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這篇文章主要講解了“如何解決Mybatis #foreach中相同的變量名導致值覆蓋的問題”，文中的講解內容簡單清晰，易于學習與理解，下面請大家跟著小編的思路慢慢深入，一起來研究和學習“如何解決Mybatis #foreach中相同的變量名導致值覆蓋的問題”吧！
背景
問題原因（簡略版）
Mybatis流程源碼解析（長文警告，按需自取）
一、獲取SqlSessionFactory
二、獲取SqlSession
三、執行SQL
使用Mybatis中執行如下查詢：
單元測試
@Test
public void test1() {
String resource = "mybatis-config.xml";
InputStream inputStream = null;
try {
inputStream = Resources.getResourceAsStream(resource);
} catch (IOException e) {
e.printStackTrace();
}
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
CommonMapper mapper = sqlSession.getMapper(CommonMapper.class);
QueryCondition queryCondition = new QueryCondition();
List<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(3);
queryCondition.setWidthList(list);
System.out.println(mapper.findByCondition(queryCondition));
}
}XML
<select id="findByCondition" parameterType="cn.liupjie.pojo.QueryCondition" resultType="cn.liupjie.pojo.Test">
select * from test
<where>
<if test="id != null">
and id = #{id,jdbcType=INTEGER}
</if>
<if test="widthList != null and widthList.size > 0">
<foreach collection="widthList" open="and width in (" close=")" item="width" separator=",">
#{width,jdbcType=INTEGER}
</foreach>
</if>
<if test="width != null">
and width = #{width,jdbcType=INTEGER}
</if>
</where>
</select>打印的SQL：
DEBUG [main] - ==> Preparing: select * from test WHERE width in ( ? , ? , ? ) and width = ?
DEBUG [main] - ==> Parameters: 1(Integer), 2(Integer), 3(Integer), 3(Integer)
Mybatis版本
<dependency> <groupId>org.mybatis</groupId> <artifactId>mybatis</artifactId> <version>3.4.1</version> </dependency>
這是公司的老項目，在迭代的過程中遇到了此問題，以此記錄！
PS: 此bug在mybatis-3.4.5版本中已經解決。并且Mybatis維護者也建議不要在item/index中使用重復的變量名。
在獲取到DefaultSqlSession之后，會獲取到Mapper接口的代理類，通過調用代理類的方法來執行查詢
真正執行數據庫查詢之前，需要將可執行的SQL拼接好，此操作在DynamicSqlSource#getBoundSql方法中執行
當解析到foreach標簽時，每次循環都會緩存一個item屬性值與變量值之間的映射（如：width:1），當foreach標簽解析完成后，緩存的參數映射關系中就保留了一個（width:3）
當解析到最后一個if標簽時，由于width變量有值，因此if判斷為true，正常執行拼接，導致出錯
3.4.5版本中，在foreach標簽解析完成后，增加了兩行代碼來解決這個問題。
//foreach標簽解析完成后，從bindings中移除item context.getBindings().remove(item); context.getBindings().remove(index);
入口，跟著build方法走
//獲取SqlSessionFactory， 解析完成后，將XML中的內容封裝到一個Configuration對象中， //使用此對象構造一個DefaultSqlSessionFactory對象，并返回 SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
來到SqlSessionFactoryBuilder#build方法
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
//獲取XMLConfigBuilder，在XMLConfigBuilder的構造方法中，會創建XPathParser對象
//在創建XPathParser對象時，會將mybatis-config.xml文件轉換成Document對象
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
//調用XMLConfigBuilder#parse方法開始解析Mybatis的配置文件
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
inputStream.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}跟著parse方法走，來到XMLConfigBuilder#parseConfiguration方法
private void parseConfiguration(XNode root) {
try {
Properties settings = settingsAsPropertiess(root.evalNode("settings"));
//issue #117 read properties first
propertiesElement(root.evalNode("properties"));
loadCustomVfs(settings);
typeAliasesElement(root.evalNode("typeAliases"));
pluginElement(root.evalNode("plugins"));
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
reflectorFactoryElement(root.evalNode("reflectorFactory"));
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
environmentsElement(root.evalNode("environments"));
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
typeHandlerElement(root.evalNode("typeHandlers"));
//這里解析mapper
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}來到mapperElement方法
//本次mappers配置：<mapper resource="xml/CommomMapper.xml"/>
private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
if ("package".equals(child.getName())) {
String mapperPackage = child.getStringAttribute("name");
configuration.addMappers(mapperPackage);
} else {
String resource = child.getStringAttribute("resource");
