【Mybatis原始碼分析】12-外掛PageHelper機制
PageInterceptor是Mybatis的外掛,用於攔截Executor的query方法,增強這個方法用於分頁查詢。
@Intercepts( { @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}), @Signature(type = Executor.class, method = "query", args = {MappedStatement.class,Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class}), } ) public class PageInterceptor implements Interceptor { //快取count查詢的ms protected Cache<String, MappedStatement> msCountMap = null; private Dialect dialect; private String default_dialect_class = "com.github.pagehelper.PageHelper"; private Field additionalParametersField; private String countSuffix = "_COUNT";
對於Mybatis配置了外掛後,例項化外掛後就會呼叫Interceper的setProperties方法。
public void setProperties(Properties properties) { //快取 count ms msCountMap = CacheFactory.createCache(properties.getProperty("msCountCache"), "ms", properties);String dialectClass = properties.getProperty("dialect"); if (StringUtil.isEmpty(dialectClass)) { dialectClass = default_dialect_class; } try { Class<?> aClass = Class.forName(dialectClass); dialect = (Dialect) aClass.newInstance(); } catch (Exception e) { throw new PageException(e); } dialect.setProperties(properties);String countSuffix = properties.getProperty("countSuffix"); if (StringUtil.isNotEmpty(countSuffix)) { this.countSuffix = countSuffix; } try { //反射獲取 BoundSql 中的 additionalParameters 屬性 additionalParametersField = BoundSql.class.getDeclaredField("additionalParameters"); additionalParametersField.setAccessible(true); } catch (NoSuchFieldException e) { throw new PageException(e); } }
這個方法中首先會例項化一個Cache例項,用於快取用於查詢資料數量的MappedStatement,如果沒有配置dialect就會使用PageHelper,然後呼叫dialect的setProperties()方法。
public void setProperties(Properties properties) { setStaticProperties(properties); pageParams = new PageParams(); autoDialect = new PageAutoDialect(); pageParams.setProperties(properties); autoDialect.setProperties(properties); }
PageHelper中有一個PageParams物件和一個AutoDialect物件,都會呼叫自身的setProperties()方法。看一下autoDialect的:
public void setProperties(Properties properties) { //多資料來源時,獲取 jdbcurl 後是否關閉資料來源 String closeConn = properties.getProperty("closeConn"); if (StringUtil.isNotEmpty(closeConn)) { this.closeConn = Boolean.parseBoolean(closeConn); } String dialectAlias = properties.getProperty("dialectAlias"); if (StringUtil.isNotEmpty(dialectAlias)) { String[] alias = dialectAlias.split(";"); for (int i = 0; i < alias.length; i++) { String[] kv = alias[i].split("="); if(kv.length != 2){ throw new IllegalArgumentException("dialectAlias 引數配置錯誤," + "請按照 alias1=xx.dialectClass;alias2=dialectClass2 的形式進行配置!"); } for (int j = 0; j < kv.length; j++) { try { Class<? extends Dialect> diallectClass = (Class<? extends Dialect>) Class.forName(kv[1]); //允許覆蓋已有的實現 registerDialectAlias(kv[0], diallectClass); } catch (ClassNotFoundException e) { throw new IllegalArgumentException("請確保 dialectAlias 配置的 Dialect 實現類存在!", e); } } } } //指定的 Helper 資料庫方言,和 不同 String dialect = properties.getProperty("helperDialect"); //執行時獲取資料來源 String runtimeDialect = properties.getProperty("autoRuntimeDialect"); //1.動態多資料來源 if (StringUtil.isNotEmpty(runtimeDialect) && "TRUE".equalsIgnoreCase(runtimeDialect)) { this.autoDialect = false; this.properties = properties; } //2.動態獲取方言 else if (StringUtil.isEmpty(dialect)) { autoDialect = true; this.properties = properties; } //3.指定方言 else { autoDialect = false; this.delegate = initDialect(dialect, properties); } }
預設情況下的結果為autoDialect = true。此類還有個方法initDelegateDialect()會在PageHelper的skip方法中呼叫(馬上會說到)。
initDelegateDialect方法會根據jdbcurl自動選取合適的Dialect例項賦值給一個委託變數delegate,PageHelper是一個委派模式,此後對PageHelper物件方法的呼叫都會委託delegate物件。
接下來就是看Intecepter的核心方法intercept()方法。
