Mybatis执行原理
MyBatis 是一款优秀的持久层框架,MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以使用简单的 XML 或注解来配置和映射原生信息,将接口和 Java 的 POJOs(Plain Ordinary Java Object,普通的 Java对象)映射成数据库中的记录。
Mybatis中的mapper接口并没有具体实现,那么他是如何执行SQL的?
mybatis-config.xml配置
xml
<!-- mybatis-config.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>
<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://127.0.0.1:3306/demo?characterEncoding=utf8"/>
<property name="username" value="" />
<property name="password" value="" />
</dataSource>
</environment>
</environments>
<!-- 加载mapper.xml -->
<mappers>
<!-- <package name=""> -->
<mapper resource="mapper/DemoMapper.xml"></mapper>
</mappers>
</configuration>
从一个简单的mybatis示例开始源码分析:
java
public static void main(String[] args) throws Exception {
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
//创建SqlSessionFacory
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
//获取Mapper
DemoMapper mapper = sqlSession.getMapper(DemoMapper.class);
Map<String,Object> map = new HashMap<>();
map.put("id","1");
System.out.println(mapper.selectAll(map));
sqlSession.close();
}
1.获取SqlSessionFactory
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
new SqlSessionFactoryBuilder().build()方法得到SqlSessionFactory实例,并解析mybatis-config.xml文件configuration节点数据源配置及mapper配置到Configuration对象,Configuration是SqlSessionFactory的一个属性。
java
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 {
if (inputStream != null) {
inputStream.close();
}
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
XMLConfigBuilder解析配置并组装SQL语句:
java
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
// 解析mybatis-config.xml文件configuration节点数据源配置及mapper配置到Configuration对象
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
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"));
// 读取mapper配置
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
2.创建SqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();
通过SqlSession工厂类创建SqlSession,并增加事务处理、执行器等。这里将SqlSessionFactory中的Environment传递给了SqlSession,Environment中包含DataSource,用于创建数据库连接。
java
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);
final Executor executor = configuration.newExecutor(tx, execType);
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();
}
}
3.创建Mapper、执行SQL
DemoMapper mapper = sqlSession.getMapper(DemoMapper.class);
java
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
在XMLConfigBuilder.parseConfiguration()中configuration.addMapper(mapperInterface)方法对knownMappers初始化,将所有Mapper的代理类放入knownMappers。
knownMappers.put(type, new MapperProxyFactory<>(type));
sqlSession.getMapper()从knownMappers获取代理类对象。
mapper.selectAll(map);
代理类通过MapperProxy.invoke()方法执行,自定义代理类会执行cachedInvoker()方法。
java
public class MapperProxy<T> implements InvocationHandler, Serializable {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
}
return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
}
最终通过mapperMethod.execute()方法执行对应的SQL语句。
java
private static class PlainMethodInvoker implements MapperMethodInvoker {
private final MapperMethod mapperMethod;
public PlainMethodInvoker(MapperMethod mapperMethod) {
this.mapperMethod = mapperMethod;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {
return mapperMethod.execute(sqlSession, args);
}
}