更改class文件 常量池中的某一个类, 更改成一个新的类

/**
 * @author:         吴永行
 * @dateTime:      2014-1-20 下午9:47:03
 * @description:
 *
 */
package 深入理解java虚拟机.第九章类加载即执行子系统的案例与实战;

public class ClassModifier {
    private static final int CONSTANT_POOL_COUNT_INDEX = 8;

    private static final int CONSTANT_Utf8_info = 1;

    private static final int[] CONSTANT_ITEM_LENGTH = { -1, -1, 5 - 1, 5, 9, 9,
            3, 3, 5, 5, 5, 5,5 };

    private static final int u1 = 1;
    private static final int u2 = 2;
    private byte[] classByte;

    public ClassModifier(byte[] classByte) {
        this.classByte = classByte;
    }

    public byte[] modifyUTF8Constant(String olderStr, String newStr) {
        int cpc = getConstantPoolCount();
        int offset = CONSTANT_POOL_COUNT_INDEX + u2;
        for (int i = 0; i < cpc; i++) {
            int tag = ByteUtils.bytes2Int(classByte, offset, u1);
            if (tag == CONSTANT_Utf8_info) {
                int len = ByteUtils.bytes2Int(classByte, offset + u1, u2);
                offset += (u1 + u2);
                String str = ByteUtils.bytes2String(classByte, offset, len);
                if (str.equalsIgnoreCase(olderStr)) {
                    byte[] strBytes = ByteUtils.string2Bytes(newStr);
                    byte[] strLen = ByteUtils.int2Bytes(strBytes.length, u2);
                    classByte = ByteUtils.bytesReplace(classByte, offset - u2,
                            u2, strLen);
                    classByte = ByteUtils.bytesReplace(classByte, offset, len,
                            strBytes);
                    return classByte;
                } else {
                    offset += len;
                }

            } else {
                offset += CONSTANT_ITEM_LENGTH[tag];
            }
        }
        return classByte;
    }

    /**
     * @author:         吴永行
     * @dateTime:      2014-1-20 下午9:53:44
     * @description:
     * @return
     *
     */
    private int getConstantPoolCount() {

        return ByteUtils.bytes2Int(classByte, CONSTANT_POOL_COUNT_INDEX, u2);
    }
}