PageHelper

@PostMapping("/list")
public PageInfo<ProductInfo> list(@RequestBody BasePage basePage){
    PageHelper.startPage(basePage.getPageNum(),basePage.getPageSize());
    List<ProductInfo> list = productInfoService.list(Wrappers.emptyWrapper());
    PageInfo<ProductInfo> productInfoPageInfo = new PageInfo<>(list);
    return productInfoPageInfo;
}

/**
* 分页调用的最终方法
**/
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;
}

//里边最重要的就是Page<E> oldPage = getLocalPage();和setLocalPage(page);方法，他俩是看当前线程中的
//ThreadLocal.ThreadLocalMap中是否存在该page对象，若存在直接取出，若不存在则设置一个，我们以第一个为例继续深入


protected 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 T get() {
    //获取当前线程
    Thread t = Thread.currentThread();
    //获取当前线程中的ThreadLocalMap
    ThreadLocalMap map = getMap(t);//ThreadLocal.ThreadLocalMap threadLocals = null;
    if (map != null) {
        //getEntry(ThreadLocal<?> key)源码在下边
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();//=> t.threadLocals = new ThreadLocalMap(this, firstValue);
}

private Entry getEntry(ThreadLocal<?> key) {
    //通过hashCode与length位运算确定出一个索引值i，这个i就是被存储在table数组中的位置
    int i = key.threadLocalHashCode & (table.length - 1);
    Entry e = table[i];
    if (e != null && e.get() == key)
        return e;
    else
        return getEntryAfterMiss(key, i, e);
}

static class Entry extends WeakReference<ThreadLocal<?>> {
    /** The value associated with this ThreadLocal. */
    Object value;

    Entry(ThreadLocal<?> k, Object v) {
        super(k);
        value = v;
    }
}

@Override
public Object intercept(Invocation invocation) throws Throwable {
    try {
        ...

        resultList = ExecutorUtil.pageQuery(dialect, executor,
                                            ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);
        ...
        }
}

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);
            ...
        }
}
获取到ThreadLocal中的page对象

@Override
public Object processParameterObject(MappedStatement ms, Object parameterObject, BoundSql boundSql, CacheKey pageKey) {
     ...
     return processPageParameter(ms, paramMap, page, boundSql, pageKey);
     }