Mybatis分页工具PageHelper详解

关于spring boot中使用PageHelper类

首先引入依赖

    com.github.pagehelper
    pagehelper-spring-boot-starter
    1.2.3

PageHelper使用时比较简单的,调用startPage方法,第一个参数:当前页数,第二个参数:每页查询展示记录数
PageHelper.startPage(pageNum,pageSize);
但是为了更好的理解和使用,我去看了下源码,看了下大概是如何实现分页的,话不多说上代码

测试代码

/**
 * 通过PageHelper实现分页
 * @param page
 * @param animal
 * @return
 */
@Override
public PageHelperCustom queryAnimalByPageHelper(PageHelperCustom page, Animal animal) {

    PageHelper.startPage(page.getPageNum(),page.getPageSize());
    List list = animalMapperCustom.queryAnimalAndTypeByPageHelperList(animal);

    long count = new PageInfo<>(list).getTotal();//获取总数
    page.setCount(count);
    page.setList(list);

    /*Page page1 = (Page)list;  这种方式也可以直接得到 count总数
    System.out.println("page1>>>>>>>>>>"+page1.getTotal());*/

    return page;
}

mapper中sql,就是简单的查询,这里并没有做什么分页

  select t.*,atype.type_text as typeText from animal t
  join animal_type atype on atype.id = t.type
  where 1=1

 打断点,当执行 List list = animalMapperCustom.queryAnimalAndTypeByPageHelperList(animal);查询时,进入了

MapperProxy代理类的invoke方法

public class PageInterceptor implements Interceptor {
....
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
        if (Object.class.equals(method.getDeclaringClass())) {
            return method.invoke(this, args);
        }

        if (this.isDefaultMethod(method)) {
            return this.invokeDefaultMethod(proxy, method, args);
        }
    } catch (Throwable var5) {
        throw ExceptionUtil.unwrapThrowable(var5);
    }

    MapperMethod mapperMethod = this.cachedMapperMethod(method);
    return mapperMethod.execute(this.sqlSession, args);
}

继续往下执行进入excute

public Object execute(SqlSession sqlSession, Object[] args) {
    Object param;
    Object result;
    switch(this.command.getType()) {
    case INSERT:
        param = this.method.convertArgsToSqlCommandParam(args);
        result = this.rowCountResult(sqlSession.insert(this.command.getName(), param));
        break;
    case UPDATE:
        param = this.method.convertArgsToSqlCommandParam(args);
        result = this.rowCountResult(sqlSession.update(this.command.getName(), param));
        break;
    case DELETE:
        param = this.method.convertArgsToSqlCommandParam(args);
        result = this.rowCountResult(sqlSession.delete(this.command.getName(), param));
        break;
    case SELECT:
        if (this.method.returnsVoid() && this.method.hasResultHandler()) {
            this.executeWithResultHandler(sqlSession, args);
            result = null;
        } else if (this.method.returnsMany()) {
            result = this.executeForMany(sqlSession, args);
        } else if (this.method.returnsMap()) {
            result = this.executeForMap(sqlSession, args);
        } else if (this.method.returnsCursor()) {
            result = this.executeForCursor(sqlSession, args);
        } else {
            param = this.method.convertArgsToSqlCommandParam(args);
            result = sqlSession.selectOne(this.command.getName(), param);
        }
        break;
    case FLUSH:
        result = sqlSession.flushStatements();
        break;
    default:
        throw new BindingException("Unknown execution method for: " + this.command.getName());
    }

    if (result == null && this.method.getReturnType().isPrimitive() && !this.method.returnsVoid()) {
        throw new BindingException("Mapper method '" + this.command.getName() + " attempted to return null from a method with a primitive return type (" + this.method.getReturnType() + ").");
    } else {
        return result;
    }
}

这里是执行select并且是查询多结果executeForMany

继续执行,中间仍有许多过程,直接看到最后执行查询的地方PageInterceptor,我们可以看到PageInterceptor 实现了Interceptor 拦截器的,到这里差不多就知道分页的基本原理了,最后是通过拦截器实现的,当然中间还有一些处理比如边界的设置等

public class PageInterceptor implements Interceptor {

    ......

