Click here to close now.




















Welcome!

Java IoT Authors: Elizabeth White, Carmen Gonzalez, Liz McMillan, Pat Romanski, Bob Gourley

Related Topics: Java IoT

Java IoT: Article

Reflection & Introspection: Objects Exposed

Reflection & Introspection: Objects Exposed

One of the salient aspects of the Java language is the control it gives to developers for dynamically generating and reusing code. This allows the language to offer Java programmers the ability to write code in which the actual behavior is determined at runtime. Of the eleven buzzwords used to define Java, this article is going to focus on the dynamic nature of the Java programming language.

One of the salient aspects of the Java language is the control it gives to developers for dynamically generating and reusing code. This allows the language to offer Java programmers the ability to write code in which the actual behavior is determined at runtime. Of the eleven buzzwords used to define Java, this article is going to focus on the dynamic nature of the Java programming language.Introspection Uses Reflection
Reflection and introspection are very closely related. Reflection is a low-level facility that allows the code to examine the internals of any class or object at runtime. Introspection builds on this facility and provides a more convenient interface for examining Beans. In fact, the relationship between reflection and introspection is very similar to the relationship between JavaBeans and other Java classes. JavaBeans are simply normal Java objects with certain design patterns enforced in their nomenclature. Introspection assumes these design patterns on the object that it is inspecting and uses low-level reflection to examine the object's internals.

The Reflection API
The Reflection API became a part of core Java with release 1.1 of the JDK. The API is defined across the following:

  • The new methods added to the java.lang.Class class in JDK 1.1
  • The java.lang.reflect package defined in JDK1.1

    The class java.lang.Class contains methods that return instances of classes/interfaces defined in the java.lang.reflect package. A detailed description of the API is beyond the scope of this article and can be found in any standard Java text. However, the classes that comprise the Reflection API are listed in Table 1.

    The Introspection API
    The Introspection API consists of several classes in the java.beans package. Again, a detailed description of the API is beyond the scope of this article and can be found in any standard Java text. The main classes in the Introspection API are listed in Table 2.

    The Costs of Usage
    Reflection and Introspection are powerful tools that contribute to the flexibility provided by the Java language. However, these APIs should be used only as needed and after taking into account the costs associated with their usage:

  • Reflection and Introspection method calls have a substantial performance overhead.
  • Using reflection makes the code much more complex and harder to understand than using direct method calls.
  • Errors in method invocation are discovered at runtime instead of being caught by the compiler.
  • The code becomes type-unsafe.

    The Reflection and Introspection APIs should be used only when other forms of object-oriented programming are not appropriate.

    The following examples demonstrate the use of Reflection and Introspection to develop some useful Java utilities.

    Cookie Factory
    Our first example illustrates the use of Reflection to build a utility that allows us to instantiate objects of types derived from a "Cookie" interface. The actual type of the object instantiated is determined by a String parameter, which contains the name of the actual class. The code for the example is shown in Listings 1and 2.

    Listing 1 defines the Cookie interface and the derived classes. The Cookie interface is simply a marker interface which is implemented by the classes FortuneCookie and MisFortuneCookie. Both these classes define a single static method which prints out a string and returns a new instance of the respective class.

    Listing 2 shows the CookieFactory class which is capable of producing objects derived from the "Cookie" interface. It defines a single method createCookie that takes a String parameter, className. The Class corresponding to this name is obtained from the Class class by calling

    c = Class.forName(className);

    Once we have the class, we need to obtain the method to be called on it. The name of the method is "newCookie." In this example, we are assuming that the name of the method is available at this point. The parameter types for the method are filled in an array of type Class and this is used to get the actual Method object as follows:

    method = c.getMethod("newCookie", pTypes);

    Once the Method object is available, the static method is invoked on the class after constructing an array of Objects that contains the actual parameter instances:

    cookie = (Cookie)(method.invoke(c, params));

    A simple tester for the class is provided in the main() method. This first constructs the CookieFactory and then creates instances of the FortuneCookie and MisFortuneCookie class. The output from the program is shown in Figure 3.

    An X-Ray Class
    Our second example illustrates the use of reflection to build a utility that allows us to view all the methods, constructors, fields, interfaces and inheritance for a supplied class. The class being X-rayed is specified by a String parameter which contains the name of the actual class. The code for the example is shown in Listing 4.

