上一篇文章介绍了 MyBatis 执行 SQL 查询的流程，对源码中的关键类如 Configuration、Executor、StatementHandler 有了一定认识之后，本篇聊聊 MyBatis 的插件机制。
1. 简介
MyBatis 插件，简单理解为拦截器，它采用动态代理的方式，实现对目标方法的拦截，在前后做一些操作。
基于插件机制，基本上可以控制 SQL 执行的各个阶段，如执行阶段，参数处理阶段，语法构建阶段，结果集处理阶段，具体可以根据项目业务来实现对应业务逻辑。
支持拦截的方法：
- 执行器 Executor：update、query、commit、rollback 等方法；
- 参数处理器 ParameterHandler：getParameterObject、setParameters 等方法；
- 结果集处理器 ResultSetHandler：handleResultSets、handleOutputParameters 等方；
- SQL语法构建器 StatementHandler：prepare、parameterize、batch、update、query 等方法；
2. 插件示例
MyBatis XML 配置文件中定义分页插件。
自定义分页插件 PageInterceptor 代码如下，用于拦截 Executor#query 方法，修改 MappedStatement 对象中的 SQL 语句。
package com.sumkor.plugin;
import com.sumkor.plugin.page.BoundSqlSqlSource;
import com.sumkor.plugin.page.Page;
import com.sumkor.plugin.page.PageUtil;
import lombok.extern.slf4j.Slf4j;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.plugin.*;
import org.apache.ibatis.reflection.DefaultReflectorFactory;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.reflection.factory.DefaultObjectFactory;
import org.apache.ibatis.reflection.wrapper.DefaultObjectWrapperFactory;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import java.util.Properties;
import java.util.StringJoiner;
/**
* 拦截 Executor#query 方法
*
* @author Sumkor
* @since 2021/7/26
*/
@Intercepts({
@Signature(
type = Executor.class,
method = "query",
args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}
)
})
@Slf4j
public class PageInterceptor implements Interceptor {
private static final int MAPPED_STATEMENT_INDEX = 0;
private static final int PARAMETER_INDEX = 1;
private static final int ROW_BOUNDS_INDEX = 2;
/**
* 通过反射工具类 MetaObject 来修改 MappedStatement 对象中的 SQL 语句
*/
@Override
public Object intercept(Invocation invocation) throws Throwable {
log.info("------------------PageInterceptor#intercept 开始------------------");
final Object[] queryArgs = invocation.getArgs();
final MappedStatement ms = (MappedStatement) queryArgs[MAPPED_STATEMENT_INDEX];
final Object parameter = queryArgs[PARAMETER_INDEX];
// 获取分页参数
Page pagingParam = PageUtil.getPagingParam();
try {
if (pagingParam != null) {
// 构造新的分页查询 SQL 字符串
final BoundSql boundSql = ms.getBoundSql(parameter);
String pagingSql = getPagingSql(boundSql.getSql(), pagingParam.getOffset(), pagingParam.getLimit());
BoundSql newBoundSql = new BoundSql(ms.getConfiguration(), pagingSql, boundSql.getParameterMappings(), boundSql.getParameterObject());
// 通过反射工具类，重置 MappedStatement 中的 SQL 语句
// MetaObject metaObject = MetaObject.forObject(ms, new DefaultObjectFactory(), new DefaultObjectWrapperFactory(), new DefaultReflectorFactory());
MetaObject metaObject = SystemMetaObject.forObject(ms);
metaObject.setValue("sqlSource", new BoundSqlSqlSource(newBoundSql));
// 重置 RowBound
queryArgs[ROW_BOUNDS_INDEX] = new RowBounds(RowBounds.NO_ROW_OFFSET, RowBounds.NO_ROW_LIMIT);
}
} catch (Exception e) {
log.error("PageInterceptor#intercept 异常", e);
} finally {
log.info("------------------PageInterceptor#intercept 结束------------------");
PageUtil.removePagingParam();
}
return invocation.proceed();
}
/**
* 使得当前插件生效
*/
@Override
public Object plugin(Object o) {
return Plugin.wrap(o, this);
}
/**
* 构造新的 sql： select xxx from xxx where yyy limit offset,limit
*/
public String getPagingSql(String sql, int offset, int limit) {
StringBuilder result = new StringBuilder(sql.length() + 100);
result.append(sql).append(" limit ");
