mybatis insert后返回主键ID实现原理及源码分析

引子:

mybatis这类ORM在往数据库insert对象后,会顺带将数据库中的自增主键值赋值给对象的id,这个功能给我们的开发带来了很多方便,那它是怎么实现的呢?

源码分析:

利用mybatis实现这一功能非常简单,网络上有一大把资料,今天我们主要看它是怎么实现的?

通过断点insert可以跟踪到这个类:
PreparedStatementHandler.java的update方法。

public int update(Statement statement) throws SQLException {
  PreparedStatement ps = (PreparedStatement) statement;
//执行insert操作
  ps.execute();
//获得执行行数
  int rows = ps.getUpdateCount();
  Object parameterObject = boundSql.getParameterObject();
    //获得id
  KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
  keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject);
  return rows;
}

进一步跟踪getKeyGenerator()获得id的方法, 会进入Jdbc3KeyGenerator类的processBatch方法,如下:

  public void processBatch(MappedStatement ms, Statement stmt, Object parameter) {
    final String[] keyProperties = ms.getKeyProperties();
    if (keyProperties == null || keyProperties.length == 0) {
      return;
    }
        //利用了statement的 getGeneratedKeys()方法
    try (ResultSet rs = stmt.getGeneratedKeys()) {
      final ResultSetMetaData rsmd = rs.getMetaData();
      final Configuration configuration = ms.getConfiguration();
      if (rsmd.getColumnCount() < keyProperties.length) {
        // Error?
      } else {
        assignKeys(configuration, rs, rsmd, keyProperties, parameter);
      }
    } catch (Exception e) {
      throw new ExecutorException("Error getting generated key or setting result to parameter object. Cause: " + e, e);
    }
  }

通过代码的注释我们可以看到,mybatis就是利用了Jdbc的Statement来获得会话insert id的,那我们可不可以自己直接利用jdbc来实现呢?

jdbc statement示例

首先创建一个test表:

create table test id int  not null auto_increment, td int, primary key(id);

然后执行以下代码就可以批量获得id了。

        Class.forName("com.mysql.jdbc.Driver");
        Connection connection = DriverManager.getConnection(url, userName, pwd);
        String sql = "insert into test(td) values(5)";
        Statement statement = connection.createStatement();
        statement.execute(sql, 1);

        ResultSet resultSet = statement.getGeneratedKeys();
        while (resultSet.next()){
            System.out.println(resultSet.getObject(1));
        }

        connection.close();

原理:

既然jdbc能获得insert后的id,那它是怎么实现的呢? 通过断点继续跟踪到这个类:StatementImpl.java

   protected ResultSetInternalMethods getGeneratedKeysInternal(long numKeys) throws SQLException {
        synchronized (checkClosed().getConnectionMutex()) {
            Field[] fields = new Field[1];
            fields[0] = new Field("", "GENERATED_KEY", Types.BIGINT, 20);
            fields[0].setConnection(this.connection);
            fields[0].setUseOldNameMetadata(true);

            ArrayList rowSet = new ArrayList();

            //获得上一次获得insert后的id
            long beginAt = getLastInsertID();

            if (beginAt < 0) { // looking at an UNSIGNED BIGINT that has overflowed
                fields[0].setUnsigned();
            }

            if (this.results != null) {
                String serverInfo = this.results.getServerInfo();

                //
                // Only parse server info messages for 'REPLACE' queries
                //
                if ((numKeys > 0) && (this.results.getFirstCharOfQuery() == 'R') && (serverInfo != null) && (serverInfo.length() > 0)) {
                    //计算有多少行数据
                    numKeys = getRecordCountFromInfo(serverInfo);
                }
                //生成批量id
                if ((beginAt != 0 /* BIGINT UNSIGNED can wrap the protocol representation */) && (numKeys > 0)) {
                    for (int i = 0; i < numKeys; i++) {
                        byte[][] row = new byte[1][];
                        if (beginAt > 0) {
                            row[0] = StringUtils.getBytes(Long.toString(beginAt));
                        } else {
                            byte[] asBytes = new byte[8];
                            asBytes[7] = (byte) (beginAt & 0xff);
                            asBytes[6] = (byte) (beginAt >>> 8);
                            asBytes[5] = (byte) (beginAt >>> 16);
                            asBytes[4] = (byte) (beginAt >>> 24);
                            asBytes[3] = (byte) (beginAt >>> 32);
                            asBytes[2] = (byte) (beginAt >>> 40);
                            asBytes[1] = (byte) (beginAt >>> 48);
                            asBytes[0] = (byte) (beginAt >>> 56);

                            BigInteger val = new BigInteger(1, asBytes);

                            row[0] = val.toString().getBytes();
                        }
                        rowSet.add(new ByteArrayRow(row, getExceptionInterceptor()));
                        beginAt += this.connection.getAutoIncrementIncrement();
                    }
                }
            }

            com.mysql.jdbc.ResultSetImpl gkRs = com.mysql.jdbc.ResultSetImpl.getInstance(this.currentCatalog, fields, new RowDataStatic(rowSet),
                    this.connection, this, false);

            return gkRs;
        }
    }

代码的流程是这样的:获得上一次insert后的id,再计算本次插入数据的行数,最后自己批量生成,也就是说jdbc并没有一行一行的去数据库查询id.然后我们再看下它是怎么获得上一次insert后的Id的?

    /**
//只支持自增主键
     * getLastInsertID returns the value of the auto_incremented key after an
     * executeQuery() or excute() call.
     * 
     * 

* This gets around the un-threadsafe behavior of "select LAST_INSERT_ID()" which is tied to the Connection that created this Statement, and therefore could * have had many INSERTS performed before one gets a chance to call "select LAST_INSERT_ID()". *

* * @return the last update ID. */ public long getLastInsertID() { try { synchronized (checkClosed().getConnectionMutex()) { return this.lastInsertId; } } catch (SQLException e) { throw new RuntimeException(e); // evolve interface to throw SQLException } }

光看上面的代码注释就明白了它的逻辑,通过select LAST_INSERT_ID()来获得会话内的insert后Id,并且只支持自增主键。

mysql client获得id

mybatis insert后返回主键ID实现原理及源码分析

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