JDBC API 事务的实践

使用了持久化框架几乎没有使用过原生的jdbc API ，发现原来使用jdbc API来实现事务也是很简单的。

数据库的链接connection具有一个属性autocommit，这个属性默认是true，作用是控制是否把执行的命令提交给数据库。一旦命令被提交就无法回滚数据库。

而我们实现事务的方式也是很简单，就是手动设置属性autocommit的值为false，等执行完全部命令之后再手动提交所有命令就可以了。

try (Connection connection = DriverManager.getConnection(url, username, password)) {
            System.out.println("数据库链接成功！");

            try (Statement statement = connection.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_READ_ONLY)) {
                String command = "insert into a_dept(name) values('1'),('2')";
                int count = statement.executeUpdate(command);
                System.out.println("受影响行数：" + count);

                String selectSql = "select * from a_dept";
                String updateSql = "update a_dept set name=? where id=?";
                String insertSql = "insert into a_dept(name) values(?)";
                try (ResultSet resultSet = statement.executeQuery(selectSql)) {
                    while (resultSet.next()) {
                        System.out.print("修改前id " + resultSet.getInt("id") + "的值是: ");
                        System.out.println(resultSet.getString("name"));
                    }
                    resultSet.beforeFirst();
                    connection.setAutoCommit(false);
                    while (resultSet.next()) {
                        PreparedStatement updateState = connection.prepareStatement(updateSql);
                        updateState.setString(1, "I am new" + resultSet.getInt("id"));
                        updateState.setInt(2, resultSet.getInt("id"));
                        updateState.executeUpdate();
                    }

//                    Savepoint savepoint = connection.setSavepoint();
                    PreparedStatement updateState = connection.prepareStatement(insertSql);
                    updateState.setString(1, "我是插入的");
                    updateState.executeUpdate();
                    try {
                        PreparedStatement updateStateFail = connection.prepareStatement(insertSql);
                        updateStateFail.setString(1, "我是插入的,但是我太长了，所以我是插不进去的，会报错！");
                        updateStateFail.executeUpdate();

                    } catch (Exception e) {
                        connection.rollback();
                        System.err.println(e.getMessage());
                    }
                    connection.commit();
                    Statement newStatement = connection.createStatement();
                    ResultSet newSet = newStatement.executeQuery(selectSql);
                    System.out.println("打印出所有的值：");
                    while (newSet.next()) {
                        System.out.print(newSet.getInt("id") + ": ");
                        System.out.println(newSet.getString("name"));
                    }
                }
            }
        }

　　贴运行结果：

红色字体就是报错的信息。我们可以发现，在这个结果集当中没有进行修改，也没有进行插入，所有的修改和插入命令因为插入操作的错误都被回滚了。

但是我们发现我们的修改操作和插入操作都被回滚了，这在一些情况下我们不希望所有的操作都被回滚，那么我们可以设置回滚的节点，设置回滚的节点也很简单，在想要的地方添加如下代码，

Savepoint savepoint = connection.setSavepoint();

那么就会在该地生成一个节点，回滚把这个节点作为参数传递给回滚的方法 rollback() ,那么回滚的时候会放过该节点之前的操作，需要注意的是节点使用完之后必须释放它。

结果如下（不要在意Id亲，我清了数据库了）：

Connected to the target VM, address: '127.0.0.1:17576', transport: 'socket'
数据库链接成功！
受影响行数：2
修改前id 479的值是: 1
修改前id 480的值是: 2
Data truncation: Data too long for column 'name' at row 1
打印出所有的值：
479: I am new479
480: I am new480
Disconnected from the target VM, address: '127.0.0.1:17576', transport: 'socket'

简单吧：

最后附上使用节点回滚的全部代码，可以不看~:

        try (Connection connection = DriverManager.getConnection(url, username, password)) {
            System.out.println("数据库链接成功！");

            try (Statement statement = connection.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_READ_ONLY)) {
                String command = "insert into a_dept(name) values('1'),('2')";
                int count = statement.executeUpdate(command);
                System.out.println("受影响行数：" + count);

                String selectSql = "select * from a_dept";
                String updateSql = "update a_dept set name=? where id=?";
                String insertSql = "insert into a_dept(name) values(?)";
                try (ResultSet resultSet = statement.executeQuery(selectSql)) {
                    while (resultSet.next()) {
                        System.out.print("修改前id " + resultSet.getInt("id") + "的值是: ");
                        System.out.println(resultSet.getString("name"));
                    }
                    resultSet.beforeFirst();
                    connection.setAutoCommit(false);
                    while (resultSet.next()) {
                        PreparedStatement updateState = connection.prepareStatement(updateSql);
                        updateState.setString(1, "I am new" + resultSet.getInt("id"));
                        updateState.setInt(2, resultSet.getInt("id"));
                        updateState.executeUpdate();
                    }

                    Savepoint savepoint = connection.setSavepoint();
                    PreparedStatement updateState = connection.prepareStatement(insertSql);
                    updateState.setString(1, "我是插入的");
                    updateState.executeUpdate();
                    try {
                        PreparedStatement updateStateFail = connection.prepareStatement(insertSql);
                        updateStateFail.setString(1, "我是插入的,但是我太长了，所以我是插不进去的，会报错！");
                        updateStateFail.executeUpdate();

                    } catch (Exception e) {
                        connection.rollback(savepoint);

　　　　　　　　　　　　　　 connection.releaseSavepoint(savepoint);
                        System.err.println(e.getMessage());
                    }
                    connection.commit();
                    Statement newStatement = connection.createStatement();
                    ResultSet newSet = newStatement.executeQuery(selectSql);
                    System.out.println("打印出所有的值：");
                    while (newSet.next()) {
                        System.out.print(newSet.getInt("id") + ": ");
                        System.out.println(newSet.getString("name"));
                    }
                }
            }
        }