这周的分享准备时间太短了，总结比较潦草，还望见谅！

学习jvm的class文件结构基础的就不说了，参照书上的内容简单了解下，感觉写个例子更能加深对class文件结构的了解，于是写了两个例子：

1、替换方法的调用，和书上的那个例子差不多，自己改了场景实践了一下，主要实现思路就是修改常量池中原class的字面值，比较简单。

2、BeanCopy的实现，自己实现的工具类，相当的容易读懂（你懂的），只是为了实践而已，鲁棒性自然很差了（很多地方写死了，比如说是有int类型的字段的Bean才可以复制，Bean的字段名字只能是一个英文字母，都是为了方便哈~~谅解）。主要实现思路是增加常量池常量，修改常量池常量数，修改code区域的jvm指令，修改code的length，最后生成class。

一、方法替换

【工具类】

package jvm;public class ByteUtil {    public static byte[] int2Bytes(int value, int len) {        byte[] b = new byte[len];        for (int i = 0; i < len; i++) {            b[len - i - 1] = (byte) ((value >> 8 * i) & 0xff);        }        return b;    }    public static int byte2Int(byte[] b, int start, int len) {        int sum = 0;        int end = start + len;        for (int i = start; i < end; i++) {            int n = ((int) b[i]) & 0xff;            n <<= (--len) * 8;            sum = n + sum;        }        return sum;    }    public static String byte2String(byte[] b, int start, int len) {        return new String(b, start, len);    }    public static byte[] string2Bytes(String str) {        return str.getBytes();    }    public static byte[] byteReplace(byte[] originalBytes, int offset, int len, byte[] replaceBytes) {        byte[] newBytes = new byte[originalBytes.length + replaceBytes.length - len];        System.arraycopy(originalBytes, 0, newBytes, 0, offset);        System.arraycopy(replaceBytes, 0, newBytes, offset, replaceBytes.length);        System.arraycopy(originalBytes, offset + len, newBytes, offset + replaceBytes.length, originalBytes.length                                                                                              - offset - len);        return newBytes;    }    public static void main(String[] args) {        byte[] res = int2Bytes(29, 2);        System.out.println(res);    }}

package jvm;public class TestA {    public static int fuc() {        return 1;    }}

package jvm;public class TestB {    public static int fuc() {        return 2;    }}

package jvm;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, -1, 5, 5, 9, 9, 3, 3, 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 int getConstantPoolCount() {        return ByteUtil.byte2Int(classByte, CONSTANT_POOL_COUNT_INDEX, u2);    }    public byte[] modify(String oldStr, String newStr) {        int cpc = getConstantPoolCount();        int offset = CONSTANT_POOL_COUNT_INDEX + u2;        for (int i = 0; i < cpc; i++) {            int tag = ByteUtil.byte2Int(classByte, offset, u1);            if (tag == CONSTANT_UTF8_INFO) {                int len = ByteUtil.byte2Int(classByte, offset + u1, u2);                offset += (u1 + u2);                String str = ByteUtil.byte2String(classByte, offset, len);                if (str.equalsIgnoreCase(oldStr)) {                    byte[] strBytes = ByteUtil.string2Bytes(newStr);                    byte[] strLen = ByteUtil.int2Bytes(newStr.length(), u2);                    classByte = ByteUtil.byteReplace(classByte, offset - u2, u2, strLen);                    classByte = ByteUtil.byteReplace(classByte, offset, len, strBytes);                    return classByte;                } else {                    offset += len;                }            } else {                offset += CONSTANT_ITEM_LENGTH[tag];            }        }        return classByte;    }}

package jvm;public class TestClassLoader extends ClassLoader {    public TestClassLoader(){        super(TestClassLoader.class.getClassLoader());    }    public Class loadByte(byte[] classByte) {        return defineClass(null, classByte, 0, classByte.length);    }}

package jvm;import java.io.FileInputStream;import java.io.InputStream;import java.lang.reflect.Method;import java.util.Date;public class TestClass {    private static final int m = staticInc();    private Date             curr = new Date();    public int inc() {        return m + 1;    }    public static int staticInc() {        return TestA.fuc();    }    public static void main(String[] args) throws Exception {        InputStream is = new FileInputStream("/Users/apple/workspace/CommonTest/bin/jvm/TestClass.class");        byte[] b = new byte[is.available()];        is.read(b);        is.close();        ClassModifier cm = new ClassModifier(b);        byte[] newBytes = cm.modify("jvm/TestA", "jvm/TestB");        TestClassLoader loader = new TestClassLoader();        Class clazz = loader.loadByte(newBytes);        Method method = clazz.getMethod("staticInc");        System.out.println((Integer) method.invoke(null));    }}

二、BeanCopy的实现

【测试Bean，其实可有可无】

package jvm;public class Bean {    int    i;    int    j;    public int getI() {        return i;    }    public void setI(int i) {        this.i = i;    }    public int getJ() {        return j;    }    public void setJ(int j) {        this.j = j;    }}

package jvm;public class BeanCopierModel {    public static Object copy(Object bean) {        return null;    }}

【实现类，比较臭，但是可读性还是可以，那些偏移量不需要关注，主要是流程】 

package jvm;import java.io.FileInputStream;import java.io.FileNotFoundException;import java.io.FileOutputStream;import java.io.InputStream;import java.lang.reflect.Method;import java.util.ArrayList;import java.util.Arrays;import java.util.Collections;import java.util.HashMap;import java.util.List;import java.util.Map;public class BeanCopyClient {    private static final int CODE_LENGTH   = 15;    private static final int CODE_PER_STEP = 8;    private static final int CONSTANT_COUNT_OFF          = 8;    private static final int CONSTANT_NAME_AND_TYPE_INIT = 9;    private static final int CONSTANT_COUNT              = 19;    private static final int CODE_BEGIN                  = 358;    private static final int CODE_LEN                    = 10;    private static final int CONSTANT_END                = 270;    private static final int METHOD_LEN                  = 14;    private static final int ATTRIBUTE_OFF               = 371;    private static final int ATTRIBUTE_LEN               = 31;    private static final int U1                          = 1;    private static final int U2                          = 2;    private static final int U4                          = 4;    String                   className;    List
    
