Can I dynamically load, unload or reload a JAR?

/*
 * ClassLoader - JarFileLoader.java, Oct 3, 2011 1:06:13 PM
 * 
 * Copyright 2011 Varra Ltd, Inc. All rights reserved.
 * Varra proprietary/confidential. Use is subject to license terms.
 */
package com.varra.net;

import java.io.File;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.Collection;
import java.util.jar.JarFile;

import com.varra.util.EnhancedTimerTask;
import com.varra.util.GlobalThread;

/**
 * The Class JarFileLoader.
 * 
 * @author Rajakrishna V. Reddy
 * @version 1.0
 */
public class JarFileLoader extends URLClassLoader
{
   
    /**
     * Instantiates a new jar file loader.
     * 
     * @param urls
     *            the urls
     */
    public JarFileLoader()
    {
        super(new URL[] {});
    }
   
    /**
     * Adds the file.
     * 
     * @param file
     *            the file
     * @throws MalformedURLException
     *             the malformed url exception
     */
    public void addFile(File file) throws MalformedURLException
    {
        addURL(file.toURI().toURL());
    }
   
    /**
     * Closes all open jar files.
     */
    public void close()
    {
        try
        {
            Class<?> clazz = java.net.URLClassLoader.class;
            Field ucp = clazz.getDeclaredField("ucp");
            ucp.setAccessible(true);
            Object sunMiscURLClassPath = ucp.get(this);
            Field loaders = sunMiscURLClassPath.getClass().getDeclaredField("loaders");
            loaders.setAccessible(true);
            Object collection = loaders.get(sunMiscURLClassPath);
            for (Object sunMiscURLClassPathJarLoader : ((Collection<?>) collection).toArray())
            {
                try
                {
                    Field loader = sunMiscURLClassPathJarLoader.getClass().getDeclaredField("jar");
                    loader.setAccessible(true);
                    Object jarFile = loader.get(sunMiscURLClassPathJarLoader);
                    ((JarFile) jarFile).close();
                }
                catch (Throwable t)
                {
                    // if we got this far, this is probably not a JAR loader so
                    // skip it
                }
            }
        }
        catch (Throwable t)
        {
            // probably not a SUN VM
        }
        return;
    }
   
    /**
     * The main method.
     * 
     * @param args
     *            the arguments
     */
    public static void main(String args[])
    {
        try
        {
            System.out.println("First attempt...");
            Class.forName("com.varra.temp.Class1");
        }
        catch (Exception ex)
        {
            System.out.println("Failed.");
        }
        try
        {
            JarFileLoader clazzLoader = new JarFileLoader();
            clazzLoader.addFile(new File("/krishna/RapidHealthAgent/FileWatcher/bin/filewatcher-1.0.jar"));
            clazzLoader.addFile(new File("/krishna/RapidHealthAgent/FileTailer/bin/filetailer-1.0.jar"));
           
            Package[] packages = clazzLoader.getPackages();
            for (Package package1 : packages)
            {
                if (package1.getName().startsWith("com"))
                {
                    System.out.println("B4 Paks: "+package1.getName());
                }
            }
            System.out.println("Second attempt...");
            Class<?> fileWatcherClass = clazzLoader.loadClass("com.mt.filewatcher.FileWatcher");
            final Method method = fileWatcherClass.getMethod("getFileWatcher", null);
            final Object fileWatcher = method.invoke(null, null);
            final GlobalThread globalThread = GlobalThread.getGlobalThread(1);
            globalThread.start();
            globalThread.onTimerTask((EnhancedTimerTask) fileWatcher);
           
            Class<?> testTailer = clazzLoader.loadClass("com.mt.filetailer.TestTailer");
            testTailer.newInstance();
            System.out.println("loadClass: " + testTailer);
            packages = clazzLoader.getPackages();
            for (Package package1 : packages)
            {
                if (package1.getName().startsWith("com"))
                {
                    System.out.println("B4 Paks: "+package1.getName());
                }
            }
            System.out.println("Success!");
        }
        catch (Exception ex)
        {
            System.out.println("Failed.");
            ex.printStackTrace();
        }
    }
}

