经验 - javassist【动态改字节码】 javassist.CannotCompileException: by java.lang.LinkageError

素颜马尾好姑娘i 2022-03-18 01:53 476阅读 0赞

这里举个简单的例子，来记录下如何用CtClass创建一个类，并且往这个类里面新加方法，

下面是代码，可以直接运行：  
Java代码

package seeeyou.app.test;

import java.io.IOException;  
import java.lang.reflect.InvocationTargetException;  
import java.lang.reflect.Method;

import javassist.CannotCompileException;  
import javassist.ClassPool;  
import javassist.CtClass;  
import javassist.CtMethod;  
import javassist.CtNewMethod;  
import javassist.NotFoundException;

/\*\*  
\*  
\* @author seeeyou  
\*  
\*/  
public class TestHelloWorld2 \{  
public static void main(String\[\] args) throws NotFoundException,  
IOException, CannotCompileException, InstantiationException,  
IllegalAccessException, SecurityException, NoSuchMethodException,  
IllegalArgumentException, InvocationTargetException \{  
// 用于取得字节码类，必须在当前的classpath中，使用全称  
ClassPool pool = ClassPool.getDefault();  
/\*\*  
\* makeClass() cannot create a new interface; makeInterface() in  
\* ClassPool can do. Member methods in an interface can be created with  
\* abstractMethod() in CtNewMethod. Note that an interface method is an  
\* abstract method.  
\*/  
CtClass ccClass = pool.makeClass("Point");  
String bodyString = "\{System.out.println(\\"Call to method \\");\}";  
//为新创建的类新加一个方法execute，无任何参数  
CtMethod n1 = CtNewMethod.make(CtClass.voidType, "execute", null, null,  
bodyString, ccClass);  
ccClass.addMethod(n1);  
/\*\*  
\* 这里无法用new的形式来创建一个对象，因为已经classloader中不能有两个相同的对象,否则会报异常如下：  
\*Caused by: java.lang.LinkageError: loader (instance of sun/misc/Launcher$AppClassLoader):  
\*attempted duplicate class definition for name: "Point"  
\*\*/  
Object oo = ccClass.toClass().newInstance();  
Method mms = oo.getClass().getMethod("execute", null);  
System.out.println("new class name is : " + oo.getClass().getName());  
System.out.println("new class's method is : " + mms.invoke(oo, null));  
System.out.println("---------------------------------------------");  
//这一行代码将class冻结了，下面无法再对类多编辑或者修改，下面的setName会报异常如：  
//Exception in thread "main" java.lang.RuntimeException: Point class is frozen  
ccClass.freeze();  
try\{  
ccClass.setName("Point2");  
\}catch (Exception e) \{  
System.out.println(e);  
\}  
//对已经冻结的class解冻之后还可以继续编辑修改  
ccClass.defrost();  
System.out.println("------------- 上面的代码是对的，下面的代码将会无法执行出结果，会报错------------------------");  
//第二个方法  
bodyString = "public int getNumber(Integer num)\{System.out.println(\\"Point2 Call to method \\");return 10+num;\}";  
CtMethod n2 = CtNewMethod.make(bodyString, ccClass);//直接创建一个方法，带有一个int的参数和返回值  
ccClass.addMethod(n2);  
Class\[\] params = new Class\[1\];  
Integer num = new Integer(15);  
params\[0\] = num.getClass();  
mms = oo.getClass().getMethod("getNumber", params);  
System.out.println("new class name is : " + oo.getClass().getName());  
System.out.println("new class's method is : " + mms.invoke(oo, 100));  
System.out.println("---------------------------------------------");  
\}  
\}

Java代码

上面的结果是：new class name is : Point  
Call to method  
new class's method is : null  
\---------------------------------------------  
java.lang.RuntimeException: Point class is frozen  
\---------------------------------------------  
Exception in thread "main" java.lang.NoSuchMethodException: Point.getNumber(java.lang.Integer)  
at java.lang.Class.getMethod(Class.java:1605)  
at seeeyou.app.test.TestHelloWorld2.main(TestHelloWorld2.java:66)

错误的原因其实和简单，因为我第二次新加一个方法后，没有再次实例化一个对象，所以oo还是原来的对象，他的成员函数肯定没有新加的方法。

那我可以再次实例化下试试，代码和结果如下：  
Java代码

package seeeyou.app.test;

import java.io.IOException;  
import java.lang.reflect.InvocationTargetException;  
import java.lang.reflect.Method;

import javassist.CannotCompileException;  
import javassist.ClassPool;  
import javassist.CtClass;  
import javassist.CtMethod;  
import javassist.CtNewMethod;  
import javassist.NotFoundException;