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; //由於邏輯關係,只會進入一次 if(args.length == 4){ //4 個引數時 boundSql = ms.getBoundSql(parameter); cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql); } else { //6 個引數時 cacheKey = (CacheKey) args[4]; boundSql = (BoundSql) args[5]; } List resultList; //呼叫方法判斷是否需要進行分頁,如果不需要,直接返回結果 if (!dialect.skip(ms, parameter, rowBounds)) { //反射獲取動態引數 String msId = ms.getId(); Configuration configuration = ms.getConfiguration(); Map<String, Object> additionalParameters = (Map<String, Object>) additionalParametersField.get(boundSql); //判斷是否需要進行 count 查詢 if (dialect.beforeCount(ms, parameter, rowBounds)) { String countMsId = msId + countSuffix; Long count; //先判斷是否存在手寫的 count 查詢 MappedStatement countMs = getExistedMappedStatement(configuration, countMsId); if(countMs != null){ count = executeManualCount(executor, countMs, parameter, boundSql, resultHandler); } else { countMs = msCountMap.get(countMsId); //自動建立 if (countMs == null) { //根據當前的 ms 建立一個返回值為 Long 型別的 ms countMs = MSUtils.newCountMappedStatement(ms, countMsId); msCountMap.put(countMsId, countMs); } count = executeAutoCount(executor, countMs, parameter, boundSql, rowBounds, resultHandler); } //處理查詢總數 //返回 true 時繼續分頁查詢,false 時直接返回 if (!dialect.afterCount(count, parameter, rowBounds)) { //當查詢總數為 0 時,直接返回空的結果 return dialect.afterPage(new ArrayList(), parameter, rowBounds); } } //判斷是否需要進行分頁查詢 if (dialect.beforePage(ms, parameter, rowBounds)) { //生成分頁的快取 key CacheKey pageKey = cacheKey; //處理引數物件 parameter = dialect.processParameterObject(ms, parameter, boundSql, pageKey); //呼叫方言獲取分頁 sql String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey); BoundSql pageBoundSql = new BoundSql(configuration, pageSql, boundSql.getParameterMappings(), parameter); //設定動態引數 for (String key : additionalParameters.keySet()) { pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key)); } //執行分頁查詢 resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql); } else { //不執行分頁的情況下,也不執行記憶體分頁 resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql); } } else { //rowBounds用引數值,不使用分頁外掛處理時,仍然支援預設的記憶體分頁 resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql); } return dialect.afterPage(resultList, parameter, rowBounds); } finally { dialect.afterAll(); } }
dialect.skip()方法用於判斷當前查詢是否跳過分頁,PageHelper的skip()方法。
public boolean skip(MappedStatement ms, Object parameterObject, RowBounds rowBounds) { if(ms.getId().endsWith(MSUtils.COUNT)){ throw new RuntimeException("在系統中發現了多個分頁外掛,請檢查系統配置!"); } Page page = pageParams.getPage(parameterObject, rowBounds); if (page == null) { return true; } else { //設定預設的 count 列 if(StringUtil.isEmpty(page.getCountColumn())){ page.setCountColumn(pageParams.getCountColumn()); } autoDialect.initDelegateDialect(ms);//注意這裡哦 return false; } }
PageParams的getPage方法如果返回一個Page物件則不可以跳過分頁,說明我們使用了查詢分頁。
public Page getPage(Object parameterObject, RowBounds rowBounds) { Page page = PageHelper.getLocalPage(); if (page == null) { if (rowBounds != RowBounds.DEFAULT) { if (offsetAsPageNum) { page = new Page(rowBounds.getOffset(), rowBounds.getLimit(), rowBoundsWithCount); } else { page = new Page(new int[]{rowBounds.getOffset(), rowBounds.getLimit()}, rowBoundsWithCount); //offsetAsPageNum=false的時候,由於PageNum問題,不能使用reasonable,這裡會強制為false page.setReasonable(false); } if(rowBounds instanceof PageRowBounds){ PageRowBounds pageRowBounds = (PageRowBounds)rowBounds; page.setCount(pageRowBounds.getCount() == null || pageRowBounds.getCount()); } } else if(supportMethodsArguments){ try { page = PageObjectUtil.getPageFromObject(parameterObject, false); } catch (Exception e) { return null; } } if(page == null){ return null; } PageHelper.setLocalPage(page); } //分頁合理化 if (page.getReasonable() == null) { page.setReasonable(reasonable); } //當設定為true的時候,如果pagesize設定為0(或RowBounds的limit=0),就不執行分頁,返回全部結果 if (page.getPageSizeZero() == null) { page.setPageSizeZero(pageSizeZero); } return page; }
1.PageHelper的getLocalPage()方法是呼叫ThreadLocal的get()方法,如果返回一個Page物件,說明我們在呼叫查詢介面之前使用了PageHelper的startPage等可以呼叫 ThreadLocal的set()方法。
2.如果page==null,在判斷是否滿足rowBounds != RowBounds.DEFAULT,因為該分頁外掛也支援RowBounds分頁的這也是比較建議的一種方式,因為侵入性小。在判斷該rowBounds instanceof PageRowBounds,因為PageRowBounds繼承RowBounds,使用它可以支援對該分頁查詢的count查詢,先做個標記page.setCount(pageRowBounds.getCount() == null || pageRowBounds.getCount());