Arrays

Working with arrays in Java Reflection can be a bit tricky at times. Especially if you need to obtain the Class object for a certain type of array, like int[] etc. This text will discuss how to both create arrays and get their class objects via Java Reflection.
Note: This text has been updated after reading Eyal Lupu's blog post
"Two Side Notes About Arrays and Reflection" which commented on the first edition of this text. The current edition takes his comments into consideration.
Here is a list of the topics covered:

1. java.lang.reflect.Array
2. Creating Arrays
3. Accessing Arrays
4. Obtaining the Class Object of an Array
5. Obtaining the Component Type of an Array

java.lang.reflect.Array

Working with arrays via Java Reflection is done using the java.lang.reflect.Array class. Do not confuse this class with the java.util.Arrays class in the Java Collections suite, which contains utility methods for sorting arrays, converting them to collections etc.

Creating Arrays

Creating arrays via Java Reflection is done using the java.lang.reflect.Array class. Here is an example showing how to create an array:

int[] intArray = (int[]) Array.newInstance(int.class, 3);

This code sample creates an array of int. The first parameter int.class given to theArray.newInstance() method tells what type each element in the array should be of. The second parameter states how many elements the array should have space for.

Accessing Arrays

It is also possible to access the elements of an array using Java Reflection. This is done via theArray.get(...) and Array.set(...) methods. Here is an example:

int[] intArray = (int[]) Array.newInstance(int.class, 3);

Array.set(intArray, 0, 123);
Array.set(intArray, 1, 456);
Array.set(intArray, 2, 789);

System.out.println("intArray[0] = " + Array.get(intArray, 0));
System.out.println("intArray[1] = " + Array.get(intArray, 1));
System.out.println("intArray[2] = " + Array.get(intArray, 2));

This code sample will print out this:

intArray[0] = 123
intArray[1] = 456
intArray[2] = 789

Obtaining the Class Object of an Array

One of the problems I ran into when implementing the script language in Butterfly DI Containerwas how to obtain the Class object for arrays via Java Reflection. Using non-reflection code you can do like this:

Class stringArrayClass = String[].class;

Doing this using Class.forName() is not quite straightforward. For instance, you can access the primitive int array class object like this:

Class intArray = Class.forName("[I");

The JVM represents an int via the letter I. The [ on the left means it is the class of an int array I am interested in. This works for all other primitives too.
For objects you need to use a slightly different notation:

Class stringArrayClass = Class.forName("[Ljava.lang.String;");

Notice the [L to the left of the class name, and the ; to the right. This means an array of objects with the given type.
As a side note, you cannot obtain the class object of primitives using Class.forName(). Both of the examples below result in a ClassNotFoundException:

Class intClass1 = Class.forName("I");
Class intClass2 = Class.forName("int");

I usually do something like this to obtain the class name for primitives as well as objects:

public Class getClass(String className){
  if("int" .equals(className)) return int .class;
  if("long".equals(className)) return long.class;
  ...
  return Class.forName(className);
}

Once you have obtained the Class object of a type there is a simple way to obtain the Class of an array of that type. The solution, or workaround as you might call it, is to create an empty array of the desired type and obtain the class object from that empty array. It's a bit of a cheat, but it works. Here is how that looks:

Class theClass = getClass(theClassName);
Class stringArrayClass = Array.newInstance(theClass, 0).getClass();

This presents a single, uniform method to access the array class of arrays of any type. No fiddling with class names etc.
To make sure that the Class object really is an array, you can call the Class.isArray() method to check:

Class stringArrayClass = Array.newInstance(String.class, 0).getClass();
System.out.println("is array: " + stringArrayClass.isArray());

Obtaining the Component Type of an Array

Once you have obtained the Class object for an array you can access its component type via theClass.getComponentType() method. The component type is the type of the items in the array. For instance, the component type of an int[] array is the int.class Class object. The component type of a String[] array is the java.lang.String Class object.
Here is an example of accessing the component type array:

String[] strings = new String[3];
Class stringArrayClass = strings.getClass();
Class stringArrayComponentType = stringArrayClass.getComponentType();
System.out.println(stringArrayComponentType);

This example will print out the text "java.lang.String" which is the component type of the Stringarray.

