一、前言
不知道大家有没有遇到过这种需求，比如现在有一个 mapper 标签：
<select id="query" resultType="HashMap">
${sql}
</select>
很显然，这个标签的作用就是直接执行传入的拼接好的 SQL 语句，比如：
select * from table where condition
一般情况下，这个 SQL 的条件是我们手动拼接好的。那么如果我们直接让 mybatis 帮我们拼接会怎么样呢？比如假设我们传来的 SQL 是这样的：
select * from table
<where>
<if test="condition">
and condition
</if>
</where>
那么 mybatis 能不能正确解析并执行呢？
答案是否定的！
因为 mybatis 是在启动的时就已经解析好了全部的 mapper 标签，而不是运行时解析，所以上述例子并不能正确执行。
但是我们想让它能够运行要怎么办呢？那就只有我们自己手动去解析了！
二、动态解析 mapper 标签
1、寻找解析方法
想要动态的解析 mapper 标签，别无他法，只能从源码入手，看看官方是怎么解析的，然后照猫画虎。下面就看看 mybatis 是怎么做的：
首先，毫无疑问从 SqlSessionFactoryBuilder.java 入手：
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
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.
}
}
}
可以看到其实是创建了一个 XMLConfigBuilder 对象，让它来进行解析，跟进去看一下它的 parse 方法：
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
里面又调了 parseConfiguration 方法，再跟：
private void parseConfiguration(XNode root) {
try {
//issue #117 read properties first
propertiesElement(root.evalNode("properties"));
Properties settings = settingsAsProperties(root.evalNode("settings"));
loadCustomVfs(settings);
loadCustomLogImpl(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"));
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
这个方法好长，我也看不懂，但我看到了最后一行有个方法 mapperElement，有点怀疑，跟进去：
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) {
ErrorContext.instance().resource(resource);
InputStream inputStream = Resources.getResourceAsStream(resource);
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
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.");
}
}
}
}
}
又是一个好长的方法，不过仔细看看里面有一个对象很显眼 XMLMapperBuilder，看名字就知道就是它的，看看它的 parse 方法：
public void parse() {
if (!configuration.isResourceLoaded(resource)) {
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
bindMapperForNamespace();
}
parsePendingResultMaps();
parsePendingCacheRefs();
parsePendingStatements();
}
从这段代码中，我一眼就看出第三行是开始解析 mapper 标签了，跟进去：
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"));
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
} catch (Exception e) {
throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
}
}
不知道为啥，看到 “select|insert|update|delete” 我就感觉越来越接近真相，跟：
private void buildStatementFromContext(List<XNode> list) {
if (configuration.getDatabaseId() != null) {
buildStatementFromContext(list, configuration.getDatabaseId());
}
buildStatementFromContext(list, null);
}
就这？真不知道加判断干嘛，直接调嘛，获取不到自然就是空，我估计是为了可读性，赞一个！接着跟：
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
for (XNode context : list) {
final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
try {
statementParser.parseStatementNode();
} catch (IncompleteElementException e) {
configuration.addIncompleteStatement(statementParser);
}
}
}
废话不说，直接跟：
public void parseStatementNode() {
String id = context.getStringAttribute("id");
String databaseId = context.getStringAttribute("databaseId");
if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
return;
}
Integer fetchSize = context.getIntAttribute("fetchSize");
Integer timeout = context.getIntAttribute("timeout");
String parameterMap = context.getStringAttribute("parameterMap");
String parameterType = context.getStringAttribute("parameterType");
Class<?> parameterTypeClass = resolveClass(parameterType);
String resultMap = context.getStringAttribute("resultMap");
String resultType = context.getStringAttribute("resultType");
String lang = context.getStringAttribute("lang");
LanguageDriver langDriver = getLanguageDriver(lang);
Class<?> resultTypeClass = resolveClass(resultType);
String resultSetType = context.getStringAttribute("resultSetType");
StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
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)
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))
? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
}
builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}
这。。。就是真相吧~~~
好吧，其实到这里就可以了，因为已经开始解析了。下面我们就想办法看看怎么能调到这个方法，那么毫无疑问，我们回退一步：
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
for (XNode context : list) {
final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
try {
statementParser.parseStatementNode();
} catch (IncompleteElementException e) {
configuration.addIncompleteStatement(statementParser);
}
}
}
很明显，我们需要一个 XMLStatementBuilder 对象，而这个对象又需要 Configuration、MapperBuilderAssistant、XNode、requiredDatabaseId。
而我们动态解析一个 mapper 肯定是基于一个我们自己配置的已经存在的 SqlSessionFactory 的，所以 Configuration、requiredDatabaseId 我们都可以通过 SqlSessionFactory 获取到。
那么还有两个参数，先看 MapperBuilderAssistant：
它是 XMLMapperBuilder 的一个属性，MapperBuilderAssistant 正是在实例化 XMLMapperBuilder 的时候被创建的，看一下 XMLMapperBuilder 的构造方法：
public XMLMapperBuilder(InputStream inputStream, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {
this(new XPathParser(inputStream, true, configuration.getVariables(), new XMLMapperEntityResolver()),
configuration, resource, sqlFragments);
}
private XMLMapperBuilder(XPathParser parser, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {
super(configuration);
this.builderAssistant = new MapperBuilderAssistant(configuration, resource);
this.parser = parser;
this.sqlFragments = sqlFragments;
this.resource = resource;
}
可以看到创建 MapperBuilderAssistant 只需要两个参数，configuration、resource。
其中 Configuration 已不是问题，而 resource 呢，更简单了，它正是我们的 mapper 标签字符串！
再看 XNode：
细心的同学应该会注意到，前面的代码中很多地方都有一个 evalNode 方法，这个方法正是解析 xml 节点的方法，也就是说 XNode 对象就是 mapper 标签作为 xml 节点被解析后的对象！
终于可以看到最终结果代码了！
2、实现动态解析
直接上代码：
/**
*
* @param sqlSessionFactory sqlSessionFactory
* @param executeClass 执行动态 mapper 的代理接口类
* @param xml mapper 标签，比如：<select id="dynamicQuery">select * from table</select>
* @param nodeType mapper 节点类型，比如：/select
*/
public void dynamicParse(SqlSessionFactory sqlSessionFactory, Class<?> executeClass, String xml, String nodeType) {
Configuration configuration = sqlSessionFactory.getConfiguration();
MapperBuilderAssistant mapperBuilderAssistant = new MapperBuilderAssistant(configuration, xml);
mapperBuilderAssistant.setCurrentNamespace(executeClass.getName());
XNode xNode = new XPathParser(xml).evalNode(nodeType);
XMLStatementBuilder xmlStatementBuilder = new XMLStatementBuilder(configuration, mapperBuilderAssistant, xNode, configuration.getDatabaseId());
try {
xmlStatementBuilder.parseStatementNode();
} catch (IncompleteElementException e) {
/**
* 这里值得注意，此异常是元素不完整异常，应该是 mapper 中存在元素依赖导致的
* 官方是先保存起来，后面再解析一次，保存代码如下所示:
* this.configuration.addIncompleteStatement(xmlStatementBuilder);
*
* 再次解析的方法是：
* parsePendingStatements();
*
* 这里就直接抛异常了
*/
throw new RuntimeException();
}
}
}
完结散花！
转载：https://blog.csdn.net/kfyty725/article/details/106729240
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