一、最先我们操作数据库就是这么几步
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二、框架该如何做到优化
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三、看看具体的MyBatis
MyBatis感觉核心就两步
- 构建，就是解析我们写的xml配置（mybatis-config.xml和mapper.xml），将其变成它所需要的对象
- 执行，在构建完成的基础上，执行我们的SQL，完成与Jdbc的交互
1、mybatis-config.xml配置文件
mybatis-config.xml配置文件中包含了对 MyBatis 系统的核心设置，包括获取数据库连接实例的数据源（DataSource）以及决定事务作用域和控制方式的事务管理器（TransactionManager）、加载mapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<!-- 和spring整合后 environments配置将废除 -->
<environments default="development">
<environment id="development">
<!-- 使用jdbc事务管理 -->
<transactionManager type="JDBC"/>
<!-- 数据库连接池 -->
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://xxxxxxx:3306/test?characterEncoding=utf8"/>
<property name="username" value="root"/>
<property name="password" value="root"/>
</dataSource>
</environment>
</environments>
<!-- 加载mapper.xml -->
<mappers>
<!-- 注意 mapper节点中，可以使用resource/url/class三种方式获取mapper-->
<!-- 通过配置文件路径 -->
<mapper resource="org/mybatis/example/BlogMapper.xml"/>
<!-- 通过Java全限定类名 -->
<mapper class="org.mybatis.example.TestMapper"/>
<!-- 通过url， 通常是mapper不在本地时用 -->
<mapper url=""/>
<!-- 通过包名 -->
<package name="org.mybatis.example"/>
</mappers>
</configuration>
2、BlogMapper.xml
这个文件中写了我们查询数据库所用的SQL
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="org.mybatis.example.BlogMapper">
<select id="selectBlog" resultType="Blog">
select * from Blog where id = #{id}
</select>
</mapper>
3、如何将这两个配置文件读取成Java对象的
- 对于mybatis-config.xml
String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
- 对于BlogMapper.xml
//第一种方式读取BlogMapper.xml
try (SqlSession session = sqlSessionFactory.openSession()) {
Blog blog = (Blog) session.selectOne("org.mybatis.example.BlogMapper.selectBlog", 101);
}
//第二种方式读取BlogMapper.xml
try (SqlSession session = sqlSessionFactory.openSession()) {
BlogMapper mapper = session.getMapper(BlogMapper.class);
Blog blog = mapper.selectBlog(101);
}
第二种代码不仅更清晰，更加类型安全，还不用担心可能出错的字符串字面值以及强制类型转换。
4、最基本完整的MyBatis代码
public static void main(String[] args) throws Exception {
//第一步读取mybatis-config.xml
String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
//第二步利用SqlSessionFactoryBuilder创建SqlSessionFactory（（这里用到了工厂模式和构建者模式））
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
//第三步：利用sqlSessionFactory去获取 SqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();
//第四步：利用SqlSession 获取Mapper
BlogMapper mapper = session.getMapper(BlogMapper.class);
//传入参数，进行具体的sql查询
Blog blog = mapper.selectBlog(101);
//关闭连接
sqlSession.close();
sqlSession.commit();
}
以上四部的流程图大概如下
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四、MyBatis3.5.6源码分析
第一步：解析mybatis-config.xml
- 看SqlSessionFactoryBuilder.build()里面的方法
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
//解析mybatis-config.xml
//XMLConfigBuilder 构造者
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
//parse(): 解析mybatis-config.xml里面的节点
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.
