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StatementHandler接口

StatementHandler封裝了Mybatis連接數(shù)據(jù)庫操作最基礎(chǔ)的部分。因?yàn)椋瑹o論怎么封裝，最終我們都是要使用JDBC和數(shù)據(jù)庫打交道的。

最早我們學(xué)習(xí)java連接數(shù)據(jù)庫時(shí)的代碼就像下面寫的那樣：：

import java.sql.*;

public class FirstExample {

   // JDBC driver name and database URL

   static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";

   static final String DB_URL = "jdbc:mysql://localhost/emp";

   //  Database credentials

   static final String USER = "root";

   static final String PASS = "123456";

   public static void main(String[] args) {

   Connection conn = null;

   Statement stmt = null;

   try{

      //STEP 2: Register JDBC driver

      Class.forName("com.mysql.jdbc.Driver");

      //STEP 3: Open a connection

      System.out.println("Connecting to database...");

      conn = DriverManager.getConnection(DB_URL,USER,PASS);

      //STEP 4: Execute a query

      System.out.println("Creating statement...");

      stmt = conn.createStatement();

      String sql;

      sql = "SELECT id, first, last, age FROM Employees";

      ResultSet rs = stmt.executeQuery(sql);

      //STEP 5: Extract data from result set

      while(rs.next()){

         //Retrieve by column name

         int id  = rs.getInt("id");

         int age = rs.getInt("age");

         String first = rs.getString("first");

         String last = rs.getString("last");

         //Display values

         System.out.print("ID: " + id);

         System.out.print(", Age: " + age);

         System.out.print(", First: " + first);

         System.out.println(", Last: " + last);

      }

      //STEP 6: Clean-up environment

      rs.close();

      stmt.close();

      conn.close();

   }catch(SQLException se){

      //Handle errors for JDBC

      se.printStackTrace();

   }catch(Exception e){

      //Handle errors for Class.forName

      e.printStackTrace();

   }finally{

      //finally block used to close resources

      try{

         if(stmt!=null)

            stmt.close();

      }catch(SQLException se2){

      }// nothing we can do

      try{

         if(conn!=null)

            conn.close();

      }catch(SQLException se){

         se.printStackTrace();

      }//end finally try

   }//end try

   System.out.println("There are so thing wrong!");

}//end main

}//end FirstExample

而對于StatementHandler來說就是將下面的代碼進(jìn)行了封裝和抽象，將和數(shù)據(jù)庫交互的能力提供給Mybatis上層應(yīng)用

StatementHandler接口：

public interface StatementHandler {

  // 從connection中獲取statement

  Statement prepare(Connection connection, Integer transactionTimeout)

      throws SQLException;

  // 對sql進(jìn)行設(shè)置參數(shù)

  void parameterize(Statement statement)

      throws SQLException;

  // 批量執(zhí)行

  void batch(Statement statement)

      throws SQLException;

  // 執(zhí)行預(yù)編譯后的sql語句（update,delete,insert）

  int update(Statement statement)

      throws SQLException;

  // 執(zhí)行查詢sql

  <E> List<E> query(Statement statement, ResultHandler resultHandler)

      throws SQLException;

  // 使用游標(biāo)執(zhí)行查詢sql

  <E> Cursor<E> queryCursor(Statement statement)

      throws SQLException;

  // 獲取執(zhí)行SQL語句的封裝類BoundSql

  BoundSql getBoundSql();

  // 參數(shù)處理器

  ParameterHandler getParameterHandler();

}

先看一下接口下面的實(shí)現(xiàn)類關(guān)系：

BaseStatementHandler

BaseStatementHandler作為繼承StatementHandler接口的抽象類存在。

public abstract class BaseStatementHandler implements StatementHandler {

  protected final Configuration configuration;

  protected final ObjectFactory objectFactory;

  protected final TypeHandlerRegistry typeHandlerRegistry;

  protected final ResultSetHandler resultSetHandler;

  protected final ParameterHandler parameterHandler;

  protected final Executor executor;

  protected final MappedStatement mappedStatement;

  protected final RowBounds rowBounds;

  protected BoundSql boundSql;

  protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    this.configuration = mappedStatement.getConfiguration();

    this.executor = executor;

    this.mappedStatement = mappedStatement;

    this.rowBounds = rowBounds;

