Mybatis中判斷集合的size,可以用下面的方法來做。

<if test='null != staffCodeList and staffCodeList.size > 0'>and gui.USER_CODE not in<foreach collection='staffCodeList' item='staffCode' open='(' separator=',' close=')'>#{staffCode}</foreach></if>

補充：警惕，MyBatis的size()方法竟然有坑！

Mybatis是一個開源的輕量級半自動化ORM框架，使得面向對象應用程序與關系數據庫的映射變得更加容易。

MyBatis使用xml描述符或注解將對象與存儲過程或SQL語句相結合。 Mybatis最大優(yōu)點是應用程序與Sql進行解耦，sql語句是寫在Xml Mapper文件中。

OGNL表達式在Mybatis當中應用非常廣泛，其表達式的靈活性使得動態(tài)Sql功能的非常強大。

OGNL是Object-Graph Navigation Language的縮寫，代表對象圖導航語言。

OGNL是一種EL表達式語言，用于設置和獲取Java對象的屬性，并且可以對列表進行投影選擇以及執(zhí)行l(wèi)ambda表達式。

Ognl類提供了許多簡便方法用于執(zhí)行表達式的。 Struts2發(fā)布的每個版本都會出現的新的高危可執(zhí)行漏洞也是因為它使用了靈活的OGNL表達式。

公司后端采用Mybatis作為數據訪問層,所使用版本為3.2.3。

線上環(huán)境業(yè)務系統在運行過程中出現了一個令人困惑的異常, 該異常時而出現時而不出現，構造各種OGNL表達式為空等特殊情況均不會重現該異常。

### Error querying database. Cause: org.apache.ibatis.builder.BuilderException: Error evaluating expression ’list != null and list.size() > 0’. Cause: org.apache.ibatis.ognl.MethodFailedException: Method 'size' failed for object [1] [java.lang.IllegalAccessException: Class org.apache.ibatis.ognl.OgnlRuntime can not access a member of class java.util.Collections$SingletonList with modifiers 'public']### Cause: org.apache.ibatis.builder.BuilderException: Error evaluating expression ’list != null and list.size() > 0’. Cause: org.apache.ibatis.ognl.MethodFailedException: Method 'size' failed for object [1] [java.lang.IllegalAccessException: Class org.apache.ibatis.ognl.OgnlRuntime can not access a member of class java.util.Collections$SingletonList with modifiers 'public'] at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:23) org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:107) at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:98) at cn.com.shaobingmm.MybatisBugTest$2.run(MybatisBugTest.java:88) at java.lang.Thread.run(Thread.java:745)Caused by: org.apache.ibatis.builder.BuilderException: Error evaluating expression ’list != null and list.size() > 0’. Cause: org.apache.ibatis.ognl.MethodFailedException: Method 'size' failed for object [1] [java.lang.IllegalAccessException: Class org.apache.ibatis.ognl.OgnlRuntime can not access a member of class java.util.Collections$SingletonList with modifiers 'public'] at org.apache.ibatis.scripting.xmltags.OgnlCache.getValue(OgnlCache.java at:47) at org.apache.ibatis.scripting.xmltags.ExpressionEvaluator.evaluateBoolean(ExpressionEvaluator.java:29) at org.apache.ibatis.scripting.xmltags.IfSqlNode.apply(IfSqlNode.java:30) at org.apache.ibatis.scripting.xmltags.MixedSqlNode.apply(MixedSqlNode.java:29) at org.apache.ibatis.scripting.xmltags.TrimSqlNode.apply(TrimSqlNode.java:51) at org.apache.ibatis.scripting.xmltags.MixedSqlNode.apply(MixedSqlNode.java:29) at org.apache.ibatis.scripting.xmltags.DynamicSqlSource.getBoundSql(DynamicSqlSource.java:37) at org.apache.ibatis.mapping.MappedStatement.getBoundSql(MappedStatement.java:275) at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:79) at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:104) ... 3 moreCaused by: org.apache.ibatis.ognl.MethodFailedException: Method 'size' failed for object [1] [java.lang.IllegalAccessException: Class org.apache.ibatis.ognl.OgnlRuntime can not access a member of class java.util.Collections$SingletonList with modifiers 'public'] at org.apache.ibatis.ognl.OgnlRuntime.callAppropriateMethod(OgnlRuntime.java:837) at org.apache.ibatis.ognl.ObjectMethodAccessor.callMethod(ObjectMethodAccessor.java:61) at org.apache.ibatis.ognl.OgnlRuntime.callMethod(OgnlRuntime.java:860) at org.apache.ibatis.ognl.ASTMethod.getValueBody(ASTMethod.java:73) at org.apache.ibatis.ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:170) at org.apache.ibatis.ognl.SimpleNode.getValue(SimpleNode.java:210) at org.apache.ibatis.ognl.ASTChain.getValueBody(ASTChain.java:109) at org.apache.ibatis.ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:170) at org.apache.ibatis.ognl.SimpleNode.getValue(SimpleNode.java:210) at org.apache.ibatis.ognl.ASTGreater.getValueBody(ASTGreater.java:49) at org.apache.ibatis.ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:170) at org.apache.ibatis.ognl.SimpleNode.getValue(SimpleNode.java:210) at org.apache.ibatis.ognl.ASTAnd.getValueBody(ASTAnd.java:56) at org.apache.ibatis.ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:170) at org.apache.ibatis.ognl.SimpleNode.getValue(SimpleNode.java:210) at org.apache.ibatis.ognl.Ognl.getValue(Ognl.java:333) at org.apache.ibatis.ognl.Ognl.getValue(Ognl.java:413) at org.apache.ibatis.ognl.Ognl.getValue(Ognl.java:395) at org.apache.ibatis.scripting.xmltags.OgnlCache.getValue(OgnlCache.java:45) ... 12 more

