简化版Mybatis实现思路
- 1.创建SqlSessionFactory实例.
- 2.实例化过程中，加载配置文件创建configuration对象.
- 3.通过factory创建SqlSession对象,把configuaration传入SqlSession.
- 4.通过SqlSession获取mapper接口动态代理
- 5.通过代理对调sqlsession中查询方法;
- 6.sqlsession将查询方法转发给executor;
- 7.executor基于JDBC访问数据库获取数据;
- 8.executor通过反射将数据转换成POJO并返回给sqlsession;
- 9.数据返回给调用者
上节讲到快速入门mybatis的demo三大阶段
// 1.读取mybatis配置文件创SqlSessionFactory
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
inputStream.close();
//-------------第二阶段-------------
// 2.获取sqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();
// 3.获取对应mapper
TUserMapper mapper = sqlSession.getMapper(TUserMapper.class);
//-------------第三阶段-------------
// 4.执行查询语句并返回结果
TUser user = mapper.selectByPrimaryKey(1);
System.out.println(user.toString());
第一阶段：
第一阶段先把配置文件加载到内存，包括数据库信息和mapper.xml。
针对mapper.xml我们定义一个MappedStatement类来存入相应信息.
public class MappedStatement {
//此处忽略getset方法
private String namespace;//xml里面的namespace即mapper接口路径
private String sourceId;//mapper接口路径+xml里面的每一个id
private String sql;//sql语句
private String resultType;//返回类型
}再定义一个全局配置信息即Configuration存放所有配置信息：
public class Configuration {
//记录mapper xml文件存放的位置
public static final String MAPPER_CONFIG_LOCATION = "config";
//记录数据库连接信息文件存放位置
public static final String DB_CONFIG_FILE = "db.properties";
private String dbUrl;
private String dbUserName;
private String dbPassword;
private String dbDriver;
//mapper xml解析完以后select节点的信息存放在mappedStatements,key为MappedStatement里面
//的sourceId
protected final Map<String, MappedStatement> mappedStatements = new HashMap<String, MappedStatement>();
//为mapper接口生成动态代理的方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return MapperProxyFactory.getMapperProxy(sqlSession, type);
}
}
SqlSessionFactory实例化，并加载configuaration对象信息,这样就把所有的配置信息加载到内存里
public class SqlSessionFactory {
//配置对象全局唯一 加载数据库信息和mapper文件信息
private Configuration conf = new Configuration();
public SqlSessionFactory() {
//加载数据库信息
loadDbInfo();
//加载mapper文件信息
loadMappersInfo();
}
private void loadMappersInfo() {
URL resources =null;
resources = SqlSessionFactory.class.getClassLoader().getResource(conf.MAPPER_CONFIG_LOCATION);
File mappers = new File(resources.getFile());
if(mappers.isDirectory()){
File[] listFiles = mappers.listFiles();
for (File file : listFiles) {
loadMapperInfo(file);
}
}
}
private void loadMapperInfo(File file) {
// 创建saxReader对象
SAXReader reader = new SAXReader();
// 通过read方法读取一个文件 转换成Document对象
Document document=null;
try {
document = reader.read(file);
} catch (DocumentException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
//获取根节点元素对象
Element node = document.getRootElement();
//获取命名空间
String namespace = node.attribute("namespace").getData().toString();
//获取select子节点列表
List<Element> selects = node.elements("select");
for (Element element : selects) {//遍历select节点，将信息记录到MappedStatement对象，并登记到configuration对象中
MappedStatement mappedStatement = new MappedStatement();
String id = element.attribute("id").getData().toString();
String resultType = element.attribute("resultType").getData().toString();
String sql = element.getData().toString();
String sourceId = namespace+"."+id;
mappedStatement.setSourceId(sourceId);
mappedStatement.setResultType(resultType);
mappedStatement.setSql(sql);
mappedStatement.setNamespace(namespace);
conf.getMappedStatements().put(sourceId, mappedStatement);//登记到configuration对象中
}
}
private void loadDbInfo() {
InputStream dbIn = SqlSessionFactory.class.getClassLoader().getResourceAsStream(conf.DB_CONFIG_FILE);
Properties p = new Properties();
try {
p.load(dbIn);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
conf.setDbDriver(p.get("jdbc.driver").toString());
conf.setDbPassword(p.get("jdbc.password").toString());
conf.setDbUrl(p.get("jdbc.url").toString());
conf.setDbUserName(p.get("jdbc.username").toString());
}
public SqlSession openSession(){
SqlSession sqlSession = new DefaultSqlSession(conf);
return sqlSession;
}
}
第二阶段
第二阶段为获取Sqlsession并且从sqlsession获取mapper动态代理.