                  fields                      = new ArrayList
     
      ();    Map
      
            constantIndex               = new HashMap
       
        ();        private void init(){        //准备测试数据     // 测试数据        fields.add("I");        fields.add("J");        className = "jvm/Bean";    }    private byte[] modifyConstantCount(byte[] b) {        return ByteUtil.byteReplace(b, CONSTANT_COUNT_OFF, 2, ByteUtil.int2Bytes(constantNum(), U2));    }    private byte[] modifyCodeLen(byte[] b) {        int len = codeLen();        return ByteUtil.byteReplace(b, CODE_BEGIN - 4, 4, ByteUtil.int2Bytes(len, U4));    }    private byte[] generateConstant() {        int len = coutLen();        int index = CONSTANT_COUNT;        byte[] b = new byte[len];        int offset = 0;        // class 的utf8-info        byte[] tag = ByteUtil.int2Bytes(1, U1);        byte[] length = ByteUtil.int2Bytes(className.length(), U2);        byte[] bytes = ByteUtil.string2Bytes(className);        System.arraycopy(tag, 0, b, offset, U1);        offset = offset + U1;        System.arraycopy(length, 0, b, offset, U2);        offset = offset + U2;        System.arraycopy(bytes, 0, b, offset, className.length());        offset = offset + className.length();        constantIndex.put("utf8_class", ++index);        // class info        System.arraycopy(ByteUtil.int2Bytes(7, U1), 0, b, offset, U1);        offset = offset + U1;        int class_index = constantIndex.get("utf8_class");        System.arraycopy(ByteUtil.int2Bytes(class_index, U2), 0, b, offset, U2);        offset = offset + U2;        constantIndex.put("class_info", ++index);        // get set 的utf8-info        for (String field : fields) {            String set = "set" + field;            String get = "get" + field;            byte[] tag_ = ByteUtil.int2Bytes(1, U1);            byte[] length_ = ByteUtil.int2Bytes(field.length() + 3, U2);            byte[] bytes_ = ByteUtil.string2Bytes(set);            byte[] bytes__ = ByteUtil.string2Bytes(get);            System.arraycopy(tag_, 0, b, offset, U1);            offset = offset + U1;            System.arraycopy(length_, 0, b, offset, U2);            offset = offset + U2;            System.arraycopy(bytes_, 0, b, offset, field.length() + 3);            offset = offset + field.length() + 3;            constantIndex.put("utf8_set" + field, ++index);            System.arraycopy(tag_, 0, b, offset, U1);            offset = offset + U1;            System.arraycopy(length_, 0, b, offset, U2);            offset = offset + U2;            System.arraycopy(bytes__, 0, b, offset, field.length() + 3);            offset = offset + field.length() + 3;            constantIndex.put("utf8_get" + field, ++index);        }        // get set的描述字段        byte[] tag_setDesc = ByteUtil.int2Bytes(1, U1);        byte[] length_setDesc = ByteUtil.int2Bytes(4, U2);        byte[] bytes_setDesc = ByteUtil.string2Bytes("(I)V");        System.arraycopy(tag_setDesc, 0, b, offset, U1);        offset = offset + U1;        System.arraycopy(length_setDesc, 0, b, offset, U2);        offset = offset + U2;        System.arraycopy(bytes_setDesc, 0, b, offset, 4);        offset = offset + 4;        constantIndex.put("utf8_setDesc", ++index);        byte[] tag_getDesc = ByteUtil.int2Bytes(1, U1);        byte[] length_getDesc = ByteUtil.int2Bytes(3, U2);        byte[] bytes_getDesc = ByteUtil.string2Bytes("()I");        System.arraycopy(tag_getDesc, 0, b, offset, U1);        offset = offset + U1;        System.arraycopy(length_getDesc, 0, b, offset, U2);        offset = offset + U2;        System.arraycopy(bytes_getDesc, 0, b, offset, 3);        offset = offset + 3;        constantIndex.put("utf8_getDesc", ++index);        // get set 的NameType        for (String field : fields) {            int field_name_index = constantIndex.get("utf8_set"+field);            int field_desc_index = constantIndex.get("utf8_setDesc");            System.arraycopy(ByteUtil.int2Bytes(12, U1), 0, b, offset, U1);            offset = offset + U1;            System.arraycopy(ByteUtil.int2Bytes(field_name_index, U2), 0, b, offset, U2);            offset = offset + U2;            System.arraycopy(ByteUtil.int2Bytes(field_desc_index, U2), 0, b, offset, U2);            offset = offset + U2;            constantIndex.put("name_type_set" + field, ++index);            int field_name_index_ = constantIndex.get("utf8_get" + field);            int field_desc_index_ = constantIndex.get("utf8_getDesc");            System.arraycopy(ByteUtil.int2Bytes(12, U1), 0, b, offset, U1);            offset = offset + U1;            System.arraycopy(ByteUtil.int2Bytes(field_name_index_, U2), 0, b, offset, U2);            offset = offset + U2;            System.arraycopy(ByteUtil.int2Bytes(field_desc_index_, U2), 0, b, offset, U2);            offset = offset + U2;            constantIndex.put("name_type_get" + field, ++index);        }        // get set method        int class_info_index = constantIndex.get("class_info");        byte[] tag_method = ByteUtil.int2Bytes(10, U1);        byte[] class_ref = ByteUtil.int2Bytes(class_info_index, U2);        for (String field : fields) {            int name_type_set = constantIndex.get("name_type_set" + field);            System.arraycopy(tag_method, 0, b, offset, U1);            offset = offset + U1;            System.arraycopy(class_ref, 0, b, offset, U2);            offset = offset + U2;            System.arraycopy(ByteUtil.int2Bytes(name_type_set, U2), 0, b, offset, U2);            offset = offset + U2;            constantIndex.put("method_set" + field, ++index);            int name_type_get = constantIndex.get("name_type_get" + field);            System.arraycopy(tag_method, 0, b, offset, U1);            offset = offset + U1;            System.arraycopy(class_ref, 0, b, offset, U2);            offset = offset + U2;            System.arraycopy(ByteUtil.int2Bytes(name_type_get, U2), 0, b, offset, U2);            offset = offset + U2;            