整体思路
如果远程系统需要执行一段代码，但是我们现在又没有入口。
可以通过这种方式。
实际上对于日志信息，可能某个调用信息没有打印，也可以通过这个方式。
代码实现
核心代码
- ByteUtils.java
package org.jvm;
/**
* Bytes数组处理工具
*/
public class ByteUtils {
public static int bytes2Int(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 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 String bytes2String(byte[] b, int start, int len) {
return new String(b, start, len);
}
public static byte[] string2Bytes(String str) {
return str.getBytes();
}
public static byte[] bytesReplace(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;
}
}
- ClassModifier.java
package org.jvm;
/**
* 修改Class文件，暂时只提供修改常量池常量的功能
*/
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 byte[] modifyUTF8Constant(String oldStr, 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(oldStr)){
byte[] strBytes = ByteUtils.string2Bytes(newStr);
byte[] strLen = ByteUtils.int2Bytes(newStr.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;
}
public int getConstantPoolCount(){
return ByteUtils.bytes2Int(classByte, CONSTANT_POOL_COUNT_INDEX, u2);
}
}
- HackSystem.java
package org.jvm;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.io.PrintStream;
/**
* 为JavaClass劫持java.lang.System提供支持
* 除了out和err外，其余的都直接转发给System处理
*/
public class HackSystem {
public final static InputStream in = System.in;
private static ByteArrayOutputStream buffer = new ByteArrayOutputStream();
public final static PrintStream out = new PrintStream(buffer);
public final static PrintStream err = out;
public static String getBufferString(){
return buffer.toString();
}
public static void clearBuffer(){
buffer.reset();
}
public static void setSecurityManager(final SecurityManager s){
System.setSecurityManager(s);
}
public static SecurityManager getSecurityManager(){
return System.getSecurityManager();
}
public static long currentTimeMills(){
return System.currentTimeMillis();
}
public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length){
System.arraycopy(src, srcPos, dest, destPos, length);
}
public static int identityHashCode(Object x){
return System.identityHashCode(x);
}
}
- HotSwapClassLoader.java
package org.jvm;
/**
* 为了多次载入执行类而加入的加载器
* 把defineClass方法开放出来，只有外部显式调用的时候才会使用到loadByte方法
* 由虚拟机调用时，仍然按照原有的双亲委派规则使用loadClass方法进行类加载
*/
public class HotSwapClassLoader extends ClassLoader{
public HotSwapClassLoader() {
super(HotSwapClassLoader.class.getClassLoader());
}
public Class<?> loadByte(byte[] classByte){
return defineClass(null,classByte,0,classByte.length);
}
}
- JavaClassExecuter.java
public class JavaClassExecuter {
@SuppressWarnings({ "unused", "rawtypes", "unchecked" })
public static String execute(byte[] classByte){
HackSystem.clearBuffer();
ClassModifier cm=new ClassModifier(classByte);
byte[] modiBytes=cm.modifyUTF8Constant("java/lang/System", "test/HackSystem");
HotSwapClassLoader loader=new HotSwapClassLoader();
Class clazz=loader.loadByte(modiBytes);
try{
Method method=clazz.getMethod("main", new Class[]{String[].class});
method.invoke(null, new String[]{null});
}catch(Throwable e){
e.printStackTrace(HackSystem.out);
}
return HackSystem.getBufferString();
}
}
测试类
package org.jvm;
/**
* 测试类，在此类中打印想要在页面看到的内容，System.out输出的内容会存在HackSystem的字节数组输出流中
*/
public class TestClass {
public static void main(String[] args) {
System.out.println("-----this is test class out println----");
}
}
3、jsp页面
- index.jsp
<%@ page import="java.lang.*" %>
<%@ page import="java.io.*" %>
<%@ page import="org.jvm.*" %>
<%
InputStream inputStream=new FileInputStream("/opt/TestClass.class");
byte[] b=new byte[inputStream.available()];
inputStream.read(b);
inputStream.close();
out.println(JavaClassExecuter.executer(b));
%>
替换步骤
1、将 ByteUtils ClassModifier HackSystem HotSwapClassloader JavaClassExecuter TestClass 这六个.java文件上传到服务器通过javac进行编译成.class 文件
2、将编译好的TestClass放在/opt目录中
3、在tomcat的项目位置的WEB-INF/classes/中新建org/jvm文件夹，再将编译好的 ByteUtils ClassModifier HackSystem HotSwapClassloader JavaClassExecuter 放在WEB-INF/classes/org/jvm中
4、将 index.jsp 放在项目中能访问到的位置，如项目的根路径中
5、在浏览器中访问jsp页面即可。
参考资料
《深入理解 jvm》