    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();

    this.objectFactory = configuration.getObjectFactory();

    if (boundSql == null) { // issue #435, get the key before calculating the statement

      generateKeys(parameterObject);

      boundSql = mappedStatement.getBoundSql(parameterObject);

    }

    this.boundSql = boundSql;

    this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);

    this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);

  }

  @Override

  public BoundSql getBoundSql() {

    return boundSql;

  }

  @Override

  public ParameterHandler getParameterHandler() {

    return parameterHandler;

  }

  @Override

  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {

    ErrorContext.instance().sql(boundSql.getSql());

    Statement statement = null;

    try {

      statement = instantiateStatement(connection);

      setStatementTimeout(statement, transactionTimeout);

      setFetchSize(statement);

      return statement;

    } catch (SQLException e) {

      closeStatement(statement);

      throw e;

    } catch (Exception e) {

      closeStatement(statement);

      throw new ExecutorException("Error preparing statement.  Cause: " + e, e);

    }

  }

  protected abstract Statement instantiateStatement(Connection connection) throws SQLException;

  protected void setStatementTimeout(Statement stmt, Integer transactionTimeout) throws SQLException {

    Integer queryTimeout = null;

    if (mappedStatement.getTimeout() != null) {

      queryTimeout = mappedStatement.getTimeout();

    } else if (configuration.getDefaultStatementTimeout() != null) {

      queryTimeout = configuration.getDefaultStatementTimeout();

    }

    if (queryTimeout != null) {

      stmt.setQueryTimeout(queryTimeout);

    }

    StatementUtil.applyTransactionTimeout(stmt, queryTimeout, transactionTimeout);

  }

  protected void setFetchSize(Statement stmt) throws SQLException {

    Integer fetchSize = mappedStatement.getFetchSize();

    if (fetchSize != null) {

      stmt.setFetchSize(fetchSize);

      return;

    }

    Integer defaultFetchSize = configuration.getDefaultFetchSize();

    if (defaultFetchSize != null) {

      stmt.setFetchSize(defaultFetchSize);

    }

  }

  protected void closeStatement(Statement statement) {

    try {

      if (statement != null) {

        statement.close();

      }

    } catch (SQLException e) {

      //ignore

    }

  }

  protected void generateKeys(Object parameter) {

    KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();

    ErrorContext.instance().store();

    keyGenerator.processBefore(executor, mappedStatement, null, parameter);

    ErrorContext.instance().recall();

  }

}

它內(nèi)部維護(hù)了核心字段：

• parameterHandler 作用就是將參數(shù)替換sql中的占位符的功能
• resultSetHandler 將sql結(jié)果集映射成結(jié)果對象
• executor 執(zhí)行sql的抽象，后續(xù)詳細(xì)看

只實(shí)現(xiàn)了三個(gè)接口方法：

• getBoundSql
• getParameterHandler
• prepare 其中prepare的實(shí)現(xiàn)使用了模版模式，子類必須實(shí)現(xiàn)instantiateStatement方法來完成父類prepare方法的模版。

在BaseStatementHandler的構(gòu)造函數(shù)中我們可以看到一段調(diào)用generateKeys的代碼，它和生成主鍵key有關(guān)。

關(guān)于獲得主鍵key和插入數(shù)據(jù)時(shí)放入主鍵key牽涉到以下幾個(gè)配置：

• selectKey
• useGeneratedKeys 設(shè)置true使用自動生成的主鍵
• keyProperty 指定主鍵是(javaBean的)哪個(gè)屬性。

對于支持自動生成記錄主鍵的數(shù)據(jù)庫，如：MySQL，SQL Server，此時(shí)設(shè)置useGeneratedKeys參數(shù)值為true，在執(zhí)行添加記錄之后可以獲取到數(shù)據(jù)庫自動生成的主鍵ID，合keyProperty指定主鍵。

例子：

  <insert id="saveMsg" parameterType="cn.com.tt.e.nano.Notice"

        useGeneratedKeys="true" keyProperty="msgId">

        insert into notice(msg_type,title,content,rec_time,send_time,user_id,deleted,viewed)

        values(#{msgType,jdbcType=INTEGER},#{title,jdbcType=VARCHAR},#{content,jdbcType=VARCHAR},

