Mybatis分页切入点
Mybatis内部有个plugins(插件)概念，本质上属于拦截器的思想。具体的解析可见他文MyBatis拦截器原理探究。本文将在此基础上直接展示实际项目的实现代码和其他的相关解析
分页具体代码实现
首先我们可以定义方言抽象类，用于实现分页AbstractDialect.java
public abstract class AbstractDialect{
/**
* 是否支持limit和偏移量
* @return
*/
public abstract boolean supportsLimitOffset();
/**
* 是否支持limit
* @return
*/
public abstract boolean supportsLimit();
/**
* 获取增加了分页属性之后的SQL
* @param sql
* @param offset
* @param limit
* @return
*/
public abstract String getLimitString(String sql, int offset, int limit);
}
再而我们就以Oracle与Mysql数据库的分页技术作下分别的实现
MySQLDialect.java-Mysql分页方言
public class MySQLDialect extends AbstractDialect {
public boolean supportsLimitOffset() {
return true;
}
public boolean supportsLimit() {
return true;
}
public String getLimitString(String sql, int offset, int limit) {
if (offset > 0) {
return sql + " limit " + offset + "," + limit;
} else {
return sql + " limit " + limit;
}
}
}
OracleDialect.java-Oracle方言实现
public class OracleDialect extends ADialect{
@Override
public boolean supportsLimitOffset() {
return false;
}
@Override
public boolean supportsLimit() {
return false;
}
@Override
public String getLimitString(String sql, int start, int limit) {
if(start < 0){
start = 0;
}
if(limit < 0){
limit = 10;
}
StringBuilder pageSql = new StringBuilder(100);
pageSql.append("select * from ( select temp.*, rownum row_id from ( ");
pageSql.append(sql);
pageSql.append(" ) temp where rownum <= ").append(start+limit);
pageSql.append(") where row_id > ").append(start);
return pageSql.toString();
}
}
对应的Mybatis插件拦截器实现如下，拦截StatementHandler#prepare(Connection con)创建SQL语句对象方法
PaginationInterceptor.java
@Intercepts({ @Signature(type = StatementHandler.class, method = "prepare", args = { Connection.class }) })
public final class PaginationInterceptor implements Interceptor {
private final static Logger log = LoggerFactory
.getLogger(PaginationInterceptor.class);
private ADialect dialect;
public void setDialect(ADialect dialect) {
this.dialect = dialect;
}
@Override
public Object intercept(Invocation invocation) throws Throwable {
// 直接获取拦截的对象，其实现类RoutingStatementHandler
StatementHandler statementHandler = (StatementHandler) invocation
.getTarget();
// 获取元对象，主要用于获取statementHandler所关联的对象及属性
MetaObject metaStatementHandler = MetaObject.forObject(
statementHandler, new DefaultObjectFactory(),
new DefaultObjectWrapperFactory());
MappedStatement mappedStmt= (MappedStatement) metaStatementHandler
.getValue("delegate.mappedStatement".intern());
// 只对queryPagination()方法进行分页操作
if(mappedStmt.getId().indexOf("queryPagination")==-1){
return invocation.proceed();
}
// 重新构造分页的sql
String originalSql = (String) metaStatementHandler
.getValue("delegate.boundSql.sql".intern());
// 注意RowBounds是必须被提前指定的，用于分页语句的拼装
RowBounds rowBounds = metaStatementHandler.getValue("delegate.rowBounds") ;
metaStatementHandler.setValue("delegate.boundSql.sql".intern(), dialect
.getLimitString(originalSql, rowBounds.getOffset(),
rowBounds.getLimit()));
metaStatementHandler.setValue("delegate.rowBounds.offset".intern(),
RowBounds.NO_ROW_OFFSET);
metaStatementHandler.setValue("delegate.rowBounds.limit".intern(),
RowBounds.NO_ROW_LIMIT);
log.debug("page sql : " + boundSql.getSql());
return invocation.proceed();
}
// 拦截对象
@Override
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}
@Override
public void setProperties(Properties properties) {
}
}
Spring对应的xml配置可如下，以oracle分页为例子
<!-- oracle方言配置，用于oracle的分页 -->
<bean id="paginationInterceptor" class="com.hsnet.winner.cas.admin.core.dao.mybatis.interceptor.PaginationInterceptor">
<property name="dialect">
<bean class="cn.cloud.winner.oss.manager.mybatis.page.OracleDialect" />
</property>
</bean>
<!--sqlSessionFactoryBean配置plugins插件用于拦截-->
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="dataSource" ref="dataSource" />
<property name="mapperLocations" value="classpath*:mybatis/*DAO.xml" />
<property name="plugins">
<array>
<ref bean="paginationInterceptor" />
</array>
</property>
</bean>
使用以上的代码以及配置即可完成对oracle数据库以及mysql数据库的分页操作。并且博主对其中的某个点作下解析
Mybatis#MetaObject-元数据对象解析
以上的代码博主之前在使用的时候，对其中的MetaObject这个类很费解，其直接通过getValue()方法便可以将所代理的对象的所关联的属性全都拿取到。我们可以跟随源码深入了解下
MetaObject#forObject()
代理对象均通过此静态方法进入
public static MetaObject forObject(Object object, ObjectFactory objectFactory, ObjectWrapperFactory objectWrapperFactory) {
if (object == null) {
return SystemMetaObject.NULL_META_OBJECT;
} else {
return new MetaObject(object, objectFactory, objectWrapperFactory);
}
}
我们可以直接观察其中的构造函数，玄机就在此处
private MetaObject(Object object, ObjectFactory objectFactory, ObjectWrapperFactory objectWrapperFactory) {
this.originalObject = object;
this.objectFactory = objectFactory;
this.objectWrapperFactory = objectWrapperFactory;
// 所有的属性获取均通过objectWrapper类来获取，此处主要对所代理的object对象类型进行判断
if (object instanceof ObjectWrapper) {
this.objectWrapper = (ObjectWrapper) object;
} else if (objectWrapperFactory.hasWrapperFor(object)) {
this.objectWrapper = objectWrapperFactory.getWrapperFor(this, object);
} else if (object instanceof Map) {
this.objectWrapper = new MapWrapper(this, (Map) object);
} else if (object instanceof Collection) {
this.objectWrapper = new CollectionWrapper(this, (Collection) object);
} else {
// 我们常用的便是BeanWrapper
this.objectWrapper = new BeanWrapper(this, object);
}
}
为了理解的更为渗透，我们继续跟进，最后我们得知其会调用Reflector类的构造函数
private Reflector(Class<?> clazz) {
type = clazz;
// 获取构造类
addDefaultConstructor(clazz);
// 获取get方法集合
addGetMethods(clazz);
// 获取set方法集合
addSetMethods(clazz);
// 获取内部属性集合
addFields(clazz);
readablePropertyNames = getMethods.keySet().toArray(new String[getMethods.keySet().size()]);
writeablePropertyNames = setMethods.keySet().toArray(new String[setMethods.keySet().size()]);
for (String propName : readablePropertyNames) {
caseInsensitivePropertyMap.put(propName.toUpperCase(Locale.ENGLISH), propName);
}
for (String propName : writeablePropertyNames) {
caseInsensitivePropertyMap.put(propName.toUpperCase(Locale.ENGLISH), propName);
}
}
由此我们便可知使用Reflector代理类以及MetaObject便可以遍历代理被代理类的所关联的所有属性，就拿RoutingStatementHandler类来说，经过上述操作后其便可以访问内部属性delegate以及delegate的内部属性configuration/objectFactory/typeHandlerRegistry/resultSetHandler/parameterHandler/mappedStatement等属性
MetaObject#getValue()
上述阐述的是如何代理被代理类的内部属性，我们也简单的看下是如何正确的调用
public Object getValue(String name) {
// PropertyTokenizer与StringTokenizer类似，只是前者写死以.为分隔符
PropertyTokenizer prop = new PropertyTokenizer(name);
if (prop.hasNext()) {
MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
if (metaValue == SystemMetaObject.NULL_META_OBJECT) {
return null;
} else {
return metaValue.getValue(prop.getChildren());
}
} else {
return objectWrapper.get(prop);
}
}
具体的解析就不在此阐述了，如何用户想获取StatementHandler所拥有的sql字符串，可通过getValue("delegate.boundSql.sql")其中以.为分隔符并其中的属性必须是内部属性(区分大小写)。
MetaObject#setValue()
原理同MetaObject#getValue()方法
总结
结合实例以及源码帮助大家更透彻的了解Mybatis是如何进行分页操作的，欢迎指正