With the following source code:
package util.abc;
public class Test{
public String out(){
return "Hello World!";
}
}
I can use:
Class c = Class.forName("util.abc.Test");
to instantiate this Class, but I must put this source file(Test.java) in the classpath /util/abc/
I want to dynamically load this class from the database (store the source code as string or binary)
Is this possible ?
Assuming you already compiled the class, you can create a DatabaseClassLoader, which loads the class from a database.
public class DatabaseClassLoader extends ClassLoader {
public DatabaseClassLoader(ClassLoader parent, ... /* connection to database */ ) {
super(parent);
// store the connection
}
public Class findClass(String name) {
byte[] data = loadDataFromDatabase(name);
return defineClass(name, data, 0, data.length);
}
private byte[] loadDataFromDatabase(String name) {
// this is your job.
}
}
If the database only contains the source code, you'll have to compile it first - look into the Java compiler API for how to do this without any files.
Pay attention, the class loaded this way will stay alive as long as the classloader is alive, so you'll need a new class loader to be able to reload the class in case of changes.
Also, if you want to interact with the class by other ways than reflection, you would better let it implement some interface (which itself is in your class path), and let the application class loader be the parent of your database class loader.
Ah, and how to load:
Class<?> c = Class.forName("util.abc.Test", myClassLoader);
or directly
Class<?> c = myClassLoader.loadClass("util.abc.Test");
Here is a method which creates objects of your interface (of any interface, in fact):
public <X> X getImplementingObject(Class<X> interfaceClass, String className)
throws ClassNotFoundException, IllegalAccessException, InstantiationException
{
ClassLoader loader = new DatabaseClassLoader(interfaceClass.getClassLoader(), ...);
Class<?> cl = loader.loadClass(className);
Class<? extends X> c = cl.asSubclass(interfaceClass);
return c.newInstance();
}
(It needs the class to have a no-argument constructor which does not throw any exceptions, of course (if it does, you'll get this exception thrown, too).)
This creates a new ClassLoader for every such class, so they can only cooperate with each other by means of the interface (or reflection).
For on-the-fly compiling you should look at the Java Compiler API, as mentioned in the answer from dcn. But I think it would be better to do the compiling on the side which puts the classes into the database than the side who pulls them out.
If you want to store the source code in you DB, you can use the Java 6 Compiler API to compile it at runtime. See here for an example.
In order to load classes at runtime, you can either use an URLClassLoader if you can specify the location of the bytecode with a URL, or use ClassLoader.defineClass and supply the bytecode as an array of bytes.
Either way you should pay attention, that in order to use your dynamically loaded class, it should implement an interface that is known at compile time.
Related
I was hoping for someone to explain this item since I might be getting this wrong:
I was reading about Java Agent Instrumentation which says that the agent can start after VM startup. So if I want to dynamically replace some class (without brining down the app) is this what I am going to go for using agent-main? Or do I need to do something more here?
I know people might ask "Are you talking about JRebel" - not really because I want to do something simple and JRebel is an overkill.
instrument docs - Java docs for Instrumentation
I understand all the instrumentation overrides, but I am slightly confused how I can hook this agent with -agent argument after the app has started.
First your agent class needs to specify an agentmain method like:
public class MyAgent {
public static void agentmain(final String args, final Instrumentation inst) {
try {
System.out.println("Agent loaded.");
} catch (Exception e) {
// Catch and handle every exception as they would
// otherwise be ignored in an agentmain method
e.printStackTrace();
}
}
}
Compile it and pack it inside a jar-file for example. If you choose the jar-variant then it must specify the Agent-Class key in its manifest-file (MANIFEST.MF). It points to the class implementing the agentmain method. It could look like:
Manifest-Version: 1.0
Agent-Class: package1.package2.MyAgent
If it is located inside those packages, as an example.
After that you can load the agent via the VirtualMachine#loadAgent method (documentation). Note that the mechanism used by those classes are part of the Attach library of Java. They decided, as most users don't need it, to not directly add it to the systems path but you can just add it. It is located at
pathToYourJDKInstallation\jre\bin\attach.dll
And it needs to be somewhere where the system property java.library.path is pointing at. You could for example just copy it to your .../Windows/System32 folder or adjust the property or stuff like that.
