Java Reflection - Dynamic Class Loading and Reloading
Jakob Jenkov |
You can load and reload classes in Java at runtime, though it is not as straightforward as one might have hoped. This is done via the Java platform's built-in classloader system. 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.
The ClassLoader
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.
The ClassLoader Hierarchy
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.
Class Loading
The steps a given class loader uses when loading classes are:
- Check if the class was already loaded.
- If not loaded, ask parent class loader to load the class.
- If parent class loader cannot load class, attempt to load it in this class loader.
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.
Dynamic Class Loading
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
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 your ClassLoader 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
ClassLoader subclass. This is not impossible, but necessary to know when designing
for class reloading.
Designing your Code 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 the ClassLoader 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 the MyObject 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 MyObject
type 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:
- Use an interface as the variable type, and just reload the implementing class.
- Use a superclass as the variable type, and just reload a subclass.
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 the MyObjectInterface class, or the
MyObjectSuperclass class, since these are referenced from within the MyObject
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.
ClassLoader Load / Reload Example
The text above has contained a lot of talk. Let's look at a simple example.
Below is an example of a simple ClassLoader 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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Jakob Jenkov | |
