java - 如何使用 Java 检测用新类替换类？

Question

我需要创建一个 java 代理，当它被启用时，它获取一个 jar 文件的路径作为参数，然后它替换任何加载的类，如果它们的名称匹配的话，jar 文件中的类。

例如，我们有一个应用程序，其中包含一个名为 com.something.ClassTest 的类。现在，如果提到的 jar(不在类路径中)有一个与 com.something.ClassTest 完全相同名称的类，我想用 jar 中的那个替换它。

我有这个类转换器，但不确定它是否正确。我收到 IOException 消息 Class not found。

    @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 的那一行。我猜这个错误是因为 urlClassloader 在当前类加载器的层次结构下不知何故...但我不知道我还能怎么做。

这是初始化加载的 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() 方法中实例化转换器时设置仪器。我试图避免使用 Javassist。你能帮我解决这个问题吗？

这是我得到的异常:

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 ====

在解决了 url 类加载器的问题后，我现在得到了当 Spring 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

解决您的问题的另一种方法是使用认可的目录。 将要加载的类保留在库中并提供 -Djava.endorsed.dirs=<directory_path>作为程序的 JVM 参数。

加载类时，JVM 首先检查此目录中的类可用性，如果未找到，它将检查应用程序类。 这工作得很好，没有任何问题，也没有任何编码。