PageHelper分页原理（源码）

PageHelper分页原理

PageHelper是我们经常使用的一个分页插件，之前咱们一直处于使用阶段的，今天咱们去探究一下其中的原理。
SQL语句实现分页查询知识，就不在赘述了。

LIMIT i,a;i:是指查询的索引值（默认是0）a:是指查询的数量值SELECT id FROM '表' WHERE '条件' LIMIT (i-1)*a,a;

首先,咱们使用PageHelper分页首先要在查询数据库数据方法前先调用PageHelper.startPage方法。

pageNum 当前页数
pageSize  每页有多少条数据
orderBy    数据的排序条件
PageHelper.startPage(pageNum, pageSize, orderBy);

该方法的三个参数都是前端传递过来的，咱们debug进入这个方法一探究竟,发现这个方法根据，pageNum、pageSize创建了一个page类，然后给他set了一个排序的条件，至此创建出一个page分页类，在这个方法之后调用的查询数据库方法都会进行分页查询。

public static <E> Page<E> startPage(int pageNum, int pageSize, String orderBy) {Page<E> page = startPage(pageNum, pageSize);page.setOrderBy(orderBy);return page;
}

咱们继续来看为啥会实现分页的，直接debug进入startPage方法下的查询数据库的方法。进入了一个cglib代理的分页拦截器，咱们这个不需要aop代理之类的操作直接调用方法就可以。

@Nullable
public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {Object oldProxy = null;boolean setProxyContext = false;Object target = null;TargetSource targetSource = this.advised.getTargetSource();Object var16;try {if (this.advised.exposeProxy) {oldProxy = AopContext.setCurrentProxy(proxy);setProxyContext = true;}target = targetSource.getTarget();Class<?> targetClass = target != null ? target.getClass() : null;List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);Object retVal;if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) {Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);retVal = methodProxy.invoke(target, argsToUse);} else {retVal = (new CglibAopProxy.CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy)).proceed();}retVal = CglibAopProxy.processReturnType(proxy, target, method, retVal);var16 = retVal;} finally {if (target != null && !targetSource.isStatic()) {targetSource.releaseTarget(target);}if (setProxyContext) {AopContext.setCurrentProxy(oldProxy);}}return var16;
}

直接执行这个方法 retVal = methodProxy.invoke(target, argsToUse);调用原方法，现在还和分页原理无关，请看官耐心等待

public Object invoke(Object obj, Object[] args) throws Throwable {try {this.init();MethodProxy.FastClassInfo fci = this.fastClassInfo;return fci.f1.invoke(fci.i1, obj, args);} catch (InvocationTargetException var4) {throw var4.getTargetException();} catch (IllegalArgumentException var5) {if (this.fastClassInfo.i1 < 0) {throw new IllegalArgumentException("Protected method: " + this.sig1);} else {throw var5;}}
}

return fci.f1.invoke(fci.i1, obj, args);执行原方法

/*** 根据条件分页查询字典类型* * @param dictType 字典类型信息* @return 字典类型集合信息*/
@Override
public List<SysDictType> selectDictTypeList(SysDictType dictType)
{return dictTypeMapper.selectDictTypeList(dictType);
}

来到这个方法，method.getDeclaringClass()判断这个类是否是object基础类型的类是的话直接调基础类型，不是的话调用this.cachedInvoker(method).invoke(proxy, method, args, this.sqlSession)

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {return Object.class.equals(method.getDeclaringClass()) ? method.invoke(this, args) : this.cachedInvoker(method).invoke(proxy, method, args, this.sqlSession);} catch (Throwable var5) {throw ExceptionUtil.unwrapThrowable(var5);}
}

来到这个方法sqlSession和 args继续调用，sqlSession是操作数据库的类

public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {return this.mapperMethod.execute(sqlSession, args);
}

进入这个方法，因为是 SELECT 所以执行result = this.executeForMany(sqlSession, args);

public Object execute(SqlSession sqlSession, Object[] args) {Object result;Object param;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);if (this.method.returnsOptional() && (result == null || !this.method.getReturnType().equals(result.getClass()))) {result = Optional.ofNullable(result);}}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;}
}

来到executeForMany方法

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {Object param = this.method.convertArgsToSqlCommandParam(args);List result;if (this.method.hasRowBounds()) {RowBounds rowBounds = this.method.extractRowBounds(args);result = sqlSession.selectList(this.command.getName(), param, rowBounds);} else {result = sqlSession.selectList(this.command.getName(), param);}if (!this.method.getReturnType().isAssignableFrom(result.getClass())) {return this.method.getReturnType().isArray() ? this.convertToArray(result) : this.convertToDeclaredCollection(sqlSession.getConfiguration(), result);} else {return result;}
}

执行result = sqlSession.selectList(this.command.getName(), param);

public <E> List<E> selectList(String statement, Object parameter) {return this.sqlSessionProxy.selectList(statement, parameter);
}

又invoke晕了，可能因为是selectList方法是代理出来的

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {SqlSession sqlSession = SqlSessionUtils.getSqlSession(SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);Object unwrapped;try {Object result = method.invoke(sqlSession, args);if (!SqlSessionUtils.isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {sqlSession.commit(true);}unwrapped = result;} catch (Throwable var11) {unwrapped = ExceptionUtil.unwrapThrowable(var11);if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);sqlSession = null;Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException)unwrapped);if (translated != null) {unwrapped = translated;}}throw (Throwable)unwrapped;} finally {if (sqlSession != null) {SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);}}return unwrapped;
}

Object result = method.invoke(sqlSession, args);调用原方法DefaultSqlSession 中selectList方法

public <E> List<E> selectList(String statement, Object parameter) {return this.selectList(statement, parameter, RowBounds.DEFAULT);
}

