方法进行拦截代理，实现一些特定的通用功能，如分页插件或者租户插件，允许进行拦截的类及对应方法基本如下
- Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)
- ParameterHandler (getParameterObject, setParameters)
- ResultSetHandler (handleResultSets, handleOutputParameters)
- StatementHandler (prepare, parameterize, batch, update, query)
这部分可以在源码中找到
// Configuration.java
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
// 对类进行代理拦截
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);
// 对类进行代理拦截
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);
// 对类进行代理拦截
statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
return statementHandler;
}
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : 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);
}
// 对类进行代理拦截
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}要想更好的使用MyBatis的插件功能，首先需要对拦截到的类和方法进行了解，以一次查询为例我们看一下相关流程
Executor: Executor是SqlSession中执行功能使用到的类，如查询时，首先从Configuration或获取MappedStatement（可以认为是对应的mapper.xml中的 SELECT节点信息），之后和具体的参数值一起传递给Executor的query方法使用
// Executor.java
<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;StatementHandler: Executor具体执行时，默认会使用StatementHandler的prepare对要执行的SQL创建预编译的Statement，之后使用parameterize设置参数
// SimpleExecutor.java
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
Connection connection = getConnection(statementLog);
// 预编译
stmt = handler.prepare(connection, transaction.getTimeout());
// 设置参数
handler.parameterize(stmt);
return stmt;
}在StatementHandler执行parameterize，默认会使用ParameterHandler#setParameters来对预编译结果继续进行参数设置
之后再调用StatementHandler#query方法来进行查询，最后使用ResultSetHandler#handleResultSets对查询结果进行处理
分析了流程之后，我们就可以根据自己的需求在指定方法上面进行拦截处理，如我们想要统一在数据库层面设置某一个字段值(如创建人信息)，那么就可以拦截Executor#update方法，对其中的参数进行修改赋值
下面我们通过例子来具体使用感受一下
首先创建一个表（这里是MySQL自带的表）
CREATE TABLE `employees` (
`emp_no` int NOT NULL,
`birth_date` date NOT NULL,
`first_name` varchar(14) NOT NULL,
`last_name` varchar(16) NOT NULL,
`gender` enum('M','F') NOT NULL,
`hire_date` date NOT NULL,
PRIMARY KEY (`emp_no`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;插入一些数据（MySQL自带的数据）
INSERT INTO `employees` (`emp_no`,`birth_date`,`first_name`,`last_name`,`gender`,`hire_date`) VALUES (10001,'1953-09-02','Georgi','Facello','M','1986-06-26');
INSERT INTO `employees` (`emp_no`,`birth_date`,`first_name`,`last_name`,`gender`,`hire_date`) VALUES (10002,'1964-06-02','Bezalel','Simmel','F','1985-11-21');
INSERT INTO `employees` (`emp_no`,`birth_date`,`first_name`,`last_name`,`gender`,`hire_date`) VALUES (10003,'1959-12-03','Parto','Bamford','M','1986-08-28');
INSERT INTO `employees` (`emp_no`,`birth_date`,`first_name`,`last_name`,`gender`,`hire_date`) VALUES (10004,'1954-05-01','Chirstian','Koblick','M','1986-12-01');
INSERT INTO `employees` (`emp_no`,`birth_date`,`first_name`,`last_name`,`gender`,`hire_date`) VALUES (10005,'1955-01-21','Kyoichi','Maliniak','M','1989-09-12');
INSERT INTO `employees` (`emp_no`,`birth_date`,`first_name`,`last_name`,`gender`,`hire_date`) VALUES (10006,'1953-04-20','Anneke','Preusig','F','1989-06-02');
INSERT INTO `employees` (`emp_no`,`birth_date`,`first_name`,`last_name`,`gender`,`hire_date`) VALUES (10007,'1957-05-23','Tzvetan','Zielinski','F','1989-02-10');
INSERT INTO `employees` (`emp_no`,`birth_date`,`first_name`,`last_name`,`gender`,`hire_date`) VALUES (10008,'1958-02-19','Saniya','Kalloufi','M','1994-09-15');
INSERT INTO `employees` (`emp_no`,`birth_date`,`first_name`,`last_name`,`gender`,`hire_date`) VALUES (10009,'1952-04-19','Sumant','Peac','F','1985-02-18');
INSERT INTO `employees` (`emp_no`,`birth_date`,`first_name`,`last_name`,`gender`,`hire_date`) VALUES (10010,'1963-06-01','Duangkaew','Piveteau','F','1989-08-24');