Sqlsession
- mybatis暴露给外部的接口，实现增删改查的能力
- 1.对外提供数据访问的api
- 2.对内将请求转发给executor
-
3.executor基于JDBC访问数据库
public class DefaultSqlSession implements SqlSession { //配置对象全局唯一 加载数据库信息和mapper文件信息 private Configuration conf; //真正提供数据库访问能力的对象 private Executor executor; public DefaultSqlSession(Configuration conf) { super(); this.conf = conf; executor = new SimpleExecutor(conf); } public <T> T selectOne(String statement, Object parameter) { List<Object> selectList = this.selectList(statement, parameter); if(selectList==null||selectList.size()==0){ return null; } if(selectList.size()==1){ return (T) selectList.get(0); }else { throw new RuntimeException("Too Many Result!"); } } public <E> List<E> selectList(String statement, Object parameter) { MappedStatement mappedStatement = conf.getMappedStatement(statement); try { return executor.query(mappedStatement, parameter); } catch (SQLException e) { // TODO Auto-generated catch block e.printStackTrace(); } return null; } @Override //获取当前mapper接口的动态代理 public <T> T getMapper(Class<T> type) { return conf.<T>getMapper(type, this); } }Executor是Mybatis核心接口定义了数据库操作的基本方法,Sqlsession都是基于它来实现的
public interface Executor { <E> List<E> query(MappedStatement ms, Object parameter) throws SQLException; <T> T selectOne(String statement,Object parameter);}
Executor实现类:
public class SimpleExecutor implements Executor {
private Configuration conf;
public SimpleExecutor(Configuration conf) {
this.conf = conf;
}
public <E> List<E> query(MappedStatement ms, Object parameter)
throws SQLException {
//获取mappedStatement对象，里面包含sql语句和目标对象等信息；
MappedStatement mappedStatement = conf.getMappedStatement(ms.getSourceId());
//1.获取Connection对象
Connection conn = getConnect();
//2.实例化StatementHandler对象，准备实例化Statement
StatementHandler statementHandler = new DefaultStatementHandler(mappedStatement);
//3.通过statementHandler和Connection获取PreparedStatement
PreparedStatement prepare = statementHandler.prepare(conn);
//4.实例化ParameterHandler对象，对Statement中sql语句的占位符进行处理
ParameterHandler parameterHandler = new DefaultParameterHandler(parameter);
parameterHandler.setParameters(prepare);
//5.执行查询语句，获取结果集resultSet
ResultSet resultSet = statementHandler.query(prepare);
//6.实例化ResultSetHandler对象，对resultSet中的结果集进行处理，转化成目标对象
ResultSetHandler resultSetHandler = new DefaultResultSetHandler(mappedStatement);
return resultSetHandler.handleResultSets(resultSet);
}
@Override
public <T> T selectOne(String statement, Object parameter) {
MappedStatement mappedStatement =conf.getMappedStatements().get(statement);
return null;
}
private Connection getConnect() {
Connection conn =null;
try {
Class.forName(conf.getDbDriver());
conn = DriverManager.getConnection(conf.getDbUrl(), conf.getDbUserName(), conf.getDbPassword());
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return conn;
}
public Configuration getConf() {
return conf;
}
public void setConf(Configuration conf) {
this.conf = conf;
}
}
mapper接口在我们工程里面没有实现类，是通过动态代理来执行方法的.
/**
* mapper接口生成动态代理的工程类
*
*/
public class MapperProxyFactory<T> {
public static <T> T getMapperProxy(SqlSession sqlSession,Class<T> mapperInterface){
MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface);
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
InvocationHandler实现类：
public class MapperProxy<T> implements InvocationHandler {
private SqlSession sqlSession;
private final Class<T> mapperInterface;
public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface) {
super();
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
}
private <T> boolean isCollection(Class<T> type) {
return Collection.class.isAssignableFrom(type);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable {
if (Object.class.equals(method.getDeclaringClass())) {// 如果是Object本身的方法不增强
return method.invoke(this, args);
}
Class<?> returnType = method.getReturnType();// 获取方法的返回参数class对象
Object ret = null;
if (isCollection(returnType)) {// 根据不同的返回参数类型调用不同的sqlsession不同的方法
ret = sqlSession.selectList(mapperInterface.getName()+"."+ method.getName(), args);
} else {
ret = sqlSession.selectOne(mapperInterface.getName()+"."+ method.getName(), args);
}
return ret;
}
}
第三阶段
第三阶段执行查询并返回结果.刚刚讲过我们执行数据库操作实际上是executor基于jdbc执行的。
jdbc三大巨头,Connection,PreparedStatement,ResultSet,
结果集Result再通过反射机制映射到对象上面，便做好了数据的映射(关于映射具体内容可查阅资料及源码),到这我们已经完成了一个简易的Mybatis框架了.
通过手写一个简单的Mybatis框架，我们就可以看得懂源码了，学习框架设计的思路并且增强我们Java的内功.
java
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