如何做系列(1)- mybatis 如何实现分页？

第一个做法，就是直接使用我们的sql语句进行分页，也就是在mapper里面加上分页的语句就好了。

<select id="" parameterType="" resultType="" resultMap="">
Select ROWNUM,ID,NAME FROM(Select ROWNUM as ROWNO, ID,NAME from CHANGED_CONTENT
<where>
<![CDATA[ROWNUM <= #{endRow}]]>
</where>
)
<where>
<![CDATA[ROWNO > #{startRow}]]>
</where>
</select>

然后我们在使用这个dao的时候，传入我们的分页的参数，就可以实现我们的分页需求了。其实很简单。

第二个做法，是一个更为通用的做法，那就是利用mybatis的拦截器，拦截每一个sql，在需要分页的sql加上分页的语句，就可以实现我们的分页功能。

我们可以查看 org.mybatis.spring.SqlSessionFactoryBean，其实他有一个plugin属性，我们可以配置实现一个插件来实现我们的需求。

首先说明一下实现的流程

例如我们有一条sql

String sql =select * from tableXXX where a=xxxx

我们的 拦截器拦截这条sql，然后判断是否是需要分页的，然后将这条sql转化为分页的sql

<span style="font-family: 微软雅黑; background-color: rgb(255, 255, 255);">    String countSql = "select count(1) from (" + sql    + ") tmp_count"  </span>

这样我们就可以得到分页的总页数了。

然后， 拼上我们传入的分页数据，哪一页，每一页几条数据：

select * from ( select row_.*, rownum rownum_ from ( sql ) row_ ) where rownum_ <= endString and rownum_ > offsetPlaceholder

这样我们就可以得到我们本页的返回了。

具体的代码如下：

首先是实现了ibatis intercepter的pageplugin

public class PagePlugin implements Interceptor {

private static Logger log = Logger.getLogger("page plugin");
private static Dialect dialectObject = null; // 数据库方言
private static String pageSqlId = ""; // mybaits的数据库xml映射文件中需要拦截的ID(正则匹配)

public Object intercept(Invocation ivk) throws Throwable {
if (ivk.getTarget() instanceof RoutingStatementHandler) {
log.info("com.dragon.dao.pulgin.mybatis.plugin.PagePlugin.intercept() enter*****************");
RoutingStatementHandler statementHandler = (RoutingStatementHandler) ivk
.getTarget();
BaseStatementHandler delegate = (BaseStatementHandler) ReflectHelper
.getValueByFieldName(statementHandler, "delegate");
MappedStatement mappedStatement = (MappedStatement) ReflectHelper
.getValueByFieldName(delegate, "mappedStatement");
/**
* 方法1：通过ＩＤ来区分是否需要分页．.*query.* 方法2：传入的参数是否有page参数，如果有，则分页，
*/
if (mappedStatement.getId().matches(pageSqlId)) { // 拦截需要分页的SQL
BoundSql boundSql = delegate.getBoundSql();
Object parameterObject = boundSql.getParameterObject();// 分页SQL<select>中parameterType属性对应的实体参数，即Mapper接口中执行分页方法的参数,该参数不得为空
if (parameterObject == null) {
//throw new NullPointerException("boundSql.getParameterObject() is null!");
return ivk.proceed();
} else {

PageView pageView = null;
if (parameterObject instanceof PageView) { // 参数就是Pages实体
pageView = (PageView) parameterObject;
} else if (parameterObject instanceof Map) {
for (Entry entry : (Set<Entry>) ((Map) parameterObject).entrySet()) {
if (entry.getValue() instanceof PageView) {
pageView = (PageView) entry.getValue();
break;
}
}
} else { // 参数为某个实体，该实体拥有Pages属性
pageView = ReflectHelper.getValueByFieldType(
parameterObject, PageView.class);
if (pageView == null) {
return ivk.proceed();
}
}

