http://mybatis.org/spring/zh/getting-started.html https://mybatis.net.cn/getting-started.html

一、回顾 jdbc

获取链接—》预编译sql—》设置参数—》执行sql

public class JdbcReview {
    public static void main(String[] args) {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver'");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        Connection connection = null;
        Statement statement = null;
        ResultSet resultSet = null;
        try {
            connection = DriverManager.getConnection("");
            statement = connection.createStatement();
            resultSet = statement.executeQuery("select * from user");
            List<User> users = new ArrayList<>();
            while (resultSet.next()) {
                User user = new User();
                user.setId(resultSet.getInt(1));
                user.setAge(resultSet.getInt(2));
                user.setName(resultSet.getString(3));
                users.add(user);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (resultSet != null) {
                try {
                    resultSet.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
            if (statement != null) {
                try {
                    statement.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
            if (connection != null) {
                try {
                    connection.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
        }
    }
}

• 1、statement中的execute方法
  • boolean execute(String sql) throws SQLException;
    • true:第一个结果是一个结果集
    • false:更新操作或者无结果集
• 2、executeUpdate
  • int executeUpdate(String sql) throws SQLException;
    • (1) DML语言的操作行数
• 3、executeQuery
  • ResultSet executeQuery(String sql) throws SQLException;
    • 查询返回结果集

二、整体架构设计

三、sql解析过程

interface UserMapper {
    @Select("SELECT * FROM user WHERE age = #{age} and name = #{name}")
    List<User> selectUserList(int age, String name);
}
public class MyBatisMapperProxy {
    public static void main(String[] args) {
        UserMapper userMapper = (UserMapper) Proxy.newProxyInstance(MyBatisMapperProxy.class.getClassLoader(), new Class<?>[]{UserMapper.class}, new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                Select annotation = method.getAnnotation(Select.class);
                Map<String, Object> nameArgMap = buildMethodArgNameMap(method, args);
                if (annotation != null) {
                    String[] value = annotation.value();
                    String sql = value[0];
                    // 解析sql，用参数替换 #{}
                    sql = parseSql(sql, nameArgMap);
                    // 这就拿到组装好的sql了
                    System.out.println(sql);
                    System.out.println(method.getReturnType());
                    System.out.println(method.getGenericReturnType());
                    System.out.println(method.getGenericReturnType().getClass());
                }
                return null;
            }
        });
        userMapper.selectUserList(1, "king");
    }
    public static String parseSql(String sql, Map<String, Object> nameArgMap) {
        StringBuilder stringBuilder = new StringBuilder();
        int length = sql.length();
        for (int i = 0; i < length; i++) {
            char c = sql.charAt(i);
            if (c == '#') {
                int nextIndex = i + 1;
                char nextChar = sql.charAt(nextIndex);
                if (nextChar != '{') {
                    throw new RuntimeException(String.format("这里应该为#{\nsql:%s\nindex:%d", stringBuilder.toString(), nextIndex));
                }
                StringBuilder argSB = new StringBuilder();
                i = parseSQLArg(argSB, sql, nextIndex + 1);
                String argName = argSB.toString();
                Object argValue = nameArgMap.get(argName);
                stringBuilder.append(argValue.toString());
                continue;
            }
            stringBuilder.append(c);
        }
        return stringBuilder.toString();
    }
    private static int parseSQLArg(StringBuilder argSB, String sql, int nextIndex) {
        for (; nextIndex < sql.length(); nextIndex++) {
            char c = sql.charAt(nextIndex);
            if (c != '}') {
                argSB.append(c);
                continue;
            }
            // 找到右括号就返回
            if (c == '}') {
                return nextIndex;
            }
        }
        throw new RuntimeException(String.format("缺少右括号#{\nsql:%s\nindex:%d", sql, nextIndex));
    }
    public static Map<String, Object> buildMethodArgNameMap(Method method, Object[] args) {
        HashMap<String, Object> map = new HashMap<>();
        Parameter[] parameters = method.getParameters();
        if (parameters.length != args.length) {
            System.out.println("s错误。。。");
            throw new RuntimeException("error,,.....");
        }
        int index[] = {0};
        Arrays.stream(parameters).forEach(parameter -> {
            // 注意：使用此方法需要开启IDEA支持带参数名编译，否则会出现 arg0，arg1
            // https://blog.csdn.net/lcgoing/article/details/88537836
//            Preferences -> "Build, Execution, Deployment" -> Compiler -> "Java Compiler"
//            "Project bytecode version"设置为1.8
//            "Additional command line parameters"添加"-parameters"
            String name = parameter.getName();
            System.out.println(name);
            map.put(name, args[index[0]]);
            index[0]++;
        });
        return map;
    }
}

这里拿到了解析完的sql，然后将这个sql放到 第一节 里执行即可。
这就是一个简化简化的mybatis。。。。