MyBatis攔截器的原理與使用
MyBatis攔截器的作用是在于Dao到DB中間進行額外的處理。大部分情況下通過mybatis的xml配置sql都可以達到想要的DB操作效果，然而存在一些類似或者相同的查詢條件或者查詢要求，這些可以通過攔截器的實現可以提升開發效率，比如：分頁、插入和更新時間/人、數據權限、SQL監控日志等。
Mybatis支持四種對象攔截Executor、StatementHandler、PameterHandler和ResultSetHandler Executor：攔截執行器的方法。 StatementHandler：攔截Sql語法構建的處理。 ParameterHandler：攔截參數的處理。 ResultHandler：攔截結果集的處理。public interface Executor { ResultHandler NO_RESULT_HANDLER = null; int update(MappedStatement var1, Object var2) throws SQLException; <E> List<E> query(MappedStatement var1, Object var2, RowBounds var3, ResultHandler var4, CacheKey var5, BoundSql var6) throws SQLException; <E> List<E> query(MappedStatement var1, Object var2, RowBounds var3, ResultHandler var4) throws SQLException; <E> Cursor<E> queryCursor(MappedStatement var1, Object var2, RowBounds var3) throws SQLException; List<BatchResult> flushStatements() throws SQLException; void commit(boolean var1) throws SQLException; void rollback(boolean var1) throws SQLException; CacheKey createCacheKey(MappedStatement var1, Object var2, RowBounds var3, BoundSql var4); boolean isCached(MappedStatement var1, CacheKey var2); void clearLocalCache(); void deferLoad(MappedStatement var1, MetaObject var2, String var3, CacheKey var4, Class<?> var5); Transaction getTransaction(); void close(boolean var1); boolean isClosed(); void setExecutorWrapper(Executor var1);}public interface StatementHandler { Statement prepare(Connection var1, Integer var2) throws SQLException; void parameterize(Statement var1) throws SQLException; void batch(Statement var1) throws SQLException; int update(Statement var1) throws SQLException; <E> List<E> query(Statement var1, ResultHandler var2) throws SQLException; <E> Cursor<E> queryCursor(Statement var1) throws SQLException; BoundSql getBoundSql(); ParameterHandler getParameterHandler();}public interface ParameterHandler { Object getParameterObject(); void setParameters(PreparedStatement var1) throws SQLException;}public interface ResultHandler<T> { void handleResult(ResultContext<? extends T> var1);}
攔截的執行順序是Executor->StatementHandler->ParameterHandler->ResultHandler
MyBatis提供的攔截器接口：public interface Interceptor { Object intercept(Invocation var1) throws Throwable; default Object plugin(Object target) {return Plugin.wrap(target, this); } default void setProperties(Properties properties) {}}
Object intercept方法用于攔截器的實現；
Object plugin方法用于判斷執行攔截器的類型；
void setProperties方法用于獲取配置項的屬性。
攔截對象和攔截器接口的結合，自定義的攔截器類需要實現攔截器接口，并通過注解@Intercepts和參數@Signature來聲明要攔截的對象。@Signature參數type是攔截對象，method是攔截的方法，即上面的四個類對應的方法，args是攔截方法對應的參數（方法存在重載因此需要指明參數個數和類型）
@Intercepts可以有多個@Signature，即一個攔截器實現類可以同時攔截多個對象及方法，示例如下：
Executor->intercept StatementHandler->intercept ParameterHandler->intercept ResultHandler->intercept@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) {}}@Intercepts({@Signature(type = StatementHandler.class, method = 'prepare', args = {Connection.class, Integer.class})})public class StatementPlugin implements Interceptor { @Override public Object intercept(Invocation invocation) throws Throwable {if (invocation.getTarget() instanceof StatementHandler) { System.out.println('StatementPlugin');}return invocation.proceed(); } @Override public Object plugin(Object target) {if (target instanceof StatementHandler) { return Plugin.wrap(target, this);}return target; } @Override public void setProperties(Properties properties) {}}@Intercepts({@Signature(type = ParameterHandler.class,method = 'setParameters',args = {PreparedStatement.class})})public class ParameterPlugin implements Interceptor { @Override public Object intercept(Invocation invocation) throws Throwable {if (invocation.getTarget() instanceof ParameterHandler) { System.out.println('ParameterPlugin');}return invocation.proceed(); } @Override public Object plugin(Object target) {if (target instanceof ParameterHandler) { return Plugin.wrap(target, this);}return