PageHelper.startPage(1, 10);
PageHelper.orderBy("id");
这是 PageHelper 平常分页排序的普遍用法，然而 PageHelper 还支持隐式分页和排序用法，开启 pagehelper.supportMethodsArguments即可，supportMethodsArguments默认是 false
下面以一个例子分析
....
query.setOrderBy("id");
query.setPageNum(1);
query.setPageSize(10);
itemMapper.selectItems(query); //myBatis mapper查询接口
....
itemMapper.selectItems 就是简单的一个 SQL，大概就是
select * from item
下面进行代码跟踪，进入itemMapper.selectItems方法，首先跳转到的是ibatis的方法
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (method.isDefault()) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
一路跟下去会进到
@Override
public <E> List<E> selectList(String statement, Object parameter) {
return this.sqlSessionProxy.selectList(statement, parameter);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
if (methods != null && methods.contains(method)) {
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
之后进入interceptor.intercept方法，会跳到 PageHelper 包的intercept方法
List resultList;
//调用方法判断是否需要进行分页，如果不需要，直接返回结果
if (!dialect.skip(ms, parameter, rowBounds)) {
//判断是否需要进行 count 查询
if (dialect.beforeCount(ms, parameter, rowBounds)) {
//查询总数
Long count = count(executor, ms, parameter, rowBounds, resultHandler, boundSql);
//处理查询总数，返回 true 时继续分页查询，false 时直接返回
if (!dialect.afterCount(count, parameter, rowBounds)) {
//当查询总数为 0 时，直接返回空的结果
return dialect.afterPage(new ArrayList(), parameter, rowBounds);
}
}
resultList = ExecutorUtil.pageQuery(dialect, executor,
ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);
} else {
//rowBounds用参数值，不使用分页插件处理时，仍然支持默认的内存分页
resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
}
由此可见，查询方法被 PageHelper 给代理了，可是上面的业务代码并没有显式使用 PageHelper，就是说，PageHelper 是开箱即用了，只要引入了，就会拦截使用，然后再进去dialect.skip方法
@Override
public boolean skip(MappedStatement ms, Object parameterObject, RowBounds rowBounds) {
if (ms.getId().endsWith(MSUtils.COUNT)) {
throw new RuntimeException("在系统中发现了多个分页插件，请检查系统配置!");
}
Page page = pageParams.getPage(parameterObject, rowBounds);
if (page == null) {
return true;
} else {
//设置默认的 count 列
if (StringUtil.isEmpty(page.getCountColumn())) {
page.setCountColumn(pageParams.getCountColumn());
}
autoDialect.initDelegateDialect(ms);
return false;
}
}
再进去Page page = pageParams.getPage(parameterObject, rowBounds);这一行
Page page = PageHelper.getLocalPage();
if (page == null) {
if (rowBounds != RowBounds.DEFAULT) {
if (offsetAsPageNum) {
page = new Page(rowBounds.getOffset(), rowBounds.getLimit(), rowBoundsWithCount);
} else {
page = new Page(new int[]{rowBounds.getOffset(), rowBounds.getLimit()}, rowBoundsWithCount);
//offsetAsPageNum=false的时候，由于PageNum问题，不能使用reasonable，这里会强制为false
page.setReasonable(false);
}
if(rowBounds instanceof PageRowBounds){
PageRowBounds pageRowBounds = (PageRowBounds)rowBounds;
page.setCount(pageRowBounds.getCount() == null || pageRowBounds.getCount());
}
} else if(parameterObject instanceof IPage || supportMethodsArguments){
try {
page = PageObjectUtil.getPageFromObject(parameterObject, false);
} catch (Exception e) {
return null;
}
}
if(page == null){
return null;
}
PageHelper.setLocalPage(page);
}
从else if(parameterObject instanceof IPage || supportMethodsArguments) 看出，假如supportMethodsArguments开启了，会进到PageObjectUtil.getPageFromObject方法
Object orderBy = getParamValue(paramsObject, "orderBy", false);
boolean hasOrderBy = false;
if (orderBy != null && orderBy.toString().length() > 0) {
hasOrderBy = true;
}
try {
Object _pageNum = getParamValue(paramsObject, "pageNum", required);
Object _pageSize = getParamValue(paramsObject, "pageSize", required);
if (_pageNum == null || _pageSize == null) {
if(hasOrderBy){
Page page = new Page();
page.setOrderBy(orderBy.toString());
page.setOrderByOnly(true);
return page;
}
return null;
}
pageNum = Integer.parseInt(String.valueOf(_pageNum));
pageSize = Integer.parseInt(String.valueOf(_pageSize));
} catch (NumberFormatException e) {
throw new PageException("分页参数不是合法的数字类型!", e);
}
Page page = new Page(pageNum, pageSize);
//count查询
Object _count = getParamValue(paramsObject, "count", false);
if (_count != null) {
page.setCount(Boolean.valueOf(String.valueOf(_count)));
}
//排序
if (hasOrderBy) {
page.setOrderBy(orderBy.toString());
}
从上面可以看到，代码会从接口参数里获取orderBy，pageNum，pageSize的值，然后设置到当前线程的page里
回到 PageHelper 包的intercept方法，进入ExecutorUtil.pageQuery方法，最后看到封装sql的方法
@Override
public String getPageSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {
String sql = boundSql.getSql();
Page page = getLocalPage();
//支持 order by
String orderBy = page.getOrderBy();
if (StringUtil.isNotEmpty(orderBy)) {
pageKey.update(orderBy);
sql = OrderByParser.converToOrderBySql(sql, orderBy);
}
if (page.isOrderByOnly()) {
return sql;
}
return getPageSql(sql, page, pageKey);
}
从上面可以看出，假如当前线程的page有orderBy值，那么就会拼接到执行sql上去，也就是一开始的query里的orderBy参数的值就这样用起来了，假如还有pageNum，pageSize，就进入到getPageSql(sql, page, pageKey)
@Override
public String getPageSql(String sql, Page page, CacheKey pageKey) {
StringBuilder sqlBuilder = new StringBuilder(sql.length() + 120);
sqlBuilder.append("SELECT * FROM ( ");
sqlBuilder.append(" SELECT TMP_PAGE.*, ROWNUM ROW_ID FROM ( ");
sqlBuilder.append(sql);
sqlBuilder.append(" ) TMP_PAGE)");
sqlBuilder.append(" WHERE ROW_ID <= ? AND ROW_ID > ?");
return sqlBuilder.toString();
}
这里用的数据库是Oracle，所以上面是相对应的sql拼接实现，可以看出这里拼接了分页的相关sql，当然之后还有一系列的赋值操作，进入对应的分页参数操作方法
@Override
public Object processPageParameter(MappedStatement ms, Map<String, Object> paramMap, Page page, BoundSql boundSql, CacheKey pageKey) {
paramMap.put(PAGEPARAMETER_FIRST, page.getEndRow());
paramMap.put(PAGEPARAMETER_SECOND, page.getStartRow());
//处理pageKey
pageKey.update(page.getEndRow());
pageKey.update(page.getStartRow());
//处理参数配置
handleParameter(boundSql, ms);
return paramMap;
}
这里就把page里的分页相关参数值设置到了paramMap，PAGEPARAMETER_FIRST，PAGEPARAMETER_SECOND就是对应上面sql最后的那两个分页占位符
总结：pagehelper.supportMethodsArguments设置为 true，那么 Mybatis 的 mapper 接口参数对象里的orderBy，pageNum，pageSize这三个关键参数值会被使用到具体的执行sql里。