    In order for the X-ray class program to determine the methods, constructors, fields and interfaces contained in the requested class, it must instantiate an object of the class by calling:

    c = Class.forName(className);

    After instantiating the class, the utility determines the selected operation on that class based on a second user-supplied string parameter, which can have one of the following values:

    localMethods
    allMethods
    Constructors
    fields
    interface
    inheritance

    The local methods contained in the specified class may be found by calling:

    methodList = c.getDeclaredMethods();

    This call returns a Method[] that contains all the methods declared in the local class including private, protected and public. This call excludes inherited methods. A list of all the public methods, both inherited and local, may be obtained by calling:

    methodList = c.getMethods();

    Constructor methods are not included in the return Method[] of this call. Retrieving a list of constructors for a specific class can be accomplished by calling:

    constructorList = c.getDeclaredConstructors();

    This call returns a constructor[] that contains all of the private, protected and public constructors declared on the local class. A list that includes only the public constructors can be constructed by calling:

    constructorList = c.getConstructors();

    Class variables can be retrieved as Field[] information. To access a complete list of fields from a class including private, protected and public, we call:

    fieldList = c.getDeclaredFields();A list that includes only the public fields can be retrieved by calling:

    fieldList = c.getFields();

    Information concerning the interfaces implemented by a class can be accessed by calling:

    interfaceList = c.getInterfaces();

    This call returns a Class[] that contains all of the interfaces implemented by the local class. Notice that this call doesn't have a getDeclaredInterfaces() counterpart like the other methods.

    To access the inheritance information in the class, we get the name of each one of the superclasses in the inheritance tree. This is done in a while loop by calling:

    classRef = c.getSuperclass();

    We use this mechanism to return a class[] with all of the classes that participate in the extension of the local class. The output of the program for obtaining the inheritance hierarchy of the java.applet.Applet class is given in Figure 4.

    Notice that of the various methods presented on this section, the only method that recursively provided information contained in its inheritance tree was the c.getMethods() call. The other methods only provided information contained by the local class.

    An X-Ray Bean
    Our third example illustrates the use of Introspection to build a utility that allows us to view all the methods, properties and events for a supplied JavaBean class. The Bean being X-rayed is specified by a String parameter which contains the name of the actual Bean class. The code for the example is shown in Listing 3.

    In order for the X-ray Bean program to determine the methods, properties and events contained in the requested class, it must instantiate an object of the class by calling:

    c = Class.forName(className);

    After instantiating the class, the utility must access the BeanInfo for the instantiated Bean. BeanInfo data can be accessed via the Introspector class by calling:

    bi = Introspector.getBeanInfo(c);

    Next, the utility determines the selected operation on that class based on the second user-supplied parameter entered at the command line (i.e., methods, properties and events). The localMethods contained by the specified Bean can be found by calling:

    methodDescriptorList = bi.getMethodDescriptors();

    This call returns a MethodDescriptor[] that contains a description of all of the methods contained by this Bean. The type of method descriptor returned by this call contains a complete list of all the public methods contained within the inheritance tree of this Bean. In order to access the actual method instances, we need to iterate through the methodDescriptionList and obtain the method by calling:

    methodRef = methodDescriptorList[i].getMethod();

    From each one of these values, we are able to build a Method[] list that can be displayed by the utility. This includes no constructor method information. To access the Bean constructor information, you must use reflection.

    Bean properties can be retrieved via PropertyDescriptors by calling:

    PropertyDescriptorList = bi.getPropertyDescriptors();

    This call returns a PropertyDescriptor[] that contains a description of all of the properties contained by this Bean. This includes name, readMethod, writeMethod, type, EditorClass, etc. Our utility uses this information to get the readMethod, writeMethod and property name via the PropertyDescriptor superclass (i.e., FeatureDescriptor) by calling:

    methodRef = PropertyDescriptorList[i].getReadMethod();
    methodRef = PropertyDescriptorList[i].getWriteMethod();
    propertyName = PropertyDescriptorList[i].getName();

    Bean events can be retrieved via EventSetDescriptors by calling:

    eventSetDescriptorList = bi.getEventSetDescriptors();

    This call returns an EventSetDescriptor[] that contains a description of all the methods associated with each event for this Bean. Our utility uses this information to get a Method[] for each one of the returned events in the eventSetDescriptorList. This is accomplished by calling:

    methodList = eventSetDescriptorList[i].getListenerMethods();

    This information is used to identify the methods associated to each one of the listener methods.

