MyBatis的自定义插件

前置知识

MyBatis 可以拦截的四大组件

• Executor - 执行器
• StatementHandler - SQL 语句构造器
• ParameterHandler - 参数处理器
• ResultSetHandler - 结果集处理器

自定义 MyBatis 插件

/*** 打印 sql 执行的时间插件*/
@Intercepts(// 指定拦截器拦截的对象、方法和参数类型{@Signature(type = StatementHandler.class, method = "update", args = {Statement.class}),@Signature(type = StatementHandler.class, method = "query", args = {Statement.class, ResultHandler.class}),@Signature(type = StatementHandler.class, method = "batch", args = {Statement.class})}
)
// 注册到 Spring 容器，不是 Spring 环境的话可以用 mybatis 的 config 配置进去
@Component
public class SqlExecuteTimePrintMybatisPlugin implements Interceptor {protected Logger logger = LoggerFactory.getLogger(SqlExecuteTimePrintMybatisPlugin.class);@Overridepublic Object intercept(Invocation invocation) throws Throwable {// 获取代理对象StatementHandler statementHandler = (StatementHandler) invocation.getTarget();// 获取执行 sqlBoundSql boundSql = statementHandler.getBoundSql();// 此处简单处理一下，只打印参数替换前的 sql，目的是演示自定义插件String sql = boundSql.getSql();long start = System.currentTimeMillis();try {return invocation.proceed();} finally {logger.info("sql -> {}, takes time -> {}", sql, System.currentTimeMillis() - start);}}
}

效果如下

2023-10-14 17:18:39.297  INFO 25972 --- [p-nio-80-exec-1] c.y.m.c.SqlExecuteTimePrintMybatisPlugin : sql -> SELECT * FROM `INFO` WHERE `id` = ? , takes time -> 57
2023-10-14 17:18:39.324  INFO 25972 --- [p-nio-80-exec-1] c.y.m.c.SqlExecuteTimePrintMybatisPlugin : sql -> SELECT `id`, `info_id`, `extend_info` FROM `INFO_DETAIL` WHERE `info_id` = ?, takes time -> 4

源码解析

创建四大对象的代码如下

public class Configuration {public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);// 此处增加拦截器责任链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);// 此处增加拦截器责任链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);// 此处增加拦截器责任链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;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);}// 此处增加拦截器责任链executor = (Executor) interceptorChain.pluginAll(executor);return executor;}
}

1. 首先在创建 Executor、StatementHandler、ParameterHandler、ResultSetHandler 四个对象时，将插件（plugins）注入

2. 调用 InterceptorChain.pluginAll() 方法将插件增加到责任链，并返回代理后的 target 包装对象，InterceptorChain 保存了所有的拦截器（Interceptors）

3. 最终在执行的时候调用的其实是 JDK 动态代理的对象，执行 MyBatis 中 InvocationHandler 的实现 org.apache.ibatis.plugin.Plugin 的 invoke 方法

public class InterceptorChain {private final List<Interceptor> interceptors = new ArrayList<>();public Object pluginAll(Object target) {for (Interceptor interceptor : interceptors) {target = interceptor.plugin(target);}return target;}
}

public interface Interceptor {// 拦截器增强方法Object intercept(Invocation invocation) throws Throwable;// 包装原来的对象default Object plugin(Object target) {return Plugin.wrap(target, this);}default void setProperties(Properties properties) {// NOP}}

public class Plugin implements InvocationHandler {private final Object target;private final Interceptor interceptor;private final Map<Class<?>, Set<Method>> signatureMap;private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {this.target = target;this.interceptor = interceptor;this.signatureMap = signatureMap;}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) {// jdk 动态代理return Proxy.newProxyInstance(type.getClassLoader(),interfaces,new Plugin(target, interceptor, signatureMap));}return target;}@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {// 获取插件中生命要增强的方法Set<Method> methods = signatureMap.get(method.getDeclaringClass());// 如果命中该方法，就使用执行插件中增强的方法if (methods != null && methods.contains(method)) {return interceptor.intercept(new Invocation(target, method, args));}// 没有命中就不对方法进行增强return method.invoke(target, args);} catch (Exception e) {throw ExceptionUtil.unwrapThrowable(e);}}...
}

备注

不明白的需要去看下 JDK 动态代理实现原理，概括的来讲就是 Proxy#newProxyInstance 时，通过字节码增强的方法，生成一个实现了跟被代理类相同接口并继承了 java.lang.reflect.Proxy 的类并返回其实例，调用这个代理类的方法时，实际上调用的是 Proxy.InvocationHandler.invoke(this, method, new Object[]{args}) 方法