Java Architecture for XML Binding (JAXB)

Overview:
 - Java Architecture for XML Binding (JAXB) is a Java standard that defines how Java objects are converted to/from XML (specified using a standard set of mappings).

 - It defines a programmer API for reading and writing Java objects to / from XML documents and a service provider which / from from XML documents allows the selection of the JAXB implementation

 - It makes reading  and writing of XML via Java very easy.

 - It allows Java developers to access and process XML data without having to know XML or XML processing

 - It is used heavily by JAX-WS
 
 - It provides ways to generate XML content from a Java representation , to generate a Java representation from XML file , to generate XML schema from Java Objects

 - The JAXBContext class provides the client's entry point to the JAXB API. It provides an abstraction for managing the XML/Java binding information necessary to implement the JAXB binding framework operations: unmarshal, marshal and validate.

- A client application obtains new instances of this class via the newInstance(contextPath) method.            
          JAXBContext jc = JAXBContext.newInstance( "customer.class" );
The contextPath contains a list of Java package names that contain schema derived interfaces



Marshalling :

The Marshaller class provides the client application the ability to convert a Java content tree back into XML data. 

Here is a simple example that unmarshals an XML document and then marshals it back out:
          JAXBContext jc = JAXBContext.newInstance( "Customer.class" );
          Unmarshaller u = jc.createUnmarshaller(); 
          Customer customer = (Customer)u.unmarshal( new File( "test.xml" ) );
          Marshaller m = jc.createMarshaller(); m.marshal( customer, System.out );


Unmarshalling :

The Unmarshaller class provides the client application the ability to convert XML data into a tree of Java content objects. 

For example: 
       JAXBContext jc = JAXBContext.newInstance( "customer.class" ); 
       Unmarshaller u = jc.createUnmarshaller();
       Customer customer = (Customer)u.unmarshal( new File( "test.xml" ) ); // ok                                                                                                            
                                                                                          to be continued...

References:
http://docs.oracle.com/javase/6/docs/api/javax/xml/bind/JAXBContext.html
http://stackoverflow.com/questions/5964996/convert-java-object-to-xml
http://ooxs-be.goracer.nl/EN/java/Java%20and%20XML%20Binding.html

Tutorial:
https://github.com/abdulwaheed18/JAXB-Basic-Tutorial

OSGi for Beginners

Open Services Gateway initiative framework (OSGi)

1. What is OSGi ?

 
OSGi is a specification. The core of the OSGi specification defines a component and service model for Java. The components and services can be dynamically activated, de-activated, updated and de-installed.
A very practical advantage of OSGi is that every bundle must define its exported Java packages and its required dependencies. This way you can effectively control the provided API and the dependencies of your plug-ins.

OSGi bundles:

The OSGi specification defines the OSGi bundle as the unit of modularization.
A bundle is a cohesive, self-contained unit, which explicitly defines its dependencies to other modules and services. It also explicitly defines its external API.

Technically OSGi bundles are .jar files with additional meta information. This meta 

information is stored in the "META-INF" folder in the "MANIFEST.MF" file.

bundle = identity & dependency info + jar

The "MANIFEST.MF" file is part of a standard jar specification to which OSGi adds additional metadata. Any non-OSGi runtime will ignore the OSGi metadata. Therefore OSGi bundles can be used without restrictions in non-OSGi Java environments.

Bundle-SymbolicName and Version:

Each bundle has a symbolic name which is defined via the Bundle-SymbolicName property. and a version number in the Bundle-Version property.

The Bundle-Version and the Bundle-SymbolicName uniquely identifies a bundle in OSGi.Both properties are defined in the "MANIFEST.MF" file.
 

Bundle dependencies and public API:

Via the "MANIFEST.MF" file a bundle can define its dependency to other bundles or packages. OSGi will throw a ClassNotFoundException, if a class from a bundle tries to access a class without a defined dependency to it.
 

In the MANIFEST.MF file a bundle also defines the Java packages which should be exported and therefore available to other bundles (as API). Packages which are not exported are not visible to other bundles.

All these restrictions are enforced via a specific OSGi classloader. Each bundle has its own classloader. Access to restricted classes is not possible, also not via reflection.

A bundle can define that it depends on a certain version (or a range) of another bundle, e.g. bundle A can define that it depends on bundle C in version 2.0, while bundle B defines that it depends on version 1.0 of bundle C.

Bundle Lifecycle:

With the installation of a bundle in the OSGi runtime this bundle is persisted in a local bundle cache. The OSGi runtime is then trying to resolve all dependencies of the bundle.
If all required dependencies are resolved the bundle is in the status "RESOLVED" otherwise it is in the status "INSTALLED".
If several bundles exist which would satisfy the dependency, then the bundle with the highest version is taking. If the versions are the same, then the bundle with the lowest ID will be taken. If the bundle is started, its status is "STARTING". Afterwards it gets the "ACTIVE" status.


OSGi offers the following advantages:

Reduced complexity: Developing OSGi means developing modules ie bundles. They hide their internals from other bundles and the communication is through well defined interfaces.Hence the later changes can be easily accommodated without affecting other modules.

Reuse : The OSGi component model makes the integration of third party components very easy.

You can install, uninstall, start, and stop different modules of your application dynamically without restarting the container.

Your application can have more than one version of a particular module running at the same time.

OSGi provides very good infrastructure for developing service-oriented applications, as well as embedded, mobile, and rich internet apps. 

Some of the currently popular implementations of the OSGi specs :

Apache Felix:
Eclipse Equinox
Knopflerfish

Popular application servers which use OSGi technology :

Weblogic – Oracle Weblogic Application Server
WebSphere – IBM Websphere JEE Application Server

REFERENCE:

http://www.osgi.org
http://www.eclipse.org/equinox
http://felix.apache.org

Java Reflection: 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. The annotation can be attached to Classes, Methods,Parameters,Fields etc.

How to create custom Annotation?

To create an annotation we use the interface keyword and add an @ symbol infront of it. The @ symbol will tell the compiler that it doing some business with an annotation.

@Retention(RetentionPolicy.RUNTIME)

public @interface MyAnnotation {

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.Here is an example of class annotation:

@MyAnnotation(value="Class Annotation")
public class MyClass {
}

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

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

You can also access a specific class annotation like this:

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

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

Method Annotations

Here is an example of a method with annotations:

public class MyClass {
  @MyAnnotation(value = "Method Annotation")
  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("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("value: " + myAnnotation.value());
}

Parameter Annotations

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

public class MyClass {
  public static void doSomethingElse(
        @MyAnnotation(value="Parameter Annotation") 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("value: " + myAnnotation.value());
    }
  }
}

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

Field Annotations

Here is an example of a field with annotations:

public class MyClass {
  @MyAnnotation(value = "Field Annotation")
  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("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("value: " + myAnnotation.value());
}

The whole tutorial is on the github:
https://github.com/abdulwaheed18/Annotation-Tutorial