public Object intercept(Invocation invocation) throws Throwable {
    Object var22;
    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;
        if (args.length == 4) {
            boundSql = ms.getBoundSql(parameter);
            cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
        } else {
            cacheKey = (CacheKey)args[4];
            boundSql = (BoundSql)args[5];
        }

        List resultList;
        if (this.dialect.skip(ms, parameter, rowBounds)) {
            resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
        } else {
            String msId = ms.getId();
            Configuration configuration = ms.getConfiguration();
            Map additionalParameters = (Map)this.additionalParametersField.get(boundSql);
            if (this.dialect.beforeCount(ms, parameter, rowBounds)) {
                String countMsId = msId + this.countSuffix;
                MappedStatement countMs = this.getExistedMappedStatement(configuration, countMsId);
                Long count;
                if (countMs != null) {
                    count = this.executeManualCount(executor, countMs, parameter, boundSql, resultHandler);
                } else {
                    countMs = (MappedStatement)this.msCountMap.get(countMsId);
                    if (countMs == null) {
                        countMs = MSUtils.newCountMappedStatement(ms, countMsId);
                        this.msCountMap.put(countMsId, countMs);
                    }

                    count = this.executeAutoCount(executor, countMs, parameter, boundSql, rowBounds, resultHandler);
                }

                if (!this.dialect.afterCount(count, parameter, rowBounds)) {
                    Object var24 = this.dialect.afterPage(new ArrayList(), parameter, rowBounds);
                    return var24;
                }
            }

            if (!this.dialect.beforePage(ms, parameter, rowBounds)) {
                resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);
            } else {
                parameter = this.dialect.processParameterObject(ms, parameter, boundSql, cacheKey);
                String pageSql = this.dialect.getPageSql(ms, boundSql, parameter, rowBounds, cacheKey);
                BoundSql pageBoundSql = new BoundSql(configuration, pageSql, boundSql.getParameterMappings(), parameter);
                Iterator var17 = additionalParameters.keySet().iterator();

                while(true) {
                    if (!var17.hasNext()) {
                        resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, pageBoundSql);
                        break;
                    }

                    String key = (String)var17.next();
                    pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
                }
            }
        }

        var22 = this.dialect.afterPage(resultList, parameter, rowBounds);
    } finally {
        this.dialect.afterAll();
    }

    return var22;
}

执行完 intercept可以看到控制台有打印sql,这里用的数据库是mysql,所以用的LIMIT 分页,起始也是通过了数据源判断之后,对sql进行了相应的拼接

select t.*,atype.type_text as typeText from animal t join animal_type atype on atype.id = t.type where 1=1  LIMIT ?, ?

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 ?, ? ");
    }

    pageKey.update(page.getPageSize());
    return sqlBuilder.toString();
}

如果是Oracle

public String getPageSql(String sql, Page page, CacheKey pageKey) {
    StringBuilder sqlBuilder = new StringBuilder(sql.length() + 120);
    if (page.getStartRow() > 0) {
        sqlBuilder.append("SELECT * FROM ( ");
    }

    if (page.getEndRow() > 0) {
        sqlBuilder.append(" SELECT TMP_PAGE.*, ROWNUM ROW_ID FROM ( ");
    }

    sqlBuilder.append(sql);
    if (page.getEndRow() > 0) {
        sqlBuilder.append(" ) TMP_PAGE WHERE ROWNUM <= ? ");
    }

    if (page.getStartRow() > 0) {
        sqlBuilder.append(" ) WHERE ROW_ID > ? ");
    }

    return sqlBuilder.toString();
}

不同的数据库,分页都有不同的拼接实现

最后上面还可以获取查询的记录总数,如若不想查询可以这样设置

PageHelper.startPage(pageNum,pageSize,false);

public static  Page startPage(int pageNum, int pageSize) {
    return startPage(pageNum, pageSize, true);
}

public static  Page startPage(int pageNum, int pageSize, boolean count) {
    return startPage(pageNum, pageSize, count, (Boolean)null, (Boolean)null);
}

总之PageHelper作为分页工具是很不错的,支持多种数据库