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。
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);
}
拦截的执行顺序是Executor->StatementHandler->ParameterHandler->ResultHandler
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。
@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：
@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) {}
}
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。
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：
<?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>
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：
@Configuration
public class MyBatisConfig {
@Bean
public String MyBatisInterceptor(SqlSessionFactory sqlSessionFactory) {
UpdatePlugin executorInterceptor = new UpdatePlugin();
Properties properties = new Properties();
properties.setProperty("prop1", "value1");
// 给拦截器添加自定义参数
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";
}
// 与sqlSessionFactory.getConfiguration().addInterceptor(new SelectPlugin());效果一致
@Bean
public SelectPlugin SelectInterceptor() {
SelectPlugin interceptor = new SelectPlugin();
Properties properties = new Properties();
// 调用properties.setProperty方法给拦截器设置自定义参数
interceptor.setProperties(properties);
return interceptor;
}
}
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) {
}
}
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：
@Component
@Intercepts({
@Signature(type = ParameterHandler.class, method = "setParameters", args = {PreparedStatement.class}),
})
public class MyBatisInterceptor implements Interceptor {
@Override
public Object intercept(Invocation invocation) throws Throwable {
// 参数代理
if (invocation.getTarget() instanceof ParameterHandler) {
System.out.println("ParameterHandler");
// 自动添加操作员信息
autoAddOperatorInfo(invocation);
}
return invocation.proceed();
}
@Override
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}
@Override
public void setProperties(Properties properties) {
}
/**
* 自动添加操作员信息
*
* @param invocation 代理对象
* @throws Throwable 异常
*/
private void autoAddOperatorInfo(Invocation invocation) throws Throwable {
System.out.println("autoInsertCreatorInfo");
// 获取代理的参数对象ParameterHandler
ParameterHandler ph = (ParameterHandler) invocation.getTarget();
// 通过MetaObject获取ParameterHandler的反射内容
MetaObject metaObject = MetaObject.forObject(ph,
SystemMetaObject.DEFAULT_OBJECT_FACTORY,
SystemMetaObject.DEFAULT_OBJECT_WRAPPER_FACTORY,
new DefaultReflectorFactory());
// 通过MetaObject反射的内容获取MappedStatement
MappedStatement mappedStatement = (MappedStatement) metaObject.getValue("mappedStatement");
// 当sql类型为INSERT或UPDATE时，自动插入操作员信息
if (mappedStatement.getSqlCommandType() == SqlCommandType.INSERT ||
mappedStatement.getSqlCommandType() == SqlCommandType.UPDATE) {
// 获取参数对象
Object obj = ph.getParameterObject();
if (null != obj) {
// 通过反射获取参数对象的属性
Field[] fields = obj.getClass().getDeclaredFields();
// 遍历参数对象的属性
for (Field f : fields) {
// 如果sql是INSERT,且存在createdAt属性
if ("createdAt".equals(f.getName()) && mappedStatement.getSqlCommandType() == SqlCommandType.INSERT) {
// 设置允许访问反射属性
f.setAccessible(true);
// 如果没有设置createdAt属性，则自动为createdAt属性添加当前的时间
if (null == f.get(obj)) {
// 设置createdAt属性为当前时间
f.set(obj, LocalDateTime.now());
}
}
// 如果sql是INSERT,且存在createdBy属性
if ("createdBy".equals(f.getName()) && mappedStatement.getSqlCommandType() == SqlCommandType.INSERT) {
// 设置允许访问反射属性
f.setAccessible(true);
// 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
if (null == f.get(obj)) {
// 设置createdBy属性为当前登录的人员
f.set(obj, 0);
}
}
// sql为INSERT或UPDATE时均需要设置updatedAt属性
if ("updatedAt".equals(f.getName())) {
f.setAccessible(true);
if (null == f.get(obj)) {
f.set(obj, LocalDateTime.now());
}
}
// sql为INSERT或UPDATE时均需要设置updatedBy属性
if ("updatedBy".equals(f.getName())) {
f.setAccessible(true);
if (null == f.get(obj)) {
f.set(obj, 0);
}
}
}
// 通过反射获取ParameterHandler的parameterObject属性
Field parameterObject = ph.getClass().getDeclaredField("parameterObject");
// 设置允许访问parameterObject属性
parameterObject.setAccessible(true);
// 将上面设置的新参数对象设置到ParameterHandler的parameterObject属性
parameterObject.set(ph, obj);
}
}
}
}
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：
public interface ParameterHandler {
Object getParameterObject();
void setParameters(PreparedStatement var1) throws SQLException;
}
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：
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：
// mapper配置文件名
private String resource;
// mybatis的全局信息，如jdbc
private Configuration configuration;
// 节点的id属性加命名空间,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语句的类型：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。
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
@Mapper
public interface AdminProjectDao {
@DeletedAt(has = false)
List<AdminProjectPo> selectProjects(AdminProjectPo po);
}
拦截器通过删除注解标记判断是否添加删除标志
@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) {
}
/**
* 检查查询是否需要添加删除标志字段
*
* @param invocation 代理对象
* @throws Throwable 异常
*/
private void checkHasDeletedAtField(Invocation invocation) throws Throwable {
System.out.println("checkHasDeletedAtField");
StatementHandler statementHandler = (StatementHandler) invocation.getTarget();
// 通过MetaObject访问对象的属性
MetaObject metaObject = MetaObject.forObject(
statementHandler,
SystemMetaObject.DEFAULT_OBJECT_FACTORY,
SystemMetaObject.DEFAULT_OBJECT_WRAPPER_FACTORY,
new DefaultReflectorFactory());
// 获取成员变量mappedStatement
MappedStatement mappedStatement = (MappedStatement) metaObject.getValue("delegate.mappedStatement");
// 如果sql类型是查询
if (mappedStatement.getSqlCommandType() == SqlCommandType.SELECT) {
// 获取删除注解标志
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DeletedAt
if (methodName.equals(declaredMethod.getName()) && declaredMethod.isAnnotationPresent(DeletedAt.class)) {
annotation = declaredMethod.getAnnotation(DeletedAt.class);
}
}
// 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
if (annotation == null || annotation.has()) {
BoundSql boundSql = statementHandler.getBoundSql();
//获取到原始sql语句
String sql = boundSql.getSql();
//通过反射修改sql语句
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。
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