Question

我需要创建一个java代理，当启用它时，它获取jar文件的路径作为参数，然后它将所有已加载的类替换为jar文件中的一个，如果它们的名称匹配。

例如，我们有一个名为com.something.ClassTest的类的应用程序。现在，如果提到的jar（不在类路径中）有一个与com.something.ClassTest完全相同的类，我想用jar中的那个替换它。

我有这类变压器，但不确定这是否正确。我收到IOException并且找不到消息Class。

    @Override
public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {

    if(classNames.contains(className.replace("/", "."))) {
        System.out.format("\n==> Found %s \n", className);
        try {
            Class c = urlClassLoader.loadClass(className.replace("/", "."));
            InputStream is = urlClassLoader.getResourceAsStream(className.replace("/", "."));
            System.out.println("Loaded class " + c);

            ClassReader reader = new ClassReader(is);
            ClassWriter writer = new ClassWriter(reader, ClassWriter.COMPUTE_FRAMES | ClassWriter.COMPUTE_MAXS);
            byte[] content = writer.toByteArray();

            System.out.println("Redifned " + new String(content));
            System.out.println("Orig " + new String(classfileBuffer));
            ClassDefinition cd = new ClassDefinition(c, content);
            instrumentation.redefineClasses(cd);

            return content;
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (UnmodifiableClassException e) {
            e.printStackTrace();
        }

    }
    return classfileBuffer;
}

我得到的错误是在实例化ClassReader的行。我猜错误是因为urlClassloader在某种程度上位于当前类加载器的层次结构之下......但我不知道我还能怎么做。

以下是初始化URL类的代码

    public SimpleClassTransformer(Instrumentation instrumentation, String jarFileName) {

    this.jarFileName = jarFileName;

    if(jarFileName != null) {
        JarFile jarFile = null;
        try {
            jarFile = new JarFile(this.jarFileName);
            Enumeration e = jarFile.entries();

            System.out.println("Jar file: " + this.jarFileName);
            URL[] urls = { new URL("jar:file:" + this.jarFileName+"!/") };
            urlClassLoader = URLClassLoader.newInstance(urls);

            while (e.hasMoreElements()) {
                JarEntry je = (JarEntry) e.nextElement();
                if(je.isDirectory() || !je.getName().endsWith(".class")){
                    continue;
                }
                // -6 because of .class
                String jarClassName = je.getName().substring(0,je.getName().length()-6);
                jarClassName = jarClassName.replace('/', '.');
                System.out.println("Adding class " + jarClassName);
                this.classNames.add(jarClassName);

            }
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}

在premain（）方法中实例化变换器时设置了Instrumentation。我试图避免使用Javassist。你能帮我解决这个问题。

这是我得到的例外：

java.io.IOException: Class not found
at jdk.internal.org.objectweb.asm.ClassReader.readClass(ClassReader.java:484)
at jdk.internal.org.objectweb.asm.ClassReader.<init>(ClassReader.java:453)
at com.agent.SimpleClassTransformer.transform(SimpleClassTransformer.java:79)
at sun.instrument.TransformerManager.transform(TransformerManager.java:188)
at sun.instrument.InstrumentationImpl.transform(InstrumentationImpl.java:428)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at org.springframework.util.ClassUtils.forName(ClassUtils.java:250)
at org.springframework.beans.factory.support.AbstractBeanDefinition.resolveBeanClass(AbstractBeanDefinition.java:394)
at org.springframework.beans.factory.support.AbstractBeanFactory.doResolveBeanClass(AbstractBeanFactory.java:1397)
at org.springframework.beans.factory.support.AbstractBeanFactory.resolveBeanClass(AbstractBeanFactory.java:1344)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.determineTargetType(AbstractAutowireCapableBeanFactory.java:628)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.predictBeanType(AbstractAutowireCapableBeanFactory.java:597)
at org.springframework.beans.factory.support.AbstractBeanFactory.isFactoryBean(AbstractBeanFactory.java:1445)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doGetBeanNamesForType(DefaultListableBeanFactory.java:445)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeanNamesForType(DefaultListableBeanFactory.java:415)
at org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration$DefaultDispatcherServletCondition.checkServlets(DispatcherServletAutoConfiguration.java:141)
at org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration$DefaultDispatcherServletCondition.getMatchOutcome(DispatcherServletAutoConfiguration.java:131)
at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:47)
at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:102)
at org.springframework.context.annotation.ConfigurationClassParser.processConfigurationClass(ConfigurationClassParser.java:203)
at org.springframework.context.annotation.ConfigurationClassParser.processMemberClasses(ConfigurationClassParser.java:336)
at org.springframework.context.annotation.ConfigurationClassParser.doProcessConfigurationClass(ConfigurationClassParser.java:248)
at org.springframework.context.annotation.ConfigurationClassParser.processConfigurationClass(ConfigurationClassParser.java:231)
at org.springframework.context.annotation.ConfigurationClassParser.processImports(ConfigurationClassParser.java:509)
at org.springframework.context.annotation.ConfigurationClassParser.processDeferredImportSelectors(ConfigurationClassParser.java:454)
at org.springframework.context.annotation.ConfigurationClassParser.parse(ConfigurationClassParser.java:185)
at org.springframework.context.annotation.ConfigurationClassPostProcessor.processConfigBeanDefinitions(ConfigurationClassPostProcessor.java:321)
at org.springframework.context.annotation.ConfigurationClassPostProcessor.postProcessBeanDefinitionRegistry(ConfigurationClassPostProcessor.java:243)
at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanDefinitionRegistryPostProcessors(PostProcessorRegistrationDelegate.java:273)
at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:98)
at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:677)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:519)
at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:118)
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:752)
at org.springframework.boot.SpringApplication.doRun(SpringApplication.java:347)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:295)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1112)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1101)