As an example, if you want to inject an agent-jar inside another currently running jar, you could use a method like this:
public void injectJarIntoJar(final String processIdOfTargetJar,
final String pathToAgentJar, final String[] argumentsToPass) {
try {
final VirtualMachine vm = VirtualMachine.attach(processIdOfTargetJar);
vm.loadAgent(pathToAgentJar, argumentsToPass.toString());
vm.detach();
} catch (AttachNotSupportedException | AgentLoadException
| AgentInitializationException | IOException e) {
System.err.println("Unable to inject jar into target jar.");
}
}
With the same technique you can inject dll-libraries (if they implement the corresponding agent-methods via the native agent interface) into jars.
Actually, if that helps you, I have written some small library for that kind of stuff some time ago. See Mem-Eater-Bug, the corresponding class is Injector.java and the whole project has a small Wiki.
It has an example showing how to use that technique to manipulate a SpaceInvaders game written as Java application.
So apparently you want to reload classes at runtime. Such that your project can react to changes of the code without restarting.
To achieve this you need to prepare your project and write a very clean architecture, it involves using interfaces, factory-patterns, proxy-patterns and a routine that checks for updates and then destroys and rebuilds all current objects.
Unfortunately this might not be an easy task, but it is doable, depending on the size of your project and the amount of code that should react dynamically to changes.
I found this article helpful, let me explain how it works. You can easily load a class with ClassLoader.loadClass(...) and you can also use that to reload a class, very easy. However at the time you have compiled your code classes are some kind of hardwired already. So your old code will continue to create instances of the old classes although you have reloaded the class.
This is the reason why we need some kind of architecture that allows exchanging the old class with the new class. Also it is pretty obvious that current instances of the old class can not automatically be transferred to the new version as everything could have changed. So you will also need a custom method that collects and rebuilds those instances.
The approach described in the article uses an Interface instead of an actual class in the first place. This allows to easily exchange the class behind that interface without breaking the code that uses the interface.
Then you need a factory where you ask for instances of that Interface. The factory can now check if the underlying class-file has changed, if so it reloads it and obtains a reference to the new class version. It can now always create an instance of the interface which uses the up-to-date class.
The factory, by that, is also able to collect all created instances in order to exchange them later, if the code base has changed. But the factory should reference them using WeakReference (documentation), else you have a big memory leak because the Garbage Collector would not be able to delete instances because the factory holds references to them.
Okay, now we are able to always obtain up-to-date implementations of an Interface. But how can we easily exchange existing instances. The answer is by using a proxy-pattern (explanation).
It is simple, you have a proxy class which is the actual object you are working with. It has all the methods of the Interface and upon calling methods it simply forwards to the real class.
Your factory, as it has a list of all current instances using WeakReference, can now iterate the list of proxies and exchange their real class with a new up-to-date version of the object.
Existing proxies that are used all around your project will now automatically use the new real version as the proxy itself has not changed, only its internal reference to the real target has changed.
Now some sample code to give you a rough idea.
The interface for the objects you want to monitor:
public interface IExample {
void example();
}
The real class, which you want to rebuild:
public class RealExample implements IExample {
#Override
public void example() {
System.out.println("Hi there.");
}
}
The proxy class that you will actually use:
public class ProxyExample implements IExample {
private IExample mTarget;
public ProxyExample(final IExample target) {
this.mTarget = target;
}
#Override
public void example() {
// Forward to the real implementation
this.mRealExample.example();
}
public void exchangeTarget(final IExample target) {
this.mTarget = target;
}
}
The factory you will mainly use:
public class ExampleFactory {
private static final String CLASS_NAME_TO_MONITOR = "somePackage.RealExample";
private final List<WeakReference<ProxyExample>> mInstances;
private final URLClassLoader mClassLoader;
public ExampleFactory() {
mInstances = new LinkedList<>();
// Classloader that will always load the up-to-date version of the class to monitor
mClassLoader = new URLClassLoader(new URL[] {getClassPath()}) {
public Class loadClass(final String name) {
if (CLASS_NAME_TO_MONITOR.equals(name)) {
return findClass(name);
}
return super.loadClass(name);
}
};
}
private IExample createRealInstance() {
return (IExample) this.mClassLoader.loadClass(CLASS_NAME_TO_MONITOR).newInstance();
}
public IExample createInstance() {
// Create an up-to-date instance
final IExample instance = createRealInstance();
// Create a proxy around it
final ProxyExample proxy = new ProxyExample(instance);
// Add the proxy to the monitor
this.mInstances.add(proxy);
return proxy;
}
public void updateAllInstances() {
// Iterate the proxies and update their references
// Use a ListIterator to easily remove instances that have been cleared
final ListIterator<WeakReference<ProxyExample>> instanceIter =
this.mInstances.listIterator();
while (instanceIter.hasNext()) {
final WeakReference<ProxyExample> reference = instanceIter.next();
final ProxyExample proxy = reference.get();
// Remove the instance if it was already cleared,
// for example by the garbage collector
if (proxy == null) {
instanceIter.remove();
continue;
}
// Create an up-to-date instance for exchange
final IExample instance = createRealInstance();
// Update the target of the proxy instance
proxy.exchangeTarget(instance);
}
}
}
And finally how to use it:
public static void main(final String[] args) {
final ExampleFactory factory = new ExampleFactory();
// Get some instances using the factory
final IExample example1 = factory.createInstance();
final IExample example2 = factory.createInstance();
// Prints "Hi there."
