一、拦截对象和接口实现示例
MyBatis拦截器的作用是在于Dao到DB中间进行额外的处理。大部分情况下通过mybatis的xml配置sql都可以达到想要的DB操作效果，然而存在一些类似或者相同的查询条件或者查询要求，这些可以通过拦截器的实现可以提升开发效率，比如：分页、插入和更新时间/人、数据权限、SQL监控日志等。
Mybatis支持四种对象拦截Executor、StatementHandler、PameterHandler和ResultSetHandler
Executor：拦截执行器的方法。
StatementHandler：拦截Sql语法构建的处理。
ParameterHandler：拦截参数的处理。
ResultHandler：拦截结果集的处理。
1 public interface Executor { 2 ResultHandler NO_RESULT_HANDLER = null; 3 int update(MappedStatement var1, Object var2) throws SQLException; 4 <E> List<E> query(MappedStatement var1, Object var2, RowBounds var3, ResultHandler var4, CacheKey var5, BoundSql var6) throws SQLException; 5 <E> List<E> query(MappedStatement var1, Object var2, RowBounds var3, ResultHandler var4) throws SQLException; 6 <E> Cursor<E> queryCursor(MappedStatement var1, Object var2, RowBounds var3) throws SQLException; 7 List<BatchResult> flushStatements() throws SQLException; 8 void commit(boolean var1) throws SQLException; 9 void rollback(boolean var1) throws SQLException; 10 CacheKey createCacheKey(MappedStatement var1, Object var2, RowBounds var3, BoundSql var4); 11 boolean isCached(MappedStatement var1, CacheKey var2); 12 void clearLocalCache(); 13 void deferLoad(MappedStatement var1, MetaObject var2, String var3, CacheKey var4, Class<?> var5); 14 Transaction getTransaction(); 15 void close(boolean var1); 16 boolean isClosed(); 17 void setExecutorWrapper(Executor var1); 18 } 19 public interface StatementHandler { 20 Statement prepare(Connection var1, Integer var2) throws SQLException; 21 void parameterize(Statement var1) throws SQLException; 22 void batch(Statement var1) throws SQLException; 23 int update(Statement var1) throws SQLException; 24 <E> List<E> query(Statement var1, ResultHandler var2) throws SQLException; 25 <E> Cursor<E> queryCursor(Statement var1) throws SQLException; 26 BoundSql getBoundSql(); 27 ParameterHandler getParameterHandler(); 28 } 29 public interface ParameterHandler { 30 Object getParameterObject(); 31 void setParameters(PreparedStatement var1) throws SQLException; 32 } 33 public interface ResultHandler<T> { 34 void handleResult(ResultContext<? extends T> var1); 35 }
拦截的执行顺序是Executor->StatementHandler->ParameterHandler->ResultHandler
MyBatis提供的拦截器接口：
1 public interface Interceptor { 2 Object intercept(Invocation var1) throws Throwable; 3 default Object plugin(Object target) { 4 return Plugin.wrap(target, this); 5 } 6 default void setProperties(Properties properties) {} 7 }
Object intercept方法用于拦截器的实现；
Object plugin方法用于判断执行拦截器的类型；
void setProperties方法用于获取配置项的属性。
拦截对象和拦截器接口的结合，自定义的拦截器类需要实现拦截器接口，并通过注解@Intercepts和参数@Signature来声明要拦截的对象。
@Signature参数type是拦截对象，method是拦截的方法，即上面的四个类对应的方法，args是拦截方法对应的参数（方法存在重载因此需要指明参数个数和类型）
@Intercepts可以有多个@Signature，即一个拦截器实现类可以同时拦截多个对象及方法，示例如下：
Executor->intercept
StatementHandler->intercept
ParameterHandler->intercept
ResultHandler->intercept
1 @Intercepts({ 2 @Signature( 3 type = Executor.class, 4 method = "query", 5 args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class} 6 ) 7 }) 8 public class SelectPlugin implements Interceptor { 9 @Override 10 public Object intercept(Invocation invocation) throws Throwable { 11 if (invocation.getTarget() instanceof Executor) { 12 System.out.println("SelectPlugin"); 13 } 14 return invocation.proceed(); 15 } 16 @Override 17 public Object plugin(Object target) { 18 if (target instanceof Executor) { 19 return Plugin.wrap(target, this); 20 } 21 return target; 22 } 23 @Override 24 public void setProperties(Properties properties) {} 25 } 26 @Intercepts({@Signature(type = StatementHandler.class, method = "prepare", args = {Connection.class, Integer.class})}) 27 public class StatementPlugin implements Interceptor { 28 @Override 29 public Object intercept(Invocation invocation) throws Throwable { 30 if (invocation.getTarget() instanceof StatementHandler) { 31 System.out.println("StatementPlugin"); 32 } 33 return invocation.proceed(); 34 } 35 @Override 36 public Object plugin(Object target) { 37 if (target instanceof StatementHandler) { 38 return Plugin.wrap(target, this); 39 } 40 return target; 41 } 42 @Override 43 public void setProperties(Properties properties) {} 44 } 45 @Intercepts({@Signature(type = ParameterHandler.class,method = "setParameters",args = {PreparedStatement.class})}) 46 public class ParameterPlugin implements Interceptor { 47 @Override 48 public Object intercept(Invocation invocation) throws Throwable { 49 if (invocation.getTarget() instanceof ParameterHandler) { 50 System.out.println("ParameterPlugin"); 51 } 52 return invocation.proceed(); 53 } 54 @Override 55 public Object plugin(Object target) { 56 if (target instanceof ParameterHandler) { 57 return Plugin.wrap(target, this); 58 } 59 return target; 60 } 61 @Override 62 public void setProperties(Properties properties) {} 63 } 64 @Intercepts({@Signature(type = ResultHandler.class,method = "handleResult",args = {ResultContext.class})}) 65 public class ResultPlugin implements Interceptor { 66 @Override 67 public Object intercept(Invocation invocation) throws Throwable { 68 if (invocation.getTarget() instanceof ResultHandler) { 69 System.out.println("ResultPlugin"); 70 } 71 return invocation.proceed(); 72 } 73 @Override 74 public Object plugin(Object target) { 75 if (target instanceof ResultHandler) { 76 return Plugin.wrap(target, this); 77 } 78 return target; 79 } 80 @Override 81 public void setProperties(Properties properties) {} 82 }
二、拦截器注册的三种方式
前面介绍了Mybatis的拦截对象及其接口的实现方式，那么在项目中如何注册拦截器呢？本文中给出三种注册方式。
1.XML注册
xml注册是最基本的方式，是通过在Mybatis配置文件中plugins元素来进行注册的。一个plugin对应着一个拦截器，在plugin元素可以指定property子元素，在注册定义拦截器时把对应拦截器的所有property通过Interceptor的setProperties方法注入给拦截器。因此拦截器注册xml方式如下：
1 <?xml version="1.0" encoding="UTF-8" ?> 2 <!DOCTYPE configuration 3 PUBLIC "-//mybatis.org//DTD Config 3.0//EN" 4 "http://mybatis.org/dtd/mybatis-3-config.dtd"> 5 <configuration> 6 <!-- ...... --> 7 <plugins> 8 <plugin interceptor="com.tiantian.mybatis.interceptor.MyInterceptor"> 9 <property name="prop1" value="prop1"/> 10 <property name="prop2" value="prop2"/> 11 </plugin> 12 </plugins> 13 <!-- ...... --> 14 </configuration>
2.配置类注册
配置类注册是指通过Mybatis的配置类中声明注册拦截器，配置类注册也可以通过Properties类给Interceptor的setProperties方法注入参数。具体参考如下：
1 @Configuration 2 public class MyBatisConfig { 3 @Bean 4 public String MyBatisInterceptor(SqlSessionFactory sqlSessionFactory) { 5 UpdatePlugin executorInterceptor = new UpdatePlugin(); 6 Properties properties = new Properties(); 7 properties.setProperty("prop1", "value1"); 8 // 给拦截器添加自定义参数 9 executorInterceptor.setProperties(properties); 10 sqlSessionFactory.getConfiguration().addInterceptor(executorInterceptor); 11 sqlSessionFactory.getConfiguration().addInterceptor(new StatementPlugin()); 12 sqlSessionFactory.getConfiguration().addInterceptor(new ResultPlugin()); 13 sqlSessionFactory.getConfiguration().addInterceptor(new ParameterPlugin()); 14 // sqlSessionFactory.getConfiguration().addInterceptor(new SelectPlugin()); 15 return "interceptor"; 16 } 17 18 // 与sqlSessionFactory.getConfiguration().addInterceptor(new SelectPlugin());效果一致 19 @Bean 20 public SelectPlugin SelectInterceptor() { 21 SelectPlugin interceptor = new SelectPlugin(); 22 Properties properties = new Properties(); 23 // 调用properties.setProperty方法给拦截器设置自定义参数 24 interceptor.setProperties(properties); 25 return interceptor; 26 } 27 }
