从class文件中获取包名(fast版本)

纠错   27 Jan 2015

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上文中，采用的是ClassFile来获取常量池信息，从而获得包名。

缺点是，依赖tools.jar包。

本文代码是将ClassFile解析代码抽取出来，进行简化，提升性能。

package org.hot2hot.utils;

import java.io.BufferedInputStream;
import java.io.DataInputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import org.apache.commons.lang.StringUtils;

/**
 * fast
 * @author jiaojianfeng
 *
 */
public class PackageFromClassFast {
    private final static int MAGIC = 0xCAFEBABE;

    public static String getPackage(String fileName) {
        DataInputStream in = null;

        try {
            in = new DataInputStream(new BufferedInputStream(new FileInputStream(fileName)));
            if (in.readInt() != MAGIC) {
                throw new IOException(fileName + "非Java class文件!");
            }

            in.readUnsignedShort();// 次版本号
            in.readUnsignedShort();// 主版本号
            int i = in.readUnsignedShort();// 常量池长度
            Map<Integer, Integer> classInfoMap = new HashMap<Integer, Integer>();
            Map<Integer, String> classNameMap = new HashMap<Integer, String>();

            //常量池
            for (int j = 1; j < i; ++j) {
                int k = in.readUnsignedByte();
                switch (k) {
                case 7:
                    classInfoMap.put(j, in.readUnsignedShort());
                    break;
                case 6:
                    in.readDouble();
                    ++j;
                    break;
                case 9:
                    in.readUnsignedShort();
                    in.readUnsignedShort();
                    break;
                case 4:
                    in.readFloat();
                    break;
                case 3:
                    in.readInt();
                    break;
                case 11:
                    in.readUnsignedShort();
                    in.readUnsignedShort();
                    break;
                case 18:
                    in.readUnsignedShort();
                    in.readUnsignedShort();
                    break;
                case 5:
                    in.readLong();
                    ++j;
                    break;
                case 15:
                    in.readUnsignedByte();
                    in.readUnsignedShort();
                    break;
                case 16:
                    in.readUnsignedShort();
                    break;
                case 10:
                    in.readUnsignedShort();
                    in.readUnsignedShort();
                    break;
                case 12:
                    in.readUnsignedShort();
                    in.readUnsignedShort();
                    break;
                case 8:
                    in.readUnsignedShort();
                    break;
                case 1:
                    classNameMap.put(j, in.readUTF());
                    break;
                case 2:
                case 13:
                case 14:
                case 17:
                default:
                    throw new RuntimeException("class文件错误！");
                }
            }
            
            //access flag
            in.readUnsignedShort();
            
            //this class
            int thisClazz = in.readUnsignedShort();
            
            return StringUtils.replace(classNameMap.get(classInfoMap.get(thisClazz)), "/", ".");

        } catch (IOException ioe) {
            ioe.printStackTrace();
        } finally {
            try {
                in.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    public static void main(String[] args) {
        System.out.println(getPackage("/Users/jiaojianfeng/Documents/empleyment/clazz/A.class"));
    }
}

还是那句话，

如果有更简单的方法，望告知。

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