               #{recTime,jdbcType=BIGINT},#{sendTime,jdbcType=BIGINT},#{userId,jdbcType=VARCHAR},

               #{deleted,jdbcType=TINYINT},#{viewed,jdbcType=INTEGER})

    </insert>

如果使用selectKey，可以設(shè)置order屬性為AFTER。

例子：

<insert id="insertAndgetkey" parameterType="com.soft.mybatis.model.User">

        <selectKey keyProperty="id" order="AFTER" resultType="java.lang.Integer">

            SELECT LAST_INSERT_ID()

        </selectKey>

        insert into t_user (username,password,create_date) values(#{username},#{password},#{createDate})

    </insert>

對于Oracle數(shù)據(jù)庫，當(dāng)要用到自增字段時(shí)，需要用到Sequence或者使用外界傳入的唯一值比如uuid。則也使用selectKey，設(shè)置order為before。

例子：

<insert id="insert"  parameterType="com.lzumetal.mybatis.entity.Employee">

    <selectKey keyProperty="id" resultType="long" order="BEFORE">

    SELECT SEQ_ADMIN.NEXTVAL FROM DUAL

    </selectKey>

    INSERT INTO tbl_employee(id, name, age, create_time)

    VALUES(#{id}, #{name}, #{age}, #{createTime})

</insert>

KeyGenerator

KeyGenerator接口的實(shí)現(xiàn)有：Jdbc3KeyGenerator，SelectKeyGenerator，NoKeyGenerator

useGeneratedKeys設(shè)置在settings配置文件中的相關(guān)源碼在MappedStatement的org.apache.ibatis.mapping.MappedStatement.Builder#Builder中：

mappedStatement.keyGenerator = configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType) ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;

默認(rèn)是NoKeyGenerator，如果配置了useGeneratedKeys=true并且是insert操作則使用Jdbc3KeyGenerator。

useGeneratedKeys設(shè)置在mapper文件中的相關(guān)源碼：

if (configuration.hasKeyGenerator(keyStatementId)) {

  keyGenerator = configuration.getKeyGenerator(keyStatementId);

} else {

  keyGenerator = context.getBooleanAttribute("useGeneratedKeys",

      configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))

      ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;

}

如果設(shè)置了selectKey則使用SelectKeyGenerator（這部分可以更下去看到），useGeneratedKeys邏輯和前面一樣。

selectKey標(biāo)簽中的order分別對應(yīng)著KeyGenerator中的processBefore方法和processAfter方法。

而前面提到的BaseStatementHandler中g(shù)enerateKeys方法就是觸發(fā)processBefore的地方，也是唯一一個(gè)地方。

  protected void generateKeys(Object parameter) {

    KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();

    ErrorContext.instance().store();

    keyGenerator.processBefore(executor, mappedStatement, null, parameter);

    ErrorContext.instance().recall();

  }

在org.apache.ibatis.executor.statement.StatementHandler#update中的各個(gè)實(shí)現(xiàn)中都可以看到執(zhí)行processAfter的代碼，比如SimpleStatementHandler的代碼：

  public int update(Statement statement) throws SQLException {

    String sql = boundSql.getSql();

    Object parameterObject = boundSql.getParameterObject();

    KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();

    int rows;

    if (keyGenerator instanceof Jdbc3KeyGenerator) {

      statement.execute(sql, Statement.RETURN_GENERATED_KEYS);

      rows = statement.getUpdateCount();

      keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);

    } else if (keyGenerator instanceof SelectKeyGenerator) {

      statement.execute(sql);

      rows = statement.getUpdateCount();

      keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);

    } else {

      statement.execute(sql);

      rows = statement.getUpdateCount();

    }

    return rows;