3.注解方式
通过@Component注解方式是最简单的方式，在不需要转递自定义参数时可以使用，方便快捷。
@Component @Intercepts({ @Signature( type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class} ) }) public class SelectPlugin implements Interceptor { @Override public Object intercept(Invocation invocation) throws Throwable { if (invocation.getTarget() instanceof Executor) { System.out.println("SelectPlugin"); } return invocation.proceed(); } @Override public Object plugin(Object target) { if (target instanceof Executor) { return Plugin.wrap(target, this); } return target; } @Override public void setProperties(Properties properties) { } }
三、ParameterHandler参数改写-修改时间和修改人统一插入
针对具体的拦截器实现进行描述。日常编码需求中会碰到修改时需要插入修改的时间和人员，如果要用xml的方式去写非常麻烦，而通过拦截器的方式可以快速实现全局的插入修改时间和人员。先看代码：
1 @Component 2 @Intercepts({ 3 @Signature(type = ParameterHandler.class, method = "setParameters", args = {PreparedStatement.class}), 4 }) 5 public class MyBatisInterceptor implements Interceptor { 6 @Override 7 public Object intercept(Invocation invocation) throws Throwable { 8 // 参数代理 9 if (invocation.getTarget() instanceof ParameterHandler) { 10 System.out.println("ParameterHandler"); 11 // 自动添加操作员信息 12 autoAddOperatorInfo(invocation); 13 } 14 return invocation.proceed(); 15 } 16 17 @Override 18 public Object plugin(Object target) { 19 return Plugin.wrap(target, this); 20 } 21 22 @Override 23 public void setProperties(Properties properties) { 24 25 } 26 27 /** 28 * 自动添加操作员信息 29 * 30 * @param invocation 代理对象 31 * @throws Throwable 异常 32 */ 33 private void autoAddOperatorInfo(Invocation invocation) throws Throwable { 34 System.out.println("autoInsertCreatorInfo"); 35 // 获取代理的参数对象ParameterHandler 36 ParameterHandler ph = (ParameterHandler) invocation.getTarget(); 37 // 通过MetaObject获取ParameterHandler的反射内容 38 MetaObject metaObject = MetaObject.forObject(ph, 39 SystemMetaObject.DEFAULT_OBJECT_FACTORY, 40 SystemMetaObject.DEFAULT_OBJECT_WRAPPER_FACTORY, 41 new DefaultReflectorFactory()); 42 // 通过MetaObject反射的内容获取MappedStatement 43 MappedStatement mappedStatement = (MappedStatement) metaObject.getValue("mappedStatement"); 44 // 当sql类型为INSERT或UPDATE时，自动插入操作员信息 45 if (mappedStatement.getSqlCommandType() == SqlCommandType.INSERT || 46 mappedStatement.getSqlCommandType() == SqlCommandType.UPDATE) { 47 // 获取参数对象 48 Object obj = ph.getParameterObject(); 49 if (null != obj) { 50 // 通过反射获取参数对象的属性 51 Field[] fields = obj.getClass().getDeclaredFields(); 52 // 遍历参数对象的属性 53 for (Field f : fields) { 54 // 如果sql是INSERT,且存在createdAt属性 55 if ("createdAt".equals(f.getName()) && mappedStatement.getSqlCommandType() == SqlCommandType.INSERT) { 56 // 设置允许访问反射属性 57 f.setAccessible(true); 58 // 如果没有设置createdAt属性，则自动为createdAt属性添加当前的时间 59 if (null == f.get(obj)) { 60 // 设置createdAt属性为当前时间 61 f.set(obj, LocalDateTime.now()); 62 } 63 } 64 // 如果sql是INSERT,且存在createdBy属性 65 if ("createdBy".equals(f.getName()) && mappedStatement.getSqlCommandType() == SqlCommandType.INSERT) { 66 // 设置允许访问反射属性 67 f.setAccessible(true); 68 // 如果没有设置createdBy属性，则自动为createdBy属性添加当前登录的人员 69 if (null == f.get(obj)) { 70 // 设置createdBy属性为当前登录的人员 71 f.set(obj, 0); 72 } 73 } 74 // sql为INSERT或UPDATE时均需要设置updatedAt属性 75 if ("updatedAt".equals(f.getName())) { 76 f.setAccessible(true); 77 if (null == f.get(obj)) { 78 f.set(obj, LocalDateTime.now()); 79 } 80 } 81 // sql为INSERT或UPDATE时均需要设置updatedBy属性 82 if ("updatedBy".equals(f.getName())) { 83 f.setAccessible(true); 84 if (null == f.get(obj)) { 85 f.set(obj, 0); 86 } 87 } 88 } 89 90 // 通过反射获取ParameterHandler的parameterObject属性 91 Field parameterObject = ph.getClass().getDeclaredField("parameterObject"); 92 // 设置允许访问parameterObject属性 93 parameterObject.setAccessible(true); 94 // 将上面设置的新参数对象设置到ParameterHandler的parameterObject属性 95 parameterObject.set(ph, obj); 96 } 97 } 98 } 99 }