====编辑====

用url类加载器修复问题后，我现在得到Spring在尝试刷新其上下文时发生的错误：

TestClass has been compiled by a more recent version of the Java Runtime (class file version 0.0), this version of the Java Runtime only recognizes class file versions up to 52.0

Answer 1

您的问题的另一种方法是使用背书目录。保留要在库中加载的类，并将-Djava.endorsed.dirs=<directory_path>作为JVM参数提供给程序。

在加载类时，JVM首先检查此目录中的类可用性，如果没有找到，则它将检查应用程序类。这完全没问题，也没有任何编码。

Answer 2

您的UrlClassLoader应包含要从中加载课程的jar文件。
如果该类已加载，则无法重新加载。
只有在将类加载到JVM时，检测和重新定义类才有效。

代码看起来不错，但是您需要仔细检查urlClassLoader是否包含要从中加载类的jar文件，并且jar具有所需的类。

您可以调试应用程序以确保上述条件。

Answer 3

我成功解决了这个问题。如果有人在这里有同样的问题是问题：

我使用的是ClassReader和ClassWriter。由于某种原因，ClassWriter填充了字节代码，或许将已编译的类传递给类编写器是错误的，但无论如何以下代码：

@RequestMapping(value="{user}", method=RequestMethod.GET)
public String userPageGet (ModelMap model, @AuthenticationPrincipal User user)
{
    List<User> usersList = userRepo.findAll();
    model.addAttribute("usersList", usersList); 

    List<StudySet> studySets = studySetRepo.findByUser(user);
    model.addAttribute("studySets", studySets);

    List<Course> courses = courseRepo.findByUser(user);
    model.addAttribute("courses", courses);

    return "user";
}

替换为：

    ClassReader reader = new ClassReader(is);
    ClassWriter writer = new ClassWriter(reader, ClassWriter.COMPUTE_FRAMES | ClassWriter.COMPUTE_MAXS);
    byte[] content = writer.toByteArray();

正如您所看到的，我正在使用InputStream直接检索字节代码。这解决了这个问题，如果你感兴趣的话，Spring会很好地发现差异并刷新上下文。

====编辑====

我注意到在这里使用URLClassLoader是不可靠的，因为某些原因，它可能会返回应用程序本身已经加载的类而不是JAR文件中的类。它是随机的，有时返回jar中的类，有时返回原始类，所以我决定删除URLClassLoader，而是在遍历jar文件时将类文件作为InputStream。对于需要它的人来说，这是我的变换器的最终代码：

        InputStream is = urlClassLoader.getResourceAsStream(className + ".class");

        byte[] content = new byte[is.available()];
        is.read(content);

        System.out.println ("Original class version: " + ((classfileBuffer[6]&0xff)<<8 | (classfileBuffer[7]&0xff)));
        System.out.println ("Redefined class version: " + ((content[6]&0xff)<<8 | (content[7]&0xff)));

}

如何使用Java检测替换新类？

3 个答案: