Tomcat example source code file (ELParser.java)

This example Tomcat source code file (ELParser.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Tomcat tags/keywords

char, el, elexpr, elparser, elparser, function, id, quotedstring, string, string, stringbuffer, stringbuffer, token, token

The Tomcat ELParser.java source code

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.jasper.compiler;

/**
 * This class implements a parser for EL expressions.
 * 
 * It takes strings of the form xxx${..}yyy${..}zzz etc, and turn it into a
 * ELNode.Nodes.
 * 
 * Currently, it only handles text outside ${..} and functions in ${ ..}.
 * 
 * @author Kin-man Chung
 */

public class ELParser {

    private Token curToken; // current token

    private ELNode.Nodes expr;

    private ELNode.Nodes ELexpr;

    private int index; // Current index of the expression

    private String expression; // The EL expression
    
    private char type;

    private boolean escapeBS; // is '\' an escape char in text outside EL?

    private static final String reservedWords[] = { "and", "div", "empty",
            "eq", "false", "ge", "gt", "instanceof", "le", "lt", "mod", "ne",
            "not", "null", "or", "true" };

    public ELParser(String expression) {
        index = 0;
        this.expression = expression;
        expr = new ELNode.Nodes();
    }

    /**
     * Parse an EL expression
     * 
     * @param expression
     *            The input expression string of the form Char* ('${' Char*
     *            '}')* Char*
     * @return Parsed EL expression in ELNode.Nodes
     */
    public static ELNode.Nodes parse(String expression) {
        ELParser parser = new ELParser(expression);
        while (parser.hasNextChar()) {
            String text = parser.skipUntilEL();
            if (text.length() > 0) {
                parser.expr.add(new ELNode.Text(text));
            }
            ELNode.Nodes elexpr = parser.parseEL();
            if (!elexpr.isEmpty()) {
                parser.expr.add(new ELNode.Root(elexpr, parser.type));
            }
        }
        return parser.expr;
    }

    /**
     * Parse an EL expression string '${...}'
     * 
     * @return An ELNode.Nodes representing the EL expression TODO: Currently
     *         only parsed into functions and text strings. This should be
     *         rewritten for a full parser.
     */
    private ELNode.Nodes parseEL() {

        StringBuffer buf = new StringBuffer();
        ELexpr = new ELNode.Nodes();
        while (hasNext()) {
            curToken = nextToken();
            if (curToken instanceof Char) {
                if (curToken.toChar() == '}') {
                    break;
                }
                buf.append(curToken.toChar());
            } else {
                // Output whatever is in buffer
                if (buf.length() > 0) {
                    ELexpr.add(new ELNode.ELText(buf.toString()));
                }
                if (!parseFunction()) {
                    ELexpr.add(new ELNode.ELText(curToken.toString()));
                }
            }
        }
        if (buf.length() > 0) {
            ELexpr.add(new ELNode.ELText(buf.toString()));
        }

        return ELexpr;
    }

    /**
     * Parse for a function FunctionInvokation ::= (identifier ':')? identifier
     * '(' (Expression (,Expression)*)? ')' Note: currently we don't parse
     * arguments
     */
    private boolean parseFunction() {
        if (!(curToken instanceof Id) || isELReserved(curToken.toString())) {
            return false;
        }
        String s1 = null; // Function prefix
        String s2 = curToken.toString(); // Function name
        int mark = getIndex();
        if (hasNext()) {
            Token t = nextToken();
            if (t.toChar() == ':') {
                if (hasNext()) {
                    Token t2 = nextToken();
                    if (t2 instanceof Id) {
                        s1 = s2;
                        s2 = t2.toString();
                        if (hasNext()) {
                            t = nextToken();
                        }
                    }
                }
            }
            if (t.toChar() == '(') {
                ELexpr.add(new ELNode.Function(s1, s2));
                return true;
            }
        }
        setIndex(mark);
        return false;
    }

    /**
     * Test if an id is a reserved word in EL
     */
    private boolean isELReserved(String id) {
        int i = 0;
        int j = reservedWords.length;
        while (i < j) {
            int k = (i + j) / 2;
            int result = reservedWords[k].compareTo(id);
            if (result == 0) {
                return true;
            }
            if (result < 0) {
                i = k + 1;
            } else {
                j = k;
            }
        }
        return false;
    }