constantIndex.put("method_get" + field, ++index);        }        // init method        System.arraycopy(tag_method, 0, b, offset, U1);        offset = offset + U1;        System.arraycopy(class_ref, 0, b, offset, U2);        offset = offset + U2;        System.arraycopy(ByteUtil.int2Bytes(CONSTANT_NAME_AND_TYPE_INIT, U2), 0, b, offset, U2);        offset = offset + U2;        constantIndex.put("method_init", ++index);        return b;    }    private byte[] generateCode() {        int len = 8 + CODE_LENGTH + CODE_PER_STEP * fields.size();        int offset = 0;        byte[] b = new byte[len];        System.arraycopy(ByteUtil.int2Bytes(2, U2), 0, b, offset, U2);        offset = offset + U2;        System.arraycopy(ByteUtil.int2Bytes(3, U2), 0, b, offset, U2);        offset = offset + U2;        System.arraycopy(ByteUtil.int2Bytes(CODE_LENGTH + CODE_PER_STEP * fields.size(), U4), 0, b, offset, U4);        offset = offset + U4;        // 代码体        int class_info_index = constantIndex.get("class_info");        int init_index = constantIndex.get("method_init");        System.arraycopy(ByteUtil.int2Bytes(0x2A, U1), 0, b, offset, U1);        offset = offset + U1;        System.arraycopy(ByteUtil.int2Bytes(0xC0, U1), 0, b, offset, U1);        offset = offset + U1;        System.arraycopy(ByteUtil.int2Bytes(class_info_index, U2), 0, b, offset, U2);        offset = offset + U2;        System.arraycopy(ByteUtil.int2Bytes(0x4C, U1), 0, b, offset, U1);        offset = offset + U1;        System.arraycopy(ByteUtil.int2Bytes(0xBB, U1), 0, b, offset, U1);        offset = offset + U1;        System.arraycopy(ByteUtil.int2Bytes(class_info_index, U2), 0, b, offset, U2);        offset = offset + U2;        System.arraycopy(ByteUtil.int2Bytes(0x59, U1), 0, b, offset, U1);        offset = offset + U1;        System.arraycopy(ByteUtil.int2Bytes(0xB7, U1), 0, b, offset, U1);        offset = offset + U1;        System.arraycopy(ByteUtil.int2Bytes(init_index, U2), 0, b, offset, U2);        offset = offset + U2;        System.arraycopy(ByteUtil.int2Bytes(0x4D, U1), 0, b, offset, U1);        offset = offset + U1;        for (String field : fields) {            int get_index = constantIndex.get("method_get" + field);            int set_index = constantIndex.get("method_set" + field);            System.arraycopy(ByteUtil.int2Bytes(0x2C, U1), 0, b, offset, U1);            offset = offset + U1;            System.arraycopy(ByteUtil.int2Bytes(0x2B, U1), 0, b, offset, U1);            offset = offset + U1;            System.arraycopy(ByteUtil.int2Bytes(0xB6, U1), 0, b, offset, U1);            offset = offset + U1;            System.arraycopy(ByteUtil.int2Bytes(get_index, U2), 0, b, offset, U2);            offset = offset + U2;            System.arraycopy(ByteUtil.int2Bytes(0xB6, U1), 0, b, offset, U1);            offset = offset + U1;            System.arraycopy(ByteUtil.int2Bytes(set_index, U2), 0, b, offset, U2);            offset = offset + U2;        }        System.arraycopy(ByteUtil.int2Bytes(0x2C, U1), 0, b, offset, U1);        offset = offset + U1;        System.arraycopy(ByteUtil.int2Bytes(0xB0, U1), 0, b, offset, U1);        offset = offset + U1;        return b;    }    private byte[] replace(byte[] o, byte[] constant, byte[] code) {        byte[] step1 = ByteUtil.byteReplace(o, CODE_BEGIN, CODE_LEN, code);        byte[] step2 = ByteUtil.byteReplace(step1, CONSTANT_END + 1, 0, constant);        return step2;    }    private int coutLen() {        // class 的utf8-info        int utf_class = className.length() + 3;        // get set 的utf8-info        int utf_getSet = fields.size() * 2 * 7;        // get set的描述字段        int utf_getSetDesc = 6 + 7;        // get set 的NameType        int name_type = fields.size() * 2 * 5;        // class info        int class_info = 3;        // init method        int method_init = 5;        // get set method        int getSet_method = fields.size() * 2 * 5;        return utf_class + utf_getSet + utf_getSetDesc + name_type + class_info + method_init + getSet_method;    }    private int constantNum() {        return fields.size() * 6 + 5 + CONSTANT_COUNT + 1;    }    private int codeLen() {        return CODE_LENGTH + fields.size() * CODE_PER_STEP - 2 + METHOD_LEN;    }    private byte[] delAttribute(byte[] b) {        return ByteUtil.byteReplace(b, ATTRIBUTE_OFF, ATTRIBUTE_LEN, new byte[] { 0 });    }    private byte[] byteModify(byte[] b) {        byte[] midRes = modifyCodeLen(b);        byte[] finalRes = modifyConstantCount(midRes);        return finalRes;    }    public static void main(String[] args) throws Exception {        String name = "/Users/apple/workspace/CommonTest/bin/jvm/BeanCopierModel.class";        BeanCopyClient bc = new BeanCopyClient();        // 准备些测试数据        bc.init();        InputStream is = new FileInputStream(name);        byte[] a = new byte[is.available()];        is.read(a);        is.close();        // 删掉Code区块里面没用的属性,只是为了学习，可以不删        byte[] b = bc.delAttribute(a);        // 修改一些count值        byte[] bm = bc.byteModify(b);        // 开始做替换        byte[] constant = bc.generateConstant();        byte[] code = bc.generateCode();        byte[] res = bc.replace(bm, constant, code);        // 打印出来反编译看看而已        FileOutputStream out = new FileOutputStream("/Users/apple/Downloads/BeanCopierModel.class");        out.write(res);        out.close();        TestClassLoader loader = new TestClassLoader();        Class clazz = loader.loadByte(res);        Method method = clazz.getMethod("copy", Object.class);        Bean bean = new Bean();        bean.setI(2);        bean.setJ(3);        Bean newBean = (Bean) method.invoke(null, bean);        System.out.println(newBean != bean && newBean.getI() == 2);    }}
       
      
     
    

package jvm;public class BeanCopier {    public static Object copy(Object bean) {        Bean plah = (Bean) bean;        Bean newBean = new Bean();        newBean.setI(plah.getI());        newBean.setJ(plah.getJ());        return newBean;    }}