本文针对PageHelper 4.1.3版本源码,与5.X版本源码差异较大
使用
- pom文件中加入PageHelper插件
<dependency>
<groupId>com.github.pagehelper</groupId>
<artifactId>pagehelper</artifactId>
<version>4.1.3</version>
</dependency>
- 在mybatis-config.xml中配置拦截器PageHelper,此为PageHelper的核心类
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<settings>
<setting name="mapUnderscoreToCamelCase" value="true"/>
</settings>
<plugins>
<plugin interceptor="com.github.pagehelper.PageHelper">
<property name="dialect" value="mysql"/>
<property name="offsetAsPageNum" value="false"/>
<property name="rowBoundsWithCount" value="false"/>
<property name="pageSizeZero" value="true"/>
<property name="reasonable" value="false"/>
<property name="params" value="pageNum=start;pageSize=limit;"/>
<property name="supportMethodsArguments" value="true"/>
<property name="returnPageInfo" value="none"/>
</plugin>
</plugins>
</configuration>
配置里面的节点顺序是properties->settings->typeAliases->typeHandlers->objectFactory->objectWrapperFactory->plugins->environments->databaseIdProvider->mappers
- 代码中直接使用PageHelper.startPage(1, count)即可自动拼接limit参数进行分页查询
Example itemExample = new Example(Item.class);
Example.Criteria itemCriteria = itemExample.createCriteria();
itemCriteria.andBetween("createDate", startTime, endTime);
itemCriteria.andEqualTo("delFlag", BizzConstants.DEL_FLAG_UN_DELETED);
itemExample.orderBy("orderId").asc();
PageHelper.startPage(1, 10);
List<Item> incomeItemList = ItemMapper.selectByExample(itemExample);
PageHelper.startPage()的作用?
- 从PageHelper.startPage()方法入手
@SuppressWarnings("rawtypes")
@Intercepts(@Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}))
public class PageHelper implements Interceptor {
/**
* 开始分页
*
* @param pageNum 页码
* @param pageSize 每页显示数量
*/
public static <E> Page<E> startPage(int pageNum, int pageSize) {
return startPage(pageNum, pageSize, true);
}
/**
* 开始分页
*
* @param pageNum 页码
* @param pageSize 每页显示数量
* @param count 是否进行count查询
*/
public static <E> Page<E> startPage(int pageNum, int pageSize, boolean count) {
return startPage(pageNum, pageSize, count, null);
}
/**
* 开始分页
*
* @param pageNum 页码
* @param pageSize 每页显示数量
* @param count 是否进行count查询
* @param reasonable 分页合理化,null时用默认配置
*/
public static <E> Page<E> startPage(int pageNum, int pageSize, boolean count, Boolean reasonable) {
return startPage(pageNum, pageSize, count, reasonable, null);
}
/**
* 开始分页
*
* @param pageNum 页码
* @param pageSize 每页显示数量
* @param count 是否进行count查询
* @param reasonable 分页合理化,null时用默认配置
* @param pageSizeZero true且pageSize=0时返回全部结果,false时分页,null时用默认配置
*/
public static <E> Page<E> startPage(int pageNum, int pageSize, boolean count, Boolean reasonable, Boolean pageSizeZero) {
Page<E> page = new Page<E>(pageNum, pageSize, count);
page.setReasonable(reasonable);
page.setPageSizeZero(pageSizeZero);
//当已经执行过orderBy的时候
Page<E> oldPage = SqlUtil.getLocalPage();
if (oldPage != null && oldPage.isOrderByOnly()) {
page.setOrderBy(oldPage.getOrderBy());
}
SqlUtil.setLocalPage(page);
return page;
}
}
PageHelper.startPage()方法创建了一个Page对象,将该Page对象保存到SqlUtil中;创建Page对象时,沿用之前Page对象的orderBy
- Page对象保存了分页参数
public class Page<E> extends ArrayList<E> {
public Page(int pageNum, int pageSize, boolean count) {
this(pageNum, pageSize, count, null);
}
private Page(int pageNum, int pageSize, boolean count, Boolean reasonable) {
super(0);
if (pageNum == 1 && pageSize == Integer.MAX_VALUE) {
pageSizeZero = true;
pageSize = 0;
}
this.pageNum = pageNum;
this.pageSize = pageSize;
this.count = count;
calculateStartAndEndRow();
setReasonable(reasonable);
}
}
注意:Page对象继承了ArrayList
- SqlUtil中LOCAL_PAGE变量存储该Page对象,ThreadLocal保持线程封闭
public class SqlUtil implements Constant {
private static final ThreadLocal<Page> LOCAL_PAGE = new ThreadLocal<Page>();
/**
* 获取Page参数
*
* @return
*/
public static <T> Page<T> getLocalPage() {
return LOCAL_PAGE.get();
}
public static void setLocalPage(Page page) {
LOCAL_PAGE.set(page);
}
/**
* 移除本地变量
*/
public static void clearLocalPage() {
LOCAL_PAGE.remove();
}
}
到此已经可以理解PageHelper.startPage(1, count)方法的具体原理:
1、创建Page对象,保存pageNum、pageSize信息
2、将该对象保存到SqlUtil中,以便后续执行SQL时获取
执行时如何使用Page?
