本文转载自微信公众号「肌肉码农」，返回方法作者邹学。主键转载本文请联系肌肉码农公众号。实现
引子：
mybatis这类ORM在往数据库insert对象后，及源会顺带将数据库中的码分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，那它是返回方法怎么实现的呢?
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，主键今天我们主要看它是实现怎么实现的?
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,及源 mappedStatement, ps, parameterObject); return rows; } 进一步跟踪getKeyGenerator()获得id的网站模板方法， 会进入Jdbc3KeyGenerator类的码分processBatch方法，如下：
public void processBatch(MappedStatement ms,返回方法 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就是主键利用了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?实现
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,及源 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<ResultSetRow> rowSet = new ArrayList<ResultSetRow>(); //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; } } 代码的源码下载流程是这样的：获得上一次insert后的id，再计算本次插入数据的码分行数，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?
/** 支持自增主键 * getLastInsertID returns the value of the auto_incremented key after an * executeQuery() or excute() call. * * <p> * 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()". * </p> * * @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。亿华云计算
mysql client获得id