String sql = boundSql.getSql();
PreparedStatement countStmt = null;
ResultSet rs = null;
try {
Connection connection = (Connection) ivk.getArgs()[0];
String countSql = "select count(1) from (" + sql
+ ") tmp_count"; // 记录统计
countStmt = connection.prepareStatement(countSql);
ReflectHelper.setValueByFieldName(boundSql, "sql",
countSql);
DefaultParameterHandler parameterHandler = new DefaultParameterHandler(
mappedStatement, parameterObject, boundSql);
parameterHandler.setParameters(countStmt);
rs = countStmt.executeQuery();
Long count = 0L;
if (rs.next()) {
count = ((Number) rs.getObject(1)).longValue();
}
pageView.setRowCount(count);
} finally {
try {
rs.close();
} catch (Exception e) {
}
try {
countStmt.close();
} catch (Exception e) {
}
}
String pageSql = generatePagesSql(sql, pageView);
ReflectHelper.setValueByFieldName(boundSql, "sql", pageSql); // 将分页sql语句反射回BoundSql.
}
}
}
return ivk.proceed();
}

/**
* 根据数据库方言，生成特定的分页sql
*
* @param sql
* @param page
* @return
*/
private String generatePagesSql(String sql, PageView page) {
if (page != null && dialectObject != null) {
//pageNow默认是从1，而已数据库是从0开始计算的．所以(page.getPageNow()-1)
int pageNow = page.getPageNow();
return dialectObject.getLimitString(sql, (pageNow <= 0 ? 0 : pageNow - 1)
* page.getPageSize(), page.getPageSize());
}
return sql;
}

public Object plugin(Object target) {
return Plugin.wrap(target, this);
}

public void setProperties(Properties p) {
String dialect = ""; // 数据库方言
dialect = p.getProperty("dialect");
if (StringUtils.isBlank(dialect)) {
try {
throw new PropertyException("dialect property is not found!");
} catch (PropertyException e) {
log.error(e);
}
} else {
try {
dialectObject = (Dialect) Class.forName(dialect)
.getDeclaredConstructor().newInstance();
} catch (Exception e) {
throw new RuntimeException(dialect + ", init fail!\n" + e);
}
}
pageSqlId = p.getProperty("pageSqlId");//根据id来区分是否需要分页
if (StringUtils.isBlank(pageSqlId)) {
try {
throw new PropertyException("pageSqlId property is not found!");
} catch (PropertyException e) {
log.error(e);
}
}
}
}

然后是实现了dialect的oracledialect 数据库方言让我们区分不同的数据库不同的sql语句

public class OracleDialect extends Dialect {

public boolean supportsLimit() {
return true;
}

public boolean supportsLimitOffset() {
return true;
}

public String getLimitString(String sql, int offset, String offsetPlaceholder, int limit, String limitPlaceholder) {
sql = sql.trim();
boolean isForUpdate = false;
if (sql.toLowerCase().endsWith(" for update")) {
sql = sql.substring(0, sql.length() - 11);
isForUpdate = true;
}

StringBuffer pagingSelect = new StringBuffer(sql.length() + 100);
if (offset > 0) {
pagingSelect.append("select * from ( select row_.*, rownum rownum_ from ( ");
} else {
pagingSelect.append("select * from ( ");
}
pagingSelect.append(sql);
if (offset > 0) {
//			int end = offset+limit;
String endString = offsetPlaceholder + "+" + limitPlaceholder;
pagingSelect.append(" ) row_ ) where rownum_ <= " + endString + " and rownum_ > " + offsetPlaceholder);
} else {
pagingSelect.append(" ) where rownum <= " + limitPlaceholder);
}

if (isForUpdate) {
pagingSelect.append(" for update");
}

return pagingSelect.toString();
}

}

配置mybatis.xml

我们需要在sqlSessionFactory中注册我们的插件，发挥效果

<bean id="pagePlugin" class="com.xxxxx.pulgin.mybatis.plugin.PagePlugin">
<property name="properties">
<props>
<prop key="dialect">com.dragon.dao.pulgin.jdbc.dialet.OracleDialect</prop>
<prop key="pageSqlId">.*query.*</prop>
</props>
</property>
</bean>

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="dataSource" ref="dataSource" />
<property name="configLocation" value="classpath:spring/mybatis.xml" />
<property name="plugins">
<array>
<ref bean="pagePlugin" />

</array>
</property>
<property name="mapperLocations">
<list>
<!-- 自动匹配Mapper映射文件  -->
<value>classpath:com/xxxxx/mapper/*-mapper.xml</value>
</list>
</property>
</bean>

至此，我们的分页效果就做出来了。

charles at P.P 2016-7-14