mybatis source code analysis (seven) KeyGenerator Detailed

A, KeyGenerator Overview

When the usual development often have such a demand, inserting the data back to the main key, or insert the data needs to obtain the primary key before this demand mybatis is also supported, wherein the main logic section in KeyGenerator, the following is his class Figure:

among them:

• NoKeyGenerator: default empty implementation, the primary key is not necessary to separately process;
• Jdbc3KeyGenerator: auto-increment primary key for the primary database, such as MySQL, PostgreSQL;
• SelectKeyGenerator: mainly used for database does not support case increment primary keys, such as Oracle, DB2;

Interface follows:

public interface KeyGenerator {
  void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter);
  void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter);
}

As seen KeyGenerator the code is very simple, mainly achieved by intercepting two methods:

• Jdbc3KeyGenerator: java.sql.Statement.getGeneratedKeys mainly based on the primary interface to return, so he does not need processBefore method, only need to use processAfter knockdown after obtaining a result, and then reflected to the primary key parameters can be set;
• SelectKeyGenerator: mainly through XML configuration or annotation provided the selectKey , then issues a single query, using the primary key is provided in the return reflection interception method, which can only use one of two methods to intercept, in selectKey.order set attribute AFTER|BEFOREdetermined;

Interception timing:

processBefore is generated at the time of StatementHandler;

protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
  ...
  if (boundSql == null) { // issue #435, get the key before calculating the statement
    generateKeys(parameterObject);
    boundSql = mappedStatement.getBoundSql(parameterObject);
  }
  ...
}

protected void generateKeys(Object parameter) {
  KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
  ErrorContext.instance().store();
  keyGenerator.processBefore(executor, mappedStatement, null, parameter);
  ErrorContext.instance().recall();
}

processAfter is inserted before completing the result to be returned PreparedStatementHandler, SimpleStatementHandler, CallableStatementHandler code a little bit different, but the position is unchanged, PreparedStatementHandler herein by way of example:

@Override
public int update(Statement statement) throws SQLException {
  PreparedStatement ps = (PreparedStatement) statement;
  ps.execute();
  int rows = ps.getUpdateCount();
  Object parameterObject = boundSql.getParameterObject();
  KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
  keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject);
  return rows;
}

二、Jdbc3KeyGenerator

The above will be based primarily on the Jdbc3KeyGenerator java.sql.Statement.getGeneratedKeys primary key returned in the interface, and PreparedStatement Statement but slightly different, resulting in a PreparedStatementHandler, update method SimpleStatementHandler slightly different:

// java.sql.Connection
PreparedStatement prepareStatement(String sql, int autoGeneratedKeys) throws SQLException;
PreparedStatement prepareStatement(String sql, String columnNames[]) throws SQLException;
PreparedStatement prepareStatement(String sql, int columnIndexes[]) throws SQLException;

// java.sql.Statement
boolean execute(String sql, int autoGeneratedKeys) throws SQLException;
boolean execute(String sql, int columnIndexes[]) throws SQLException;
boolean execute(String sql, String columnNames[]) throws SQLException;
// 其中 autoGenerateKeys - Statement.RETURN_GENERATED_KEYS、Statement.NO_GENERATED_KEYS

PreparedStatement can see is instantiated when he designated, and Statement is only specified in the implementation of sql but the essence is the same, here to PreparedStatement example:

public void testJDBC3() {
  try {
    String url = "jdbc:mysql://localhost:3306/mybatis?serverTimezone=GMT";
    String sql = "INSERT INTO user(username,password,address) VALUES (?,?,?)";
    Class.forName("com.mysql.jdbc.Driver");
    Connection conn = DriverManager.getConnection(url, "root", "root");
    String[] columnNames = {"ids", "name"};
    PreparedStatement stmt = conn.prepareStatement(sql, columnNames);
    stmt.setString(1, "test");
    stmt.setString(2, "123456");
    stmt.setString(3, "test");
    stmt.executeUpdate();
    ResultSet rs = stmt.getGeneratedKeys();
    int id = 0;
    if (rs.next()) {
      id = rs.getInt(1);
      System.out.println("----------" + id);
    }
  } catch (Exception e) {
    e.printStackTrace();
  }
}

Here's User table to id primary key, but code I pass columnNames not met, and the result can still correct return the primary key, mainly because of the long drive mybatis columnNames.length> 1 can be, so in the specific also note that when used to achieve different effects of different database driver brought;

Statement of the above and returns to the main PreparedStatement specify different key positions, we can clearly see the following:

// org.apache.ibatis.executor.statement.SimpleStatementHandler
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 {
    //如果没有keyGenerator,直接调用Statement.execute和Statement.getUpdateCount
    statement.execute(sql);
    rows = statement.getUpdateCount();
  }
  return rows;
}

// org.apache.ibatis.executor.statement.PreparedStatementHandler
protected Statement instantiateStatement(Connection connection) throws SQLException {
  String sql = boundSql.getSql();
  if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
    String[] keyColumnNames = mappedStatement.getKeyColumns();
    if (keyColumnNames == null) {
      return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
    } else {
      return connection.prepareStatement(sql, keyColumnNames);
    }
  } else if (mappedStatement.getResultSetType() != null) {
    return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
  } else {
    return connection.prepareStatement(sql);
  }
}

Upon completion of initialization, let's look Jdbc3KeyGenerator most important intercept method:

public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
  List<Object> parameters = new ArrayList<Object>();
  parameters.add(parameter);
  processBatch(ms, stmt, parameters);
}

public void processBatch(MappedStatement ms, Statement stmt, List<Object> parameters) {
  ResultSet rs = null;
  try {
    //核心是使用JDBC3的Statement.getGeneratedKeys
    rs = stmt.getGeneratedKeys();
    final Configuration configuration = ms.getConfiguration();
    final TypeHandlerRegistry typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    final String[] keyProperties = ms.getKeyProperties();
    final ResultSetMetaData rsmd = rs.getMetaData();
    TypeHandler<?>[] typeHandlers = null;
    if (keyProperties != null && rsmd.getColumnCount() >= keyProperties.length) {
      for (Object parameter : parameters) {
        // there should be one row for each statement (also one for each parameter)
        if (!rs.next()) {
          break;
        }
        final MetaObject metaParam = configuration.newMetaObject(parameter);
        if (typeHandlers == null) {
          //先取得类型处理器
          typeHandlers = getTypeHandlers(typeHandlerRegistry, metaParam, keyProperties);
        }
        //填充键值
        populateKeys(rs, metaParam, keyProperties, typeHandlers);
      }
    }
  } catch (Exception e) {
    ...
  }
}

Here it is clear, direct access to the primary key returned, and then provided to the reflection time using the parameters;

三、SelectKeyGenerator

The above also spoke SelectKeyGenerator is mainly used to configure selectKey use processBefore default, but can be configured to order property (AFTER | BEFORE);

<insert id="insertUser2" parameterType="u" useGeneratedKeys="true" keyProperty="id">
  <selectKey keyProperty="id" resultType="long" order="BEFORE">
    SELECT if(max(id) is null,1,max(id)+2) as newId FROM user2
  </selectKey>
  INSERT INTO user2(id,username,password,address) VALUES (#{id},#{userName},#{password},#{address})
</insert>

Looking directly at the source code here:

public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
  if (executeBefore) processGeneratedKeys(executor, ms, parameter);
}

public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
  if (!executeBefore) processGeneratedKeys(executor, ms, parameter);
}

private void processGeneratedKeys(Executor executor, MappedStatement ms, Object parameter) {
  try {
    if (parameter != null && keyStatement != null && keyStatement.getKeyProperties() != null) {
      String[] keyProperties = keyStatement.getKeyProperties();
      final Configuration configuration = ms.getConfiguration();
      final MetaObject metaParam = configuration.newMetaObject(parameter);
      if (keyProperties != null) {
        // Do not close keyExecutor.
        // The transaction will be closed by parent executor.
        Executor keyExecutor = configuration.newExecutor(executor.getTransaction(), ExecutorType.SIMPLE);
        List<Object> values = keyExecutor.query(keyStatement, parameter, RowBounds.DEFAULT, Executor.NO_RESULT_HANDLER);
        if (values.size() == 0) {
          throw new ExecutorException("SelectKey returned no data.");            
        } else if (values.size() > 1) {
          throw new ExecutorException("SelectKey returned more than one value.");
        } else {
          MetaObject metaResult = configuration.newMetaObject(values.get(0));
          if (keyProperties.length == 1) {
            if (metaResult.hasGetter(keyProperties[0])) {
              setValue(metaParam, keyProperties[0], metaResult.getValue(keyProperties[0]));
            } else {
              // no getter for the property - maybe just a single value object
              // so try that
              setValue(metaParam, keyProperties[0], values.get(0));
            }
          } else {
            handleMultipleProperties(keyProperties, metaParam, metaResult);
          }
        }
      }
    }
  } catch (ExecutorException e) {
    ...
  }
}

This code is also very simple, is to use a new recurrence an Executor the SQL, is then reflected to the parameter set;