解密Mybatis,手寫Mybatis框架（二）

摘要： 簡化版Mybatis實現思路 1.建立SqlSessionFactory例項. 2.例項化過程中，載入配置檔案建立configuration物件. 3.通過factory建立SqlSession物件,把configuaration傳入SqlSession. 4.通...

簡化版Mybatis實現思路

• 1.建立SqlSessionFactory例項.
• 2.例項化過程中，載入配置檔案建立configuration物件.
• 3.通過factory建立SqlSession物件,把configuaration傳入SqlSession.
• 4.通過SqlSession獲取mapper介面動態代理
• 5.通過代理對調sqlsession中查詢方法;
• 6.sqlsession將查詢方法轉發給executor;
• 7.executor基於JDBC訪問資料庫獲取資料;
• 8.executor通過反射將資料轉換成POJO並返回給sqlsession;
• 9.資料返回給呼叫者

上節講到快速入門mybatis的demo三大階段

// 1.讀取mybatis配置檔案創SqlSessionFactory
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
inputStream.close();
//-------------第二階段-------------
// 2.獲取sqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();
// 3.獲取對應mapper
TUserMapper mapper = sqlSession.getMapper(TUserMapper.class);

//-------------第三階段-------------
// 4.執行查詢語句並返回結果
TUser user = mapper.selectByPrimaryKey(1);
System.out.println(user.toString());

第一階段：

第一階段先把配置檔案載入到記憶體，包括資料庫資訊和mapper.xml。

針對mapper.xml我們定義一個MappedStatement類來存入相應資訊.

public class MappedStatement {
//此處忽略getset方法
private String namespace;

private String sourceId;//mapper介面路徑+xml裡面的每一個id

private String sql;//sql語句

private String resultType;//返回型別

}

再定義一個全域性配置資訊即Configuration存放所有配置資訊：

public class Configuration {
//記錄mapper xml檔案存放的位置
public static final String MAPPER_CONFIG_LOCATION = "config";
//記錄資料庫連線資訊檔案存放位置
public static final String DB_CONFIG_FILE = "db.properties";

private String dbUrl;

private String dbUserName;

private String dbPassword;

private String dbDriver;

//mapper xml解析完以後select節點的資訊存放在mappedStatements,key為MappedStatement裡面 
//的sourceId
protected final Map<String, MappedStatement> mappedStatements = new HashMap<String, MappedStatement>();

//為mapper介面生成動態代理的方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return MapperProxyFactory.getMapperProxy(sqlSession, type);
}
}

SqlSessionFactory例項化，並載入configuaration物件資訊,這樣就把所有的配置資訊載入到記憶體裡

public class SqlSessionFactory {
//配置物件全域性唯一 載入資料庫資訊和mapper檔案資訊
private Configuration conf = new Configuration();

public SqlSessionFactory() {
//載入資料庫資訊
loadDbInfo();
//載入mapper檔案資訊
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) {
// 建立saxReader物件
SAXReader reader = new SAXReader();
// 通過read方法讀取一個檔案 轉換成Document物件
Document document=null;
try {
document = reader.read(file);
} catch (DocumentException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
//獲取根節點元素物件
Element node = document.getRootElement();
//獲取名稱空間
String namespace = node.attribute("namespace").getData().toString();
//獲取select子節點列表
List<Element> selects = node.elements("select");
for (Element element : selects) {//遍歷select節點，將資訊記錄到MappedStatement物件，並登記到configuration物件中
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);//登記到configuration物件中
}
}

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;
}
}

第二階段

第二階段為獲取Sqlsession並且從sqlsession獲取mapper動態代理.

Sqlsession

• mybatis暴露給外部的介面，實現增刪改查的能力
• 1.對外提供資料訪問的api
• 2.對內將請求轉發給executor

• 3.executor基於JDBC訪問資料庫

  public class DefaultSqlSession implements SqlSession {
  
  //配置物件全域性唯一 載入資料庫資訊和mapper檔案資訊
  private Configuration conf;
  
  //真正提供資料庫訪問能力的物件
  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
  //獲取當前mapper介面的動態代理
  public <T> T getMapper(Class<T> type) {
  return conf.<T>getMapper(type, this);
  }
  
  }

  Executor是Mybatis核心介面定義了資料庫操作的基本方法,Sqlsession都是基於它來實現的

  public interface Executor {
  
  
   <E> List<E> query(MappedStatement ms, Object parameter) throws SQLException;
  
  
   <T> T selectOne(String statement,Object parameter);

  }

  Executor實現類:

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 {
//獲取mappedStatement物件，裡面包含sql語句和目標物件等資訊；
MappedStatement mappedStatement = conf.getMappedStatement(ms.getSourceId());
//1.獲取Connection物件
Connection conn = getConnect();
//2.例項化StatementHandler物件，準備例項化Statement
StatementHandler statementHandler = new DefaultStatementHandler(mappedStatement);
//3.通過statementHandler和Connection獲取PreparedStatement
PreparedStatement prepare = statementHandler.prepare(conn);
//4.例項化ParameterHandler物件，對Statement中sql語句的佔位符進行處理
ParameterHandler parameterHandler = new DefaultParameterHandler(parameter);
parameterHandler.setParameters(prepare);
//5.執行查詢語句，獲取結果集resultSet
ResultSet resultSet = statementHandler.query(prepare);
//6.例項化ResultSetHandler物件，對resultSet中的結果集進行處理，轉化成目標物件
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介面在我們工程裡面沒有實現類，是通過動態代理來執行方法的.

/**
 * mapper介面生成動態代理的工程類
 * 
 */
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實現類：

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本身的方法不增強
return method.invoke(this, args);
}

Class<?> returnType = method.getReturnType();// 獲取方法的返回引數class物件
Object ret = null;
if (isCollection(returnType)) {// 根據不同的返回引數型別呼叫不同的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;
}

}

第三階段

第三階段執行查詢並返回結果.剛剛講過我們執行資料庫操作實際上是executor基於jdbc執行的。

jdbc三大巨頭,Connection,PreparedStatement,ResultSet,

結果集 Result 再通過反射機制對映到物件上面，便做好了資料的對映(關於對映具體內容可查閱資料及原始碼),到這我們已經完成了一個簡易的Mybatis框架了.

通過手寫一個簡單的Mybatis框架，我們就可以看得懂原始碼了，學習框架設計的思路並且增強我們Java的內功.