target; } @Override public void setProperties(Properties properties) {}}@Intercepts({@Signature(type = ResultHandler.class,method = 'handleResult',args = {ResultContext.class})})public class ResultPlugin implements Interceptor { @Override public Object intercept(Invocation invocation) throws Throwable {if (invocation.getTarget() instanceof ResultHandler) { System.out.println('ResultPlugin');}return invocation.proceed(); } @Override public Object plugin(Object target) {if (target instanceof ResultHandler) { return Plugin.wrap(target, this);}return target; } @Override public void setProperties(Properties properties) {}}二、攔截器注冊的三種方式
前面介紹了Mybatis的攔截對象及其接口的實現方式，那么在項目中如何注冊攔截器呢？本文中給出三種注冊方式。
1.XML注冊xml注冊是最基本的方式，是通過在Mybatis配置文件中plugins元素來進行注冊的。一個plugin對應著一個攔截器，在plugin元素可以指定property子元素，在注冊定義攔截器時把對應攔截器的所有property通過Interceptor的setProperties方法注入給攔截器。因此攔截器注冊xml方式如下：
<?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.tiantian.mybatis.interceptor.MyInterceptor'> <property name='prop1' value='prop1'/> <property name='prop2' value='prop2'/> </plugin> </plugins> <!-- ...... --></configuration>2.配置類注冊
配置類注冊是指通過Mybatis的配置類中聲明注冊攔截器，配置類注冊也可以通過Properties類給Interceptor的setProperties方法注入參數。具體參考如下：
@Configurationpublic class MyBatisConfig { @Bean public String MyBatisInterceptor(SqlSessionFactory sqlSessionFactory) {UpdatePlugin executorInterceptor = new UpdatePlugin();Properties properties = new Properties();properties.setProperty('prop1', 'value1');// 給攔截器添加自定義參數executorInterceptor.setProperties(properties);sqlSessionFactory.getConfiguration().addInterceptor(executorInterceptor);sqlSessionFactory.getConfiguration().addInterceptor(new StatementPlugin());sqlSessionFactory.getConfiguration().addInterceptor(new ResultPlugin());sqlSessionFactory.getConfiguration().addInterceptor(new ParameterPlugin());// sqlSessionFactory.getConfiguration().addInterceptor(new SelectPlugin());return 'interceptor'; } // 與sqlSessionFactory.getConfiguration().addInterceptor(new SelectPlugin());效果一致 @Bean public SelectPlugin SelectInterceptor() {SelectPlugin interceptor = new SelectPlugin();Properties properties = new Properties();// 調用properties.setProperty方法給攔截器設置自定義參數interceptor.setProperties(properties);return interceptor; }}3.注解方式
通過@Component注解方式是最簡單的方式，在不需要轉遞自定義參數時可以使用，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參數改寫-修改時間和修改人統一插入
針對具體的攔截器實現進行描述。日常編碼需求中會碰到修改時需要插入修改的時間和人員，如果要用xml的方式去寫非常麻煩，而通過攔截器的方式可以快速實現全局的插入修改時間和人員。先看代碼：
@Component@Intercepts({@Signature(type = ParameterHandler.class, method = 'setParameters', args = {PreparedStatement.class}),})public class MyBatisInterceptor implements Interceptor { @Override public Object intercept(Invocation invocation) throws Throwable {// 參數代理if (invocation.getTarget() instanceof ParameterHandler) { System.out.println('ParameterHandler'); // 自動添加操作員信息 autoAddOperatorInfo(invocation);}return invocation.proceed(); } @Override public Object plugin(Object target) {return Plugin.wrap(target, this); } @Override public void setProperties(Properties properties) { } /** * 自動添加操作員信息 * * @param invocation 代理對象 * @throws Throwable 異常 */ private void autoAddOperatorInfo(Invocation invocation) throws Throwable {System.out.println('autoInsertCreatorInfo');// 獲取代理的參數對象ParameterHandlerParameterHandler ph = (ParameterHandler) invocation.getTarget();// 通過MetaObject獲取ParameterHandler的反射內容MetaObject metaObject = MetaObject.forObject(ph,SystemMetaObject.DEFAULT_OBJECT_FACTORY,SystemMetaObject.DEFAULT_OBJECT_WRAPPER_FACTORY,new DefaultReflectorFactory());// 通過MetaObject反射的內容獲取MappedStatementMappedStatement mappedStatement = (MappedStatement) metaObject.getValue('mappedStatement');// 當sql類型為INSERT或UPDATE時，自動插入操作員信息if (mappedStatement.getSqlCommandType() == SqlCommandType.INSERT ||mappedStatement.getSqlCommandType() == SqlCommandType.UPDATE) { // 獲取參數對象 Object obj = ph.getParameterObject(); if (null != obj) {// 通過反射獲取參數對象的屬性Field[] fields = obj.getClass().getDeclaredFields();// 遍歷參數對象的屬性for (Field f : fields) { // 如果sql是INSERT,且存在createdAt屬性 if ('createdAt'.equals(f.getName()) && mappedStatement.getSqlCommandType() == SqlCommandType.INSERT) {// 設置允許訪問反射屬性f.setAccessible(true);// 如果沒有設置createdAt屬性，則自動為createdAt屬性添加當前的時間if (null == f.get(obj)) { // 設置createdAt屬性為當前時間 f.set(obj, LocalDateTime.now());} } // 如果sql是INSERT,且存在createdBy屬性 if ('createdBy'.equals(f.getName()) && mappedStatement.getSqlCommandType() == SqlCommandType.INSERT) {// 設置允許訪問反射屬性f.setAccessible(true);// 如果沒有設置createdBy屬性，則自動為createdBy屬性添加當前登錄的人員if (null == f.get(obj)) { // 設置createdBy屬性為當前登錄的人員 f.set(obj, 0);} } // sql為INSERT或UPDATE時均需要設置updatedAt屬性 if ('updatedAt'.equals(f.getName())) {f.setAccessible(true);if (null == f.get(obj)) { f.set(obj, LocalDateTime.now());} } // sql為INSERT或UPDATE時均需要設置updatedBy屬性 if ('updatedBy'.equals(f.getName())) {f.setAccessible(true);if (null == f.get(obj)) { f.set(obj, 0);} }}// 通過反射獲取ParameterHandler的parameterObject屬性Field parameterObject = ph.getClass().getDeclaredField('parameterObject');// 設置允許訪問parameterObject屬性parameterObject.setAccessible(true);// 將上面設置的新參數對象設置到ParameterHandler的parameterObject屬性parameterObject.set(ph, obj); }} }}
攔截器的接口實現參考前文，這里著重介紹autoAddOperatorInfo方法里的相關類。
1.ParameterHandler
接口源碼：
public interface ParameterHandler { Object getParameterObject(); void setParameters(PreparedStatement var1) throws SQLException; }
提供兩個方法：
getParameterObject是獲取參數對象，可能存在null，需要注意null指針。
setParameters是控制如何設置SQL參數，即sql語句中配置的java對象和jdbc類型對應的關系，例如#{id,jdbcType=INTEGER}，id默認類型是javaType=class java.lang.Integer。
該接口有一個默認的實現類，源碼如下：
public class DefaultParameterHandler implements ParameterHandler { private final TypeHandlerRegistry typeHandlerRegistry; private final MappedStatement mappedStatement; private final Object parameterObject; private final BoundSql boundSql; private final Configuration configuration; public DefaultParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {this.mappedStatement = mappedStatement;this.configuration = mappedStatement.getConfiguration();this.typeHandlerRegistry = mappedStatement.getConfiguration().getTypeHandlerRegistry();this.parameterObject = parameterObject;this.boundSql = boundSql; } public Object getParameterObject() {return this.parameterObject; } public void setParameters(PreparedStatement ps) {ErrorContext.instance().activity('setting parameters').object(this.mappedStatement.getParameterMap().getId());List<ParameterMapping> parameterMappings = this.boundSql.getParameterMappings();if (parameterMappings != null) { for(int i = 0; i < parameterMappings.size(); ++i) {ParameterMapping parameterMapping = (ParameterMapping)parameterMappings.get(i);if (parameterMapping.getMode() != ParameterMode.OUT) { String propertyName = parameterMapping.getProperty(); Object value; if (this.boundSql.hasAdditionalParameter(propertyName)) {value = this.boundSql.getAdditionalParameter(propertyName); } else if (this.parameterObject == null) {value = null; } else if (this.typeHandlerRegistry.hasTypeHandler(this.parameterObject.getClass())) {value = this.parameterObject; } else {MetaObject metaObject = this.configuration.newMetaObject(this.parameterObject);value = metaObject.getValue(propertyName); } TypeHandler typeHandler = parameterMapping.getTypeHandler(); JdbcType jdbcType = parameterMapping.getJdbcType(); if (value == null && jdbcType == null) {jdbcType = this.configuration.getJdbcTypeForNull(); } try {typeHandler.setParameter(ps, i + 1, value, jdbcType); } catch (SQLException | TypeException var10) {throw new TypeException('Could not set parameters for mapping: ' + parameterMapping + '. Cause: ' + var10, var10); }} }} }}
通過DefaultParameterHandler實現類我們知道通過ParameterHandler可以獲取到哪些屬性和方法，其中包括我們下面一個重要的類MappedStatement。
2.MappedStatement
