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How to replace a class with a new one using Java toolkit?

I need to create a java agent that, when it is turned on, takes the path to the jar file as an argument, and then replaces any loaded class with the one inside the jar file, if their names match.

For example, we have an application with class com.something.ClassTest. Now, if the jar mentioned (not in the class path) has a class with the same name as com.something.ClassTest, I want to replace it with the one in the bank.

I have this class transformer, but not sure if this is correct or not. I get an IOException with the message 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;
}

, , , ClassReader. , , urlClassloader - ... , .

URL

    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();
        }

    }

}

premain(). Javassist. .

, :

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)

==== EDIT ====

url , , Spring :

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
+3
3

. , - ​​ , :

ClassReader ClassWriter. - ClassWriter , , , :

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

:

        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)));

, InputStream . , , Spring .

==== EDIT ====

, URLClassLoader - , , JAR . , jar, , URLClassLoader InputStream jar. , :

public class JarFileClassTransformer implements ClassFileTransformer {

private String jarFileName = null;
protected Map<String, InputStream> classNames = new HashMap<>();
static Instrumentation instrumentation = null;

/**
 * Constructor.
 * @param jarFileName
 */
public JarFileClassTransformer(String jarFileName) {
    this.jarFileName = jarFileName;

    File file  = new File(jarFileName);
    System.out.println("Jar file '" + this.jarFileName + "' " + (file.exists() ? "exists" : "doesn't exists!"));

    if(file.exists()) {
        try {
            JarFile jarFile = new JarFile(file);
            Enumeration e = jarFile.entries();

            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.put(jarClassName, jarFile.getInputStream(je));

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

    }

}

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

    if(classNames.containsKey(className.replace("/", "."))) {
        System.out.format("\n==> Found %s to replace with the existing version\n", className);
        try {

            Class c = loader.loadClass(className.replace("/", "."));
            System.out.println("Existing class: " + c);
            InputStream is = classNames.get(className.replace("/", "."));

            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)));

            System.out.println("Original bytecode: " + new String(classfileBuffer));
            System.out.println("Redefined byte code: " + new String(content));
            ClassDefinition cd = new ClassDefinition(c, content);
            instrumentation.redefineClasses(cd);

            return content;
        } catch (Throwable e) {
            e.printStackTrace();

        }

    }
    return classfileBuffer;
}

}

+1

dirs. , , -Djava.endorsed.dirs=<directory_path> JVM .

JVM , , . - - .

+3
  • UrlClassLoader jar, .
  • , .
  • , JVM.

, , urlClassLoader jar, , jar .

.

+1

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