Mybatis源码解析-sql执行
一、传统的jdbc操作步骤
- 获取驱动
- 获取jdbc连接
- 创建参数化预编译的sql
- 绑定参数
- 发送sql到数据库执行
- 将将获取到的结果集返回应用
- 关闭连接
传统的jdbc代码：
package com.zjp;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
public class JDBCTest {
public static void main(String[] args) {
try {
Connection con = null; //定义一个MYSQL链接对象
Class.forName("com.mysql.jdbc.Driver").newInstance(); //MYSQL驱动
con = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/mybatis?useUnicode=true", "root", "root"); //链接本地MYSQL
//更新一条数据
String updateSql = "UPDATE user_t SET user_name = 'test' WHERE id = ?";
PreparedStatement pstmt = con.prepareStatement(updateSql);
pstmt.setString(1, "1");
long updateRes = pstmt.executeUpdate();
System.out.print("UPDATE:" + updateRes);
//查询数据并输出
String sql = "select * from user_t where id = ?";
PreparedStatement pstmt2 = con.prepareStatement(sql);
pstmt2.setString(1, "1");
ResultSet rs = pstmt2.executeQuery();
while (rs.next()) { //循环输出结果集
String id = rs.getString("id");
String username = rs.getString("user_name");
System.out.print("\r\n\r\n");
System.out.print("id:" + id + ",username:" + username);
}
//关闭资源
rs.close();
pstmt.close();
pstmt2.close();
con.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}
二、mybatis 执行sql
mybatis要执行sql，同样也需要获取到数据库连接，这个在mybatis里面就是sqlSession
// 使用MyBatis提供的Resources类加载mybatis的配置文件
Reader reader = Resources.getResourceAsReader("mybatis-config.xml");
// 构建sqlSession的工厂
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
SqlSession session = sqlSessionFactory.openSession();2.1 获取mapper对象
获取到了session对象之后就是获取mapper对象了，在mybatis中是使用动态代理的方式获取
@Override
public T getMapper(Class type) {
//最后会去调用MapperRegistry.getMapper
return configuration.getMapper(type, this);
} /**
* 返回代理类
* @param type
* @param sqlSession
* @return
*/
@SuppressWarnings("unchecked")
public T getMapper(Class type, SqlSession sqlSession) {
// MapperProxyFactory去把代理类做出来
final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
// 这里就是返回代理对象
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
} @SuppressWarnings("unchecked")
protected T newInstance(MapperProxy mapperProxy) {
//用JDK自带的动态代理生成映射器
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
} - 通过session获取mapper，session通过MapperProxyFactory代理对象去获取mapper对象，最终在newInstance中我们看到了使用jdk自带的动态代理方式获取到了mapper对象。
2.2 执行sql语句
现在获取到了mapper对象，那么下一步就是执行sql语句了。
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//代理以后，所有Mapper的方法调用时，都会调用这个invoke方法
//并不是任何一个方法都需要执行调用代理对象进行执行，如果这个方法是Object中通用的方法（toString、hashCode等）无需执行
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
//这里优化了，去缓存中找MapperMethod
final MapperMethod mapperMethod = cachedMapperMethod(method);
//执行 sql
return mapperMethod.execute(sqlSession, args);
}所有的sql语句都会调用invoke方法，会后都要调用execute来执行sql语句
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//可以看到执行时就是4种情况，insert|update|delete|select，分别调用SqlSession的4大类方法
if (SqlCommandType.INSERT == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
} else if (SqlCommandType.UPDATE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
} else if (SqlCommandType.DELETE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
} else if (SqlCommandType.SELECT == command.getType()) {
if (method.returnsVoid() && method.hasResultHandler()) {
//如果有结果处理器
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
//如果结果有多条记录
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
//如果结果是map
result = executeForMap(sqlSession, args);
} else {
//否则就是一条记录
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
} else {
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}这个方法也就是通过使用枚举的方式执行insert|update|delete|select，分别执行不同的方法。这里跟着select，这个也是最复杂的。在这里我们看到了很重要的一个方法
result = sqlSession.selectOne(command.getName(), param);通过源码可知selectOne方法转而去调用selectList,很简单的，如果得到0条则返回null，得到1条则返回1条，得到多条报TooManyResultsException错，特别需要主要的是当没有查询到结果的时候就会返回null。因此一般建议在mapper中编写resultType的时候使用包装类型而不是基本类型，比如推荐使用Integer而不是int。这样就可以避免NPE
//核心selectOne
@Override
public T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
List list = this.selectList(statement, parameter);
if (list.size() == 1) {
return list.get(0);
} else if (list.size() > 1) {
throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
} else {
return null;
}
} public List selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
//根据statement id找到对应的MappedStatement
MappedStatement ms = configuration.getMappedStatement(statement);
//转而用执行器来查询结果,注意这里传入的ResultHandler是null
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
} 从selectList方法中可以看出来，最终的查询还是交给了executor
public List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
//得到绑定sql
BoundSql boundSql = ms.getBoundSql(parameter);
//创建缓存Key
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
//查询
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
} 从MappedStatement对象中获取到BoundSql对象，BoundSql对象包含了我们需要执行的sql语句。
public List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
//新建一个StatementHandler
//这里看到ResultHandler传入了
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
//准备语句
stmt = prepareStatement(handler, ms.getStatementLog());
//StatementHandler.query
return handler.query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
} 通过一些列的跟踪，定位到了doQuery方法，最终sql语句的执行交给了StatementHandler对象，这个对象也就是我们最常用的，封装的是PreparedStatement
@Override
public List query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
// 结果交给了ResultSetHandler 去处理
return resultSetHandler. handleResultSets(ps);
} 很明显这里就是使用的PreparedStatement进行处理。
public List最后就是返回结果集了。