MyBatis的mapper文件中的每個select/update/insert/delete標簽會被解析器解析成一個對應的MappedStatement對象，也就是一個MappedStatement對象描述一條SQL語句。MappedStatement對象屬性如下：
// mapper配置文件名 private String resource; // mybatis的全局信息，如jdbc private Configuration configuration; // 節點的id屬性加命名空間,如：com.example.mybatis.dao.UserMapper.selectByExample private String id; private Integer fetchSize; private Integer timeout; private StatementType statementType; private ResultSetType resultSetType; private SqlSource sqlSource; private Cache cache; private ParameterMap parameterMap; private List<ResultMap> resultMaps; private boolean flushCacheRequired; private boolean useCache; private boolean resultOrdered; // sql語句的類型：select、update、delete、insert private SqlCommandType sqlCommandType; private KeyGenerator keyGenerator; private String[] keyProperties; private String[] keyColumns; private boolean hasNestedResultMaps; private String databaseId; private Log statementLog; private LanguageDriver lang; private String[] resultSets;
在本例中通過MappedStatement對象的sqlCommandType來判斷當前的sql類型是insert、update來進行下一步的操作。
四、通過StatementHandler改寫SQLStatementHandler是用于封裝JDBC Statement操作，負責對JDBC Statement的操作，如設置參數，并將Statement結果集轉換成List集合。
實現代碼如下：
刪除注解標記
@Target({ElementType.METHOD}) //表示注解的使用范圍@Retention(RetentionPolicy.RUNTIME) //注解的保存時間@Documented //文檔顯示public @interface DeletedAt { boolean has() default true;}
Dao層添加刪除注解，為false時不添加刪除標志
@Mapper public interface AdminProjectDao { @DeletedAt(has = false) List<AdminProjectPo> selectProjects(AdminProjectPo po); }
攔截器通過刪除注解標記判斷是否添加刪除標志
@Component@Intercepts({@Signature(type = StatementHandler.class, method = 'prepare', args = {Connection.class, Integer.class}),})public class MyBatisInterceptor implements Interceptor { @Override public Object intercept(Invocation invocation) throws Throwable {if (invocation.getTarget() instanceof StatementHandler) { System.out.println('StatementHandler'); checkHasDeletedAtField(invocation);}return invocation.proceed(); } @Override public Object plugin(Object target) {return Plugin.wrap(target, this); } @Override public void setProperties(Properties properties) { } /** * 檢查查詢是否需要添加刪除標志字段 * * @param invocation 代理對象 * @throws Throwable 異常 */ private void checkHasDeletedAtField(Invocation invocation) throws Throwable {System.out.println('checkHasDeletedAtField');StatementHandler statementHandler = (StatementHandler) invocation.getTarget();// 通過MetaObject訪問對象的屬性MetaObject metaObject = MetaObject.forObject(statementHandler,SystemMetaObject.DEFAULT_OBJECT_FACTORY,SystemMetaObject.DEFAULT_OBJECT_WRAPPER_FACTORY,new DefaultReflectorFactory());// 獲取成員變量mappedStatementMappedStatement mappedStatement = (MappedStatement) metaObject.getValue('delegate.mappedStatement');// 如果sql類型是查詢if (mappedStatement.getSqlCommandType() == SqlCommandType.SELECT) { // 獲取刪除注解標志 DeletedAt annotation = null; String id = mappedStatement.getId(); String className = id.substring(0, id.lastIndexOf('.')); String methodName = id.substring(id.lastIndexOf('.') + 1); Class<?> aClass = Class.forName(className); Method[] declaredMethods = aClass.getDeclaredMethods(); for (Method declaredMethod : declaredMethods) {declaredMethod.setAccessible(true);//方法名相同，并且注解是DeletedAtif (methodName.equals(declaredMethod.getName()) && declaredMethod.isAnnotationPresent(DeletedAt.class)) { annotation = declaredMethod.getAnnotation(DeletedAt.class);} } // 如果注解不存在或者注解為true（默認為true） 則為mysql語句增加刪除標志 if (annotation == null || annotation.has()) {BoundSql boundSql = statementHandler.getBoundSql();//獲取到原始sql語句String sql = boundSql.getSql();//通過反射修改sql語句Field field = boundSql.getClass().getDeclaredField('sql');field.setAccessible(true);String newSql = sql.replaceAll('9=9', '9=9 and deleted_at is null ');field.set(boundSql, newSql); }} }}
在SQL語句替換上需要能識別到要被替換的內容，因此在xml的sql語句中加入特殊標志'9=9',該標志不影響原來SQL的執行結果，不同的過濾條件可以設置不同的標志，是一個比較巧妙的替換方式。
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