}
}
}
- 进入parse()
注意parse的返回值，Configuration，这个与mybatis-config.xml中的<configuration>节点是关联起来的
public Configuration parse() {
//改标识标识只能被解析一次
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
//解析<configuration>节点
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
- 进入parseConfiguration()
对比着mybatis-config.xml中的<configuration></configuration>去看，一些在就能明白了，configuration可以再去看看官网的，有些我们没配置，比如properties、settings、typeAliases、objectFactory、objectWrapperFactory、reflectorFactory、databaseIdProvider、typeHandlers等这些我们都没配置，具体的配置请参考官网
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"));
//这一步的解析看第二步：解析mapper.xml
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
第二步：解析mapper.xml
在上面的最后一步是解析mappers
mapperElement(root.evalNode("mappers"));
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");
//优先级 resource>url>mapperClass
if (resource != null && url == null && mapperClass == null) {
ErrorContext.instance().resource(resource);
//通过XMLMapperBuilder解析XXXMapper.xml，构建的XMLMapperBuilde传入了configuration,所以之后肯定会将mapper封装到configuration对象中去
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.");
}
}
}
}
}
- 先看通过resource来解析mapper.xml
public void parse() {
if (!configuration.isResourceLoaded(resource)) {
//解析mapper文件节点
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
//绑定Namespace里面的Class对象
bindMapperForNamespace();
}
//重新解析之前解析不了的节点
parsePendingResultMaps();
parsePendingCacheRefs();
parsePendingStatements();
}
- configurationElement(parser.evalNode("/mapper"));
拿到里面配置的配置项 ,最后封装成一个MapperedStatemanet
[图片上传中...(image.png-3fd741-1646375471539-0)]
private void configurationElement(XNode context) {
try {
//获取命名空间 namespace，这个非常重要，mybatis后面会通过namespace动态代理我们的Mapper接口
String namespace = context.getStringAttribute("namespace");
if (namespace == null || namespace.isEmpty()) {
throw new BuilderException("Mapper's namespace cannot be empty");
}
builderAssistant.setCurrentNamespace(namespace);
//解析缓存节点，平时实际写sql没写这个
cacheRefElement(context.evalNode("cache-ref"));
cacheElement(context.evalNode("cache"));
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
resultMapElements(context.evalNodes("/mapper/resultMap"));
sqlElement(context.evalNodes("/mapper/sql"));
//解析增删改查
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);
}
}
上面解析的namespace、/mapper/resultMap、/mapper/sql、select|insert|update|delete就如下面
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- 解析增删改查节点
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
根据这个名字就可以了解到是用来构造一个Statement，用过原生Jdbc的都知道，Statement就是我们操作数据库的对象。就是我们文章最先的那个流程图的第三步（创建Statement对象）
private void buildStatementFromContext(List<XNode> list) {
if (configuration.getDatabaseId() != null) {
buildStatementFromContext(list, configuration.getDatabaseId());
}
buildStatementFromContext(list, null);
}
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) {
//xml语句有问题时 存储到集合中 等解析完能解析的再重新解析
configuration.addIncompleteStatement(statementParser);
}
}
}
- 来看真正的解析工作
statementParser.parseStatementNode();这段代码比较长
public void parseStatementNode() {
//获取<select id="xxx">中的id
String id = context.getStringAttribute("id");
String databaseId = context.getStringAttribute("databaseId");
//这儿跟一下源码进去看可以看到MappedStatement ,MyBatis中很亲切的一个大对象
// MappedStatement previous = this.configuration.getMappedStatement(id, false); // issue #2
if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
return;
}
//获取节点名 select update delete insert
String nodeName = context.getNode().getNodeName();
// SqlCommandType 是一个枚举型，里面的值是UNKNOWN, INSERT, UPDATE, DELETE, SELECT,FLUSH;
SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
//判断是否是查询
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
//是否刷新缓存 默认:增删改刷新 查询不刷新
boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
//是否使用二级缓存 默认值:查询使用 增删改不使用
boolean useCache = context.getBooleanAttribute("useCache", isSelect);
boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);
// Include Fragments before parsing
XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
//看代码后的图 替换Includes标签为对应的sql标签里面的值
includeParser.applyIncludes(context.getNode());
//获取parameterType名
String parameterType = context.getStringAttribute("parameterType");
// value = (Class<T>) Resources.classForName(string);
Class<?> parameterTypeClass = resolveClass(parameterType);
//解析配置的自定义脚本语言驱动 这里为null,走进去可以看到是设置了默认的
//languageRegistry.getDefaultDriver(),是放到
// private final Map<Class<? extends LanguageDriver>, LanguageDriver> LANGUAGE_DRIVER_MAP = new HashMap<>();一个HashMap中的
String lang = context.getStringAttribute("lang");
LanguageDriver langDriver = getLanguageDriver(lang);
// Parse selectKey after includes and remove them.