  }

ParameterHandler

再來看一下前面提到的ParameterHandler，功能就是將動態(tài)的sql中的占位符替換成實(shí)參。

它的實(shí)現(xiàn)是DefaultParameterHandler：

public class DefaultParameterHandler implements ParameterHandler {

  private final TypeHandlerRegistry typeHandlerRegistry;

  private final MappedStatement mappedStatement;

  private final Object parameterObject;

  private final BoundSql boundSql;

  private final Configuration configuration;

  public DefaultParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {

    this.mappedStatement = mappedStatement;

    this.configuration = mappedStatement.getConfiguration();

    this.typeHandlerRegistry = mappedStatement.getConfiguration().getTypeHandlerRegistry();

    this.parameterObject = parameterObject;

    this.boundSql = boundSql;

  }

  @Override

  public Object getParameterObject() {

    return parameterObject;

  }

  @Override

  public void setParameters(PreparedStatement ps) {

    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());

    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();

    if (parameterMappings != null) {

      for (int i = 0; i < parameterMappings.size(); i++) {

        ParameterMapping parameterMapping = parameterMappings.get(i);

        if (parameterMapping.getMode() != ParameterMode.OUT) {

          Object value;

          String propertyName = parameterMapping.getProperty();

          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params

            value = boundSql.getAdditionalParameter(propertyName);

          } else if (parameterObject == null) {

            value = null;

          } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {

            value = parameterObject;

          } else {

            MetaObject metaObject = configuration.newMetaObject(parameterObject);

            value = metaObject.getValue(propertyName);

          }

          TypeHandler typeHandler = parameterMapping.getTypeHandler();

          JdbcType jdbcType = parameterMapping.getJdbcType();

          if (value == null && jdbcType == null) {

            jdbcType = configuration.getJdbcTypeForNull();

          }

          try {

            typeHandler.setParameter(ps, i + 1, value, jdbcType);

          } catch (TypeException e) {

            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);

          } catch (SQLException e) {

            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);

          }

        }

      }

    }

  }

}

SimpleStatementHandler類

SimpleStatementHandler是BaseStatementHandler的子類，使用Statement來完成數(shù)據(jù)庫的操作，所以Sql中不會有占位符，parameterize就是空實(shí)現(xiàn)。

query方法：

  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {

    String sql = boundSql.getSql();

    statement.execute(sql);

    return resultSetHandler.<E>handleResultSets(statement);

  }

最后一步將數(shù)據(jù)庫數(shù)據(jù)轉(zhuǎn)換成java對象，這個(gè)后續(xù)展開。

update方法：

  public int update(Statement statement) throws SQLException {

    String sql = boundSql.getSql();

    Object parameterObject = boundSql.getParameterObject();

    KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();

    int rows;

    if (keyGenerator instanceof Jdbc3KeyGenerator) {

      statement.execute(sql, Statement.RETURN_GENERATED_KEYS);

      rows = statement.getUpdateCount();

      keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);

    } else if (keyGenerator instanceof SelectKeyGenerator) {

      statement.execute(sql);

      rows = statement.getUpdateCount();

      keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);

    } else {

      statement.execute(sql);

      rows = statement.getUpdateCount();

    }

    return rows;

  }

其中對主鍵處理的代碼前面已經(jīng)作了解釋，從上面兩個(gè)代碼片段來看已經(jīng)挖到最深了，最終都是調(diào)用java.sql.*的api。

PreparedStatementHandler

PreparedStatementHandler使用PreparedStatement實(shí)現(xiàn)。

  public <E> Cursor<E> queryCursor(Statement statement) throws SQLException {

    PreparedStatement ps = (PreparedStatement) statement;

    ps.execute();

    return resultSetHandler.<E> handleCursorResultSets(ps);

  }

    public void parameterize(Statement statement) throws SQLException {

    parameterHandler.setParameters((PreparedStatement) statement);

  }

RoutingStatementHandler

看名字就可以猜到了這個(gè)是路由用的，看它的構(gòu)造函數(shù)：

  private final StatementHandler delegate;

  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    switch (ms.getStatementType()) {

      case STATEMENT:

        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);

        break;

      case PREPARED:

        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);

        break;

      case CALLABLE:

        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);

        break;

      default:

        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());

    }

  }

所以具體使用哪一個(gè)StatementHandler是由MappedStatement.getStatementType()決定的

CallableStatementHandler

依賴CallableStatement，用于調(diào)用存儲過程。

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