List的size()方法明顯是public為何還會出現不可訪問的異常。該問題并不是每一次都會出現，經過多次嘗試，該異常一直未在測試環(huán)境重現。

該接口在完整調用鏈路中的出錯次數占總調用次數的比率為0.01%,無意中聯想到并發(fā)問題在周期性時間內往往是概率性發(fā)生。

<mapper namespace='CompanyMapper'> <select id='getCompanysByIds'resultType='cn.com.shaobingmm.Company'> select * from company <where> <if test='list != null and list.size() > 0'> and id in <foreach collection='list' item='id' open='(' separator=',' close=')'>#{id}</foreach> </if> </where> </select></mapper>多線程并發(fā)環(huán)境下的壓測代碼

上訴異常堆棧信息在并發(fā)環(huán)境下果然重現出現，根據異常信息代碼執(zhí)行至該行代碼時發(fā)生異常:

異常信息表明OgnlRuntime類不能夠訪問java.util.Collections的私有成員SingletonList。

查看源代碼發(fā)現能夠拋出MethodFailedException異?？梢枣i定在invokeMethod方法內部。

public static Object callAppropriateMethod(OgnlContext context, Object source, Object target, String methodName, String propertyName, List methods, Object[] args) throws MethodFailedException { Object reason = null; Object[] actualArgs = objectArrayPool.create(args.length); try { Method e = getAppropriateMethod(context, source, target, methodName, propertyName, methods, args, actualArgs); if(e == null || !isMethodAccessible(context, source, e, propertyName)) { StringBuffer buffer = new StringBuffer(); if(args != null) { int i = 0; for(int ilast = args.length - 1; i <= ilast; ++i) { Object arg = args[i]; buffer.append(arg == null?NULL_STRING:arg.getClass().getName()); if(i < ilast) { buffer.append(', '); } } } throw new NoSuchMethodException(methodName + '(' + buffer + ')'); } Object var14 = invokeMethod(target, e, actualArgs); return var14; } catch (NoSuchMethodException var21) { reason = var21; } catch (IllegalAccessException var22) { reason = var22; } catch (InvocationTargetException var23) { reason = var23.getTargetException(); } finally { objectArrayPool.recycle(actualArgs); } throw new MethodFailedException(source, methodName, (Throwable)reason); }invokeMethod方法代碼