Java Reflection Tutorial

Java's Reflection API's makes it possible to inspect classes, interfaces, fields and methods at runtime, without knowing the names of the classes, methods etc. at compile time. It is also possible to instantiate new objects, invoke methods and get/set field values using reflection.
Java Reflection is a quite powerful and can be very useful. For instance, when mapping objects to tables in a database at runtime, like Butterfly Persistence does. Or, when mapping the statements in a script language to method calls on real objects at runtime, like Butterfly Container does when parsing its configuration scripts.
There are already numerous Java Reflection Tutorials on the internet. However, most of them, including Sun's own Java Reflection tutorial, only scratch the surface of Java Reflection and its possibilities.
This tutorial will get into Java reflection in more depth than most of the tutorials I have seen. It will explain the basics of Java Reflection including how to work with arrays, annotations, generics and dynamic proxies, and do dynamic class loading and reloading. It will also show you how to do more specific tasks, like reading all getter methods of a class, or accessing private fields and methods of a class. This tutorial will also clear up some of the confusion out there about what Generics information is available at runtime. Some people claim that all Generics information is lost at runtime. This is not true.
This tutorial is work in progress, so please be patient and check back later, if your topic isn't yet covered, or subscribe to the <. Here is a list of the topics covered so far:

Java Reflection
		Introduction to Java Reflection
		Classes
		Constructors
		Fields
		Methods
		Getters and Setters
		Private Fields and Methods
		Annotations
		Generics
		Arrays
		Dynamic Proxies
		Dynamic Class Loading and Reloading

Constructors in Java

Using Java Reflection you can inspect the constructors of classes and instantiate objects at runtime. This is done via the Java class java.lang.reflect.Constructor. This text will get into more detail about the Java Construcor object. Here is a list of the topics covered:

1. Obtaining Constructor Objects
2. Constructor Parameters
3. Instantiating Objects using Constructor Object

Obtaining Constructor Objects

The Constructor class is obtained from the Class object. Here is an example:

Class aClass = ...//obtain class object
Constructor[] constructors = aClass.getConstructors();

The Constructor[] array will have one Constructor instance for each public constructor declared in the class.
If you know the precise parameter types of the constructor you want to access, you can do so rather than obtain the array all constructors. This example returns the public constructor of the given class which takes a String as parameter:

Class aClass = ...//obtain class object
Constructor constructor =
        aClass.getConstructor(new Class[]{String.class});

If no constructor matches the given constructor arguments, in this case String.class, aNoSuchMethodException is thrown.

Constructor Parameters

You can read what parameters a given constructor takes like this:

Constructor constructor = ... // obtain constructor - see above
Class[] parameterTypes = constructor.getParameterTypes();

Instantiating Objects using Constructor Object

You can instantiate an object like this:

//get constructor that takes a String as argument
Constructor constructor = MyObject.class.getConstructor(String.class);

MyObject myObject = (MyObject)
        constructor.newInstance("constructor-arg1");

The Constructor.newInstance() method takes an optional amount of parameters, but you must supply exactly one parameter per argument in the constructor you are invoking. In this case it was a constructor taking a String, so one String must be supplied.

