com.nginious.http.xsp.expr.LeftFunction.java Source code

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Here is the source code for com.nginious.http.xsp.expr.LeftFunction.java

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/**
 * Copyright 2012 NetDigital Sweden AB
 *
 * Licensed 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 com.nginious.http.xsp.expr;

import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

/**
 * A left string function which takes a string and an integer as arguments producing a
 * substring starting from position 0 in the string. Below is an example which evaluates
 * to 'Tes'.
 * 
 * <pre>
 * left('Test', 3)
 * </pre>
 * 
 * @author Bojan Pisler, NetDigital Sweden AB
 *
 */
public class LeftFunction extends StringFunction {

    private Value value2;

    /**
     * Constructs a new left function with the specified argument values.
     * 
     * @param value1 the string.
     * @param value2 value producing integer for number of characters to extract from string
     */
    public LeftFunction(Value value1, Value value2) {
        super("left", value1);
        this.value2 = value2;
    }

    /**
     * Evaluates this left function producing a substring starting at position 0 in the
     * string.
     * 
     * @return the substring
     */
    protected String getStringValue() {
        String expr = value.getStringValue();

        if (expr != null) {
            return expr.substring(0, value2.getIntValue());
        }

        return null;
    }

    /**
     * Creates bytecode for evaluating this left function. The specified method visitor
     * and type are used for generating bytecode.
     * 
     * @param visitor the method visitor
     * @param type the type
     */
    void compile(MethodVisitor visitor, Type type) {
        value.compile(visitor, Type.STRING);
        visitor.visitVarInsn(Opcodes.ASTORE, 1);

        Label nullLabel = new Label();
        Label notNullLabel = new Label();

        visitor.visitVarInsn(Opcodes.ALOAD, 1);
        visitor.visitJumpInsn(Opcodes.IFNULL, nullLabel);

        visitor.visitVarInsn(Opcodes.ALOAD, 1);
        visitor.visitLdcInsn(0);
        value2.compile(visitor, Type.INT);
        visitor.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/lang/String", "substring", "(II)Ljava/lang/String;");
        visitor.visitJumpInsn(Opcodes.GOTO, notNullLabel);

        visitor.visitLabel(nullLabel);
        visitor.visitInsn(Opcodes.ACONST_NULL);

        visitor.visitLabel(notNullLabel);
    }
}