if (offset > 0) {
result.append(offset).append(",").append(limit);
}else{
result.append(limit);
}
return result.toString();
}
}通过反射工具类 MetaObject 来修改 MappedStatement 对象中的 SQL 语句，加上 limit m,n 的分页条件。
3. 源码分析
3.1 解析插件配置
通过 SqlSessionFactoryBuilder 来解析 mybatis-config.xml 文件：
Reader reader = Resources.getResourceAsReader("mybatis-config.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);其中会解析 plugins 标签：
org.apache.ibatis.builder.xml.XMLConfigBuilder#parseConfiguration
org.apache.ibatis.builder.xml.XMLConfigBuilder#pluginElement
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).getDeclaredConstructor().newInstance();
interceptorInstance.setProperties(properties);
configuration.addInterceptor(interceptorInstance);
}
}
}这里会将 Interceptor 的实现类进行实例化，并注册到 Configuration 对象之中的 InterceptorChain 成员变量之中。
org.apache.ibatis.session.Configuration#addInterceptor
protected final InterceptorChain interceptorChain = new InterceptorChain();
public void addInterceptor(Interceptor interceptor) {
interceptorChain.addInterceptor(interceptor);
}InterceptorChain 类完整内容如下，其中有个 List 集合存储注册给 Configuration 对象的插件实例。
org.apache.ibatis.plugin.InterceptorChain
public class InterceptorChain {
private final List interceptors = new ArrayList<>();
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}
public void addInterceptor(Interceptor interceptor) {
interceptors.add(interceptor);
}
public List getInterceptors() {
return Collections.unmodifiableList(interceptors);
}
} 3.2 插件实现机制
MyBatis 中的 Executor、ParameterHandler、ResultSetHandler、StatementHandler 类均支持插件扩展，而插件的实现机制主要是基于动态代理实现的。
由 Configuration 对象统一管理这些对象的生成。
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
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);
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);
statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
return statementHandler;
}
public Executor newExecutor(Transaction transaction) {
return newExecutor(transaction, defaultExecutorType);
}
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
// 该类型的执行器会批量执行所有更新语句，如果 SELECT 在多个更新中间执行，将在必要时将多条更新语句分隔开来，以方便理解。
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
}
// 该类型的执行器会复用预处理语句。
else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
}
// 该类型的执行器没有特别的行为。它为每个语句的执行创建一个新的预处理语句。
else {
executor = new SimpleExecutor(this, transaction);
}
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
// 使用插件来包装 executor
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}3.2.1 原始对象的生成
探究插件的动态代理机制之前，回顾一下需要被代理的原始对象的生成流程：
Executor
开启 SqlSession 会话的时候创建：
org.apache.ibatis.session.defaults.DefaultSqlSessionFactory#openSession
org.apache.ibatis.session.defaults.DefaultSqlSessionFactory#openSessionFromDataSource
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
final Executor executor = configuration.newExecutor(tx, execType); // 创建
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}StatementHandler
SqlSession#selectList 执行 SQL 的时候创建：
org.apache.ibatis.session.defaults.DefaultSqlSession#selectList
org.apache.ibatis.executor.CachingExecutor#query
org.apache.ibatis.executor.BaseExecutor#query
org.apache.ibatis.executor.BaseExecutor#queryFromDatabase
org.apache.ibatis.executor.SimpleExecutor#doQuery
@Override
public List 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); // 创建
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
} ParameterHandler、ResultSetHandler
在 StatementHandler 的构造函数中创建：