Fields in Java

Using Java Reflection you can inspect the fields (member variables) of classes and get / set them at runtime. This is done via the Java class java.lang.reflect.Field. This text will get into more detail about the Java Field object. Remember to check the JavaDoc from Sun out too. Here is a list of the topics covered:

1. Obtaining Field Objects
2. Field Name
3. Field Type
4. Getting and Setting Field Values

Obtaining Field Objects

The Field class is obtained from the Class object. Here is an example:

Class aClass = ...//obtain class object
Field[] methods = aClass.getFields();

The Field[] array will have one Field instance for each public field declared in the class.
If you know the name of the field you want to access, you can access it like this:

Class  aClass = MyObject.class
Field field = aClass.getField("someField");

The example above will return the Field instance corresponding to the field someField as declared in the MyObject below:

public class MyObject{
  public String someField = null;

}

If no field exists with the name given as parameter to the getField() method, aNoSuchFieldException is thrown.

Field Name

Once you have obtained a Field instance, you can get its field name using the Field.getName()method, like this:

Field field = ... //obtain field object
String fieldName = field.getName();

Field Type

You can determine the field type (String, int etc.) of a field using the Field.getType() method:

Field field = aClass.getField("someField");
Object fieldType = field.getType();

Getting and Setting Field Values

Once you have obtained a Field reference you can get and set its values using the Field.get()and Field.set()methods, like this:

Class  aClass = MyObject.class
Field field = aClass.getField("someField");

MyObject objectInstance = new MyObject();

Object value = field.get(objectInstance);

field.set(objetInstance, value);

The objectInstance parameter passed to the get and set method should be an instance of the class that owns the field. In the above example an instance of MyObject is used, because thesomeField is an instance member of the MyObject class.
It the field is a static field (public static ...) pass null as parameter to the get and set methods, instead of the objectInstance parameter passed above.

Methods in Java

Using Java Reflection you can inspect the methods of classes and invoke them at runtime. This is done via the Java class java.lang.reflect.Method. This text will get into more detail about the Java Method object. Here is a list of the topics covered:

1. Obtaining Method Objects
2. Method Parameters and Return Types
3. Instantiating Objects using Constructor Object

Obtaining Method Objects

The Method class is obtained from the Class object. Here is an example:

Class aClass = ...//obtain class object
Method[] methods = aClass.getMethods();

The Method[] array will have one Method instance for each public method declared in the class.
If you know the precise parameter types of the method you want to access, you can do so rather than obtain the array all methods. This example returns the public method named "doSomething", in the given class which takes a String as parameter:

Class  aClass = ...//obtain class object
Method method =
    aClass.getMethod("doSomething", new Class[]{String.class});

If no method matches the given method name and arguments, in this case String.class, aNoSuchMethodException is thrown.
If the method you are trying to access takes no parameters, pass null as the parameter type array, like this:

Class  aClass = ...//obtain class object
Method method =
    aClass.getMethod("doSomething", null);

Method Parameters and Return Types

You can read what parameters a given method takes like this:

Method method = ... // obtain method - see above
Class[] parameterTypes = method.getParameterTypes();

You can access the return type of a method like this:

Method method = ... // obtain method - see above
Class returnType = method.getReturnType();

Invoking Methods using Method Object

You can invoke a method like this:

//get method that takes a String as argument
Method method = MyObject.class.getMethod("doSomething", String.class);

Object returnValue = method.invoke(null, "parameter-value1");

The null parameter is the object you want to invoke the method on. If the method is static you supply null instead of an object instance. In this example, if doSomething(String.class) is not static, you need to supply a valid MyObject instance instead of null;
The Method.invoke(Object target, Object ... parameters) method takes an optional amount of parameters, but you must supply exactly one parameter per argument in the method you are invoking. In this case it was a method taking a String, so one String must be supplied.

