用java实现一个简单的sql select 解析器,无需第三方依赖，完全从0开始

以下是一个简单的 SQL SELECT 解析器的 Java 实现，支持单表查询和基本条件过滤。代码包含词法分析和语法分析模块，并支持以下语法：

SELECT column1, column2 FROM table WHERE column3 = 5

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完整代码

1. Token 类型定义 (TokenType.java)

public enum TokenType {

SELECT, FROM, WHERE,

IDENTIFIER, COMMA, STAR, EQUALS,

STRING, NUMBER, EOF

}

2. Token 类 (Token.java)

public class Token {

public final TokenType type;

public final String value;

public Token(TokenType type, String value) {
    this.type = type;
    this.value = value;
}

@Override
public String toString() {
    return String.format("(%s, %s)", type, value);
}

}

3. 词法分析器 (Lexer.java)

import java.util.ArrayList;

import java.util.regex.Matcher;

import java.util.regex.Pattern;

public class Lexer {

private final String input;

private int pos = 0;

private static final Pattern TOKEN_PATTERN = Pattern.compile(
    "(?i)(SELECT|FROM|WHERE)" +    // 关键字 (不区分大小写)
    "|([a-zA-Z_][a-zA-Z0-9_]*)" + // 标识符
    "|(\\*)" +                    // 星号
    "|(,)" +                      // 逗号
    "|(=)" +                      // 等于号
    "|('([^']*)')" +              // 字符串字面量
    "|(\\d+)" +                   // 数字
    "|(\\s+)"                     // 空白字符 (跳过)
);

public Lexer(String input) {
    this.input = input;
}

public ArrayList<Token> tokenize() {
    ArrayList<Token> tokens = new ArrayList<>();
    Matcher matcher = TOKEN_PATTERN.matcher(input);

    while (pos < input.length()) {
        if (!matcher.find(pos)) {
            throw new RuntimeException("Invalid token at position: " + pos);
        }

        // 跳过空白字符
        if (matcher.group(7) != null) {
            pos = matcher.end();
            continue;
        }

        // 匹配其他 Token
        for (int i = 1; i <= matcher.groupCount(); i++) {
            if (matcher.group(i) != null) {
                TokenType type = null;
                String value = matcher.group(i);
                switch (i) {
                    case 1:  // 关键字
                        type = TokenType.valueOf(value.toUpperCase());
                        break;
                    case 2:  // 标识符
                        type = TokenType.IDENTIFIER;
                        break;
                    case 3:  // *
                        type = TokenType.STAR;
                        break;
                    case 4:  // ,
                        type = TokenType.COMMA;
                        break;
                    case 5:  // =
                        type = TokenType.EQUALS;
                        break;
                    case 6:  // 字符串 (带引号)
                        type = TokenType.STRING;
                        value = matcher.group(7);  // 去掉引号
                        break;
                    case 8:  // 数字
                        type = TokenType.NUMBER;
                        break;
                }
                if(type!=null){
                    tokens.add(new Token(type, value));
                }
                pos = matcher.end();
                break;
            }
        }
    }
    tokens.add(new Token(TokenType.EOF, ""));
    return tokens;
}

}

4. AST 结构类 (SelectQuery.java)

import java.util.List;

public class SelectQuery {

public List columns;

public String table;

public Condition whereCondition;

public static class Condition {
    public String column;
    public String operator;
    public String value;
}

}

5. 语法解析器 (Parser.java)

import java.util.ArrayList;

import java.util.List;

public class Parser {

private final List tokens;

private int pos = 0;

public Parser(List<Token> tokens) {
    this.tokens = tokens;
}

public SelectQuery parse() {
    SelectQuery query = new SelectQuery();
    parseSelect(query);
    parseFrom(query);
    parseWhere(query);
    return query;
}

private void parseSelect(SelectQuery query) {
    consume(TokenType.SELECT);
    query.columns = new ArrayList<>();
    if (peek().type == TokenType.STAR) {
        consume(TokenType.STAR);
        query.columns.add("*");
    } else {
        do {
            query.columns.add(consume(TokenType.IDENTIFIER).value);
        } while (match(TokenType.COMMA));
    }
}

private void parseFrom(SelectQuery query) {
    consume(TokenType.FROM);
    query.table = consume(TokenType.IDENTIFIER).value;
}

private void parseWhere(SelectQuery query) {
    if (match(TokenType.WHERE)) {
        SelectQuery.Condition condition = new SelectQuery.Condition();
        condition.column = consume(TokenType.IDENTIFIER).value;
        consume(TokenType.EQUALS);
        Token valueToken = peek();
        if (valueToken.type == TokenType.STRING || valueToken.type == TokenType.NUMBER) {
            condition.value = valueToken.value;
            advance();
        } else {
            throw new RuntimeException("Expected string or number");
        }
        query.whereCondition = condition;
    }
}

private Token consume(TokenType expected) {
    Token token = peek();
    if (token.type != expected) {
        throw new RuntimeException("Expected " + expected + ", found " + token.type);
    }
    advance();
    return token;
}

private boolean match(TokenType type) {
    if (peek().type == type) {
        advance();
        return true;
    }
    return false;
}

private Token peek() {
    return tokens.get(pos);
}

private void advance() {
    pos++;
}

}

6. 测试主类 (Main.java)

import java.util.List;

public class Main {

public static void main(String[] args) {

String sql = "SELECT id, name FROM users WHERE age = 25";

Lexer lexer = new Lexer(sql);

List tokens = lexer.tokenize();

Parser parser = new Parser(tokens);

SelectQuery query = parser.parse();

    System.out.println("Columns: " + query.columns);
    System.out.println("Table: " + query.table);
    if (query.whereCondition != null) {
        System.out.println("WHERE " + query.whereCondition.column + 
            " = " + query.whereCondition.value);
    }
}

}

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代码说明

1. 词法分析器 (Lexer)

   • 使用正则表达式匹配 SQL 关键字、标识符、数字、字符串等 Token。
   • 跳过空白字符，返回 Token 列表。

2. 语法解析器 (Parser)

   • 递归下降解析器，依次解析 SELECT、FROM、WHERE 子句。
   • 构建 SelectQuery 对象存储解析结果。

3. AST 结构 (SelectQuery)

   • 保存查询的列、表名和过滤条件。

4. 测试示例 (Main)

   • 输入 SQL 语句，输出解析后的结构。

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运行结果

Columns: [id, name]

Table: users

WHERE age = 25

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支持特性

• 单表 SELECT 查询
• 列名列表或 *
• 简单的 WHERE 条件（仅支持 = 和字符串/数字值）

可根据需要扩展 WHERE 条件（如 >, <, AND/OR）和更复杂的数据类型。