系列

• mybatis-3.4.6 配置介绍
• mybatis-3.4.6 顶层配置解析
• mybatis-3.4.6 子配置解析
• mybatis-3.4.6 mapper解析
• mybatis-3.4.6 SQL执行流程
• mybatis-3.4.6 SqlSession执行过程
• mybatis-3.4.6 缓存介绍
• mybatis-3.4.6 自增主键
• mybatis-3.4.6 foreach 自增主键
• mybatis-3.4.6 事务管理

开篇

• 这个系列是基于mybatis-3.4.6版本的源码解析，这篇文章主要分析mybatis的主键生成的逻辑。
  
• 这篇文章会阐述JDBC本身获取自增主键的原理和mybatis处理自增主键的逻辑。
  
• 【mybatis-3.4.6|mybatis-3.4.6 自增主键】mybatis针对自增主键的处理是建立在JDBC的Statement的getGeneratedKeys基础上，所以本质还是得理解JDBC针对自增主键的处理逻辑。
  

JDBC自增主键

Class.forName("com.mysql.jdbc.Driver"); Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mydb", "root", "123"); conn.setAutoCommit(false); PreparedStatement pstm = conn.prepareStatement("insert into students(name, email) values(?, ?)", Statement.RETURN_GENERATED_KEYS); pstm.setString(1, "name1"); pstm.setString(2, "email1"); pstm.addBatch(); pstm.setString(1, "name2"); pstm.setString(2, "email2"); pstm.addBatch(); pstm.executeBatch(); // 返回自增主键值 ResultSet rs = pstm.getGeneratedKeys(); while (rs.next()) { Object value = https://www.it610.com/article/rs.getObject(1); System.out.println(value); }conn.commit(); rs.close(); pstm.close(); conn.close();

• PreparedStatement在executeBatch后通过getGeneratedKeys来返回生成的主键。

mybatis自增主键

INSERTINTO imc_user (`user_nick`) VALUES (#{userNick})

• mybatis的自增主键需要的配置如上所示，核心关键字包括useGeneratedKeys和keyProperty。

DefaultSqlSession

public class DefaultSqlSession implements SqlSession {@Override public int insert(String statement, Object parameter) { return update(statement, parameter); }@Override public int update(String statement, Object parameter) { try { dirty = true; MappedStatement ms = configuration.getMappedStatement(statement); // 通过CachingExecutor执行update操作 return executor.update(ms, wrapCollection(parameter)); } catch (Exception e) { throw ExceptionFactory.wrapException("Error updating database.Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } }

• mybaits的insert/update都是通过DefaultSqlSession的update来实现的。
• DefaultSqlSession的update执行CachingExecutor的update操作。

CachingExecutor

public class CachingExecutor implements Executor {public int update(MappedStatement ms, Object parameterObject) throws SQLException { flushCacheIfRequired(ms); return delegate.update(ms, parameterObject); }

• CachingExecutor调用SimpleExecutor的update操作执行逻辑。

SimpleExecutor

public abstract class BaseExecutor implements Executor {@Override public int update(MappedStatement ms, Object parameter) throws SQLException { ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId()); if (closed) { throw new ExecutorException("Executor was closed."); } clearLocalCache(); return doUpdate(ms, parameter); } }public class SimpleExecutor extends BaseExecutor {public SimpleExecutor(Configuration configuration, Transaction transaction) { super(configuration, transaction); }@Override public int doUpdate(MappedStatement ms, Object parameter) throws SQLException { Statement stmt = null; try { Configuration configuration = ms.getConfiguration(); // handler为PreparedStatementHandler StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null); // 生成Statement对象 stmt = prepareStatement(handler, ms.getStatementLog()); // 执行Statement的操作 return handler.update(stmt); } finally { closeStatement(stmt); } } }

• SimpleExecutor的doUpdate负责执行真正的update操作。
• 由PreparedStatementHandler来处理真正的update操作。

PreparedStatementHandler

public class PreparedStatementHandler extends BaseStatementHandler {public PreparedStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) { super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql); }@Override public int update(Statement statement) throws SQLException { PreparedStatement ps = (PreparedStatement) statement; // 执行PreparedStatement的操作 ps.execute(); // 获取影响的行数 int rows = ps.getUpdateCount(); // 获取KeyGenerator对象，处理自增主键 Object parameterObject = boundSql.getParameterObject(); // mysql使用Jdbc3KeyGenerator来进行处理 KeyGenerator keyGenerator = mappedStatement.getKeyGenerator(); keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject); return rows; }

• 本质上和mysql的交互都是由PreparedStatement的execute来实现的。

Jdbc3KeyGenerator

public class Jdbc3KeyGenerator implements KeyGenerator {public static final Jdbc3KeyGenerator INSTANCE = new Jdbc3KeyGenerator(); @Override public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) { processBatch(ms, stmt, getParameters(parameter)); }public void processBatch(MappedStatement ms, Statement stmt, Collection