Ant example source code file (ClassNameReader.java)

This example Ant source code file (ClassNameReader.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 - Ant tags/keywords

constantpool, constantpool, datainputstream, float, integer, integer, interfacemethodref, io, methodref, object, object, string, string, unused, utf8

The ClassNameReader.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.tools.ant.taskdefs.optional.jlink;

import java.io.DataInput;
import java.io.DataInputStream;
import java.io.IOException;
import java.io.InputStream;

/**
 * Reads just enough of a class file to determine the class' full name.
 *
 * <p>Extremely minimal constant pool implementation, mainly to support extracting
 * strings from a class file.
 */
class ConstantPool {
    // CheckStyle:VisibilityModifier OFF - bc
    static final
        byte UTF8 = 1, UNUSED = 2, INTEGER = 3, FLOAT = 4, LONG = 5, DOUBLE = 6,
        CLASS = 7, STRING = 8, FIELDREF = 9, METHODREF = 10,
        INTERFACEMETHODREF = 11, NAMEANDTYPE = 12;

    byte[] types;

    Object[] values;
    // CheckStyle:VisibilityModifier ON

    /**
     * Create a constant pool.
     * @param data the data input containing the class.
     * @throws IOException if there is an error.
     */
    ConstantPool(DataInput data) throws IOException {
        super();

        int count = data.readUnsignedShort();
        types = new byte [ count ];
        values = new Object [ count ];
        // read in all constant pool entries.
        for (int i = 1; i < count; i++) {
            byte type = data.readByte();
            types[i] = type;
            switch (type) {
            case UTF8 :
                values[i] = data.readUTF();
                break;

            case UNUSED :
                break;

            case INTEGER :
                values[i] = new Integer(data.readInt());
                break;

            case FLOAT :
                values[i] = new Float(data.readFloat());
                break;

            case LONG :
                values[i] = new Long(data.readLong());
                ++i;
                break;

            case DOUBLE :
                values[i] = new Double(data.readDouble());
                ++i;
                break;

            case CLASS :
            case STRING :
                values[i] = new Integer(data.readUnsignedShort());
                break;

            case FIELDREF :
            case METHODREF :
            case INTERFACEMETHODREF :
            case NAMEANDTYPE :
                values[i] = new Integer(data.readInt());
                break;
            default:
                // Do nothing
            }
        }
    }
}

/**
 * Provides a quick and dirty way to determine the true name of a class
 * given just an InputStream. Reads in just enough to perform this
 * minimal task only.
 */
public class ClassNameReader extends Object {

    /**
     * Get the class name of a class in an input stream.
     *
     * @param input an <code>InputStream value
     * @return the name of the class
     * @exception IOException if an error occurs
     */
    public static String getClassName(InputStream input) throws IOException {
        DataInputStream data = new DataInputStream(input);
        // verify this is a valid class file.
        int cookie = data.readInt();
        if (cookie != 0xCAFEBABE) {
            return null;
        }
        /* int version = */ data.readInt();
        // read the constant pool.
        ConstantPool constants = new ConstantPool(data);
        Object[] values = constants.values;
        // read access flags and class index.
        /* int accessFlags = */ data.readUnsignedShort();
        int classIndex = data.readUnsignedShort();
        Integer stringIndex = (Integer) values[classIndex];
        String className = (String) values[stringIndex.intValue()];
        return className;
    }


}