使用的插件有
<dependency>
<groupId>com.github.pagehelper</groupId>
<artifactId>pagehelper-spring-boot-starter</artifactId>
<version>1.2.13</version>
</dependency>
<dependency>
<groupId>com.baomidou</groupId>
<artifactId>mybatis-plus-boot-starter</artifactId>
<version>3.3.1</version>
</dependency>控制器中使用下面语句就能分页。
startPage();
List<SysUser> list = userService.selectUserList(user);
return getDataTable(list);方法startPage，有什么用呢？
protected void startPage()
{
PageDomain pageDomain = TableSupport.buildPageRequest();
Integer pageNum = pageDomain.getPageNum();
Integer pageSize = pageDomain.getPageSize();
if (StringUtils.isNotNull(pageNum) && StringUtils.isNotNull(pageSize))
{
String orderBy = pageDomain.getOrderBy();
PageHelper.startPage(pageNum, pageSize, orderBy);
}
}startPage首先将前台传递过来的分页信息记录下来。
public static PageDomain getPageDomain()
{
PageDomain pageDomain = new PageDomain();
pageDomain.setPageNum(ServletUtils.getParameterToInt(Constants.PAGE_NUM));
pageDomain.setPageSize(ServletUtils.getParameterToInt(Constants.PAGE_SIZE));
pageDomain.setOrderByColumn(ServletUtils.getParameter(Constants.ORDER_BY_COLUMN));
pageDomain.setIsAsc(ServletUtils.getParameter(Constants.IS_ASC));
return pageDomain;
}
public static PageDomain buildPageRequest()
{
return getPageDomain();
}那么PageHelper.startPage(pageNum, pageSize, orderBy)是什么呢？
/**
* 开始分页
*
* @param pageNum 页码
* @param pageSize 每页显示数量
* @param orderBy 排序
*/
public static <E> Page<E> startPage(int pageNum, int pageSize, String orderBy) {
Page<E> page = startPage(pageNum, pageSize);
page.setOrderBy(orderBy);
return page;
}
···
/**
* 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 = getLocalPage();
if (oldPage != null && oldPage.isOrderByOnly()) {
page.setOrderBy(oldPage.getOrderBy());
}
setLocalPage(page);
return page;
}其中getLocalPage()和setLocalPage();很奇怪：
protected static final ThreadLocal<Page> LOCAL_PAGE = new ThreadLocal<Page>();
protected static boolean DEFAULT_COUNT = true;
/**
* 设置 Page 参数
*
* @param page
*/
protected static void setLocalPage(Page page) {
LOCAL_PAGE.set(page);
}
/**
* 获取 Page 参数
*
* @return
*/
public static <T> Page<T> getLocalPage() {
return LOCAL_PAGE.get();
}原来setLocalPage是将page保存到本地线程变量里面。那么有赋值，就该有取值，不然就没什么用了，对吧，但是在哪里用呢？先跳过这里。回到selectUserList。
执行完startPage,就执行了mybatis的sql语句：
<select id="selectUserList" parameterType="SysUser" resultMap="SysUserResult">
select u.user_id, u.dept_id, u.login_name, u.user_name, u.email, u.phonenumber, u.password, u.sex, u.avatar, u.salt, u.status, u.order_num, u.del_flag, u.login_ip, u.login_date, u.create_by, u.create_time, u.update_time, u.remark
from sys_user u
where u.del_flag = '0'
group by u.user_id
order by u.order_num, u.user_name, d.order_num
</select>就是这么简单的sql语句，为甚页面会分页呢？难道有地方进行了拦截处理？
debug发现进入com.baomidou.mybatisplus.core.override的MybatisMapperProxy类中
@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()) {
if (privateLookupInMethod == null) {
return invokeDefaultMethodJava8(proxy, method, args);
} else {
return invokeDefaultMethodJava9(proxy, method, args);
}
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
final MybatisMapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}最后执行execute方法，接着进入MybatisMapperMethod类中：
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
switch (command.getType()) {
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
if (IPage.class.isAssignableFrom(method.getReturnType())) {
assert args != null;
IPage<?> page = null;
for (Object arg : args) {
if (arg instanceof IPage) {
page = (IPage) arg;
break;
}
}
assert page != null;
result = executeForIPage(sqlSession, args);
if (result instanceof PageList) {
PageList pageList = (PageList) result;
page.setRecords(pageList.getRecords());
page.setTotal(pageList.getTotal());
result = page;
} else {
List list = (List<Object>) result;
result = page.setRecords(list);
}
} else {
result = sqlSession.selectOne(command.getName(), param);
if (method.returnsOptional()
&& (result == null || !method.getReturnType().equals(result.getClass()))) {
result = Optional.ofNullable(result);
}
}
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}执行的查询多条的select，所以到了 result = executeForMany(sqlSession, args)
然后是SqlSessionTemplate和DefaultSqlSession
最后可以看到代码进入了org.apache.ibatis.plugin.Plugin这个类的invoke方法中
@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);
}
}这下明白了，interceptor是mybatis的拦截器，而com.github.pagehelper.PageInterceptor这个类就实现了interceptor接口，调用其中的intercept方法。
@Override
public Object intercept(Invocation invocation) throws Throwable {
try {
Object[] args = invocation.getArgs();
MappedStatement ms = (MappedStatement) args[0];
Object parameter = args[1];
RowBounds rowBounds = (RowBounds) args[2];
ResultHandler resultHandler = (ResultHandler) args[3];
Executor executor = (Executor) invocation.getTarget();
CacheKey cacheKey;
BoundSql boundSql;
//由于逻辑关系，只会进入一次
if (args.length == 4) {
//4 个参数时
boundSql = ms.getBoundSql(parameter);
cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
} else {
//6 个参数时
cacheKey = (CacheKey) args[4];
boundSql = (BoundSql) args[5];
}
checkDialectExists();
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);
}
return dialect.afterPage(resultList, parameter, rowBounds);
} finally {
if(dialect != null){
dialect.afterAll();//这里将销毁threadlocal
}
}
}ExecutorUtil.pageQuery()来处理分页，但是怎么处理的呢？
com.github.pagehelper.util.ExecutorUtil
public static <E> List<E> pageQuery(Dialect dialect, Executor executor, MappedStatement ms, Object parameter,
RowBounds rowBounds, ResultHandler resultHandler,
BoundSql boundSql, CacheKey cacheKey) throws SQLException {
//判断是否需要进行分页查询
if (dialect.beforePage(ms, parameter, rowBounds)) {
//生成分页的缓存 key
CacheKey pageKey = cacheKey;
//处理参数对象
parameter = dialect.processParameterObject(ms, parameter, boundSql, pageKey);
//调用方言获取分页 sql
String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);
BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);
Map<String, Object> additionalParameters = getAdditionalParameter(boundSql);
//设置动态参数
for (String key : additionalParameters.keySet()) {
pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
}
//执行分页查询
return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);
} else {
//不执行分页的情况下，也不执行内存分页
return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);
}
}getPageSql方法：
com.github.pagehelper.PageHelper
@Override
public String getPageSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {
return autoDialect.getDelegate().getPageSql(ms, boundSql, parameterObject, rowBounds, pageKey);
}然后调用com.github.pagehelper.dialect.AbstractHelperDialect
@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);
}在getPageSql中getLocalPage将ThreadLocal中page变量取出来。
com.github.pagehelper.dialect.helper.MySqlDialect
@Override
public String getPageSql(String sql, Page page, CacheKey pageKey) {
StringBuilder sqlBuilder = new StringBuilder(sql.length() + 14);
sqlBuilder.append(sql);
if (page.getStartRow() == 0) {
sqlBuilder.append(" LIMIT ? ");
} else {
sqlBuilder.append(" LIMIT ?, ? ");
}
return sqlBuilder.toString();
}PageHelper首先将前端传递的参数保存到page这个对象中，接着将page的副本存放入ThreadLocal中，这样可以保证分页的时候，参数互不影响，接着利用了mybatis提供的拦截器，取得ThreadLocal的值，重新拼装分页SQL，完成分页。我们都知道ThreadLocal使用完以后需要销毁来防止内存泄露，那么pageHelper是在哪里销毁的呢？
dialect.afterAll() 调用PageHelper类
@Override
public void afterAll() {
//这个方法即使不分页也会被执行，所以要判断 null
AbstractHelperDialect delegate = autoDialect.getDelegate();
if (delegate != null) {
delegate.afterAll();
autoDialect.clearDelegate();
}
clearPage();//就是 LOCAL_PAGE.remove();
}
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