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。
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下面是我的实现，但还是觉得有些复杂。
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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。
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@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 {
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?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();
?}
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?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) {
?}
}
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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。
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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。
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ResultSetHandlerInterceptor.java 负责修正RowBounds
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@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) {
?}
}
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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。
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其他辅助类
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public abstract class AbstractStatementHandlerInterceptor implements Interceptor, InitializingBean {
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?private Class Dialect? dialectClass;
?public void setDialectClass(Class Dialect? dialectClass) {
??this.dialectClass = dialectClass;
?}
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?protected Dialect dialect;
?public void setDialect(Dialect dialect) {
??this.dialect = dialect;
?}
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?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;
?}
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?protected RowBounds getRowBounds(StatementHandler statement) {
??return (RowBounds) FieldUtils.getFieldValue(statement,? rowBounds );
?}
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?protected boolean hasBounds(RowBounds rowBounds) {
??return (rowBounds != null
?????? rowBounds.getLimit()?? 0
?????? rowBounds.getLimit()?? RowBounds.NO_ROW_LIMIT);
?}
}
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public interface Dialect {
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?public void setLimitParamters(PreparedStatement ps, int parameterSize, int offset, int limit) throws SQLException;
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?public String getLimitString(String sql, boolean hasOffset);
?public String getLimitString(String sql, int offset, int limit);
}
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public abstract class FieldUtils {
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?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);
?}
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?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);
?}
}
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ReflectionUtils是spring里的。
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spring 集成用到的。增加interceptors集合，直接通过spring注入interceptor.
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public class MyBatisSessionFactoryBean extends SqlSessionFactoryBean {
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?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;
?}
}
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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。
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?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
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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。?
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