String url = child.getStringAttribute("url");
String mapperClass = child.getStringAttribute("class");
if (resource != null && url == null && mapperClass == null) {
//因此走這里，讀取xml文件，并開始解析
ErrorContext.instance().resource(resource);
InputStream inputStream = Resources.getResourceAsStream(resource);
//這里同上文創建XMLConfigBuilder對象一樣，在內部構造時，也將xml文件轉換為了一個Document對象
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
//解析
mapperParser.parse();
} else if (resource == null && url != null && mapperClass == null) {
ErrorContext.instance().resource(url);
InputStream inputStream = Resources.getUrlAsStream(url);
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
mapperParser.parse();
} else if (resource == null && url == null && mapperClass != null) {
Class<?> mapperInterface = Resources.classForName(mapperClass);
configuration.addMapper(mapperInterface);
} else {
throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
}
}
}
}
}XMLMapperBuilder類，負責解析SQL語句所在XML中的內容
//parse方法
public void parse() {
if (!configuration.isResourceLoaded(resource)) {
//解析mapper標簽
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
bindMapperForNamespace();
}
parsePendingResultMaps();
parsePendingChacheRefs();
parsePendingStatements();
}
//configurationElement方法
private void configurationElement(XNode context) {
try {
String namespace = context.getStringAttribute("namespace");
if (namespace == null || namespace.equals("")) {
throw new BuilderException("Mapper's namespace cannot be empty");
}
builderAssistant.setCurrentNamespace(namespace);
cacheRefElement(context.evalNode("cache-ref"));
cacheElement(context.evalNode("cache"));
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
resultMapElements(context.evalNodes("/mapper/resultMap"));
sqlElement(context.evalNodes("/mapper/sql"));
//解析各種類型的SQL語句：select|insert|update|delete
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
} catch (Exception e) {
throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
}
}
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
for (XNode context : list) {
//創建XMLStatementBuilder對象
final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
try {
//解析
statementParser.parseStatementNode();
} catch (IncompleteElementException e) {
configuration.addIncompleteStatement(statementParser);
}
}
}XMLStatementBuilder負責解析單個select|insert|update|delete節點
public void parseStatementNode() {
String id = context.getStringAttribute("id");
String databaseId = context.getStringAttribute("databaseId");
//判斷databaseId是否匹配，將namespace+'.'+id拼接，判斷是否已經存在此id
if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
return;
}
Integer fetchSize = context.getIntAttribute("fetchSize");
Integer timeout = context.getIntAttribute("timeout");
String parameterMap = context.getStringAttribute("parameterMap");
//獲取參數類型
String parameterType = context.getStringAttribute("parameterType");
//獲取參數類型的class對象
Class<?> parameterTypeClass = resolveClass(parameterType);
String resultMap = context.getStringAttribute("resultMap");
String resultType = context.getStringAttribute("resultType");
String lang = context.getStringAttribute("lang");
LanguageDriver langDriver = getLanguageDriver(lang);
//獲取resultType的class對象
Class<?> resultTypeClass = resolveClass(resultType);
String resultSetType = context.getStringAttribute("resultSetType");
StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
//獲取select|insert|update|delete類型
String nodeName = context.getNode().getNodeName();
SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
boolean useCache = context.getBooleanAttribute("useCache", isSelect);
boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);
// Include Fragments before parsing
XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
includeParser.applyIncludes(context.getNode());
// Parse selectKey after includes and remove them.