Dynamic Class Loading and Reloading in Java

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:

1. The ClassLoader
2. The ClassLoader Hierarchy
3. Class Loading
4. Dynamic Class Loading
5. Dynamic Class Reloading
6. Designing your Code for Class Reloading
7. ClassLoader Load / Reload Example

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.ClassLoadersubclass.
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 JavaClassLoader 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:

1. Check if the class was already loaded.
2. If not loaded, ask parent class loader to load the class.
3. 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 itsloadClass() 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 ClassLoadersubclass.
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 MyObjectloaded 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 reloadedMyObject 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:

1. Use an interface as the variable type, and just reload the implementing class.
2. 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 sameClassLoader 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
}

Dynamic Proxies in Java

Using Java Reflection you create dynamic implementations of interfaces at runtime. You do so using the class java.lang.reflect.Proxy. The name of this class is why I refer to these dynamic interface implementations as dynamic proxies. Dynamic proxies can be used for many different purposes, e.g. database connection and transaction management, dynamic mock objects for unit testing, and other AOP-like method intercepting purposes.
Here is a list of topics covered in this text on dynamic proxies:

1. Creating Proxies
2. InvocationHandler's
3. Known Use Cases

Creating Proxies

You create dynamic proxies using the Proxy.newProxyInstance() method. ThenewProxyInstance() methods takes 3 parameters:

1. The ClassLoader that is to "load" the dynamic proxy class.
2. An array of interfaces to implement.
3. An InvocationHandler to forward all methods calls on the proxy to.

Here is an example:

InvocationHandler handler = new MyInvocationHandler();
MyInterface proxy = (MyInterface) Proxy.newProxyInstance(
                            MyInterface.class.getClassLoader(),
                            new Class[] { MyInterface.class },
                            handler);

After running this code the proxy variable contains a dynamic implementation of the MyInterfaceinterface. All calls to the proxy will be forwarded to the handler implementation of the generalInvocationHandler interface. InvocationHandler's are covered i the next section.

InvocationHandler's

As mentioned earlier you must pass an InvocationHandler implementation to theProxy.newProxyInstance() method. All method calls to the dynamic proxy are forwarded to thisInvocationHandler implementation. Here is how the InvocationHandler interface looks:

public interface InvocationHandler{
  Object invoke(Object proxy, Method method, Object[] args)
         throws Throwable;
}

Here is an example implementation:

public class MyInvocationHandler implements InvocationHandler{

  public Object invoke(Object proxy, Method method, Object[] args)
  throws Throwable {
    //do something "dynamic"
  }
}

The proxy parameter passed to the invoke() method is the dynamic proxy object implementing the interface. Most often you don't need this object.
The Method object passed into the invoke() method represents the method called on the interface the dynamic proxy implements. From the Method object you can obtain the method name, parameter types, return type, etc. See the text on Methods for more information.
The Object[] args array contains the parameter values passed to the proxy when the method in the interface implemented was called. Note: Primitives (int, long etc) in the implemented interface are wrapped in their object counterparts (Integer, Long etc.).

Known Use Cases

Dynamic proxies are known to be used for at least the following purposes:

• Database Connection and Transaction Management
• Dynamic Mock Objects for Unit Testing
• Adaptation of DI Container to Custom Factory Interfaces
• AOP-like Method Interception

Database Connection and Transaction Management

The Spring framework has a transaction proxy that can start and commit / rollback a transaction for you. How this works is described in more detail in the text Advanced Connection and Transaction Demarcation and Propagation , so I'll only describe it briefly. The call sequence becomes something along this:

web controller --> proxy.execute(...);
  proxy --> connection.setAutoCommit(false);
  proxy --> realAction.execute();
    realAction does database work
  proxy --> connection.commit();

Dynamic Mock Objects for Unit Testing

The Butterfly Testing Tools makes use of dynamic proxies to implement dynamic stubs, mocks and proxies for unit testing. When testing a class A that uses another class B (interface really), you can pass a mock implementation of B into A instead of a real B. All method calls on B are now recorded, and you can set what return values the mock B is to return.
Furthermore Butterfly Testing Tools allow you to wrap a real B in a mock B, so that all method calls on the mock are recorded, and then forwarded to the real B. This makes it possible to check what methods were called on a real functioning B. For instance, if testing a DAO you can wrap the database connection in a mock. The DAO will not see the difference, and the DAO can read/write data to the database as usual since the mock forwards all calls to the database. But now you can check via the mock if the DAO uses the connection properly, for instance if theconnection.close() is called (or NOT called), if you expected that. This is normally not possible to determine from the return value of a DAO.

