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<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.
*
* <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。
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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