It is possible to load and reload classes at runtime in Java, though it is not as straightforward as one might have hoped. This text will explain when and how you can load and reload classes in Java.
You can argue whether Java's dynamic class loading features are really part of Java Reflection, or a part of the core Java platform. Anyways, the article has been put in the Java Reflection trail in lack of a better place to put it.
Here is a list of the topics covered in this text:
All classes in a Java application are loaded using some subclass of?java.lang.ClassLoader. Loading classes dynamically must therefore also be done using a?java.lang.ClassLoader?subclass.
When a class is loaded, all classes it references are loaded too. This class loading pattern happens recursively, until all classes needed are loaded. This may not be all classes in the application. Unreferenced classes are not loaded until the time they are referenced.
Class loaders in Java are organized into a hierarchy. When you create a new standard Java?ClassLoader?you must provide it with a parent?ClassLoader. If a?ClassLoader?is asked to load a class, it will ask its parent class loader to load it. If the parent class loader can't find the class, the child class loader then tries to load it itself.
The steps a given class loader uses when loading classes are:
When you implement a class loader that is capable of reloading classes you will need to deviate a bit from this sequence. The classes to reload should not be requested loaded by the parent class loader. More on that later.
Loading a class dynamically is easy. All you need to do is to obtain a?ClassLoader?and call its?loadClass()method. Here is an example:
public class MainClass {
public static void main(String[] args){
ClassLoader classLoader = MainClass.class.getClassLoader();
try {
Class aClass = classLoader.loadClass("com.jenkov.MyClass");
System.out.println("aClass.getName() = " + aClass.getName());
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
Dynamic class reloading is a bit more challenging. Java's builtin Class loaders always checks if a class is already loaded before loading it. Reloading the class is therefore not possible using Java's builtin class loaders. To reload a class you will have to implement your own?ClassLoader?subclass.
Even with a custom subclass of?ClassLoader?you have a challenge. Every loaded class needs to be linked. This is done using the?ClassLoader.resolve()?method. This method is final, and thus cannot be overridden in yourClassLoader?subclass. The?resolve()?method will not allow any given?ClassLoader?instance to link the same class twice. Therefore, everytime you want to reload a class you must use a new instance of your?ClassLoadersubclass. This is not impossible, but necessary to know when designing for class reloading.
As stated earlier you cannot reload a class using a?ClassLoader?that has already loaded that class once. Therefore you will have to reload the class using a different?ClassLoader?instance. But this poses som new challenges.
Every class loaded in a Java application is identified by its fully qualified name (package name + class name), and theClassLoader?instance that loaded it. That means, that a class?MyObject?loaded by class loader A, is not the same class as the?MyObject?class loaded with class loader B. Look at this code:
MyObject object = (MyObject)
myClassReloadingFactory.newInstance("com.jenkov.MyObject");
Notice how the?MyObject?class is referenced in the code, as the type of the?object?variable. This causes theMyObject?class to be loaded by the same class loader that loaded the class this code is residing in.
If the?myClassReloadingFactory?object factory reloads the?MyObject?class using a different class loader than the class the above code resides in, you cannot cast the instance of the reloaded?MyObject?class to the?MyObjecttype of the?object?variable. Since the two?MyObject?classes were loaded with different class loaders, the are regarded as different classes, even if they have the same fully qualified class name. Trying to cast an object of the one class to a reference of the other will result in a?ClassCastException.
It is possible to work around this limitation but you will have to change your code in either of two ways:
Here are two coresponding code examples:
MyObjectInterface object = (MyObjectInterface)
myClassReloadingFactory.newInstance("com.jenkov.MyObject");
MyObjectSuperclass object = (MyObjectSuperclass)
myClassReloadingFactory.newInstance("com.jenkov.MyObject");
Either of these two methods will work if the type of the variable, the interface or superclass, is not reloaded when the implementing class or subclass is reloaded.
To make this work you will of course need to implement your class loader to let the interface or superclass be loaded by its parent. When your class loader is asked to load the?MyObject?class, it will also be asked to load theMyObjectInterface?class, or the?MyObjectSuperclass?class, since these are referenced from within theMyObject?class. Your class loader must delegate the loading of those classes to the same class loader that loaded the class containing the interface or superclass typed variables.
The text above has contained a lot of talk. Let's look at a simple example. Below is an example of a simpleClassLoader?subclass. Notice how it delegates class loading to its parent except for the one class it is intended to be able to reload. If the loading of this class is delegated to the parent class loader, it cannot be reloaded later. Remember, a class can only be loaded once by the same?ClassLoader?instance.
As said earlier, this is just an example that serves to show you the basics of a?ClassLoader's behaviour. It is not a production ready template for your own class loaders. Your own class loaders should probably not be limited to a single class, but a collection of classes that you know you will need to reload. In addition, you should probably not hardcode the class paths either.
public class MyClassLoader extends ClassLoader{
public MyClassLoader(ClassLoader parent) {
super(parent);
}
public Class loadClass(String name) throws ClassNotFoundException {
if(!"reflection.MyObject".equals(name))
return super.loadClass(name);
try {
String url = "file:C:/data/projects/tutorials/web/WEB-INF/" +
"classes/reflection/MyObject.class";
URL myUrl = new URL(url);
URLConnection connection = myUrl.openConnection();
InputStream input = connection.getInputStream();
ByteArrayOutputStream buffer = new ByteArrayOutputStream();
int data = input.read();
while(data != -1){
buffer.write(data);
data = input.read();
}
input.close();
byte[] classData = buffer.toByteArray();
return defineClass("reflection.MyObject",
classData, 0, classData.length);
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
}
Below is an example use of the?MyClassLoader.
public static void main(String[] args) throws
ClassNotFoundException,
IllegalAccessException,
InstantiationException {
ClassLoader parentClassLoader = MyClassLoader.class.getClassLoader();
MyClassLoader classLoader = new MyClassLoader(parentClassLoader);
Class myObjectClass = classLoader.loadClass("reflection.MyObject");
AnInterface2 object1 =
(AnInterface2) myObjectClass.newInstance();
MyObjectSuperClass object2 =
(MyObjectSuperClass) myObjectClass.newInstance();
//create new class loader so classes can be reloaded.
classLoader = new MyClassLoader(parentClassLoader);
myObjectClass = classLoader.loadClass("reflection.MyObject");
object1 = (AnInterface2) myObjectClass.newInstance();
object2 = (MyObjectSuperClass) myObjectClass.newInstance();
}
Here is the?reflection.MyObject?class that is loaded using the class loader. Notice how it both extends a superclass and implements an interface. This is just for the sake of the example. In your own code you would only have to one of the two - extend or implement.
public class MyObject extends MyObjectSuperClass implements AnInterface2{
//... body of class ... override superclass methods
// or implement interface methods
}
?
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