    /**
     * Skip until an EL expression ('${' || '#{') is reached, allowing escape
     * sequences '\\' and '\$' and '\#'.
     * 
     * @return The text string up to the EL expression
     */
    private String skipUntilEL() {
        char prev = 0;
        StringBuffer buf = new StringBuffer();
        while (hasNextChar()) {
            char ch = nextChar();
            if (prev == '\\') {
                prev = 0;
                if (ch == '\\') {
                    buf.append('\\');
                    if (!escapeBS)
                        prev = '\\';
                } else if (ch == '$' || ch == '#') {
                    buf.append(ch);
                }
                // else error!
            } else if (prev == '$' || prev == '#') {
                if (ch == '{') {
                    this.type = prev;
                    prev = 0;
                    break;
                }
                buf.append(prev);
                prev = 0;
            }
            if (ch == '\\' || ch == '$' || ch == '#') {
                prev = ch;
            } else {
                buf.append(ch);
            }
        }
        if (prev != 0) {
            buf.append(prev);
        }
        return buf.toString();
    }

    /*
     * @return true if there is something left in EL expression buffer other
     * than white spaces.
     */
    private boolean hasNext() {
        skipSpaces();
        return hasNextChar();
    }

    /*
     * @return The next token in the EL expression buffer.
     */
    private Token nextToken() {
        skipSpaces();
        if (hasNextChar()) {
            char ch = nextChar();
            if (Character.isJavaIdentifierStart(ch)) {
                StringBuffer buf = new StringBuffer();
                buf.append(ch);
                while ((ch = peekChar()) != -1
                        && Character.isJavaIdentifierPart(ch)) {
                    buf.append(ch);
                    nextChar();
                }
                return new Id(buf.toString());
            }

            if (ch == '\'' || ch == '"') {
                return parseQuotedChars(ch);
            } else {
                // For now...
                return new Char(ch);
            }
        }
        return null;
    }

    /*
     * Parse a string in single or double quotes, allowing for escape sequences
     * '\\', and ('\"', or "\'")
     */
    private Token parseQuotedChars(char quote) {
        StringBuffer buf = new StringBuffer();
        buf.append(quote);
        while (hasNextChar()) {
            char ch = nextChar();
            if (ch == '\\') {
                ch = nextChar();
                if (ch == '\\' || ch == quote) {
                    buf.append(ch);
                }
                // else error!
            } else if (ch == quote) {
                buf.append(ch);
                break;
            } else {
                buf.append(ch);
            }
        }
        return new QuotedString(buf.toString());
    }

    /*
     * A collection of low level parse methods dealing with character in the EL
     * expression buffer.
     */

    private void skipSpaces() {
        while (hasNextChar()) {
            if (expression.charAt(index) > ' ')
                break;
            index++;
        }
    }

    private boolean hasNextChar() {
        return index < expression.length();
    }

    private char nextChar() {
        if (index >= expression.length()) {
            return (char) -1;
        }
        return expression.charAt(index++);
    }

    private char peekChar() {
        if (index >= expression.length()) {
            return (char) -1;
        }
        return expression.charAt(index);
    }

    private int getIndex() {
        return index;
    }

    private void setIndex(int i) {
        index = i;
    }

    /*
     * Represents a token in EL expression string
     */
    private static class Token {

        char toChar() {
            return 0;
        }

        public String toString() {
            return "";
        }
    }

    /*
     * Represents an ID token in EL
     */
    private static class Id extends Token {
        String id;

        Id(String id) {
            this.id = id;
        }

        public String toString() {
            return id;
        }
    }

    /*
     * Represents a character token in EL
     */
    private static class Char extends Token {

        private char ch;

        Char(char ch) {
            this.ch = ch;
        }

        char toChar() {
            return ch;
        }

        public String toString() {
            return (new Character(ch)).toString();
        }
    }

    /*
     * Represents a quoted (single or double) string token in EL
     */
    private static class QuotedString extends Token {

        private String value;

        QuotedString(String v) {
            this.value = v;
        }

        public String toString() {
            return value;
        }
    }

    public char getType() {
        return type;
    }
}