- Mybatis拦截器
public interface Interceptor {
Object intercept(Invocation invocation) throws Throwable;
Object plugin(Object target);
void setProperties(Properties properties);
}
- PageHelper实现该拦截器,拦截Executor
public class PageHelper implements Interceptor {
/**
* 只拦截Executor
*
* @param target
* @return
*/
public Object plugin(Object target) {
if (target instanceof Executor) {
return Plugin.wrap(target, this);
} else {
return target;
}
}
/**
* 设置属性值
*
* @param p 属性值
*/
public void setProperties(Properties p) {
checkVersion();
//多数据源时,获取jdbcurl后是否关闭数据源
String closeConn = p.getProperty("closeConn");
this.closeConn = Boolean.parseBoolean(closeConn);
//数据库方言
String dialect = p.getProperty("dialect");
String runtimeDialect = p.getProperty("autoRuntimeDialect");
if (StringUtil.isNotEmpty(runtimeDialect) && runtimeDialect.equalsIgnoreCase("TRUE")) {
this.autoRuntimeDialect = true;
this.autoDialect = false;
this.properties = p;
} else if (StringUtil.isEmpty(dialect)) {
autoDialect = true;
this.properties = p;
} else {
autoDialect = false;
sqlUtil = new SqlUtil(dialect);
sqlUtil.setProperties(p);
}
}
/**
* Mybatis拦截器方法
*
* @param invocation 拦截器入参
* @return 返回执行结果
* @throws Throwable 抛出异常
*/
public Object intercept(Invocation invocation) throws Throwable {
if (autoRuntimeDialect) {
SqlUtil sqlUtil = getSqlUtil(invocation);
return sqlUtil.processPage(invocation);
} else {
if (autoDialect) {
initSqlUtil(invocation);
}
return sqlUtil.processPage(invocation);
}
}
}
PageHelper拦截Executor,使用SqlUtil.processPage()方法进行处理
public class SqlUtil implements Constant {
/**
* Mybatis拦截器方法,这一步嵌套为了在出现异常时也可以清空Threadlocal
*
* @param invocation 拦截器入参
* @return 返回执行结果
* @throws Throwable 抛出异常
*/
public Object processPage(Invocation invocation) throws Throwable {
try {
Object result = _processPage(invocation);
return result;
} finally {
clearLocalPage();
}
}
/**
* Mybatis拦截器方法
*
* @param invocation 拦截器入参
* @return 返回执行结果
* @throws Throwable 抛出异常
*/
private Object _processPage(Invocation invocation) throws Throwable {
final Object[] args = invocation.getArgs();
Page page = null;
//支持方法参数时,会先尝试获取Page
if (supportMethodsArguments) {
page = getPage(args);
}
//分页信息
RowBounds rowBounds = (RowBounds) args[2];
//支持方法参数时,如果page == null就说明没有分页条件,不需要分页查询
if ((supportMethodsArguments && page == null)
//当不支持分页参数时,判断LocalPage和RowBounds判断是否需要分页
|| (!supportMethodsArguments && SqlUtil.getLocalPage() == null && rowBounds == RowBounds.DEFAULT)) {
return invocation.proceed();
} else {
//不支持分页参数时,page==null,这里需要获取
if (!supportMethodsArguments && page == null) {
page = getPage(args);
}
return doProcessPage(invocation, page, args);
}
}
/**
* Mybatis拦截器方法
*
* @param invocation 拦截器入参
* @return 返回执行结果
* @throws Throwable 抛出异常
*/