Adaptation of DI Container to Custom Factory Interfaces

The dependency injection container Butterfly Container has a powerful feature that allows you to inject the whole container into beans produced by it. But, since you don't want a dependency on the container interface, the container is capable of adapting itself to a custom factory interface of your design. You only need the interface. No implementation. Thus the factory interface and your class could look something like this:

public interface IMyFactory {
  Bean   bean1();
  Person person();
  ...
}

public class MyAction{

  protected IMyFactory myFactory= null;

  public MyAction(IMyFactory factory){
    this.myFactory = factory;
  }

  public void execute(){
    Bean bean = this.myFactory.bean();
    Person person = this.myFactory.person();
  }

}

When the MyAction class calls methods on the IMyFactory instance injected into its constructor by the container, the method calls are translated into calls to the IContainer.instance()method, which is the method you use to obtain instances from the container. That way an object can use Butterfly Container as a factory at runtime, rather than only to have dependencies injected into itself at creation time. And this without having any dependencies on any Butterfly Container specific interfaces.

AOP-like Method Interception

The Spring framework makes it possible to intercept method calls to a given bean, provided that bean implements some interface. The Spring framework wraps the bean in a dynamic proxy. All calls to the bean are then intercepted by the proxy. The proxy can decide to call other methods on other objects either before, instead of, or after delegating the method call to the bean wrapped.

Generics in Java

I have often read in articles and forums that all Java Generics information is erased at compile time so that you cannot access any of that information at runtime. This is not entirely true though. It ispossible to access generics information at runtime in a handful of cases. These cases actually cover several of our needs for Java Generics information. This text explains these cases.
Here is a list of the topics covered in this text:

1. The Generics Reflection Rule of Thumb
2. Generic Method Return Type
3. Generic Method Parameter Types
4. Generic Field Types

The Generics Reflection Rule of Thumb

Using Java Generics typically falls into one of two different situations:

1. Declaring a class/interface as being parameterizable.
2. Using a parameterizable class.

When you write a class or interface you can specify that it should be paramerizable. This is the case with the java.util.List interface. Rather than create a list of Object you can parameterizejava.util.List to create a list of say String.
When runtime inspecting a parameterizable type itself, like java.util.List, there is no way of knowing what type is has been parameterized to. This makes sense since the type can be parameterized to all kinds of types in the same application. But, when you inspect the method or field that declares the use of a parameterized type, you can see at runtime what type the paramerizable type was parameterized to. In short:
You cannot see on a type itself what type it is parameterized to a runtime, but you can see it in fields and methods where it is used and parameterized. Its concrete parameterizations in other words.
The following sections take a closer look at these situations.

Generic Method Return Types

If you have obtained a java.lang.reflect.Method object it is possible to obtain information about its generic return type. This cannot be any of the Method objects in the parameterized type, but in the class that uses the parameterized type. You can read how to obtain Method objects in the text "Java Generics: Methods". Here is an example class with a method having a parameterized return type:

public class MyClass {

  protected List<String> stringList = ...;

  pulic List<String> getStringList(){
    return this.stringList;
  }
}

In this class it is possible to obtain the generic return type of the getStringList() method. In other words, it is possible to detect that getStringList() returns a List<String> and not just a List. Here is how:

Method method = MyClass.class.getMethod("getStringList", null);

Type returnType = method.getGenericReturnType();

if(returnType instanceof ParameterizedType){
    ParameterizedType type = (ParameterizedType) returnType;
    Type[] typeArguments = type.getActualTypeArguments();
    for(Type typeArgument : typeArguments){
        Class typeArgClass = (Class) typeArgument;
        System.out.println("typeArgClass = " + typeArgClass);
    }
}

This piece of code will print out the text "typeArgClass = java.lang.String". The Type[] arraytypeArguments array will contain one item - a Class instance representing the classjava.lang.String. Class implements the Type interface.