拦截器的接口实现参考前文，这里着重介绍autoAddOperatorInfo方法里的相关类。
1.ParameterHandler
接口源码：
1 public interface ParameterHandler { 2 Object getParameterObject(); 3 void setParameters(PreparedStatement var1) throws SQLException; 4 }
提供两个方法：
getParameterObject是获取参数对象，可能存在null，需要注意null指针。
setParameters是控制如何设置SQL参数，即sql语句中配置的java对象和jdbc类型对应的关系，例如#{id,jdbcType=INTEGER}，id默认类型是javaType=class java.lang.Integer。
该接口有一个默认的实现类，源码如下：
1 public class DefaultParameterHandler implements ParameterHandler { 2 private final TypeHandlerRegistry typeHandlerRegistry; 3 private final MappedStatement mappedStatement; 4 private final Object parameterObject; 5 private final BoundSql boundSql; 6 private final Configuration configuration; 7 8 public DefaultParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) { 9 this.mappedStatement = mappedStatement; 10 this.configuration = mappedStatement.getConfiguration(); 11 this.typeHandlerRegistry = mappedStatement.getConfiguration().getTypeHandlerRegistry(); 12 this.parameterObject = parameterObject; 13 this.boundSql = boundSql; 14 } 15 16 public Object getParameterObject() { 17 return this.parameterObject; 18 } 19 20 public void setParameters(PreparedStatement ps) { 21 ErrorContext.instance().activity("setting parameters").object(this.mappedStatement.getParameterMap().getId()); 22 List<ParameterMapping> parameterMappings = this.boundSql.getParameterMappings(); 23 if (parameterMappings != null) { 24 for(int i = 0; i < parameterMappings.size(); ++i) { 25 ParameterMapping parameterMapping = (ParameterMapping)parameterMappings.get(i); 26 if (parameterMapping.getMode() != ParameterMode.OUT) { 27 String propertyName = parameterMapping.getProperty(); 28 Object value; 29 if (this.boundSql.hasAdditionalParameter(propertyName)) { 30 value = this.boundSql.getAdditionalParameter(propertyName); 31 } else if (this.parameterObject == null) { 32 value = null; 33 } else if (this.typeHandlerRegistry.hasTypeHandler(this.parameterObject.getClass())) { 34 value = this.parameterObject; 35 } else { 36 MetaObject metaObject = this.configuration.newMetaObject(this.parameterObject); 37 value = metaObject.getValue(propertyName); 38 } 39 40 TypeHandler typeHandler = parameterMapping.getTypeHandler(); 41 JdbcType jdbcType = parameterMapping.getJdbcType(); 42 if (value == null && jdbcType == null) { 43 jdbcType = this.configuration.getJdbcTypeForNull(); 44 } 45 46 try { 47 typeHandler.setParameter(ps, i + 1, value, jdbcType); 48 } catch (SQLException | TypeException var10) { 49 throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + var10, var10); 50 } 51 } 52 } 53 } 54 55 } 56 }
通过DefaultParameterHandler实现类我们知道通过ParameterHandler可以获取到哪些属性和方法，其中包括我们下面一个重要的类MappedStatement。
2.MappedStatement
MyBatis的mapper文件中的每个select/update/insert/delete标签会被解析器解析成一个对应的MappedStatement对象，也就是一个MappedStatement对象描述一条SQL语句。MappedStatement对象属性如下：
