要实现一个Java SQL解析器，可以按照以下步骤进行：

1、定义词法分析器（Lexer）

2、定义语法分析器（Parser）

3、定义SQL语句的抽象语法树（AST）

4、遍历AST并执行相应的操作

下面是详细的实现过程：

1. 定义词法分析器（Lexer）

需要定义一个词法分析器，用于将输入的SQL语句分割成一个个的词法单元，可以使用正则表达式来匹配不同的词法单元。

import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class SqlLexer {
    private static final String SQL_COMMENT = "[^
]*";
    private static final String SQL_STRING = "'[^']*'|("[^"]*")";
    private static final String SQL_WORD = "\b\w+b";
    private static final String SQL_SPACE = "\s+";
    private static final String SQL_OPERATOR = "[=><!]+";
    private static final String SQL_NUMBER = "\d+(\.\d*)?";
    private static final String SQL_IDENTIFIER = "[azAZ_][azAZ09_]*";
    private static final String SQL_QUOTED_IDENTIFIER = "[" + SQL_IDENTIFIER + "]";
    private static final String SQL_CHARACTER = "[''"][^'"]*['"]";
    private static final String SQL_DATETIME = "\d{4}\d{2}\d{2} \d{2}:\d{2}:d{2}";
    private static final String SQL_SYMBOL = ",;()[]{}<>?/\*+=&|%!";
    private static final String SQL_WHITESPACE = "[ \t]+";
    private static final String SQL_UNKNOWN = "[^\s" + SQL_COMMENT + SQL_STRING + SQL_WORD + SQL_SPACE + SQL_OPERATOR + SQL_NUMBER + SQL_IDENTIFIER + SQL_QUOTED_IDENTIFIER + SQL_CHARACTER + SQL_DATETIME + SQL_SYMBOL + "]";
    private static final String SQL_REGEX = SQL_COMMENT + "|" + SQL_STRING + "|" + SQL_WORD + "|" + SQL_SPACE + "|" + SQL_OPERATOR + "|" + SQL_NUMBER + "|" + SQL_IDENTIFIER + "|" + SQL_QUOTED_IDENTIFIER + "|" + SQL_CHARACTER + "|" + SQL_DATETIME + "|" + SQL_SYMBOL;
    private static final Pattern PATTERN = Pattern.compile(SQL_REGEX);
    public List<String> tokenize(String input) {
        List<String> tokens = new ArrayList<>();
        Matcher matcher = PATTERN.matcher(input);
        while (matcher.find()) {
            tokens.add(matcher.group());
        }
        return tokens;
    }
}

2. 定义语法分析器（Parser）

接下来，需要定义一个语法分析器，用于根据词法分析器生成的词法单元构建SQL语句的抽象语法树（AST），可以使用递归下降解析方法来实现。

import java.util.*;
import java.util.stream.*;
import org.antlr.v4.*;
import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;
import org.antlr.v4.runtime.tree.ParseTreeWalker;
import org.antlr.v4.tool.*;
import org.antlr.v4.tool.ast.*;
import org.antlr.v4.tool.lexer.*;
import org.antlr.v4.tool.parser.*;
import org.antlr.v4.tool.tree.*;
import org.antlr.v4.tool.grun.*;
import org.antlr.v4.*;
import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.*;
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