/\*\*  
\*  
\* @author seeeyou  
\*  
\*/  
public class TestHelloWorld2 \{  
public static void main(String\[\] args) throws NotFoundException,  
IOException, CannotCompileException, InstantiationException,  
IllegalAccessException, SecurityException, NoSuchMethodException,  
IllegalArgumentException, InvocationTargetException \{  
// 用于取得字节码类，必须在当前的classpath中，使用全称  
ClassPool pool = ClassPool.getDefault();  
/\*\*  
\* makeClass() cannot create a new interface; makeInterface() in  
\* ClassPool can do. Member methods in an interface can be created with  
\* abstractMethod() in CtNewMethod. Note that an interface method is an  
\* abstract method.  
\*/  
CtClass ccClass = pool.makeClass("Point");  
String bodyString = "\{System.out.println(\\"Call to method \\");\}";  
//为新创建的类新加一个方法execute，无任何参数  
CtMethod n1 = CtNewMethod.make(CtClass.voidType, "execute", null, null,  
bodyString, ccClass);  
ccClass.addMethod(n1);  
/\*\*  
\* 这里无法用new的形式来创建一个对象，因为已经classloader中不能有两个相同的对象,否则会报异常如下：  
\*Caused by: java.lang.LinkageError: loader (instance of sun/misc/Launcher$AppClassLoader):  
\*attempted duplicate class definition for name: "Point"  
\*\*/  
Object oo = ccClass.toClass().newInstance();  
Method mms = oo.getClass().getMethod("execute", null);  
System.out.println("new class name is : " + oo.getClass().getName());  
System.out.println("new class's method is : " + mms.invoke(oo, null));  
System.out.println("---------------------------------------------");  
//这一行代码将class冻结了，下面无法再对类多编辑或者修改，下面的setName会报异常如：  
//Exception in thread "main" java.lang.RuntimeException: Point class is frozen  
ccClass.freeze();  
try\{  
ccClass.setName("Point2");  
\}catch (Exception e) \{  
System.out.println(e);  
\}  
//对已经冻结的class解冻之后还可以继续编辑修改  
ccClass.defrost();  
System.out.println("------------- 上面的代码是对的，下面的代码将会无法执行出结果，会报错------------------------");  
//第二个方法  
bodyString = "public int getNumber(Integer num)\{System.out.println(\\"Point2 Call to method \\");return 10+num;\}";  
CtMethod n2 = CtNewMethod.make(bodyString, ccClass);//直接创建一个方法，带有一个int的参数和返回值  
ccClass.addMethod(n2);  
Class\[\] params = new Class\[1\];  
Integer num = new Integer(15);  
params\[0\] = num.getClass();//就多了下面这个实例化，但是这样会导致一个错误  
oo = ccClass.toClass().newInstance();  
mms = oo.getClass().getMethod("getNumber", params);  
System.out.println("new class name is : " + oo.getClass().getName());  
System.out.println("new class's method is : " + mms.invoke(oo, 100));  
System.out.println("---------------------------------------------");  
\}  
\}

Java代码

这也会导致一个错误：new class name is : Point  
Call to method  
new class's method is : null  
\---------------------------------------------  
java.lang.RuntimeException: Point class is frozen  
\------------- 上面的代码是对的，下面的代码将会无法执行出结果，会报错------------------------  
Exception in thread "main" javassist.CannotCompileException: by java.lang.LinkageError: loader (instance of sun/misc/Launcher$AppClassLoader): attempted duplicate class definition for name: "Point"  
at javassist.ClassPool.toClass(ClassPool.java:1051)  
at javassist.ClassPool.toClass(ClassPool.java:994)  
at javassist.ClassPool.toClass(ClassPool.java:952)  
at javassist.CtClass.toClass(CtClass.java:1079)  
at seeeyou.app.test.TestHelloWorld2.main(TestHelloWorld2.java:66)  
Caused by: java.lang.LinkageError: loader (instance of sun/misc/Launcher$AppClassLoader): attempted duplicate class definition for name: "Point"  
at java.lang.ClassLoader.defineClass1(Native Method)  
at java.lang.ClassLoader.defineClass(ClassLoader.java:621)  
at java.lang.ClassLoader.defineClass(ClassLoader.java:466)  
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)  
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)  
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)  
at java.lang.reflect.Method.invoke(Method.java:597)  
at javassist.ClassPool.toClass2(ClassPool.java:1063)  
at javassist.ClassPool.toClass(ClassPool.java:1045)  
... 4 more