Generic Method Parameter Types

You can also access the generic types of parameter types at runtime via Java Reflection. Here is an example class with a method taking a parameterized List as parameter:

public class MyClass {
  protected List<String> stringList = ...;

  pulic void setStringList(List<String> list){
    this.stringList = list;
  }
}

You can access the generic parameter types of the method parameters like this:

method = Myclass.class.getMethod("setStringList", List.class);

Type[] genericParameterTypes = method.getGenericParameterTypes();

for(Type genericParameterType : genericParameterTypes){
    if(genericParameterType instanceof ParameterizedType){
        ParameterizedType aType = (ParameterizedType) genericParameterType;
        Type[] parameterArgTypes = aType.getActualTypeArguments();
        for(Type parameterArgType : parameterArgTypes){
            Class parameterArgClass = (Class) parameterArgType;
            System.out.println("parameterArgClass = " + parameterArgClass);
        }
    }
}

This code will print out the text "parameterArgType = java.lang.String". The Type[] arrayparameterArgTypes array will contain one item - a Class instance representing the classjava.lang.String. Class implements the Type interface.

Generic Field Types

It is also possible to access the generic types of public fields. Fields are class member variables - either static or instance variables. You can read about obtaining Field objects in the text "Java Generics: Fields". Here is the example from earlier, with an instance field called stringList.

public class MyClass {
  public List<String> stringList = ...;
}

Field field = MyClass.class.getField("stringList");

Type genericFieldType = field.getGenericType();
    
if(genericFieldType instanceof ParameterizedType){
    ParameterizedType aType = (ParameterizedType) genericFieldType;
    Type[] fieldArgTypes = aType.getActualTypeArguments();
    for(Type fieldArgType : fieldArgTypes){
        Class fieldArgClass = (Class) fieldArgType;
        System.out.println("fieldArgClass = " + fieldArgClass);
    }
}

This code will print out the text "fieldArgClass = java.lang.String". The Type[] array fieldArgTypesarray will contain one item - a Class instance representing the class java.lang.String. Classimplements the Type interface.

Annotations in Java

Using Java Reflection you can access the annotations attached to Java classes at runtime. Here is a list of the topics covered in this text:

• What are Java Annotations?
• Class Annotations
• Method Annotations
• Parameter Annotations
• Field Annotations

What are Java Annotations?

Annotations is a new feature from Java 5. Annotations are a kind of comment or meta data you can insert in your Java code. These annotations can then be processed at compile time by pre-compiler tools, or at runtime via Java Reflection. Here is an example of class annotation:

@MyAnnotation(name="someName",  value = "Hello World")
public class TheClass {
}

The class TheClass has the annotation @MyAnnotation written ontop. Annotations are defined like interfaces. Here is the MyAnnotation definition:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)

public @interface MyAnnotation {
    public String name();
    public String value();
}

The @ in front of the interface marks it as an annotation. Once you have defined the annotation you can use it in your code, as shown in the earlier examples.
The two directives in the annotation definition, @Retention(RetentionPolicy.RUNTIME) and@Target(ElementType.TYPE), specifies how the annotation is to be used.
@Retention(RetentionPolicy.RUNTIME) means that the annotation can be accessed via reflection at runtime. If you do not set this directive, the annotation will not be preserved at runtime, and thus not available via reflection.
@Target(ElementType.TYPE) means that the annotation can only be used ontop of types (classes and interfaces typically). You can also specify METHOD or FIELD, or you can leave the target out alltogether so the annotation can be used for both classes, methods and fields.