public static Object invokeMethod(Object target, Method method, Object[] argsArray) throws InvocationTargetException, IllegalAccessException { boolean wasAccessible = true; if(securityManager != null) { try { securityManager.checkPermission(getPermission(method)); } catch (SecurityException var6) { throw new IllegalAccessException('Method [' + method + '] cannot be accessed.'); } } if((!Modifier.isPublic(method.getModifiers()) || !Modifier.isPublic(method.getDeclaringClass().getModifiers())) && !(wasAccessible = method.isAccessible())) { method.setAccessible(true); （1） } Object result = method.invoke(target, argsArray); (3) if(!wasAccessible) { method.setAccessible(false); (2) } return result; }

問題出現在method實際上是一個共享變量，也就是例子中的

public int java.util.Collections$SingletonList.size()方法

當第一個線程t1至(1)行代碼允許method方法可以被調用，第二個線程t2執(zhí)行至(2)將method的方法設置為不可以訪問。接著t1又開始執(zhí)行到(3)行的時候就會發(fā)生該異常。這是一個很典型的同步問題。

Ognl2.7已經修復了該問題，因為ognl源碼是直接打包內嵌在mybatis包中，mybatis3.3.0版本中也已經進行了修復升級。(劃重點)

public static Object invokeMethod(Object target, Method method, Object[] argsArray) throws InvocationTargetException, IllegalAccessException { boolean syncInvoke = false; boolean checkPermission = false; int mHash = method.hashCode(); synchronized(method) { if(_methodAccessCache.get(Integer.valueOf(mHash)) == null || _methodAccessCache.get(Integer.valueOf(mHash)) == Boolean.TRUE) { syncInvoke = true; } if(_securityManager != null && _methodPermCache.get(Integer.valueOf(mHash)) == null || _methodPermCache.get(Integer.valueOf(mHash)) == Boolean.FALSE) { checkPermission = true; } } boolean wasAccessible = true; Object result; if(syncInvoke) { synchronized(method) { if(checkPermission) { try { _securityManager.checkPermission(getPermission(method)); _methodPermCache.put(Integer.valueOf(mHash), Boolean.TRUE); } catch (SecurityException var12) { _methodPermCache.put(Integer.valueOf(mHash), Boolean.FALSE); throw new IllegalAccessException('Method [' + method + '] cannot be accessed.'); } } if(Modifier.isPublic(method.getModifiers()) && Modifier.isPublic(method.getDeclaringClass().getModifiers())) { _methodAccessCache.put(Integer.valueOf(mHash), Boolean.FALSE); } else if(!(wasAccessible = method.isAccessible())) { method.setAccessible(true); _methodAccessCache.put(Integer.valueOf(mHash), Boolean.TRUE); } else { _methodAccessCache.put(Integer.valueOf(mHash), Boolean.FALSE); } result = method.invoke(target, argsArray); if(!wasAccessible) { method.setAccessible(false); } } } else { if(checkPermission) { try { _securityManager.checkPermission(getPermission(method)); _methodPermCache.put(Integer.valueOf(mHash), Boolean.TRUE); } catch (SecurityException var11) { _methodPermCache.put(Integer.valueOf(mHash), Boolean.FALSE); throw new IllegalAccessException('Method [' + method + '] cannot be accessed.'); } } result = method.invoke(target, argsArray); } return result; }

以上為個人經驗，希望能給大家一個參考，也希望大家多多支持好吧啦網。如有錯誤或未考慮完全的地方，望不吝賜教。