mybatis占位符源码（mybatis通配符和占位符）
本文目录一览：
初看Mybatis 源码 SQL是怎么执行的
一条sql语句到底是怎么执行的？我们知道Mybatis其实是对JDBC的一个封装。假如我执行
session.update("com.mybatis.dao.AuthUserDao.updateAuthUserEmailByName", test@email.com);
语句，追踪下来，Executor、 BaseStatementHandler等等。在 SimpleExecutor 中有如下代码：
public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.update(stmt);
} finally {
closeStatement(stmt);
}
}
1. 首先获取相关配置信息，这个在初始化时，从配置文件中解析而来
2. 新建了一个handler
3. 做了执行statement之前的准备工作。看看准备了些什么，跟踪代码，最后进入了DataSource类的doGetConnection方法，该方法做如下操作：
private Connection doGetConnection(Properties properties) throws SQLException {
initializeDriver();
Connection connection = DriverManager.getConnection(url, properties);
configureConnection(connection);
return connection;
}
private synchronized void initializeDriver() throws SQLException {
if (!registeredDrivers.containsKey(driver)) {
Class? driverType;
try {
if (driverClassLoader != null) {
driverType = Class.forName(driver, true, driverClassLoader);
} else {
driverType = Resources.classForName(driver);
}
// DriverManager requires the driver to be loaded via the system ClassLoader.
//
Driver driverInstance = (Driver)driverType.newInstance();
DriverManager.registerDriver(new DriverProxy(driverInstance));
registeredDrivers.put(driver, driverInstance);
} catch (Exception e) {
throw new SQLException("Error setting driver on UnpooledDataSource. Cause: " + e);
}
}
}
原来是通过prepareStatement 来执行了 我们初始化jdbc的操作。Class.forName DriverManager.getConnection. 这两步是在这里面完成的。
4. 将执行sql的部分交给handler
继续跟踪handler 可以看到SimpleStatementHandler 中。如下执行这个update语句
public int update(Statement statement)
throws SQLException {
String sql = boundSql.getSql();
Object parameterObject = boundSql.getParameterObject();
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
int rows;
if (keyGenerator instanceof Jdbc3KeyGenerator) {
statement.execute(sql, Statement.RETURN_GENERATED_KEYS);
rows = statement.getUpdateCount();
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else if (keyGenerator instanceof SelectKeyGenerator) {
statement.execute(sql);
rows = statement.getUpdateCount();
keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
} else {
statement.execute(sql);
rows = statement.getUpdateCount();
}
return rows;
}
这边就完成了statement的操作，整个过程就是我们Jdbc的过程。原来真的就是对JDBC的简单封装。
其实Mybatis的整个执行过程，理解起来分为如下几个过程：
1. 加载配置文件
2. 解析配置文件，从配置文件中解析出来 datasource、mapper文件、事务配置等等。将配置信息保存在对象内
3. 调用相关语句，执行sql。在执行的方法中分别完成JDBC的一系列操作。
mybatis中的sql语句中的#占位符和$占位符有什么区别
#{}，和 ${}传参的区别如下：
使用#传入参数是，sql语句解析是会加上"",当成字符串来解析，这样相比于$的好处是比较明显对的吧，#{}传参能防止sql注入，如果你传入的参数为 单引号'，那么如果使用${},这种方式 那么是会报错的
另外一种场景是，如果要做动态的排序，比如 order by column，这个时候务必要用${}
select * from table order by 'name' ,这样是没用
目前来看，能用#就不要用$,
怎么没有mybatis源码解析相关的文档
我们还记得是这样配置sqlSessionFactory的：
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bean id="sqlSessionFactory" class="b9de-e9b9-cd12-d8f6 org.mybatis.spring.SqlSessionFactoryBean"
property name="dataSource" ref="dataSource" /
property name="configLocation" value="classpath:configuration.xml"/property
property name="mapperLocations" value="classpath:com/xxx/mybatis/mapper/*.xml"/
property name="typeAliasesPackage" value="com.tiantian.mybatis.model" /
/bean
这里配置了一个mapperLocations属性，它是一个表达式，sqlSessionFactory会根据这个表达式读取包com.xxx.mybaits.mapper下面的所有xml格式文件，那么具体是怎么根据这个属性来读取配置文件的呢？
答案就在SqlSessionFactoryBean类中的buildSqlSessionFactory方法中：
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if (!isEmpty(this.mapperLocations)) {
for (Resource mapperLocation : this.mapperLocations) {
if (mapperLocation == null) {
continue;
}
try {
XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
configuration, mapperLocation.toString(), configuration.getSqlFragments());
xmlMapperBuilder.parse();
} catch (Exception e) {
throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
} finally {
ErrorContext.instance().reset();
}
if (logger.isDebugEnabled()) {
logger.debug("Parsed mapper file: '" + mapperLocation + "'");
}
}
}
mybatis使用XMLMapperBuilder类的实例来解析mapper配置文件。
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public XMLMapperBuilder(Reader reader, Configuration configuration, String resource, MapString, XNode sqlFragments) {
this(new XPathParser(reader, true, configuration.getVariables(), new XMLMapperEntityResolver()),
configuration, resource, sqlFragments);
}
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private XMLMapperBuilder(XPathParser parser, Configuration configuration, String resource, MapString, XNode sqlFragments) {
super(configuration);
this.builderAssistant = new MapperBuilderAssistant(configuration, resource);
this.parser = parser;
this.sqlFragments = sqlFragments;
this.resource = resource;
}
接着系统调用xmlMapperBuilder的parse方法解析mapper。
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public void parse() {
//如果configuration对象还没加载xml配置文件（避免重复加载，实际上是确认是否解析了mapper节点的属性及内容，
//为解析它的子节点如cache、sql、select、resultMap、parameterMap等做准备），
//则从输入流中解析mapper节点，然后再将resource的状态置为已加载
if (!configuration.isResourceLoaded(resource)) {
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
bindMapperForNamespace();
}
//解析在configurationElement函数中处理resultMap时其extends属性指向的父对象还没被处理的resultMap节点
parsePendingResultMaps();
//解析在configurationElement函数中处理cache-ref时其指向的对象不存在的cache节点(如果cache-ref先于其指向的cache节点加载就会出现这种情况)
parsePendingChacheRefs();
//同上，如果cache没加载的话处理statement时也会抛出异常
parsePendingStatements();
}
mybatis解析mapper的xml文件的过程已经很明显了，接下来我们看看它是怎么解析mapper的：
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private void configurationElement(XNode context) {
try {
//获取mapper节点的namespace属性
String namespace = context.getStringAttribute("namespace");
if (namespace.equals("")) {
throw new BuilderException("Mapper's namespace cannot be empty");
}
//设置当前namespace
builderAssistant.setCurrentNamespace(namespace);
//解析mapper的cache-ref节点
cacheRefElement(context.evalNode("cache-ref"));
//解析mapper的cache节点
cacheElement(context.evalNode("cache"));
//解析mapper的parameterMap节点
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
//解析mapper的resultMap节点
resultMapElements(context.evalNodes("/mapper/resultMap"));
//解析mapper的sql节点
sqlElement(context.evalNodes("/mapper/sql"));
//使用XMLStatementBuilder的对象解析mapper的select、insert、update、delete节点,
//mybaits会使用MappedStatement.Builder类build一个MappedStatement对象,
//所以mybaits中一个sql对应一个MappedStatement
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
} catch (Exception e) {
throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
}
}
configurationElement函数几乎解析了mapper节点下所有子节点，至此mybaits解析了mapper中的所有节点，并将其加入到了Configuration对象中提供给sqlSessionFactory对象随时使用。这里我们需要补充讲一下mybaits是怎么使用XMLStatementBuilder类的对象的parseStatementNode函数借用MapperBuilderAssistant类对象builderAssistant的addMappedStatement解析MappedStatement并将其关联到Configuration类对象的：
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public void parseStatementNode() {
//ID属性
String id = context.getStringAttribute("id");
//databaseId属性
String databaseId = context.getStringAttribute("databaseId");
if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
return;
}
//fetchSize属性
Integer fetchSize = context.getIntAttribute("fetchSize");
//timeout属性
Integer timeout = context.getIntAttribute("timeout");
//parameterMap属性
String parameterMap = context.getStringAttribute("parameterMap");
//parameterType属性
String parameterType = context.getStringAttribute("parameterType");
Class? parameterTypeClass = resolveClass(parameterType);
//resultMap属性
String resultMap = context.getStringAttribute("resultMap");
//resultType属性
String resultType = context.getStringAttribute("resultType");
//lang属性
String lang = context.getStringAttribute("lang");
LanguageDriver langDriver = getLanguageDriver(lang);
Class? resultTypeClass = resolveClass(resultType);
//resultSetType属性
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));
//是否是select节点
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
//flushCache属性
boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
//useCache属性
boolean useCache = context.getBooleanAttribute("useCache", isSelect);
//resultOrdered属性
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);
//resultSets属性
String resultSets = context.getStringAttribute("resultSets");
//keyProperty属性
String keyProperty = context.getStringAttribute("keyProperty");
//keyColumn属性
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 {
//useGeneratedKeys属性
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);
}
由以上代码可以看出mybaits使用XPath解析mapper的配置文件后将其中的resultMap、parameterMap、cache、statement等节点使用关联的builder创建并将得到的对象关联到configuration对象中，而这个configuration对象可以从sqlSession中获取的，这就解释了我们在使用sqlSession对数据库进行操作时mybaits怎么获取到mapper并执行其中的sql语句的问题。