    Information concerning the interfaces implemented by a class can be accessed by calling:

    interfaceList = c.getInterfaces();

    This call returns a class[] that contains all the interfaces implemented by the local class. Notice that this call doesn't have a getDeclaredInterfaces() counterpart like the other methods.

    The output from the program for examining the events in the class, com.sun.swing.Jpanel is shown in Figure 5.

    Conclusion
    In this article, we took a look at the Reflection and Introspection APIs and used them to develop several useful utilities for Java development. In our next article, we will use the concepts and utilities introduced here to develop a new category of dynamically generated adapters called Dynamic Adapters.

    The traditional Adapter design pattern is defined as follows: Adapter: "Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces." [Design Patterns: Elements of Resuable Object-Oriented Software, Gamma et. al., Addison Wesley, 1995.]

    Adapters are used when the input and output interfaces are known at compile-time. Dynamic Adapters will allow a program to dynamically map the interfaces at runtime. We will examine these patterns in more detail in the next article.

  • More Stories By Ajit Sagar

    Ajit Sagar is Associate VP, Digital Transformation Practice at Infosys Limited. A seasoned IT executive with 20+ years experience across various facts of the industry including consulting, business development, architecture and design he is architecture consulting and delivery lead for Infosys's Digital Transformation practice. He was also the Founding Editor of XML Journal and Chief Editor of Java Developer's Journal.

    Comments (0)

    Share your thoughts on this story.

    Add your comment
    You must be signed in to add a comment. Sign-in | Register

    In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


    @ThingsExpo Stories
    Akana has announced the availability of the new Akana Healthcare Solution. The API-driven solution helps healthcare organizations accelerate their transition to being secure, digitally interoperable businesses. It leverages the Health Level Seven International Fast Healthcare Interoperability Resources (HL7 FHIR) standard to enable broader business use of medical data. Akana developed the Healthcare Solution in response to healthcare businesses that want to increase electronic, multi-device access to health records while reducing operating costs and complying with government regulations.
    SYS-CON Events announced today that the "Second Containers & Microservices Expo" will take place November 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. Containers and microservices have become topics of intense interest throughout the cloud developer and enterprise IT communities.
    The 17th International Cloud Expo has announced that its Call for Papers is open. 17th International Cloud Expo, to be held November 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, brings together Cloud Computing, APM, APIs, Microservices, Security, Big Data, Internet of Things, DevOps and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportunity. Submit your speaking proposal today!
    SYS-CON Events announced today that IceWarp will exhibit at the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. IceWarp, the leader of cloud and on-premise messaging, delivers secured email, chat, documents, conferencing and collaboration to today's mobile workforce, all in one unified interface
    SYS-CON Events announced today that Micron Technology, Inc., a global leader in advanced semiconductor systems, will exhibit at the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. Micron’s broad portfolio of high-performance memory technologies – including DRAM, NAND and NOR Flash – is the basis for solid state drives, modules, multichip packages and other system solutions. Backed by more than 35 years of technology leadership, Micron's memory solutions enable the world's most innovative computing, consumer,...
    SYS-CON Events announced today the Containers & Microservices Bootcamp, being held November 3-4, 2015, in conjunction with 17th Cloud Expo, @ThingsExpo, and @DevOpsSummit at the Santa Clara Convention Center in Santa Clara, CA. This is your chance to get started with the latest technology in the industry. Combined with real-world scenarios and use cases, the Containers and Microservices Bootcamp, led by Janakiram MSV, a Microsoft Regional Director, will include presentations as well as hands-on demos and comprehensive walkthroughs.
    In his session at @ThingsExpo, Lee Williams, a producer of the first smartphones and tablets, will talk about how he is now applying his experience in mobile technology to the design and development of the next generation of Environmental and Sustainability Services at ETwater. He will explain how M2M controllers work through wirelessly connected remote controls; and specifically delve into a retrofit option that reverse-engineers control codes of existing conventional controller systems so they don't have to be replaced and are instantly converted to become smart, connected devices.
    SYS-CON Events announced today that Pythian, a global IT services company specializing in helping companies leverage disruptive technologies to optimize revenue-generating systems, has been named “Bronze Sponsor” of SYS-CON's 17th Cloud Expo, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. Founded in 1997, Pythian is a global IT services company that helps companies compete by adopting disruptive technologies such as cloud, Big Data, advanced analytics, and DevOps to advance innovation and increase agility. Specializing in designing, imple...
    With the Apple Watch making its way onto wrists all over the world, it’s only a matter of time before it becomes a staple in the workplace. In fact, Forrester reported that 68 percent of technology and business decision-makers characterize wearables as a top priority for 2015. Recognizing their business value early on, FinancialForce.com was the first to bring ERP to wearables, helping streamline communication across front and back office functions. In his session at @ThingsExpo, Kevin Roberts, GM of Platform at FinancialForce.com, will discuss the value of business applications on wearable ...
    17th Cloud Expo, taking place Nov 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud strategy. Meanwhile, 94% of enterprises are using some form of XaaS – software, platform, and infrastructure as a service.
    The 3rd International WebRTC Summit, to be held Nov. 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA, announces that its Call for Papers is now open. Topics include all aspects of improving IT delivery by eliminating waste through automated business models leveraging cloud technologies. WebRTC Summit is co-located with 15th International Cloud Expo, 6th International Big Data Expo, 3rd International DevOps Summit and 2nd Internet of @ThingsExpo. WebRTC (Web-based Real-Time Communication) is an open source project supported by Google, Mozilla and Opera that aims to enable bro...
    WebRTC services have already permeated corporate communications in the form of videoconferencing solutions. However, WebRTC has the potential of going beyond and catalyzing a new class of services providing more than calls with capabilities such as mass-scale real-time media broadcasting, enriched and augmented video, person-to-machine and machine-to-machine communications. In his session at @ThingsExpo, Luis Lopez, CEO of Kurento, will introduce the technologies required for implementing these ideas and some early experiments performed in the Kurento open source software community in areas ...
    Consumer IoT applications provide data about the user that just doesn’t exist in traditional PC or mobile web applications. This rich data, or “context,” enables the highly personalized consumer experiences that characterize many consumer IoT apps. This same data is also providing brands with unprecedented insight into how their connected products are being used, while, at the same time, powering highly targeted engagement and marketing opportunities. In his session at @ThingsExpo, Nathan Treloar, President and COO of Bebaio, will explore examples of brands transforming their businesses by t...
    SYS-CON Events announced today that HPM Networks will exhibit at the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. For 20 years, HPM Networks has been integrating technology solutions that solve complex business challenges. HPM Networks has designed solutions for both SMB and enterprise customers throughout the San Francisco Bay Area.
    While many app developers are comfortable building apps for the smartphone, there is a whole new world out there. In his session at @ThingsExpo, Narayan Sainaney, Co-founder and CTO of Mojio, will discuss how the business case for connected car apps is growing and, with open platform companies having already done the heavy lifting, there really is no barrier to entry.
    Too often with compelling new technologies market participants become overly enamored with that attractiveness of the technology and neglect underlying business drivers. This tendency, what some call the “newest shiny object syndrome,” is understandable given that virtually all of us are heavily engaged in technology. But it is also mistaken. Without concrete business cases driving its deployment, IoT, like many other technologies before it, will fade into obscurity.
    With major technology companies and startups seriously embracing IoT strategies, now is the perfect time to attend @ThingsExpo in Silicon Valley. Learn what is going on, contribute to the discussions, and ensure that your enterprise is as "IoT-Ready" as it can be! Internet of @ThingsExpo, taking place Nov 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 17th Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. The Internet of Things (IoT) is the most profound change in personal an...
    With the proliferation of connected devices underpinning new Internet of Things systems, Brandon Schulz, Director of Luxoft IoT – Retail, will be looking at the transformation of the retail customer experience in brick and mortar stores in his session at @ThingsExpo. Questions he will address include: Will beacons drop to the wayside like QR codes, or be a proximity-based profit driver? How will the customer experience change in stores of all types when everything can be instrumented and analyzed? As an area of investment, how might a retail company move towards an innovation methodolo...
    As more intelligent IoT applications shift into gear, they’re merging into the ever-increasing traffic flow of the Internet. It won’t be long before we experience bottlenecks, as IoT traffic peaks during rush hours. Organizations that are unprepared will find themselves by the side of the road unable to cross back into the fast lane. As billions of new devices begin to communicate and exchange data – will your infrastructure be scalable enough to handle this new interconnected world?
    The Internet of Things is in the early stages of mainstream deployment but it promises to unlock value and rapidly transform how organizations manage, operationalize, and monetize their assets. IoT is a complex structure of hardware, sensors, applications, analytics and devices that need to be able to communicate geographically and across all functions. Once the data is collected from numerous endpoints, the challenge then becomes converting it into actionable insight.