example1.example();
// Update all instances
factory.updateAllInstances();
// Prints whatever the class now contains
example1.example();
}
Attaching an agent at runtime requires use of the attach API which is contained in the tools.jar until Java 8 and is contained in its own module starting from Java 9. The location of the tools.jar and the name of its classes is system-dependent (OS, version, vendor) and as of Java 9 it does not exist at all but must be resolved via its module.
If you are looking for an easy way to access this functionality, try out Byte Buddy which has a subproject byte-buddy-agent for this. Create a Java agent as you are used to it but add anAgent-Main entry where you would put your Pre-Main in the manifest. Also, name the entry method agentmain, not premain.
Using byte-buddy-agent, you can write a program:
class AgentLoader {
public static void main(String[] args) {
String processId = ...
File agentJar = ...
ByteBuddyAgent.attach(processId, agentJar);
}
}
and you are done.
I know Java loads Classes in first Access (creating new Instance, calling static method or static field), but in this simple example I try to execute a jar file that uses some classes which there aren't in my ClassPath at run time. I expect (because of loading classes in first access) print my messages in static block and main method before an exception occurred. but I got "Exception in thread "main" java.lang.NoClassDefFoundError: com/example/DateAbstract" and nothing printed.
This occurred when I used an abstract class or interface in main class which that classes or interfaces are in another jar file.
public class Driver {
static { System.out.println("I am first.[static block]"); }
public static void main(String[] args) {
System.out.println("I am first.[ main method]");
DateAbstract date = new CustomDate();
System.out.println(date.sayDate());
}
in my another jar :
public class CustomDate extends DateAbstract {
#Override
public String sayDate() {
return new Date().toString();
}
public abstract class DateAbstract {
public abstract String sayDate();
}
when I use this code for add my classes to classpath at runtime. nothing changed. I got execption before execute static block.
public class Driver {
static {
System.out.println("I am first.[static block]");
try {
URL url = new File("lib/DateApi.jar").toURI().toURL();
URLClassLoader urlClassLoader = (URLClassLoader) URLClassLoader.getSystemClassLoader();
Method method = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
method.setAccessible(true);
method.invoke(urlClassLoader,url);
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
System.out.println("I am first.[ main method]");
DateAbstract date = new CustomDate();
System.out.println(date.sayDate());
}
}
Questions :
why is this happening and how to solve it ?
It’s not correct to say that in Java classes are loaded on their first access. You are confusing this with the initialization of a class, which implies executing the Java code of static initializer blocks and field initializers. The loading and verification might happen at an earlier time; the specification provides some freedom to the JVMs in this regard.
The key point here is that your main method instantiates an object of type CustomDate, stores it into a variable of the compile-time type DateAbstract and then tries to invoke sayDate() on that variable. This combination of instantiating CustomDate and invoking DateAbstract.sayDate() on it requires the verification of its correctness, i.e. whether CustomDate is a subtype DateAbstract. So the loading of these two classes will already happen at verification time.
You can easily check that this is the cause. If you change the type of the local variable date to CustomDate, the instantiated type and the receiver type of the method invocation are the same, so the correctness can be proven without loading the type, so it will be indeed deferred to the actual attempt to instantiate CustomDate, hence the messages will be printed.
Still, the loading time is an implementation-specific detail. A different JVM could load the referenced classes eagerly, even if they are not required for verification. The only safe way to ensure a deferred loading, is to use dynamic loading, e.g. Class.forName(String). Note that within the class detached this way, all types might be again referenced ordinarily. So if you do the dynamic loading once after the class path has been adjusted, there is not much impact on how you have to write the code nor its performance. Of course, having the code adjusting the class path and the code depending on it within the same class won’t work reliably.
