How do I replace a class with new one with Java instrumentation?

Question

I need to create a java agent that when is enabled it gets the path to a jar file as argument and then it replaces any loaded class the the one inside the jar file if their names are matched.

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

I have this class transformer but not sure if that's correct or not. I get IOException with 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;
}

The error I get is at the line where the ClassReader is instantiated. I guess the error is because the urlClassloader is somehow under the hierarchy of the current class loader... but I don't know how else I can do it.

Here is the code where URL class loaded is initialized

    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();
        }

    }

}

Instrumentation is set while instantiating the transformer in the premain() method. I am trying to avoid using Javassist. Can you please help me on this.

This is the exception I get:

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 ====

After fixing the issue with url class loader I now get this error that happens when Spring tries to refresh its context:

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

score 3 · Answer 1 · answered Feb 16 '16 at 06:53

3

Another Approach to your problem is using endorsed dirs. Keep classes that you want to be loaded in a library and provide -Djava.endorsed.dirs=<directory_path> as JVM argument to the program.

While loading classes, JVM first checks the class availability in this directory and if not found then it will check the application classes. This works perfectly fine without any issue and without any coding.

answered Feb 16 '16 at 06:53

Vitthal Kavitake

879
5
18

How does this work? So if a class is found in both, which one will be loaded? – xbmono Feb 16 '16 at 06:56
It will be loaded from endorsed directory. The other one is simply ignored. – Vitthal Kavitake Feb 16 '16 at 06:57
This could be a good solution but seems like it doesn't work for me. When I add that I get ClassNotFoundException for other classes... it's as if Java picks the jar file in endorsed dir and it completely ignores the jar file (and all its classes) where the same class is located in the application. – xbmono Feb 16 '16 at 07:04
This is because of ClassLoaderHierarchy, the changed classes are loaded in higher hierarchy so for lower hierarchy classes, it will give ClassNotFound. In instrumentation approach also, the same thing will happen as the changed classes will be loaded by childClassLoader and hence not accessible to parent classes. Similar error expected in instrumentation approach as well. – Vitthal Kavitake Feb 16 '16 at 07:07
So how hot swapping works? For example Spring-Loaded is able to do it even if method signature is changed – xbmono Feb 16 '16 at 07:10
hot swapping is done by JVM and while hot swapping, the classLoader remains the same. ClassLoader does not get changed therefore it works. In your case the original class loader and class being redefined class loader, both are different. Hence the issue. – Vitthal Kavitake Feb 16 '16 at 07:13
1

Using instrumentation, it is possible to add methods to the class or change methods in the same class. Perfectly possible. But loading the new class from different jar, I dont think it will work. – Vitthal Kavitake Feb 16 '16 at 07:16
Vitthal ... but it seems if I can get rid of the error message about class version things will be okay. Do you know why that error comes up? I have compiled the classes in the jar file using the same jdk as my application, what is this error saying exactly? – xbmono Feb 16 '16 at 23:02
I dont think, replacing whole class works at runtime, redefining class is modifying the same class definition at runtime which can be done using BCEL or Javaassist library. Read more about redefiningClasses here http://docs.oracle.com/javase/7/docs/api/java/lang/instrument/Instrumentation.html#redefineClasses(java.lang.instrument.ClassDefinition...) – Vitthal Kavitake Feb 17 '16 at 05:09

xbmono · Accepted Answer · 2016-02-18T00:31:12.110

I managed to fix the issue. In case someone has the same issue here was the problem:

I was using ClassReader and ClassWriter. For some reason, ClassWriter were stuffing the byte code, perhaps it was my mistake to pass already compiled class to class writer but anyway the following code:

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

Replaced with:

        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)));

As you can see I am using InputStream to retrieve the byte code directly. That fixed the issue and in case you are interested, Spring detected the difference nicely and refreshed the context.

==== EDIT ====

I noticed using URLClassLoader here is not reliable as for some reason, it may return the already loaded class in the application itself and not the class inside the JAR file. It was random, sometimes returns the class inside the jar and sometimes the original class so I have decided to remove the URLClassLoader and instead get the class file as InputStream while traversing the jar file. This is the final code of my transformer for anyone who needs it:

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;
}

}

Great that it worked for you. I tried the same snippet but it did not worked for me. — Vitthal Kavitake, Feb 17 '16 at 05:20
You probably had the jar file classes compiled with a different version. In our case, it's not different version and it always is the same java version of the application itself. — xbmono, Feb 17 '16 at 05:37

score 1 · Answer 3 · answered Feb 16 '16 at 05:24

1

Your UrlClassLoader should contain the jar file from which you want to load the class.
If the class is already loaded, then it cant be reloaded.
Instrumentation, and redefining of class works only when the class is being loaded to JVM.

The code looks good, but you need to double check if the urlClassLoader contains the jar file from which you want to load the class and jar has the required class.

You can debug the application to ensure the above conditions.

answered Feb 16 '16 at 05:24

Vitthal Kavitake

879
5
18

It has. If you look at the line I put a System.out to print out the class loaded from urlClassLoader. I can see the log it's been loaded inside the url classloader. But I think the problem here is the class loaded inside url classloader is not available in ClassReader which is using the current class loader ... even so the names are exactly the same – xbmono Feb 16 '16 at 05:29
1

Do you mean this? `System.out.format("\n==> Found %s \n", className);` – Vitthal Kavitake Feb 16 '16 at 05:31
No. This line System.out.println("Loaded class " + c); – xbmono Feb 16 '16 at 05:32
Can you specify the line number where you get the exception? that will help to analyse quickly. Point to the line where you get the exception. – Vitthal Kavitake Feb 16 '16 at 05:33
Added the exception. It happens exactly at this line: ClassReader reader = new ClassReader(is); – xbmono Feb 16 '16 at 05:36
in the stack trace as you can see spring framework class loaders are involved... do you think something to do with it? Is there any other way to replace the byte code of the loaded class with the one in the jar? – xbmono Feb 16 '16 at 05:43
try this code `String classAsPath = className.replace('.', '/') + ".class"; InputStream is = urlClassLoader.getResourceAsStream(classAsPath);` instead of `InputStream is = urlClassLoader.getResourceAsStream(className.replace("/", "."));` – Vitthal Kavitake Feb 16 '16 at 06:00
InputStream might be null when it is passed to ClassReader. – Vitthal Kavitake Feb 16 '16 at 06:01
Yes. url class loader doesn't return the resource at all. I put a break point ... it doesn't work with dot in fully class name nor does it work with slash. Don't know what's wrong here! – xbmono Feb 16 '16 at 06:33
1

It returns the InputStream with this code `String classAsPath = className.replace('.', '/') + ".class"; InputStream is = urlClassLoader.getResourceAsStream(classAsPath);` – Vitthal Kavitake Feb 16 '16 at 06:38
YES. I missed .class ... it worked but now I have another problem. After this, Spring tries to refresh its context and this is the error it's giving me: "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" – xbmono Feb 16 '16 at 06:48
Yea, facing the same issue. I have another approach for your problem. See my other answer I am just posting it. – Vitthal Kavitake Feb 16 '16 at 06:51

How do I replace a class with new one with Java instrumentation?

3 Answers3

Linked