//SelectKey在Mybatis中是为了解决Insert数据时不支持主键自动生成的问题，他可以很随意的设置生成主键的方式
//SelectKey需要注意order属性，像Mysql一类支持自动增长类型的数据库中，order需要设置为after才会取到正确的值。
processSelectKeyNodes(id, parameterTypeClass, langDriver);
// Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
//设置主键自增规则
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;
}
//根据sql来判断是否需要动态解析 如果没有动态sql语句且 只有#{}参数的时候 直接静态解析，使用?占位 当有 ${} 不解析
SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
//获取StatementType，可以理解为Statement和PreparedStatement
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");
//获取返回值类型名
String resultType = context.getStringAttribute("resultType");
// 实际最后就调用的这个value = (Class<T>) Resources.classForName(string);
Class<?> resultTypeClass = resolveClass(resultType);
//获取resultMap
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");
//将上面获取到的属性，封装成MappedStatement对象，我们解析mappper.xml中的非常重要的类
builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}
include标签
这个具体的sql解析的SqlSource ，单独放一篇文章总结，方便自己好看，现在只看下SqlSource的debug调试的结果，可以看到具体的sql以及参数
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- 封装成MappedStatement对象的代码
一个MappedStatement就是一个sql，整个项目中有很多sql，是存在Configuration中的mappedStatements 这个参数里面的,她是一个StrictMap
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protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection")
.conflictMessageProducer((savedValue, targetValue) ->
". please check " + savedValue.getResource() + " and " + targetValue.getResource());
public MappedStatement addMappedStatement(
String id,
SqlSource sqlSource,
StatementType statementType,
SqlCommandType sqlCommandType,
Integer fetchSize,
Integer timeout,
String parameterMap,
Class<?> parameterType,
String resultMap,
Class<?> resultType,
ResultSetType resultSetType,
boolean flushCache,
boolean useCache,
boolean resultOrdered,
KeyGenerator keyGenerator,
String keyProperty,
String keyColumn,
String databaseId,
LanguageDriver lang,
String resultSets) {
if (unresolvedCacheRef) {
throw new IncompleteElementException("Cache-ref not yet resolved");
}
id = applyCurrentNamespace(id, false);
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
//利用构造者模式进行MappedStatement
MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
.resource(resource)
.fetchSize(fetchSize)
.timeout(timeout)
.statementType(statementType)
.keyGenerator(keyGenerator)
.keyProperty(keyProperty)
.keyColumn(keyColumn)
.databaseId(databaseId)
.lang(lang)
.resultOrdered(resultOrdered)
.resultSets(resultSets)
.resultMaps(getStatementResultMaps(resultMap, resultType, id))
.resultSetType(resultSetType)
.flushCacheRequired(valueOrDefault(flushCache, !isSelect))
.useCache(valueOrDefault(useCache, isSelect))
.cache(currentCache);
ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
if (statementParameterMap != null) {
statementBuilder.parameterMap(statementParameterMap);
}
MappedStatement statement = statementBuilder.build();
configuration.addMappedStatement(statement);
return statement;
}
至此MyBatis的查询前构建的过程大致这样子，简单地说就是
①MyBatis会在执行查询之前，对配置文件进行解析成配置对象：Configuration
②存放SQL的xml又会解析成MappedStatement对象，但是MappedStatement最后还是放入到Configuration中的一个集合中的
所以最终我们需要操作的对象就是Configuration
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