6 Mybatis的Executor介绍（二）——批处理
在程序中，有时候我们需要批量的去操作一些数据，批量的新增、修改、删除，如果是通过for循环一条记录一条记录的去更新无疑效率会比较慢。更佳的做法无疑是采用JDBC对批处理的支持。Mybatis基于JDBC对批处理的支持，也提供了进行数据的批量操作的API，BatchExecutor。下面是一段JDBC进行批量操作的示例代码。
@Test
public void testJDBCBatch() throws SQLException {
String sql = "insert into t_user(name, mobile, email) values(?,?,?)";
try (Connection conn = dataSource.getConnection(); PreparedStatement pstmt = conn.prepareStatement(sql);) {
List<User> users = this.getUsers();
for (User user : users) {
pstmt.setString(1, user.getName());
pstmt.setString(2, user.getMobile());
pstmt.setString(3, user.getEmail());
pstmt.addBatch();
}
pstmt.executeBatch();
conn.commit();
}
}
在每一次调用的时候是调用Statement.addBatch()方法，最终所有的参数都传递完了，没有更多的参数时调用Statement.executeBatch()方法进行批量操作。在上一篇博文中我们查看了BatchExecutor的源码，它的核心代码doUpdate()方法，每次被调用的时候都是以handler.batch()结束，而handler.batch()对应的底层代码是调用对应的Statement的addBatch()方法。那它是在什么时候执行executeBatch()呢？
@Override
public int doUpdate(MappedStatement ms, Object parameterObject) throws SQLException {
final Configuration configuration = ms.getConfiguration();
final StatementHandler handler = configuration.newStatementHandler(this, ms, parameterObject, RowBounds.DEFAULT, null, null);
final BoundSql boundSql = handler.getBoundSql();
final String sql = boundSql.getSql();
final Statement stmt;
if (sql.equals(currentSql) && ms.equals(currentStatement)) {
intlast = statementList.size() - 1;
stmt = statementList.get(last);
handler.parameterize(stmt);//fix Issues 322
BatchResult batchResult = batchResultList.get(last);
batchResult.addParameterObject(parameterObject);
} else {
Connection connection = getConnection(ms.getStatementLog());
stmt = handler.prepare(connection);
handler.parameterize(stmt); //fix Issues 322
currentSql = sql;
currentStatement = ms;
statementList.add(stmt);
batchResultList.add(new BatchResult(ms, sql, parameterObject));
}
// handler.parameterize(stmt);
handler.batch(stmt);
return BATCH_UPDATE_RETURN_VALUE;
}
它的executeBatch()是在doFlushStatements()方法调用中调用的，它会在当前Executor进行commit时调用，也会在当前Executor第一次执行doQuery()时调用，在这个时候都会调用Executor的flushStatements()方法，而BaseExecutor在执行flushStatements()时最终会调用其doFlushStatements()方法。当然，我们也可以手动的调用SqlSession的flushStatements()方法，其底层实现也会调用对应的BaseExecutor的flushStatements()方法。
@Override
public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
try {
List<BatchResult> results = new ArrayList<BatchResult>();
if (isRollback) {
return Collections.emptyList();
}
for (int i = 0, n = statementList.size(); i < n; i++) {
Statement stmt = statementList.get(i);
BatchResult batchResult = batchResultList.get(i);
try {
batchResult.setUpdateCounts(stmt.executeBatch());
MappedStatement ms = batchResult.getMappedStatement();
List<Object> parameterObjects = batchResult.getParameterObjects();
KeyGenerator keyGenerator = ms.getKeyGenerator();
if (Jdbc3KeyGenerator.class.equals(keyGenerator.getClass())) {
Jdbc3KeyGenerator jdbc3KeyGenerator = (Jdbc3KeyGenerator) keyGenerator;
jdbc3KeyGenerator.processBatch(ms, stmt, parameterObjects);
} else if (!NoKeyGenerator.class.equals(keyGenerator.getClass())) { //issue #141
for (Object parameter : parameterObjects) {
keyGenerator.processAfter(this, ms, stmt, parameter);
}
}
} catch (BatchUpdateException e) {
StringBuilder message = new StringBuilder();
message.append(batchResult.getMappedStatement().getId())
.append(" (batch index #")
.append(i + 1)
.append(")")
.append(" failed.");
if (i > 0) {
message.append(" ")
.append(i)
.append(" prior sub executor(s) completed successfully, but will be rolled back.");
}
throw new BatchExecutorException(message.toString(), e, results, batchResult);
}
results.add(batchResult);
}
return results;
} finally {
for (Statement stmt : statementList) {
closeStatement(stmt);
}
currentSql = null;
statementList.clear();
batchResultList.clear();
}
}
下面是一个使用Mybatis的BatchExecutor进行批量操作的示例。在示例中我们创建了一个使用BatchExecutor的SqlSession，也可以是SqlSessionTemplate，其实现了SqlSession接口，底层通过一个SqlSession代理进行相应的操作。然后在循环中一次调用对应Mapper的新增操作，相当于调用BatchExecutor的doUpdate()，最后调用SqlSession的commit()方法提交事务，在SqlSession的commit()中会调用Executor的commit()，从而导致了executeBatch()的发生。
@Test
public void testExecutor() {
SqlSession session = this.sessionFactory.openSession(ExecutorType.BATCH);
UserMapper mapper = session.getMapper(UserMapper.class);
User user = null;
for (inti = 0; i < 10; i++) {
user = new User();
user.setName("Name_" + i);
user.setEmail("email");
mapper.insert(user);
}
// 批量插入User
session.commit();// 提交事务时批量操作才会写入数据库
session.close();
}
参考文档
Mybatis源码
（注：本文是基于Mybatis3.3.1所写，写于2016年12月24日星期六）
