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The Apache Commons JEXL library is an implementation of the JSTL Expression Language with extensions.
There is a newer version: 3.4.0
/*
 * 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.commons.jexl3.parser;

import org.apache.commons.jexl3.JexlInfo;
import org.apache.commons.jexl3.introspection.JexlMethod;
import org.apache.commons.jexl3.introspection.JexlPropertyGet;
import org.apache.commons.jexl3.introspection.JexlPropertySet;

/**
 * Base class for parser nodes - holds an 'image' of the token for later use.
 *
 * @since 2.0
 */
public abstract class JexlNode extends SimpleNode {
    // line + column encoded: up to 4096 columns (ie 20 bits for line + 12 bits for column)
    private int lc = -1;

    /**
     * A marker interface for constants.
     * @param  the literal type
     */
    public interface Constant {
        T getLiteral();
    }

    public JexlNode(int id) {
        super(id);
    }

    public JexlNode(Parser p, int id) {
        super(p, id);
    }

    public void jjtSetFirstToken(Token t) {
        // 0xc = 12, 12 bits -> 4096
        // 0xfff, 12 bits mask
        this.lc = (t.beginLine << 0xc) | (0xfff & t.beginColumn);
    }

    public void jjtSetLastToken(Token t) {
        // nothing
    }

    /**
     * Gets the associated JexlInfo instance.
     *
     * @return the info
     */
    public JexlInfo jexlInfo() {
        JexlInfo info = null;
        JexlNode node = this;
        while (node != null) {
            if (node.jjtGetValue() instanceof JexlInfo) {
                info = (JexlInfo) node.jjtGetValue();
                break;
            }
            node = node.jjtGetParent();
        }
        if (lc >= 0) {
            int c = lc & 0xfff;
            int l = lc >> 0xc;
            // at least an info with line/column number
            return info != null? info.at(l, c) : new JexlInfo(null, l, c);
        } else {
            // weird though; no jjSetFirstToken(...) ever called?
            return info;
        }
    }

    /**
     * Clears any cached value of type JexlProperty{G,S}et or JexlMethod.
     * 
     * This is called when the engine detects the evaluation of a script occurs with a class loader
     * different that the one that created it.
     */
    public void clearCache() {
        final Object value = jjtGetValue();
        if (value instanceof JexlPropertyGet
            || value instanceof JexlPropertySet
            || value instanceof JexlMethod) {
            jjtSetValue(null);
        }
        for (int n = 0; n < jjtGetNumChildren(); ++n) {
            jjtGetChild(n).clearCache();
        }
    }

    /**
     * Whether this node is a constant node Its value can not change after the first evaluation and can be cached
     * indefinitely.
     *
     * @return true if constant, false otherwise
     */
    public boolean isConstant() {
        return isConstant(this instanceof JexlNode.Constant);
    }

    protected boolean isConstant(boolean literal) {
        if (literal) {
            for (int n = 0; n < jjtGetNumChildren(); ++n) {
                JexlNode child = jjtGetChild(n);
                if (child instanceof ASTReference) {
                    boolean is = child.isConstant(true);
                    if (!is) {
                        return false;
                    }
                } else if (child instanceof ASTMapEntry) {
                    boolean is = child.isConstant(true);
                    if (!is) {
                        return false;
                    }
                } else if (!child.isConstant()) {
                    return false;
                }
            }
            return true;
        }
        return false;
    }

    /**
     * Whether this node is a left value.
     * @return true if node is assignable, false otherwise
     */
    public boolean isLeftValue() {
        if (this instanceof ASTIdentifier || this instanceof ASTIdentifierAccess) {
            return true;
        }
        int nc = this.jjtGetNumChildren() - 1;
        if (nc >= 0) {
            JexlNode last = this.jjtGetChild(this.jjtGetNumChildren() - 1);
            return last.isLeftValue();
        }
        if (jjtGetParent() instanceof ASTReference || jjtGetParent() instanceof ASTArrayAccess) {
            return true;
        }
        return false;
    }
}