processSelectKeyNodes(id, parameterTypeClass, langDriver);
// Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
//獲取SqlSource對象，langDriver為默認的XMLLanguageDriver，在new Configuration時設置
//若sql中包含元素節點或$，則返回DynamicSqlSource，否則返回RawSqlSource
SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
String resultSets = context.getStringAttribute("resultSets");
String keyProperty = context.getStringAttribute("keyProperty");
String keyColumn = context.getStringAttribute("keyColumn");
KeyGenerator keyGenerator;
String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
if (configuration.hasKeyGenerator(keyStatementId)) {
keyGenerator = configuration.getKeyGenerator(keyStatementId);
} else {
keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
? new Jdbc3KeyGenerator() : new NoKeyGenerator();
}
builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}由上文可知，此處的SqlSessionFactory使用的是DefaultSqlSessionFactory
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
//創建執行器，默認是SimpleExecutor
//如果在配置文件中開啟了緩存（默認開啟），則是CachingExecutor
final Executor executor = configuration.newExecutor(tx, execType);
//返回DefaultSqlSession對象
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}這里獲取到了一個DefaultSqlSession對象
獲取CommonMapper的對象，這里CommonMapper是一個接口，因此是一個代理對象，代理類是MapperProxy
org.apache.ibatis.binding.MapperProxy@72cde7cc
執行Query方法，來到MapperProxy的invoke方法
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
//緩存
final MapperMethod mapperMethod = cachedMapperMethod(method);
//執行操作：select|insert|update|delete
return mapperMethod.execute(sqlSession, args);
}執行操作時，根據SELECT操作，以及返回值類型（反射方法獲取）確定executeForMany方法
caseSELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
} else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
break;來到executeForMany方法中，就可以看到執行查詢的操作，由于這里沒有進行分頁查詢，因此走else
if (method.hasRowBounds()) {
RowBounds rowBounds = method.extractRowBounds(args);
result = sqlSession.<E>selectList(command.getName(), param, rowBounds);
} else {
result = sqlSession.<E>selectList(command.getName(), param);
}來到DefaultSqlSession#selectList方法中
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
//根據key（namespace+"."+id）來獲取MappedStatement對象
//MappedStatement對象中封裝了解析好的SQL信息
MappedStatement ms = configuration.getMappedStatement(statement);
//通過CachingExecutor#query執行查詢
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}CachingExecutor#query
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
//解析SQL為可執行的SQL
BoundSql boundSql = ms.getBoundSql(parameter);
//獲取緩存的key
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
//執行查詢
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}MappedStatement#getBoundSql
public BoundSql getBoundSql(Object parameterObject) {
//解析SQL
BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings == null || parameterMappings.isEmpty()) {
boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject);
}
//檢查是否有嵌套的ResultMap
// check for nested result maps in parameter mappings (issue #30)
for (ParameterMapping pm : boundSql.getParameterMappings()) {
String rmId = pm.getResultMapId();
if (rmId != null) {
ResultMap rm = configuration.getResultMap(rmId);
if (rm != null) {
hasNestedResultMaps |= rm.hasNestedResultMaps();
}
}
}
return boundSql;
}由上文，此次語句由于SQL中包含元素節點，因此是DynamicSqlSource。由此來到DynamicSqlSource#getBoundSql。
rootSqlNode.apply(context);這段代碼便是在執行SQL解析。
@Override
public BoundSql getBoundSql(Object parameterObject) {
DynamicContext context = new DynamicContext(configuration, parameterObject);
//執行SQL解析
rootSqlNode.apply(context);
SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());
BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
