简单sql字段解析器实现参考

　　用例：有一段sql语句，我们需要从中截取出所有字段部分，以便进行后续的类型推断，请给出此解析方法。

　　想来很简单吧，因为 sql 中的字段列表，使用方式有限，比如 a as b, a, a b...

1. 解题思路

　　如果不想做复杂处理，最容易想到的，就是直接用某个特征做分割即可。比如，先截取出 字段列表部分，然后再用逗号',' 分割，就可以得到一个个的字段了。然后再要细分，其实只需要用 as 进行分割就可以了。

　　看起来好像可行，但是存在许多漏洞，首先，这里面有太多的假设：各种截取部分要求必须符合要求，必须没有多余的逗号，必须要有as 等等。这明显不符合要求了。

　　其二，我们可以换一种转换方式。比如先截取到field部分，然后先以 as 分割，再以逗号分割，然后取最后一个词作为field。

　　看起来好像更差了，截取到哪里已经完全不知道了。即原文已经被破坏殆尽，而且同样要求要有 as 转换标签，而且对于函数觊觎有 as 的场景，就完全错误了。

　　其三，最好还是自行一个个单词地解析，field 字段无外乎几种情况，1. 普通字段如 select a; 2. 带as的普通字段如 select a as b; 3. 带函数的字段如 select coalesce(a, b); 4. 带函数且带as的字段如 select coalesce(a, b) ab; 5. 函数内带as的字段如 select cast(a as string) b; ...   我们只需依次枚举对应的情况，就可以将字段解析出来了。

　　看起来是个不错的想法。但是具体实现如何？

2. 具体解析实现

　　主要分两个部分，1. 需要定义一个解析后的结果数据结构，以便清晰描述字段信息； 2. 分词解析sql并以结构体返回；

　　我们先来看看整个算法核心：

/**
 * 功能描述: 简单sql字段解析器
 *
 *        样例如1:
 *          select COALESCE(t1.xno, t2.xno, t3.xno) as xno,
 *             case when t1.no is not null then 1 else null end as xxk001,
 *             case when t2.no is not null then 1 else null end as xxk200,
 *             case when t3.xno is not null then 1 else null end as xx3200
 *             from xxk001 t1
 *               full join xxkj100 t2 on t1.xno = t2.xno
 *               full join xxkj200 t3 on t1.xno = t3.xno;
 *
 *        样例如2:
 *          select cast(a as string) as b from ccc;
 *
 *        样例如3:
 *          with a as(select cus,x1 from b1), b as (select cus,x2 from b2)
 *              select a.cus as a_cus from a join b on a.cus=b.cus where xxx;
 *
 *        样例如4:
 *         select a.xno,b.xx from a_tb as a join b_tb as b on a.id = b.id
 *
 *        样例如5:
 *          select cast  \t(a as string) a_str, cc (a as double) a_double from x
 *
 */
public class SimpleSqlFieldParser {

    /**
     * 解析一段次标签sql 中的字段列表
     *
     * @param sql 原始sql, 需如 select xx from xxx join ... 格式
     * @return 字段列表
     */
    public static List<SelectFieldClauseDescriptor> parse(String sql) {
        String columnPart = adaptFieldPartSql(sql);
        int deep = 0;
        List<StringBuilder> fieldTokenSwap = new ArrayList<>();
        StringBuilder currentTokenBuilder = new StringBuilder();
        List<SelectFieldClauseDescriptor> fieldList = new ArrayList<>();
        fieldTokenSwap.add(currentTokenBuilder);
        int len = columnPart.length();
        char[] columnPartChars = columnPart.toCharArray();
        for(int i = 0; i < len; i++) {
            // 空格忽略，换行忽略，tab忽略
            // 字符串相接
            // 左(号入栈，++deep；
            // 右)号出栈，--deep；
            // deep>0 忽略所有其他直接拼接
            // as 则取下一个值为fieldName
            // case 则直接取到end为止；
            //,号则重置token，构建结果集
            char currentChar = columnPartChars[i];
            switch (currentChar) {
                case '(':
                    ++deep;
                    currentTokenBuilder.append(currentChar);
                    break;
                case ')':
                    --deep;
                    currentTokenBuilder.append(currentChar);
                    break;
                case ',':
                    if(deep == 0) {
                        addNewField(fieldList, fieldTokenSwap, true);
                        fieldTokenSwap = new ArrayList<>();
                        currentTokenBuilder = new StringBuilder();
                        fieldTokenSwap.add(currentTokenBuilder);
                        break;
                    }
                    currentTokenBuilder.append(currentChar);
                    break;
                case ' ':
                case '\t':
                case '\r':
                case '\n':
                    if(deep > 0) {
                        currentTokenBuilder.append(currentChar);
                        continue;
                    }
                    if(currentTokenBuilder.length() == 0) {
                        continue;
                    }
                    // original_name as   --> alias
                    if(i + 1 < len) {
                        int j = i + 1;
                        // 收集连续的空格
                        StringBuilder spaceHolder = new StringBuilder();
                        boolean isNextLeftBracket = false;
                        do {
                            char nextChar = columnPart.charAt(j++);
                            if(nextChar == ' ' || nextChar == '\t'
                                    || nextChar == '\r' || nextChar == '\n') {
                                spaceHolder.append(nextChar);
                                continue;
                            }
                            if(nextChar == '(') {
                                isNextLeftBracket = true;
                            }
                            break;
                        } while (j < len);
                        if(isNextLeftBracket) {
                            currentTokenBuilder.append(currentChar);
                        }
                        if(spaceHolder.length() > 0) {
                            currentTokenBuilder.append(spaceHolder);
                            i += spaceHolder.length();
                        }
                        if(isNextLeftBracket) {
                            // continue next for, function begin
                            continue;
                        }
                    }
                    if(fieldTokenSwap.size() == 1) {
                        if(fieldTokenSwap.get(0).toString().equalsIgnoreCase("case")) {
                            String caseWhenPart = CommonUtil.readSplitWord(
                                    columnPartChars, i, " ", "end");
                            currentTokenBuilder.append(caseWhenPart);
                            if(caseWhenPart.length() <= 0) {
                                throw new BizException("语法错误，未找到case..when的结束符");
                            }
                            i += caseWhenPart.length();
                        }
                    }
                    addNewField(fieldList, fieldTokenSwap, false);
                    currentTokenBuilder = new StringBuilder();
                    fieldTokenSwap.add(currentTokenBuilder);
                    break;
                    // 空格忽略
                default:
                    currentTokenBuilder.append(currentChar);
                    break;
            }

        }
        // 处理剩余尚未存储的字段信息
        addNewField(fieldList, fieldTokenSwap, true);
        return fieldList;
    }

    /**
     * 新增一个字段描述
     *
     * @param fieldList 字段容器
     * @param fieldTokenSwap 候选词
     */
    private static void addNewField(List<SelectFieldClauseDescriptor> fieldList,
                                    List<StringBuilder> fieldTokenSwap,
                                    boolean forceAdd) {
        int ts = fieldTokenSwap.size();
        if(ts == 1 && forceAdd) {
            // db.original_name,
            String fieldName = fieldTokenSwap.get(0).toString();
            String alias = fieldName;
            if(fieldName.contains(".")) {
                alias = fieldName.substring(fieldName.lastIndexOf('.') + 1);
            }
            fieldList.add(new SelectFieldClauseDescriptor(fieldName, alias));
            return;
        }
        if(ts < 2) {
            return;
        }
        if(ts == 2) {
            // original_name alias,
            if(fieldTokenSwap.get(1).toString().equalsIgnoreCase("as")) {
                return;
            }
            fieldList.add(new SelectFieldClauseDescriptor(
                    fieldTokenSwap.get(0).toString(),
                    fieldTokenSwap.get(1).toString()));
        }
        else if(ts == 3) {
            // original_name as alias,
            fieldList.add(new SelectFieldClauseDescriptor(
                    fieldTokenSwap.get(0).toString(),
                    fieldTokenSwap.get(2).toString()));
        }
        else {
            throw new BizException("字段语法解析错误，超过3个以字段描述信息:" + ts);
        }
    }

    // 截取适配 field 字段信息部分
    private static String adaptFieldPartSql(String fullSql) {
        int start = fullSql.lastIndexOf("select ");
        int end = fullSql.lastIndexOf(" from");
        String columnPart = fullSql.substring(start + "select ".length(), end);
        return columnPart.trim();
    }

}

　　应该说是比较简单的，一个for, 一个 switch ，就搞定了。其他的，更多的是逻辑判定。

　　下面我们来看看字段描述类的写法，其实就是两个字段，源字段和别名。

/**
 * 功能描述: sql字段描述 select 字段描述类
 *
 */
public class SelectFieldClauseDescriptor {
    private String fieldName;
    private String alias;

    public SelectFieldClauseDescriptor(String fieldName, String alias) {
        this.fieldName = fieldName;
        this.alias = alias;
    }

    public String getFieldName() {
        return fieldName;
    }

    public String getAlias() {
        return alias;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        SelectFieldClauseDescriptor that = (SelectFieldClauseDescriptor) o;
        return Objects.equals(fieldName, that.fieldName) &&
                Objects.equals(alias, that.alias);
    }

    @Override
    public int hashCode() {
        return Objects.hash(fieldName, alias);
    }

    @Override
    public String toString() {
        return "SelectFieldClauseDescriptor{" +
                "fieldName='" + fieldName + '\'' +
                ", alias='" + alias + '\'' +
                '}';
    }
}

　　它存在的意义，仅仅是为了使用方更方便取值，以为更进一步的解析提供了依据。

3. 单元测试

　　其实像写这种工具类，最是方便简单。因为最初的结果，我们早已预料，以测试驱动开发最合适不过了。而且，基本上一出现不符合预期的值时，很快速就定位问题了。

/**
 * 功能描述: sql字段解析器测试
 **/
public class SimpleSqlFieldParserTest {

    @Test
    public void testParse() {
        String sql;
        List<SelectFieldClauseDescriptor> parsedFieldList;
        sql = "select COALESCE(t1.xno, t2.xno, t3.xno) as xno,\n" +
                "   case when t1.xno is not null then 1 else null end as xxk001,\n" +
                "   case when t2.xno is not null then 1 else null end as xxk200,\n" +
                "   case when t3.xno is not null then 1 else null end as xx3200\n" +
                "   from xxk001 t1\n" +
                "     full join xxkj100 t2 on t1.xno = t2.xno\n" +
                "     full join xxkj200 t3 on t1.xno = t3.xno;";
        parsedFieldList = SimpleSqlFieldParser.parse(sql);
        System.out.println("result:");
        parsedFieldList.forEach(System.out::println);
        Assert.assertEquals("字段个数解析不正确",
                4, parsedFieldList.size());
        Assert.assertEquals("字段别名解析不正确",
                "xno", parsedFieldList.get(0).getAlias());
        Assert.assertEquals("字段别名解析不正确",
                "xx3200", parsedFieldList.get(3).getAlias());

        sql = "select cast(a as string) as b from ccc;";
        parsedFieldList = SimpleSqlFieldParser.parse(sql);
        System.out.println("result:");
        parsedFieldList.forEach(System.out::println);
        Assert.assertEquals("字段个数解析不正确",
                1, parsedFieldList.size());
        Assert.assertEquals("字段别名解析不正确",
                "b", parsedFieldList.get(0).getAlias());

        sql = "with a as(select cus,x1 from b1), b as (select cus,x2 from b2)\n" +
                "    select a.cus as a_cus, cast(a \nas string) as a_cus2, " +
                "b.x2 b2 from a join b on a.cus=b.cus where xxx;";
        parsedFieldList = SimpleSqlFieldParser.parse(sql);
        System.out.println("result:");
        parsedFieldList.forEach(System.out::println);
        Assert.assertEquals("字段个数解析不正确",
                3, parsedFieldList.size());
        Assert.assertEquals("字段别名解析不正确",
                "a_cus", parsedFieldList.get(0).getAlias());
        Assert.assertEquals("字段别名解析不正确",
                "b2", parsedFieldList.get(2).getAlias());

        sql = "select a.xno,b.xx,qqq from a_tb as a join b_tb as b on a.id = b.id";
        parsedFieldList = SimpleSqlFieldParser.parse(sql);
        System.out.println("result:");
        parsedFieldList.forEach(System.out::println);
        Assert.assertEquals("字段个数解析不正确",
                3, parsedFieldList.size());
        Assert.assertEquals("字段别名解析不正确",
                "xno", parsedFieldList.get(0).getAlias());
        Assert.assertEquals("字段别名解析不正确",
                "qqq", parsedFieldList.get(2).getAlias());

        sql = "select cast (a.a_int as string) a_str, b.xx, coalesce  \n( a, b, c) qqq from a_tb as a join b_tb as b on a.id = b.id";
        parsedFieldList = SimpleSqlFieldParser.parse(sql);
        System.out.println("result:");
        parsedFieldList.forEach(System.out::println);
        Assert.assertEquals("字段个数解析不正确",
                3, parsedFieldList.size());
        Assert.assertEquals("字段别名解析不正确",
                "a_str", parsedFieldList.get(0).getAlias());
        Assert.assertEquals("字段原始名解析不正确",
                "cast (a.a_int as string)", parsedFieldList.get(0).getFieldName());
        Assert.assertEquals("字段别名解析不正确",
                "qqq", parsedFieldList.get(2).getAlias());
        Assert.assertEquals("字段原始名解析不正确",
                "coalesce  \n( a, b, c)", parsedFieldList.get(2).getFieldName());
    }
}

　　至此，一个简单的字段解析器完成。小工具，供参考！