Presto-SqlParser-源码浅析-AstBuilder类
- 2022-01-24
AstBuilder 浅析
语法树是一个复杂的结构，为了对这个复杂对象结构中的全部元素执行某些操作，可以使用访问者模式
Antlr为了支持访问者模式，需要让语法树中的每一个节点都支持accept方法, 因此在ParseTree接口中要求继承类实现accept方法
RuleContext 和 TerminalNodeImpl 提供了默认的实现， 默认时，Antlr生成的类重写了accept方法以便于找到访问者中正确的方法
accept 实现相关源码
public class RuleContext implements RuleNode {
...
@Override
public <T> T accept(ParseTreeVisitor<? extends T> visitor) {
return visitor.visitChildren(this);
}
...
}
public class TerminalNodeImpl implements TerminalNode {
...
@Override
public <T> T accept(ParseTreeVisitor<? extends T> visitor) {
return visitor.visitTerminal(this);
}
...
}
public static class NonReservedContext extends ParserRuleContext {
@Override
public <T> T accept(ParseTreeVisitor<? extends T> visitor) {
if ( visitor instanceof SqlBaseVisitor ) return ((SqlBaseVisitor<? extends T>)visitor).visitNonReserved(this);
else return visitor.visitChildren(this);
}
}antlr 默认生成的 SqlBaseBaseVisitor 里 visitXXX 实际上调用了 visitChildren
visitXXX 相关源码
public class SqlBaseBaseVisitor<T> extends AbstractParseTreeVisitor<T> implements SqlBaseVisitor<T> {
@Override public T visitSingleStatement(SqlBaseParser.SingleStatementContext ctx) {
return visitChildren(ctx);
}
...
}
public abstract class AbstractParseTreeVisitor<T> implements ParseTreeVisitor<T> {
@Override
public T visitChildren(RuleNode node) {
T result = defaultResult();
int n = node.getChildCount();
for (int i=0; i<n; i++) {
if (!shouldVisitNextChild(node, result)) {
break;
}
ParseTree c = node.getChild(i);
T childResult = c.accept(this);
result = aggregateResult(result, childResult);
}
return result;
}
...
}我们需要将Antlr产生的语法树进行语义分析，然后转换成Presto下的语法树，因此 AstBuilder 重写了 visitXXX 方法
SqlParser 调用 new AstBuilder(parsingOptions).visit(tree)
public class SqlParser{
private Node invokeParser(String name, String sql, Function<SqlBaseParser, ParserRuleContext> parseFunction, ParsingOptions parsingOptions) {
...
return new AstBuilder(parsingOptions).visit(tree);
...
}
...
}以后进入 AbstractParseTreeVisitor 中的 visit 方法
public abstract class AbstractParseTreeVisitor<T> implements ParseTreeVisitor<T> {
public T visit(ParseTree tree) {
return tree.accept(this);
}
...
}此处根据tree的类型，调用对应的accept
例如tree是SingleStatementContext的话, 就会调用 AstBuilder 中的 visitSingleStatement
SingleStatementContext 相关源码
public class SqlBaseParser extends Parser {
public static class SingleStatementContext extends ParserRuleContext {
public StatementContext statement() {
return getRuleContext(StatementContext.class,0);
}
public TerminalNode EOF() { return getToken(SqlBaseParser.EOF, 0); }
public SingleStatementContext(ParserRuleContext parent, int invokingState) {
super(parent, invokingState);
}
@Override
public int getRuleIndex() { return RULE_singleStatement; }
@Override
public void enterRule(ParseTreeListener listener) {
if ( listener instanceof SqlBaseListener ) ((SqlBaseListener)listener).enterSingleStatement(this);
}
@Override
public void exitRule(ParseTreeListener listener) {
if ( listener instanceof SqlBaseListener ) ((SqlBaseListener)listener).exitSingleStatement(this);
}
@Override
public <T> T accept(ParseTreeVisitor<? extends T> visitor) {
if ( visitor instanceof SqlBaseVisitor ) return ((SqlBaseVisitor<? extends T>)visitor).visitSingleStatement(this);
else return visitor.visitChildren(this);
}
}
...
}
class AstBuilder extends SqlBaseBaseVisitor<Node> {
@Override
public Node visitSingleStatement(SqlBaseParser.SingleStatementContext context) {
return visit(context.statement());
}
...
}
例如tree是Query， 就会调用 AstBuilder 中的 visitQuery，然后构造成Presto的结构
Query 相关源码
public class SqlBaseParser extends Parser {
public static class QueryContext extends ParserRuleContext {
public QueryNoWithContext queryNoWith() {
return getRuleContext(QueryNoWithContext.class,0);
}
public WithContext with() {
return getRuleContext(WithContext.class,0);
}
public QueryContext(ParserRuleContext parent, int invokingState) {
super(parent, invokingState);
}
@Override public int getRuleIndex() { return RULE_query; }
@Override
public void enterRule(ParseTreeListener listener) {
if ( listener instanceof SqlBaseListener ) ((SqlBaseListener)listener).enterQuery(this);
}
@Override
public void exitRule(ParseTreeListener listener) {
if ( listener instanceof SqlBaseListener ) ((SqlBaseListener)listener).exitQuery(this);
}
@Override
public <T> T accept(ParseTreeVisitor<? extends T> visitor) {
if ( visitor instanceof SqlBaseVisitor ) return ((SqlBaseVisitor<? extends T>)visitor).visitQuery(this);
else return visitor.visitChildren(this);
}
}
...
}
class AstBuilder extends SqlBaseBaseVisitor<Node> {
@Override
public Node visitQuery(SqlBaseParser.QueryContext context) {
Query body = (Query) visit(context.queryNoWith());
return new Query(
getLocation(context),
visitIfPresent(context.with(), With.class),
body.getQueryBody(),
body.getOrderBy(),
body.getOffset(),
body.getLimit());
}
...
}