3.如果上述條件不滿足在判斷是否supportMethodsArguments==true,這種條件下會使用引數物件的屬性作為分頁的關鍵字。這種用的方式較少暫不分析,預設值如下:
PARAMS.put("pageNum", "pageNum");
PARAMS.put("pageSize", "pageSize");
PARAMS.put("count", "countSql");
PARAMS.put("orderBy", "orderBy");
PARAMS.put("reasonable", "reasonable");
PARAMS.put("pageSizeZero", "pageSizeZero");
情況2和3是外掛為我們自動建立的Page物件,呼叫PageHelper.setLocalPage(page)儲存到當前執行緒。
下面就說一下滿足分頁查詢的情況。
1、dialect.beforeCount()方法判斷是否需要count()查詢。
beforeCount()方法定義在AbstractHelperDialect,就是檢視ThreadLocal中Page物件的count屬性。
public boolean beforeCount(MappedStatement ms, Object parameterObject, RowBounds rowBounds) { Page page = getLocalPage(); return !page.isOrderByOnly() && page.isCount(); }
需要count查詢,接下來會根據當前查詢的MappedStatement構建一個count查詢的MappedStatement。除了id與返回型別不同其餘都一樣。
public static MappedStatement newCountMappedStatement(MappedStatement ms, String newMsId) { MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(), newMsId, ms.getSqlSource(), ms.getSqlCommandType()); builder.resource(ms.getResource()); builder.fetchSize(ms.getFetchSize()); builder.statementType(ms.getStatementType()); builder.keyGenerator(ms.getKeyGenerator()); if (ms.getKeyProperties() != null && ms.getKeyProperties().length != 0) { StringBuilder keyProperties = new StringBuilder(); for (String keyProperty : ms.getKeyProperties()) { keyProperties.append(keyProperty).append(","); } keyProperties.delete(keyProperties.length() - 1, keyProperties.length()); builder.keyProperty(keyProperties.toString()); } builder.timeout(ms.getTimeout()); builder.parameterMap(ms.getParameterMap()); //count查詢返回值int List<ResultMap> resultMaps = new ArrayList<ResultMap>(); ResultMap resultMap = new ResultMap.Builder(ms.getConfiguration(), ms.getId(), Long.class, EMPTY_RESULTMAPPING).build(); resultMaps.add(resultMap); builder.resultMaps(resultMaps); builder.resultSetType(ms.getResultSetType()); builder.cache(ms.getCache()); builder.flushCacheRequired(ms.isFlushCacheRequired()); builder.useCache(ms.isUseCache()); return builder.build(); }
之後執行方法executeAutoCount(executor, countMs, parameter, boundSql, rowBounds, resultHandler)獲取count插敘的結果。
private Long executeAutoCount(Executor executor, MappedStatement countMs, Object parameter, BoundSql boundSql, RowBounds rowBounds, ResultHandler resultHandler) throws IllegalAccessException, SQLException { Map<String, Object> additionalParameters = (Map<String, Object>) additionalParametersField.get(boundSql); //建立 count 查詢的快取 key CacheKey countKey = executor.createCacheKey(countMs, parameter, RowBounds.DEFAULT, boundSql); //呼叫方言獲取 count sql String countSql = dialect.getCountSql(countMs, boundSql, parameter, rowBounds, countKey); //countKey.update(countSql); BoundSql countBoundSql = new BoundSql(countMs.getConfiguration(), countSql, boundSql.getParameterMappings(), parameter); //當使用動態 SQL 時,可能會產生臨時的引數,這些引數需要手動設定到新的 BoundSql 中 for (String key : additionalParameters.keySet()) { countBoundSql.setAdditionalParameter(key, additionalParameters.get(key)); } //執行 count 查詢 Object countResultList = executor.query(countMs, parameter, RowBounds.DEFAULT, resultHandler, countKey, countBoundSql); Long count = (Long) ((List) countResultList).get(0); return count; }
dialect.getCountSql根據當前查詢sql語句返回一個select count(0)。。。形式的sql語句,因為AbstractHelperDialect內的countSqlParser提供了這樣的能力(有興趣的自己看下具體實現,這裡不再展開)。構建好了新的BoundSql後傳入executor的query方法返回數量。
2.dialect.beforePage(ms, parameter, rowBounds)) 判斷是否需要進行分頁查詢
public boolean beforePage(MappedStatement ms, Object parameterObject, RowBounds rowBounds) { Page page = getLocalPage(); if (page.isOrderByOnly() || page.getPageSize() > 0) { return true; } return false; }
String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);這句是分頁的核心方法,用於加入分頁語句,對於mysql就是limit xx,yy。
//執行分頁查詢
resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);
最後呼叫dialect.afterPage(resultList, parameter, rowBounds);填充resultList和count到ThreadLocal的Page中。
在intercept方法中最後的finally程式碼塊中會呼叫 dialect.afterAll();內部會清除ThreadLocal的變數,所以每次呼叫分頁方法都要重新設定Page到ThreadLocal中。