private Page doProcessPage(Invocation invocation, Page page, Object[] args) throws Throwable {
//保存RowBounds状态
RowBounds rowBounds = (RowBounds) args[2];
//获取原始的ms
MappedStatement ms = (MappedStatement) args[0];
//判断并处理为PageSqlSource
if (!isPageSqlSource(ms)) {
processMappedStatement(ms);
}
//设置当前的parser,后面每次使用前都会set,ThreadLocal的值不会产生不良影响
((PageSqlSource)ms.getSqlSource()).setParser(parser);
//忽略RowBounds-否则会进行Mybatis自带的内存分页
args[2] = RowBounds.DEFAULT;
//如果只进行排序 或 pageSizeZero的判断
if (isQueryOnly(page)) {
return doQueryOnly(page, invocation);
}
//简单的通过total的值来判断是否进行count查询
if (page.isCount()) {
page.setCountSignal(Boolean.TRUE);
//替换MS
args[0] = msCountMap.get(ms.getId());
//查询总数
Object result = invocation.proceed();
//还原ms
args[0] = ms;
//设置总数
page.setTotal((Integer) ((List) result).get(0));
if (page.getTotal() == 0) {
return page;
}
} else {
page.setTotal(-1l);
}
//pageSize>0的时候执行分页查询,pageSize<=0的时候不执行相当于可能只返回了一个count
if (page.getPageSize() > 0 &&
((rowBounds == RowBounds.DEFAULT && page.getPageNum() > 0)
|| rowBounds != RowBounds.DEFAULT)) {
//将参数中的MappedStatement替换为新的qs
page.setCountSignal(null);
BoundSql boundSql = ms.getBoundSql(args[1]);
args[1] = parser.setPageParameter(ms, args[1], boundSql, page);
page.setCountSignal(Boolean.FALSE);
//执行分页查询
Object result = invocation.proceed();
//得到处理结果
page.addAll((List) result);
}
//返回结果
return page;
}
/**
* 是否只做查询
*
* @param page
* @return
*/
private boolean isQueryOnly(Page page) {
return page.isOrderByOnly()
|| ((page.getPageSizeZero() != null && page.getPageSizeZero()) && page.getPageSize() == 0);
}
/**
* 只做查询
*
* @param page
* @param invocation
* @return
* @throws Throwable
*/
private Page doQueryOnly(Page page, Invocation invocation) throws Throwable {
page.setCountSignal(null);
//执行正常(不分页)查询
Object result = invocation.proceed();
//得到处理结果
page.addAll((List) result);
//相当于查询第一页
page.setPageNum(1);
//这种情况相当于pageSize=total
page.setPageSize(page.size());
//仍然要设置total
page.setTotal(page.size());
//返回结果仍然为Page类型 - 便于后面对接收类型的统一处理
return page;
}
}
一、_processPage(Invocation invocation) 方法中,取出此前保存的Page对象
二、doProcessPage(Invocation invocation, Page page, Object[] args)方法中执行具体查询
1、如果只做查询,将会调用doQueryOnly(Page page, Invocation invocation),此方法将SQL执行返回的List结果转换为Page对象,设置pageSize、total的值
2、如果Page.isCount()为true,则先进行一次count查询统计总条数,根据count结果计算好total、pages、pageNum的值;如果count结果为0,则不进行后续分页查询(使用PageHelper.startPage(int pageNum, int pageSize)创建Page对象时,Page.isCount()值默认为true)
更多使用方法
- 不分页
PageHelper.startPage(1, -1);
- 返回的Page对象,是ArrayList的子类,包含了分页信息,如果想获取查询结果中的分页信息,需要将结果强转为Page类型
Page<User> page = (Page) userMapper.findAllUsers();
page.getPageNum();
page.getPageSize();
page.getPages();
page.getTotal();
page.size();
- PageInfo包装Page对象,可以获取更多属性
PageInfo<User> info = new PageInfo<>(userMapper.findAllUsers());