最近开始尝试用mybatis,比较郁闷其对分页的实现，看了网上一些例子，还是不满意。最好的应该是rapid-framework里的实现了，但没实现分页参数的参数化，参数都是硬编码在sql里，在oracle这样的数据库里，性能影响还是有的。
下面是我的实现，但还是觉得有些复杂。
StatementHandlerInterceptor.java 主要是负责修改sql，在rapid-framework，其实就只有这样一个Interceptor，但如果要设置参数，就不止了。
@Intercepts({
@Signature(type = StatementHandler.class, method = prepare , args = { Connection.class }),
@Signature(type = StatementHandler.class, method = parameterize , args = { Statement.class }) })
public class StatementHandlerInterceptor extends
AbstractStatementHandlerInterceptor implements Interceptor {
private Object prepare(Invocation invocation) throws Throwable {
StatementHandler statement = getStatementHandler(invocation);
if (statement instanceof SimpleStatementHandler
|| statement instanceof PreparedStatementHandler) {
RowBounds rowBounds = getRowBounds(statement);
if (hasBounds(rowBounds)) {
BoundSql boundSql = statement.getBoundSql();
String sql = boundSql.getSql();
if (statement instanceof SimpleStatementHandler) {
sql = dialect.getLimitString(sql, rowBounds.getOffset(),
rowBounds.getLimit());
}
else if (statement instanceof PreparedStatementHandler) {
sql = dialect.getLimitString(sql, rowBounds.getOffset() 0);
}
FieldUtils.setFieldValue(boundSql, sql , sql);
}
}
return invocation.proceed();
}
private Object parameterize(Invocation invocation) throws Throwable {
Statement statement = (Statement) invocation.getArgs()[0];
Object rtn = invocation.proceed();
if (statement instanceof PreparedStatement) {
PreparedStatement ps = (PreparedStatement) statement;
StatementHandler statementHandler = getStatementHandler(invocation);
RowBounds rowBounds = getRowBounds(statementHandler);
if (hasBounds(rowBounds)) {
BoundSql boundSql = statementHandler.getBoundSql();
int parameterSize = boundSql.getParameterMappings().size();
dialect.setLimitParamters(ps, parameterSize,
rowBounds.getOffset(), rowBounds.getLimit());
}
}
return rtn;
}
@Override
public Object intercept(Invocation invocation) throws Throwable {
Method m = invocation.getMethod();
if ( prepare .equals(m.getName())) {
return prepare(invocation);
}
else if ( parameterize .equals(m.getName())) {
return parameterize(invocation);
}
return invocation.proceed();
}
@Override
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}
@Override
public void setProperties(Properties properties) {
}
}
prepare和parameterize是在两个不同点调用的，一个负责生成Statement，一个负责设置参数。本来是想分开写成两个Interceptor，但后来发现多次针对同一个类进行拦截，是无法获取其内部属性的，Plugin.wrap(target, this)是生成一个代理类，下一个拦截器就是在这个基础上继续做代理。这个和struts2之类的拦截器是不一样的。
好像这样是可以了，但实际还是不够的。因为StatementHandler和FastResultSetHandler是同时生成，而且都接引用了RowBounds参数，在FastResultSetHandler还是分页形式存在，如果不修正，会导致mybatis用游标移动后取数据。
ResultSetHandlerInterceptor.java 负责修正RowBounds
@Intercepts({ @Signature(type = ResultSetHandler.class, method = handleResultSets , args = { Statement.class }) })
public class ResultSetHandlerInterceptor implements Interceptor {
public Object intercept(Invocation invocation) throws Throwable {
ResultSetHandler resultSet = (ResultSetHandler) invocation.getTarget();
// 不用浪费性能做属性存在判断
RowBounds rowBounds = (RowBounds) FieldUtils.getFieldValue(resultSet,
rowBounds );
if (rowBounds.getLimit() 0
rowBounds.getLimit() RowBounds.NO_ROW_LIMIT) {
// 强制不允许游标分页
FieldUtils.setFieldValue(resultSet, rowBounds , RowBounds.DEFAULT);
}
return invocation.proceed();
}
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}
