Mybatis如何获取sql语句

Mybatis如何获取SQL语句
一、获取映射文件xml
在我们的mybatis-config.xml文件里面，有这么一个标签

众所周知，这个绑定的是写SQL语句的文件，所以说，谁去解析的上面的这个xml文件，谁就能拿到SQL语句

那这个文件将会被哪个标签或者哪个类加载呢？我们知道UserMapper.xml文件包含在mappers标签中，那么谁去解析mappers这个标签就能拿到这个文件所以内容。

在自动查找资源方面，Java 并没有提供一个很好的解决方案，所以最好的办法是直接告诉 MyBatis 到哪里去找映射文件。

我们查看mappers标签，发现mybatis提供了4种方法找映射文件。

与此同时，查看官方文档，官方给出了具体的使用方法。

但是，这四种哪一种的优先级的级别最高呢？

我们继续查看源码。我们来到上一讲的解析 configuration 标签下的所有属性配置的函数方法下面，

/**

• 功能描述: 解析 configuration 标签下的所有属性配置
• @date 2020/2/27
  */
  private void parseConfiguration(XNode root) {
  try {
  // 读取mybatis-config.xml中的properties，加载config.properties文件中的参数
  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);
  // 加载environments节点
  environmentsElement(root.evalNode(“environments”));
  databaseIdProviderElement(root.evalNode(“databaseIdProvider”));
  typeHandlerElement(root.evalNode(“typeHandlers”));
  // 加载mapper文件
  mapperElement(root.evalNode(“mappers”));
  } catch (Exception e) {
  throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
  }
  }

找到对mappers解析的代码,mapperElement()

我们在里面可以查看到，mybatis对于映射方法的优先级确定。

private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
// 加载mapper的4种方式，分别为package、resource、url、class，优先级从前往后
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”);
// 解析resource
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();
// 解析url
} 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();
// 解析mapperClass
} 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.”);
}
}
}
}
}

我们先得到参数列表中的context值，

SELECT * FROM User where id = #{id}

发现，这正是mapper文件的内容。所以我们就已经拿到了mapper文件的内容，接下来的就是需要解析mapper文件里面的标签，拿到SQL语句。

三、得到SQL语句
我们继续查看上面对mapper文件解析的java代码，发现了两个可能与SQL语句相关的代码

sqlElement(context.evalNodes(“/mapper/sql”));
buildStatementFromContext(context.evalNodes(“select|insert|update|delete”));
1
2
我们先深入查看第一行代码，

private void sqlElement(List list) throws Exception {
//从这里开始看
if (configuration.getDatabaseId() != null) {
sqlElement(list, configuration.getDatabaseId());
}
sqlElement(list, null);
}

sqlElement(list, configuration.getDatabaseId());
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的解析过程如下：

private void sqlElement(List list, String requiredDatabaseId) throws Exception {
// 开始遍历每1个子节点
for (XNode context : list) {
// 获取databaseId属性
String databaseId = context.getStringAttribute(“databaseId”);
// 获取id属性
String id = context.getStringAttribute(“id”);
// 添加上namespace构成完整路径
id = builderAssistant.applyCurrentNamespace(id, false);
// 继续处理
if (databaseIdMatchesCurrent(id, databaseId, requiredDatabaseId)) {
// 添加到这里
sqlFragments.put(id, context);
//添加进去
}
//结束
}
//结束
}

sqlElement(list, null);
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这个一般不处理。

我们再查看第二行代码buildStatementFromContext

private void buildStatementFromContext(List list) {
if (configuration.getDatabaseId() != null) {
buildStatementFromContext(list, configuration.getDatabaseId());
}
buildStatementFromContext(list, null);
}

四、对SQL语句的处理
我们得到SQL语句，那么我们接下来的就是需要怎么执行它。

我们继续深入，发现myabtis对list中内容进行进一步的处理，一路深入，我们得到了一个核心方法

public void parseStatementNode() {
String id = context.getStringAttribute(“id”);
String databaseId = context.getStringAttribute(“databaseId”);

if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
  return;
}

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());

String parameterType = context.getStringAttribute("parameterType");
Class parameterTypeClass = resolveClass(parameterType);

String lang = context.getStringAttribute("lang");
LanguageDriver langDriver = getLanguageDriver(lang);

// Parse selectKey after includes and remove them.
processSelectKeyNodes(id, parameterTypeClass, langDriver);

// Parse the SQL (pre:  and  were parsed and removed)
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;
}
// sql语句的拼装
SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
Integer fetchSize = context.getIntAttribute("fetchSize");
Integer timeout = context.getIntAttribute("timeout");
String parameterMap = context.getStringAttribute("parameterMap");
String resultType = context.getStringAttribute("resultType");
Class resultTypeClass = resolveClass(resultType);
String resultMap = context.getStringAttribute("resultMap");
String resultSetType = context.getStringAttribute("resultSetType");
ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
if (resultSetTypeEnum == null) {
  resultSetTypeEnum = configuration.getDefaultResultSetType();
}
String keyProperty = context.getStringAttribute("keyProperty");
String keyColumn = context.getStringAttribute("keyColumn");
String resultSets = context.getStringAttribute("resultSets");

builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
    fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
    resultSetTypeEnum, flushCache, useCache, resultOrdered,
    keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);

}

上面的代码是将xml文件中标签内容解析出来，然后再调用builderAssistant.addMappedStatement()方法，将他们拼接在一起，并封装成一个类MappedStatement，然后在MapperBuilderAssistantClass中放入configuration中

MappedStatement statement = statementBuilder.build();
configuration.addMappedStatement(statement);