I've an interface implemented by classes that perform a file processing, say searching or whatever.
public interface FileProcessorInterface {
public void processFile(String fileName);
}
Then i have a different implementation for each file type:
public class TxtProcessor implements FileProcessorInterface {
#Override public void processFile(String fileName) { //do the work }
}
Thus i have the Utilizer of the processor, that has a method that allows for registering each class, something like this:
class Utilizer {
Map <String, Class> registered = new HashMap<>();
public void registerClass(String fileExt, Class clazz) {
registered.put(fileExt, clazz);
}
public void processFile(String fileName) {
//1) get the registered class from registered map (omitted because easy and not relevant)
//2) create an instance of the class using reflection (omitted because easy and not relevant)
FileProcessorInterface p = ....
p.processFile(fileName);
}
So far it's ok.
Now, i'm providing many implementations of my interface.
And i am tempted to provide each implementation class with a static initializer that register itself in the Utilizer, in the case of my previous TxtProcessor it would be:
class TxtProcessor implements FileProcessorInterface {
//previous code
static {
Utilizer.registerClass("txt", TxtProcessor.class);
}
}
The problem is that this static method will never be called because in the "statically reachable" code of the application there is no reference to my TxtProcessor class, since it is instantiated via reflection. So the jvm does not call the static initializer.
Say that i have two parts: the "generic code" that is the Utilizer and on the other side the implementations; it has to be thought as something provided dinamically and so it is not known by the Utilizer part.
Infact the idea was exactly that each class would register itself leaving the Utilizer untouched.
It is hard for me conceiving a solution that does not put some form of 'knowledge' of the implementations on the Utilizer side (and that stays simple), just because of the problem of the static initializer not called. How to overcome this?
Using reflections seems to be the best fit here. It's like geared to do this.
All you need is a small static block in Utilizer as
static {
Reflections reflections = new Reflections(
new ConfigurationBuilder()
.setUrls(ClasspathHelper.forPackage("path.to.all.processors.pkg"))
.setScanners(new SubTypesScanner())
);
reflections.getSubTypesOf(path.to.all.processors.pkg.FileProcessor.class);
}
If you don't want a third-part dependency, just add a FileProcessors.properties file to your classpath
txt=path.to.all.processors.pkg.TxtProcessor
doc=path.to.all.processors.pkg.DocProcessor
pdf=path.to.all.processors.pkg.PdfProcessor
and then register all the listed classes from Utilizer as
static {
Properties processors = new Properties();
try {
processors.load(Utilizer.class
.getResourceAsStream("FileProcessors.properties"));
} catch (IOException e) {
e.printStackTrace();
}
for (String ext : processors.stringPropertyNames()) {
Utilizer.registerClass(ext, Class.forName(processors.getProperty(ext));
}
}
This no longer requires a static block in every FileProcessor now.
You can look at Reflections library. It allow you to find all the classes which implement an interface, have an annotation or extend a class.
You Could...
Use the same concept as JDBC does for loading it's drivers. This would require you to use Class#forName to initialize the class when the program is first loaded. While this does mean that the implementation is still dynamic from the point of view of your utility class, it is specified at run time by your application...
This gives you control over which implementation you might want to use
You Could...
Use the same concept as something like java.awt.Toolkit uses when it initializes it's instance.
It basically looks up the resource (in this case a System property) and then loads the class dynamically using Class.
Personally, I normally look for a named resource (usually a properties file) and load a key from it.
Something like getClass().getResource("/some/gloabl/configFile");, which every implementation would need to provide.
Then, if available, read the properties file and find the key I'm after.
If more then one implementation is linked in though, there is no guarantee which one will be loaded.
Quick and dirty: You can statically initialize your Utilizer in main() with correct association.
Better solution: externalize in a resource file association like
txt=path.to.package.TxProcessor
load it in Utilizer and load FileProcessorInterface implementors with Class.forName()
you can force the static init by Class.forName(fqn, true, classLoader) or the short form Class.forName(fqn)
You could have a registry file (for example, some XML file), that would contain the list of all classes you support :
<item type="txt">somepackage.TxtProcessor</item>
<item type="gif">somepackage.GIFProcessor</item>
...
Your Utilizer would load this file into its registry.
