mybatis分页实现

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。