天天在写sql，一直很好奇sql到底咋解析的，于是随便整个小demo。
比如sql是：
String sql = "select name,score,sex from users where name = 'jack' and isdelete <> 1 ; ";
中间可能有1个或者多个空格，就想着怎么着也得有个分词的类，就叫做Tokenizer吧。
public class Tokenizer implements Iterator{ String[] tokens; int index = 0; public Tokenizer(String sql) throws BadSqlGrammarException { sql = sql.trim(); if (!sql.endsWith(";")) { throw new BadSqlGrammarException("SQL未正确结束！"); } //去除多余的空格 sql = sql.replace(";", " ;").replaceAll("\s+", " "); //分词 tokens = sql.split(" "); } @Override public boolean hasNext() { return index < tokens.length; } @Override public String next() { return tokens[index++]; } }
实现Iterator是因为需要去遍历维护的String数组，方便后面解析。
然后是parser类，里面聚合一个Tokenizer的引用。
public class Parser {
Tokenizer tokenizer;
DBCmd cmd;
Map cmdMap = new HashMap<>();
public Parser(String sql) throws BadSqlGrammarException {
//用查表法，代替一大堆的if else
this.cmdMap.put("select", new SelectCmd());
this.tokenizer = new Tokenizer(sql);
//根据第一个sql关键字来确定是什么sql命令
this.cmd = this.cmdMap.get(tokenizer.next());
if (cmd == null)
throw new BadSqlGrammarException("未识别的sql命令！");
}
public void query() throws BadSqlGrammarException {
cmd.query(tokenizer);
}
}
DBCmd做成一个抽象类，不同的命令需要单独设计一个类，去继承他。DBCmd就弄一些通用的方法即可。
public abstract class DBCmd {
public abstract void query(Tokenizer tokenizer) throws BadSqlGrammarException;
protected String splitUntilEnd(Tokenizer tokenizer) throws BadSqlGrammarException {
return splitUntil(tokenizer, ";");
}
protected String splitUntil(Tokenizer tokenizer, String until) throws BadSqlGrammarException {
StringBuffer sb = new StringBuffer();
boolean find = false;
while (tokenizer.hasNext()) {
String next = tokenizer.next();
if (!next.equals(until)) {
sb.append(next).append(" ");
continue;
} else {
find = true;
break;
}
}
if (!find)
throw new BadSqlGrammarException("语法不正确");
return sb.toString();
}
}
只测试一下select语法，所以只写一个select操作类。
public class SelectCmd extends DBCmd {
@Override
public void query(Tokenizer tokenizer) throws BadSqlGrammarException {
String querys = splitUntil(tokenizer, "from");
String tableName = tokenizer.next();
String condition = null;
if (tokenizer.hasNext() && tokenizer.next().equals("where")) {
condition = splitUntilEnd(tokenizer);
}
System.out.println("查询字段：" + querys);
System.out.println("查询表：" + tableName);
System.out.println("查询条件：" + condition);
}
}
最后测试一下子：
public static void main(String[]args)throws BadSqlGrammarException{
String sql="select name,score,sex from users where name = 'jack' and isdelete <> 1 ; ";
Parser parser=new Parser(sql);
parser.query();
}
效果：
查询字段：name,score,sex
查询表：users
查询条件：name = 'jack' and isdelete <> 1
仔细一想，如果涉及到连表，where条件多层嵌套，甚至子查询，就GG了。实在是够复杂的。本文仅为抛砖引玉，匿了。