For some special use-case I have a small utility to load Java classes from jars using a dynamic class loader DynamicClassLoader. This works fine for Java classes contained in jars. Loading Scala classes from a jar also works without problems. However, instantiating the loaded Scala class leads to the following exception. It looks like the Scala class has private default constructor? Note the compiled Scala class name ending with $
java.lang.IllegalAccessException: Class XXX can not access a member of class ScalaClassYYY$ with modifiers "private"
The snippet below illustrates the idea of what I'm trying to achieve and gives a bit more context. The exception happens at the annotated line:
// deploy and register the new code
byte[] jarBytes = (byte[]) ((Object) message.getAttachment("jar"));
String registerClassName = message.getAttachment("register");
logger.debug("the register is '" + registerClassName + "'");
DynamicClassLoader loader = new DynamicClassLoader(jarBytes);
Class<?> registerClass = loader.lookUp(registerClassName);
// ===> this is where the java.lang.IllegalAccessException happens
IRegisterExecutor registerExecutor = (IRegisterExecutor) registerClass.newInstance();
registerExecutor.register();
Any ideas how to fix?
Obviously, you need to make the default constructor public (it won't work for Java classes without a public default constructor either). E.g.
class ScalaClassYYY() {
...
}
or if you want primary constructor to take some arguments,
class ScalaClassYYY(arg1: Int) {
def this() = this(0)
}
But from
Note the compiled Scala class name ending with $
it seems like you are actually trying to instantiate a Scala object:
object ScalaClassYYY { ... }
In this case, you shouldn't create a new instance and instead use the existing one:
(IRegisterExecutor) registerClass.getField("MODULE$").get(null);
EDIT:
I don't see in your answer how you add a default public constructor to a Scala class that does NOT require any parameters.
A class (not an object) that doesn't require any parameters has a default public constructor already (my first example).
Actually in Java all classes by default offer a public default constructor
No. Only those classes which have no constructors which take arguments.
remove the "(it won't work for Java classes without a public default constructor either)" because it is wrong
The documentation for Class.newInstance() says
IllegalAccessException - if the class or its nullary constructor is not accessible.
So I am pretty sure it's right. If it does work for Java classes without a public default constructor, this seems to be a major bug in the class loader you use. You can test it with a Java class which looks like this:
public class TestClass implements IRegisterExecutor {
public TestClass(int dummy) {}
// some implementation for IRegisterExecutor methods to get it to compile
}
i'm very confused...
I have a class which directly implements an interface:
public class Device implements AutocompleteResult
{...}
Here is proof that I'm looking at the right variables:
Object match = ...;
log.debug(match.getClass()); // Outputs 'Device'
log.debug(match.getClass().getInterfaces()[0]); // Outputs 'AutocompleteResult'
Yet when I try to cast an instance of the class to the interface:
AutocompleteResult result = (AutocompleteResult) match;
I get a ClassCastException!
ClassCastException: Device cannot be cast to AutocompleteResult
Also, isAssignableFrom returns false and i'm not sure why:
log.debug(AutocompleteResult.class.isAssignableFrom(Device.class));
from the doc:
Determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface of, the class or interface represented by the specified Class parameter.
Shouldn't I always be able to cast a object to an interface its class implements?
Thanks.
This can happen if two different classloaders load a class named AutocompleteResult.
These two classes are then treated as entirely different classes, even if they have the same package and name (and even implementation/fields/methods).
A common cause for this is if you use some kind of plugin system and both your base classes and the plugin classes provide the same class.
To check for this issue print the value returned by Class.getClassLoader() on both offending classes (i.e. the class of the interface implemented by Device and the result of AutocompleteResult.class).
AKA when Java apparently doesn't Java.
I hit this problem recently with Play Framework 2.6.3, what helped me was this:
https://www.playframework.com/documentation/2.6.x/ThreadPools#Application-class-loader
I leave this info here for the people that might have the same problem.
To make it clearer, what helps is:
Injecting Application on an Eager Singleton and then using its classloader to load the classes I was having issues with.
To make it clearer
public class Module {
#Override
public void configure {
bind(TheClassLoaderLoader.class).asEagerSingleton()
public static class TheClassLoaderLoader {
#Inject
public TheClassLoaderLoader( Application application) {
ClassLoader classloader = application.classloader();
Class<?> interfaceClass = classloader.loadClass(InterfaceClass.class.getName());
classloader.loadClass(ImplementsInterfaceClass.class.getName()).asSubclass(interfaceClass);
The example here https://playframework.com/documentation/2.6.x/JavaDependencyInjection#Configurable-bindings
That uses Environment often throws a frustrating ClassNotFoundException
Cheers