INSERT INTO `employees` (`emp_no`,`birth_date`,`first_name`,`last_name`,`gender`,`hire_date`) VALUES (10011,'1972-02-29','Jiang','David','M','1990-02-20');之后可以使用mybatis-generator生成表对应的类及相应的mapper文件
这里我们以一个最简单的分页插件功能为例，看一下如何实现使用一个插件
因为分页查询就是涉及查询，所以一般可以拦截Executor的query方法，因为有两个query方法，这里就统一都拦截了
// 需要添加拦截器注解
@Intercepts({
// 这里需要声明拦截的类，方法名称、参数信息
@Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
@Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class})
})
public class PageInterceptor implements Interceptor {
private static ThreadLocal<Page> pageThreadLocal = new ThreadLocal<>();
public static void page(Integer page, Integer size) {
final Page p = new Page();
p.setPage(page);
p.setSize(size);
pageThreadLocal.set(p);
}
@Override
public Object intercept(Invocation invocation) throws Throwable {
final Page page = pageThreadLocal.get();
if (page == null) {
return invocation.proceed();
}
try {
// 1. 通过 invocation 获取拦截到的方法相关信息
final Executor executor = (Executor) invocation.getTarget();
final Object[] args = invocation.getArgs();
// 2. 获取全部的参数(6个)
MappedStatement mappedStatement = (MappedStatement) args[0];
Object param = args[1];
RowBounds rowBounds = (RowBounds) args[2];
ResultHandler resultHandler = (ResultHandler) args[3];
CacheKey cacheKey = null;
BoundSql boundSql = null;
// 拦截的是有boundSql参数的方法
if (args.length == 6) {
cacheKey = (CacheKey) args[4];
boundSql = (BoundSql) args[5];
// 修改boundSql中的 sql 语句
changePageBoundSql(page, boundSql);
}
// 拦截的是无boundSql参数的方法
if (args.length == 4) {
boundSql = mappedStatement.getBoundSql(param);
// 修改boundSql中的 sql 语句(先修改sql后再创建cacheKey)
changePageBoundSql(page, boundSql);
cacheKey = executor.createCacheKey(mappedStatement, param, rowBounds, boundSql);
}
// 3. 调用原方法执行
return executor.query(mappedStatement, page, rowBounds, resultHandler, cacheKey, boundSql);
} finally {
pageThreadLocal.remove();
}
}
private void changePageBoundSql(Page page, BoundSql boundSql) {
final int offset = (page.getPage() - 1) * page.getSize();
// 修改SQL，添加分页参数
final String sql = boundSql.getSql();
String newSql = sql + " limit " + page.getSize() + " offset " + offset;
// 重新设置 BoundSql 实例中的 sql 属性值
final MetaObject metaObject = SystemMetaObject.forObject(boundSql);
metaObject.setValue("sql", newSql);
}
static class Page {
private Integer page;
private Integer size;
public Integer getPage() {
return page;
}
public void setPage(Integer page) {
this.page = page;
}
public Integer getSize() {
return size;
}
public void setSize(Integer size) {
this.size = size;
}
}
}之后在mybatis的配置文件中配置上此插件
<?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.github.zavier.mybatis.plugin.PageInterceptor" />
</plugins>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="com.mysql.cj.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/test"/>
<property name="username" value="root"/>
<property name="password" value="mysqlroot"/>
</dataSource>
</environment>
</environments>
<!-- 指定mapper配置文件 -->
<mappers>
<mapper resource="mapper/EmployeesMapper.xml" />
</mappers>
</configuration>这时候就可以使用了
@Test
public void test() {
PageInterceptor.page(1, 2);
final EmployeesExample example = new EmployeesExample();
final List<Employees> employees = employeesMapper.selectByExample(example);
Assert.assertEquals(2, employees.size());
}这时候可以看到执行的SQL日志
==> Preparing: select emp_no, birth_date, first_name, last_name, gender, hire_date from employees limit 2 offset 0
==> Parameters:
<== Total: 2
对应源代码：https://github.com/zavier/mybatis-plugin-example
这里只是一个示例，分页可以使用PageHelper，或者使用如果使用MyBatis-Plus的话，它也提供了很多的插件可以使用（分页插件、乐观锁插件、多租户插件等），这里我们主要就是了解一下原理，在有需要的时候可以开发自定义的插件