Java代码

原因也很简单，一个classloader里面怎么有两个重复的对象呢，除非是两个不同的classloader。。所以爆了个重复加载类的错误

Java代码

对的方式是只实例化一次：如下：  
Java代码

package seeeyou.app.test;

import java.io.IOException;  
import java.lang.reflect.InvocationTargetException;  
import java.lang.reflect.Method;

import javassist.CannotCompileException;  
import javassist.ClassPool;  
import javassist.CtClass;  
import javassist.CtMethod;  
import javassist.CtNewMethod;  
import javassist.NotFoundException;

/\*\*  
\*  
\* @author seeeyou  
\*  
\*/  
public class TestHelloWorld3 \{

public static void main(String\[\] args) throws NotFoundException,  
IOException, CannotCompileException, InstantiationException,  
IllegalAccessException, SecurityException, NoSuchMethodException,  
IllegalArgumentException, InvocationTargetException \{  
// 用于取得字节码类，必须在当前的classpath中，使用全称  
ClassPool pool = ClassPool.getDefault();  
/\*\*  
\* makeClass() cannot create a new interface; makeInterface() in  
\* ClassPool can do. Member methods in an interface can be created with  
\* abstractMethod() in CtNewMethod. Note that an interface method is an  
\* abstract method.  
\*/  
CtClass ccClass = pool.makeClass("Point");  
String bodyString = "\{System.out.println(\\"Call to method \\");\}";  
//为新创建的类新加一个方法execute，无任何参数  
CtMethod n1 = CtNewMethod.make(CtClass.voidType, "execute", null, null,  
bodyString, ccClass);  
ccClass.addMethod(n1);  
//新加第二个方法  
bodyString = "public Integer getNumber(Integer num);";  
CtMethod n2 = CtNewMethod.make(bodyString, ccClass);//直接创建一个方法，带有一个int的参数和返回值  
n2.setBody("\{System.out.println(\\"Point Call to method \\");return $1;\}");  
ccClass.addMethod(n2);

/\*\*  
\* 这里无法用new的形式来创建一个对象，因为已经classloader中不能有两个相同的对象,否则会报异常如下：  
\*Caused by: java.lang.LinkageError: loader (instance of sun/misc/Launcher$AppClassLoader):  
\*attempted duplicate class definition for name: "Point"  
\*\*/  
Object oo = ccClass.toClass().newInstance();  
Method mms = oo.getClass().getMethod("execute", null);  
System.out.println("new class name is : " + oo.getClass().getName());  
System.out.println("new class's method is : " + mms.invoke(oo, null));  
System.out.println("---------------------------------------------");  
//这一行代码将class冻结了，下面无法再对类多编辑或者修改，下面的setName会报异常如：  
//Exception in thread "main" java.lang.RuntimeException: Point class is frozen  
ccClass.freeze();  
try\{  
ccClass.setName("Point2");  
\}catch (Exception e) \{  
System.out.println(e);  
\}  
//对已经冻结的class解冻之后还可以继续编辑修改  
ccClass.defrost();  
System.out.println("------------- 上面的代码是对的，下面的代码将会无法执行出结果，会报错------------------------");  
Class\[\] params = new Class\[1\];  
Integer num = new Integer(0);  
params\[0\] = num.getClass();  
mms = oo.getClass().getMethod("getNumber",params);  
System.out.println("new class name is : " + oo.getClass().getName());  
System.out.println("new class's method is : " + mms.invoke(oo, 100));  
System.out.println("---------------------------------------------");  
\}  
\}

Java代码

结果如下：new class name is : Point  
Call to method  
new class's method is : null  
\---------------------------------------------  
java.lang.RuntimeException: Point class is frozen  
\------------- 上面的代码是对的，下面的代码将会无法执行出结果，会报错------------------------  
new class name is : Point  
Point2 Call to method  
new class's method is : 100  
\---------------------

consult :   
[https://blog.csdn.net/caodaoxi/article/details/84277517][https_blog.csdn.net_caodaoxi_article_details_84277517]

[https_blog.csdn.net_caodaoxi_article_details_84277517]: https://blog.csdn.net/caodaoxi/article/details/84277517