执行this.selectList(statement, parameter, RowBounds.DEFAULT);

private <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {List var6;try {MappedStatement ms = this.configuration.getMappedStatement(statement);var6 = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, handler);} catch (Exception var10) {throw ExceptionFactory.wrapException("Error querying database.  Cause: " + var10, var10);} finally {ErrorContext.instance().reset();}return var6;
}

这个方法貌似没作用

private Object wrapCollection(Object object) {return ParamNameResolver.wrapToMapIfCollection(object, (String)null);
}

调用var6 = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, handler); 如果然代理的方法都会进入invoke ，这个类是个InvocationHandler

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {Set<Method> methods = (Set)this.signatureMap.get(method.getDeclaringClass());return methods != null && methods.contains(method) ? this.interceptor.intercept(new Invocation(this.target, method, args)) : method.invoke(this.target, args);} catch (Exception var5) {throw ExceptionUtil.unwrapThrowable(var5);}
}

绕来绕去，来到关键点了激动 pageintercepted类中的拦截器！！！

public Object intercept(Invocation invocation) throws Throwable {Object var16;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];}this.checkDialectExists();if (this.dialect instanceof Chain) {boundSql = ((Chain)this.dialect).doBoundSql(Type.ORIGINAL, boundSql, cacheKey);}List resultList;if (!this.dialect.skip(ms, parameter, rowBounds)) {if (this.dialect.beforeCount(ms, parameter, rowBounds)) {Long count = this.count(executor, ms, parameter, rowBounds, (ResultHandler)null, boundSql);if (!this.dialect.afterCount(count, parameter, rowBounds)) {Object var12 = this.dialect.afterPage(new ArrayList(), parameter, rowBounds);return var12;}}resultList = ExecutorUtil.pageQuery(this.dialect, executor, ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);} else {resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);}var16 = this.dialect.afterPage(resultList, parameter, rowBounds);} finally {if (this.dialect != null) {this.dialect.afterAll();}}return var16;
}

resultList = ExecutorUtil.pageQuery(this.dialect, executor, ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);获得最后结果数据

public static <E> List<E> pageQuery(Dialect dialect, Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql, CacheKey cacheKey) throws SQLException {if (!dialect.beforePage(ms, parameter, rowBounds)) {return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);} else {parameter = dialect.processParameterObject(ms, parameter, boundSql, cacheKey);String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, cacheKey);BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);Map<String, Object> additionalParameters = getAdditionalParameter(boundSql);Iterator var12 = additionalParameters.keySet().iterator();while(var12.hasNext()) {String key = (String)var12.next();pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));}if (dialect instanceof Chain) {pageBoundSql = ((Chain)dialect).doBoundSql(Type.PAGE_SQL, pageBoundSql, cacheKey);}return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, pageBoundSql);}
}

执行String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, cacheKey);

public String getPageSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {return this.autoDialect.getDelegate().getPageSql(ms, boundSql, parameterObject, rowBounds, pageKey);
}

获取分页的sql语句，Page page = this.getLocalPage();获取存储到本地线程的 page 类

public String getPageSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {String sql = boundSql.getSql();Page page = this.getLocalPage();String orderBy = page.getOrderBy();if (StringUtil.isNotEmpty(orderBy)) {pageKey.update(orderBy);sql = OrderByParser.converToOrderBySql(sql, orderBy);}return page.isOrderByOnly() ? sql : this.getPageSql(sql, page, pageKey);
}
getLocalPage在本地获取page
public static <T> Page<T> getLocalPage() {return (Page)LOCAL_PAGE.get();
}

进入converToOrderBySql，可以看到給普通的sql语句中拼接了一个" order by "排序的条件，完成了排序的需求

public static String converToOrderBySql(String sql, String orderBy) {Statement stmt = null;try {stmt = CCJSqlParserUtil.parse(sql);Select select = (Select)stmt;SelectBody selectBody = select.getSelectBody();List<OrderByElement> orderByElements = extraOrderBy(selectBody);String defaultOrderBy = PlainSelect.orderByToString(orderByElements);if (defaultOrderBy.indexOf(63) != -1) {throw new PageException("原SQL[" + sql + "]中的order by包含参数，因此不能使用OrderBy插件进行修改!");}sql = select.toString();} catch (Throwable var7) {log.warn("处理排序失败: " + var7 + "，降级为直接拼接 order by 参数");}return sql + " order by " + orderBy;
}

return page.isOrderByOnly() ? sql : this.getPageSql(sql, page, pageKey);判断是否只是orderby排序如果需要分页进入getPageSql方法，拼接 LIMIT语句
1.如果是第一页拼接LIMIT ?
2.不是的话拼接LIMIT ?, ?

   public String getPageSql(String sql, Page page, CacheKey pageKey) {StringBuilder sqlBuilder = new StringBuilder(sql.length() + 14);sqlBuilder.append(sql);if (page.getStartRow() == 0L) {sqlBuilder.append("\n LIMIT ? ");} else {sqlBuilder.append("\n LIMIT ?, ? ");}return sqlBuilder.toString();}
}

至此分页的基本功能就完成了拼接完成了一个标准的分页sql语句

SELECT dict_id, dict_name, dict_type, status, create_by, create_time, remark FROM sys_dict_type order by dict_id ascLIMIT ?, ?

分页源码代码的运行步骤

总结：pageHpler原理其实还简单，就是有一个PageInterceptor拦截器中拦截了原生的sql语句，拼接对应的排序orderby条件和limit分页位置

如果没有批评，赞美将毫无意义，欢迎指正批评。

路漫漫其修远兮，吾将上下而求索

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PageHelper分页原理（源码）

PageHelper分页原理

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