public void setProperties(Properties properties) {
}
}
不过现在想想，用反射直接修改RowBounds里的属性，不就可以不用ResultSetHandlerInterceptor了？哎！又SB了一次。后面改进。
其他辅助类
public abstract class AbstractStatementHandlerInterceptor implements Interceptor, InitializingBean {
private Class Dialect dialectClass;
public void setDialectClass(Class Dialect dialectClass) {
this.dialectClass = dialectClass;
}
protected Dialect dialect;
public void setDialect(Dialect dialect) {
this.dialect = dialect;
}
public void afterPropertiesSet() throws Exception {
setDialect(dialectClass.newInstance());
}
protected StatementHandler getStatementHandler(Invocation invocation) {
StatementHandler statement = (StatementHandler) invocation.getTarget();
if (statement instanceof RoutingStatementHandler) {
statement = (StatementHandler) FieldUtils.getFieldValue(statement,
delegate );
}
return statement;
}
protected RowBounds getRowBounds(StatementHandler statement) {
return (RowBounds) FieldUtils.getFieldValue(statement, rowBounds );
}
protected boolean hasBounds(RowBounds rowBounds) {
return (rowBounds != null
rowBounds.getLimit() 0
rowBounds.getLimit() RowBounds.NO_ROW_LIMIT);
}
}
public interface Dialect {
public void setLimitParamters(PreparedStatement ps, int parameterSize, int offset, int limit) throws SQLException;
public String getLimitString(String sql, boolean hasOffset);
public String getLimitString(String sql, int offset, int limit);
}
public abstract class FieldUtils {
public static boolean hasField(Object target, String fieldName, Class ? type) {
return ReflectionUtils.findField(target.getClass(), fieldName, type) == null;
}
public static Object getFieldValue(Object target, String fieldName) {
Field field = ReflectionUtils.findField(target.getClass(), fieldName);
ReflectionUtils.makeAccessible(field);
return ReflectionUtils.getField(field, target);
}
public static void setFieldValue(Object target, String fieldName, Object value) {
Field field = ReflectionUtils.findField(target.getClass(), fieldName);
ReflectionUtils.makeAccessible(field);
ReflectionUtils.setField(field, target, value);
}
}
ReflectionUtils是spring里的。
spring 集成用到的。增加interceptors集合，直接通过spring注入interceptor.
public class MyBatisSessionFactoryBean extends SqlSessionFactoryBean {
private List Interceptor interceptors = Collections.emptyList();
public void setInterceptors(List Interceptor interceptors) {
this.interceptors = interceptors;
}
protected SqlSessionFactory buildSqlSessionFactory() throws IOException,
IllegalAccessException, InstantiationException {
SqlSessionFactory factory = super.buildSqlSessionFactory();
Configuration config = factory.getConfiguration();
for (Interceptor interceptor : interceptors) {
config.addInterceptor(interceptor);
}
return factory;
}
}
下一次改进方向，现在目前来讲，只是实现了sql能分页，但缺少count的集成。mybatis中两条语句是避免不了，虽然可以用include来避免重复的where语句。
sql id= findAllWhere
where mm = #{love}
/sql
select id= findAll resultMap= helloResultMap
select * from HELLO
include refid= findAllWhere /
order by create_time
/select
select id= findAllCount resultType= long
select count(*) from HELLO
include refid= findAllWhere /
/select
下次希望是，能在调用findAll的时候，自动调用findAllCount，遵守命名约定或者用注释来提示。由于我打算完全不写dao层，只写xml文件和接口，需要用到SqlSession.getMapper自动生成代理。由于期望是自动调用count方法和返回Page这样的对象。 这个可能就要看，是再代理一层，还是修改代理的实现了。入手点可以是继承Configuration，然后覆盖mapperRegistry来实现。