1 // mapper配置文件名 2 private String resource; 3 // mybatis的全局信息，如jdbc 4 private Configuration configuration; 5 // 节点的id属性加命名空间,如：com.example.mybatis.dao.UserMapper.selectByExample 6 private String id; 7 private Integer fetchSize; 8 private Integer timeout; 9 private StatementType statementType; 10 private ResultSetType resultSetType; 11 private SqlSource sqlSource; 12 private Cache cache; 13 private ParameterMap parameterMap; 14 private List<ResultMap> resultMaps; 15 private boolean flushCacheRequired; 16 private boolean useCache; 17 private boolean resultOrdered; 18 // sql语句的类型：select、update、delete、insert 19 private SqlCommandType sqlCommandType; 20 private KeyGenerator keyGenerator; 21 private String[] keyProperties; 22 private String[] keyColumns; 23 private boolean hasNestedResultMaps; 24 private String databaseId; 25 private Log statementLog; 26 private LanguageDriver lang; 27 private String[] resultSets;
在本例中通过MappedStatement对象的sqlCommandType来判断当前的sql类型是insert、update来进行下一步的操作。
四、通过StatementHandler改写SQL
StatementHandler是用于封装JDBC Statement操作，负责对JDBC Statement的操作，如设置参数，并将Statement结果集转换成List集合。
实现代码如下：
删除注解标记
@Target({ElementType.METHOD}) //表示注解的使用范围
@Retention(RetentionPolicy.RUNTIME) //注解的保存时间
@Documented //文档显示
public @interface DeletedAt {
boolean has() default true;
}Dao层添加删除注解，为false时不添加删除标志
1 @Mapper 2 public interface AdminProjectDao { 3 @DeletedAt(has = false) 4 List<AdminProjectPo> selectProjects(AdminProjectPo po); 5 }
拦截器通过删除注解标记判断是否添加删除标志
1 @Component 2 @Intercepts({ 3 @Signature(type = StatementHandler.class, method = "prepare", args = {Connection.class, Integer.class}), 4 }) 5 public class MyBatisInterceptor implements Interceptor { 6 @Override 7 public Object intercept(Invocation invocation) throws Throwable { 8 if (invocation.getTarget() instanceof StatementHandler) { 9 System.out.println("StatementHandler"); 10 checkHasDeletedAtField(invocation); 11 } 12 return invocation.proceed(); 13 } 14 15 @Override 16 public Object plugin(Object target) { 17 return Plugin.wrap(target, this); 18 } 19 20 @Override 21 public void setProperties(Properties properties) { 22 23 } 24 25 /** 26 * 检查查询是否需要添加删除标志字段 27 * 28 * @param invocation 代理对象 29 * @throws Throwable 异常 30 */ 31 private void checkHasDeletedAtField(Invocation invocation) throws Throwable { 32 System.out.println("checkHasDeletedAtField"); 33 StatementHandler statementHandler = (StatementHandler) invocation.getTarget(); 34 // 通过MetaObject访问对象的属性 35 MetaObject metaObject = MetaObject.forObject( 36 statementHandler, 37 SystemMetaObject.DEFAULT_OBJECT_FACTORY, 38 SystemMetaObject.DEFAULT_OBJECT_WRAPPER_FACTORY, 39 new DefaultReflectorFactory()); 40 // 获取成员变量mappedStatement 41 MappedStatement mappedStatement = (MappedStatement) metaObject.getValue("delegate.mappedStatement"); 42 // 如果sql类型是查询 43 if (mappedStatement.getSqlCommandType() == SqlCommandType.SELECT) { 44 // 获取删除注解标志 45 DeletedAt annotation = null; 46 String id = mappedStatement.getId(); 47 String className = id.substring(0, id.lastIndexOf(".")); 48 String methodName = id.substring(id.lastIndexOf(".") + 1); 49 Class<?> aClass = Class.forName(className); 50 Method[] declaredMethods = aClass.getDeclaredMethods(); 51 for (Method declaredMethod : declaredMethods) { 52 declaredMethod.setAccessible(true); 53 //方法名相同，并且注解是DeletedAt 54 if (methodName.equals(declaredMethod.getName()) && declaredMethod.isAnnotationPresent(DeletedAt.class)) { 55 annotation = declaredMethod.getAnnotation(DeletedAt.class); 56 } 57 } 58 // 如果注解不存在或者注解为true（默认为true） 则为mysql语句增加删除标志 59 if (annotation == null || annotation.has()) { 60 BoundSql boundSql = statementHandler.getBoundSql(); 61 //获取到原始sql语句 62 String sql = boundSql.getSql(); 63 //通过反射修改sql语句 64 Field field = boundSql.getClass().getDeclaredField("sql"); 65 field.setAccessible(true); 66 String newSql = sql.replaceAll("9=9", "9=9 and deleted_at is null "); 67 field.set(boundSql, newSql); 68 } 69 } 70 } 71 }
在SQL语句替换上需要能识别到要被替换的内容，因此在xml的sql语句中加入特殊标志"9=9",该标志不影响原来SQL的执行结果，不同的过滤条件可以设置不同的标志，是一个比较巧妙的替换方式。