for (Map.Entry<String, Object> entry : context.getBindings().entrySet()) {
boundSql.setAdditionalParameter(entry.getKey(), entry.getValue());
}
return boundSql;
}打上斷點，跟著解析流程，來到解析foreach標簽的代碼，ForEachSqlNode#apply
@Override
public boolean apply(DynamicContext context) {
Map<String, Object> bindings = context.getBindings();
final Iterable<?> iterable = evaluator.evaluateIterable(collectionExpression, bindings);
if (!iterable.iterator().hasNext()) {
return true;
}
boolean first = true;
//解析open屬性
applyOpen(context);
int i = 0;
for (Object o : iterable) {
DynamicContext oldContext = context;
if (first) {
context = new PrefixedContext(context, "");
} else if (separator != null) {
context = new PrefixedContext(context, separator);
} else {
context = new PrefixedContext(context, "");
}
int uniqueNumber = context.getUniqueNumber();
// Issue #709
//集合中的元素是Integer，走else
if (o instanceof Map.Entry) {
@SuppressWarnings("unchecked")
Map.Entry<Object, Object> mapEntry = (Map.Entry<Object, Object>) o;
applyIndex(context, mapEntry.getKey(), uniqueNumber);
applyItem(context, mapEntry.getValue(), uniqueNumber);
} else {
//使用index屬性
applyIndex(context, i, uniqueNumber);
//使用item屬性
applyItem(context, o, uniqueNumber);
}
//當foreach中使用#號時，會將變量替換為占位符（類似__frch_width_0）（StaticTextSqlNode）
//當使用$符號時，會將值直接拼接到SQL中（TextSqlNode）
contents.apply(new FilteredDynamicContext(configuration, context, index, item, uniqueNumber));
if (first) {
first = !((PrefixedContext) context).isPrefixApplied();
}
context = oldContext;
i++;
}
applyClose(context);
return true;
}
private void applyItem(DynamicContext context, Object o, int i) {
if (item != null) {
//在參數映射中綁定item屬性值與集合值的關系
//第一次：（width:1）
//第二次：（width:2）
//第三次：（width:3）
context.bind(item, o);
//在參數映射中綁定處理后的item屬性值與集合值的關系
//第一次：（__frch_width_0:1）
//第二次：（__frch_width_1:2）
//第三次：（__frch_width_2:3）
context.bind(itemizeItem(item, i), o);
}
}到這里，結果就清晰了，在解析foreach標簽時，每次循環都會將item屬性值與參數集合中的值進行綁定，到最后就會保留（width:3）的映射關系，而在解析完foreach標簽后，會解析最后一個if標簽，此時在判斷if標簽是否成立時，答案是true，因此最終拼接出來一個錯誤的SQL。
在3.4.5版本中，代碼中增加了context.getBindings().remove(item);在foreach標簽解析完成后移除bindings中的參數映射。以下是源碼：
@Override
public boolean apply(DynamicContext context) {
Map<String, Object> bindings = context.getBindings();
final Iterable<?> iterable = evaluator.evaluateIterable(collectionExpression, bindings);
if (!iterable.iterator().hasNext()) {
return true;
}
boolean first = true;
applyOpen(context);
int i = 0;
for (Object o : iterable) {
DynamicContext oldContext = context;
if (first || separator == null) {
context = new PrefixedContext(context, "");
} else {
context = new PrefixedContext(context, separator);
}
int uniqueNumber = context.getUniqueNumber();
// Issue #709
if (o instanceof Map.Entry) {
@SuppressWarnings("unchecked")
Map.Entry<Object, Object> mapEntry = (Map.Entry<Object, Object>) o;
applyIndex(context, mapEntry.getKey(), uniqueNumber);
applyItem(context, mapEntry.getValue(), uniqueNumber);
} else {
applyIndex(context, i, uniqueNumber);
applyItem(context, o, uniqueNumber);
}
contents.apply(new FilteredDynamicContext(configuration, context, index, item, uniqueNumber));
if (first) {
first = !((PrefixedContext) context).isPrefixApplied();
}
context = oldContext;
i++;
}
applyClose(context);
//foreach標簽解析完成后，從bindings中移除item
context.getBindings().remove(item);
context.getBindings().remove(index);
return true;
}感謝各位的閱讀，以上就是“如何解決Mybatis #foreach中相同的變量名導致值覆蓋的問題”的內容了，經過本文的學習后，相信大家對如何解決Mybatis #foreach中相同的變量名導致值覆蓋的問題這一問題有了更深刻的體會，具體使用情況還需要大家實踐驗證。這里是億速云，小編將為大家推送更多相關知識點的文章，歡迎關注！
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