Class Annotations

You can access the annotations of a class, method or field at runtime. Here is an example that accesses the class annotations:

Class aClass = TheClass.class;
Annotation[] annotations = aClass.getAnnotations();

for(Annotation annotation : annotations){
    if(annotation instanceof MyAnnotation){
        MyAnnotation myAnnotation = (MyAnnotation) annotation;
        System.out.println("name: " + myAnnotation.name());
        System.out.println("value: " + myAnnotation.value());
    }
}

You can also access a specific class annotation like this:

Class aClass = TheClass.class;
Annotation annotation = aClass.getAnnotation(MyAnnotation.class);

if(annotation instanceof MyAnnotation){
    MyAnnotation myAnnotation = (MyAnnotation) annotation;
    System.out.println("name: " + myAnnotation.name());
    System.out.println("value: " + myAnnotation.value());
}

Method Annotations

Here is an example of a method with annotations:

public class TheClass {
  @MyAnnotation(name="someName",  value = "Hello World")
  public void doSomething(){}
}

You can access method annotations like this:

Method method = ... //obtain method object
Annotation[] annotations = method.getDeclaredAnnotations();

for(Annotation annotation : annotations){
    if(annotation instanceof MyAnnotation){
        MyAnnotation myAnnotation = (MyAnnotation) annotation;
        System.out.println("name: " + myAnnotation.name());
        System.out.println("value: " + myAnnotation.value());
    }
}

You can also access a specific method annotation like this:

Method method = ... // obtain method object
Annotation annotation = method.getAnnotation(MyAnnotation.class);

if(annotation instanceof MyAnnotation){
    MyAnnotation myAnnotation = (MyAnnotation) annotation;
    System.out.println("name: " + myAnnotation.name());
    System.out.println("value: " + myAnnotation.value());
}

Parameter Annotations

It is possible to add annotations to method parameter declarations too. Here is how that looks:

public class TheClass {
  public static void doSomethingElse(
        @MyAnnotation(name="aName", value="aValue") String parameter){
  }
}

You can access parameter annotations from the Method object like this:

Method method = ... //obtain method object
Annotation[][] parameterAnnotations = method.getParameterAnnotations();
Class[] parameterTypes = method.getParameterTypes();

int i=0;
for(Annotation[] annotations : parameterAnnotations){
  Class parameterType = parameterTypes[i++];

  for(Annotation annotation : annotations){
    if(annotation instanceof MyAnnotation){
        MyAnnotation myAnnotation = (MyAnnotation) annotation;
        System.out.println("param: " + parameterType.getName());
        System.out.println("name : " + myAnnotation.name());
        System.out.println("value: " + myAnnotation.value());
    }
  }
}

Notice how the Method.getParameterAnnotations() method returns a two-dimensionalAnnotation array, containing an array of annotations for each method parameter.

Field Annotations

Here is an example of a field with annotations:

public class TheClass {

  @MyAnnotation(name="someName",  value = "Hello World")
  public String myField = null;
}

You can access field annotations like this:

Field field = ... //obtain field object
Annotation[] annotations = field.getDeclaredAnnotations();

for(Annotation annotation : annotations){
    if(annotation instanceof MyAnnotation){
        MyAnnotation myAnnotation = (MyAnnotation) annotation;
        System.out.println("name: " + myAnnotation.name());
        System.out.println("value: " + myAnnotation.value());
    }
}

You can also access a specific field annotation like this:

Field field = ... // obtain method object
Annotation annotation = field.getAnnotation(MyAnnotation.class);

if(annotation instanceof MyAnnotation){
    MyAnnotation myAnnotation = (MyAnnotation) annotation;
    System.out.println("name: " + myAnnotation.name());
    System.out.println("value: " + myAnnotation.value());
}