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jStyleParser is a CSS parser written in Java. It has its own application interface that is designed to allow an efficient CSS processing in Java and mapping the values to the Java data types. It parses CSS 2.1 style sheets into structures that can be efficiently assigned to DOM elements. It is intended be the primary CSS parser for the CSSBox library. While handling errors, it is user agent conforming according to the CSS specification.

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package cz.vutbr.web.csskit.antlr;

import org.antlr.runtime.BitSet;
import org.antlr.runtime.Parser;
import org.antlr.runtime.RecognitionException;
import org.antlr.runtime.RecognizerSharedState;
import org.antlr.runtime.Token;
import org.antlr.runtime.TokenStream;
import org.antlr.runtime.tree.CommonTree;
import org.antlr.runtime.tree.TreeAdaptor;

import org.slf4j.Logger;

public class CSSTreeNodeRecovery {
	
	private final Parser parser;
	private final TokenStream input;
	private final RecognizerSharedState state;
	private final TreeAdaptor adaptor;
	private final Logger log;
	
	public CSSTreeNodeRecovery(Parser parser,
	                           TokenStream input,
	                           RecognizerSharedState state,
	                           TreeAdaptor adaptor,
	                           Logger log) {
		this.parser = parser;
		this.input = input;
		this.state = state;
		this.adaptor = adaptor;
		this.log = log;
	}
	
	/**
	 * Recovers and logs error, prepares tree part replacement
	 */
	public Object invalidFallback(int ttype, String ttext, RecognitionException re) {
		parser.reportError(re);
		parser.recover(input, re);
		return invalidReplacement(ttype, ttext);
	}
	
	/**
	 * Recovers and logs error, using custom follow set,
	 * prepares tree part replacement
	 */
	public Object invalidFallbackGreedy(int ttype, String ttext, BitSet follow, RecognitionException re) {
		parser.reportError(re);
		if ( state.lastErrorIndex==input.index() ) {
			// uh oh, another error at same token index; must be a case
			// where LT(1) is in the recovery token set so nothing is
			// consumed; consume a single token so at least to prevent
			// an infinite loop; this is a failsafe.
			input.consume();
		}
		state.lastErrorIndex = input.index();
		parser.beginResync();
		consumeUntilGreedy(input, follow);
		parser.endResync();
		return invalidReplacement(ttype, ttext);
		
	}
	
	/**
	 * Recovers and logs error inside a function, using custom follow set,
	 * prepares tree part replacement
	 */
	public Object invalidFallback(int ttype, String ttext, BitSet follow, CSSLexerState.RecoveryMode mode, CSSLexerState ls, RecognitionException re) {
		parser.reportError(re);
		if ( state.lastErrorIndex==input.index() ) {
			// uh oh, another error at same token index; must be a case
			// where LT(1) is in the recovery token set so nothing is
			// consumed; consume a single token so at least to prevent
			// an infinite loop; this is a failsafe.
			input.consume();
		}
		state.lastErrorIndex = input.index();
		parser.beginResync();
		consumeUntil(input, follow, mode, ls);
		parser.endResync();
		return invalidReplacement(ttype, ttext);
		
	}

	/**
	 * Recovers and logs error inside a function, using custom follow set,
	 * prepares tree part replacement
	 */
	public Object invalidFallbackGreedy(int ttype, String ttext, BitSet follow, CSSLexerState.RecoveryMode mode, CSSLexerState ls, RecognitionException re) {
		parser.reportError(re);
		if ( state.lastErrorIndex==input.index() ) {
			// uh oh, another error at same token index; must be a case
			// where LT(1) is in the recovery token set so nothing is
			// consumed; consume a single token so at least to prevent
			// an infinite loop; this is a failsafe.
			input.consume();
		}
		state.lastErrorIndex = input.index();
		parser.beginResync();
		consumeUntilGreedy(input, follow, mode, ls);
		parser.endResync();
		return invalidReplacement(ttype, ttext);
		
	}
	
	private Object invalidReplacement(int ttype, String ttext) {
		
		Object root = (Object) adaptor.nil();
		Object node = (Object) adaptor.create(ttype, ttext);
		
		adaptor.addChild(root, node);
		
		if(log.isDebugEnabled()) {
			log.debug("Invalid fallback with: {}", TreeUtil.toStringTree((CommonTree) root));
		}
		
		return root;
	}
	
	/**
	 * Consumes token until lexer state is balanced and
	 * token from follow is matched. Matched token is also consumed
	 */
	private void consumeUntilGreedy(TokenStream input, BitSet follow) {
		CSSToken t = null;
		do{
		  Token next = input.LT(1);
		  if (next instanceof CSSToken)
		      t= (CSSToken) input.LT(1);
		  else
		      break; /* not a CSSToken, probably EOF */
		  log.trace("Skipped greedy: {} follow: {}", t, follow);
		  // consume token even if it will match
		  input.consume();
		}while(!(t.getLexerState().isBalanced() && follow.member(t.getType())));
	}
	
	/**
	 * Consumes token until lexer state is function-balanced and
	 * token from follow is matched. Matched token is also consumed
	 */
	private void consumeUntilGreedy(TokenStream input, BitSet follow, CSSLexerState.RecoveryMode mode, CSSLexerState ls) {
		CSSToken t = null;
		do{
			Token next = input.LT(1);
			if (next instanceof CSSToken)
				t= (CSSToken) input.LT(1);
			else
				break; /* not a CSSToken, probably EOF */
			log.trace("Skipped greedy: {}", t);
			// consume token even if it will match
			input.consume();
		}while(!(t.getLexerState().isBalanced(mode, ls, t) && follow.member(t.getType())));
	}
	
	/**
	 * Consumes token until lexer state is function-balanced and
	 * token from follow is matched.
	 */
	private void consumeUntil(TokenStream input, BitSet follow, CSSLexerState.RecoveryMode mode, CSSLexerState ls) {
		CSSToken t = null;
		boolean finish = false;
		do{
			Token next = input.LT(1);
			if (next instanceof CSSToken)
				t= (CSSToken) input.LT(1);
			else
				break; /* not a CSSToken, probably EOF */
			// consume token if does not match
			finish = (t.getLexerState().isBalanced(mode, ls, t) && follow.member(t.getType()));
			if (!finish)
			{
				log.trace("Skipped: {}", t);
				input.consume();
			}
		}while(!finish);
	}
}