org.apache.ibatis.session.Configuration#newStatementHandler
org.apache.ibatis.executor.statement.RoutingStatementHandler#RoutingStatementHandler
org.apache.ibatis.executor.statement.PreparedStatementHandler#PreparedStatementHandler
org.apache.ibatis.executor.statement.BaseStatementHandler#BaseStatementHandler
protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
this.configuration = mappedStatement.getConfiguration();
this.executor = executor;
this.mappedStatement = mappedStatement;
this.rowBounds = rowBounds;
this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
this.objectFactory = configuration.getObjectFactory();
if (boundSql == null) { // issue #435, get the key before calculating the statement
generateKeys(parameterObject);
boundSql = mappedStatement.getBoundSql(parameterObject);
}
this.boundSql = boundSql;
this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql); // 创建 ParameterHandler，支持插件扩展
this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql); // 创建 ResultSetHandler，支持插件扩展
}3.2.2 代理对象的生成
在 Configuration 类中创建 Executor、ParameterHandler、ResultSetHandler、StatementHandler 对象的时候，都会调用 InterceptorChain#pluginAll 方法来进行插件扩展。
org.apache.ibatis.plugin.InterceptorChain#pluginAll
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {
target = interceptor.plugin(target);
}
return target;
}Interceptor 接口中的 plugin 方法默认实现如下：
org.apache.ibatis.plugin.Interceptor#plugin
default Object plugin(Object target) {
return Plugin.wrap(target, this);
}Plugin#wrap 方法的两个入参：
- target 表示 Executor、ParameterHandler、ResultSetHandler、StatementHandler 这些原始对象。
- interceptor 是自定义的插件类。
代码流程：
- 解析 Interceptor 插件类上的 @Intercepts @Signature 注解。
- 根据注解，判断是否是对 target 原始对象的拦截，是则为 target 原始对象生成代理对象。
org.apache.ibatis.plugin.Plugin#wrap
public static Object wrap(Object target, Interceptor interceptor) {
Map, Set> signatureMap = getSignatureMap(interceptor); // 解析插件类上的 @Intercepts @Signature 注解
Class type = target.getClass();
Class[] interfaces = getAllInterfaces(type, signatureMap);
if (interfaces.length > 0) { // 满足条件的，说明 target 类需要经过插件 interceptor 来拦截，因此为 target 生成代理
return Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap)); // 返回代理对象
}
return target; // 返回原始对象
} 3.2.3 代理对象的使用
以上述的自定义分页插件为例，com.sumkor.plugin.PageInterceptor 类注解声明了对 Executor#query 方法的拦截，是对 Executor 对象生成动态代理。
org.apache.ibatis.session.defaults.DefaultSqlSession#selectList
因此，对 Executor#query 方法的调用，实际是执行代理对象的方法：
org.apache.ibatis.plugin.Plugin#invoke
public class Plugin implements InvocationHandler {
private final Object target;
private final Interceptor interceptor;
private final Map, Set> signatureMap;
private Plugin(Object target, Interceptor interceptor, Map, Set> signatureMap) {
this.target = target;
this.interceptor = interceptor;
this.signatureMap = signatureMap;
}
@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)) { // 如果插件配置类，声明了对指定的方法的拦截，则符合这里的条件
return interceptor.intercept(new Invocation(target, method, args)); // 将请求转发给插件方法
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
} 再进一步调用到自定义的插件类的方法：
com.sumkor.plugin.PageInterceptor#intercept
4. 总结
MyBatis 插件对 Executor、ParameterHandler、ResultSetHandler、StatementHandler 这四个接口上的方法进行拦截，利用 JDK 动态代理机制，为这些接口的实现类创建代理对象。
在执行方法时，先去执行代理对象的方法，从而执行自己编写的拦截逻辑。
值得注意的是，Executor、ParameterHandler、ResultSetHandler、StatementHandler 这些对象的生命周期都是 Session 范围的，每一次开启 SqlSession 会话都会创建新的代理对象。
作者：Sumkor
链接：https://segmentfault.com/a/11...