Welcome!

Java IoT Authors: Elizabeth White, Carmen Gonzalez, Jyoti Bansal, Liz McMillan, Pat Romanski

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
    The Internet of Things can drive efficiency for airlines and airports. In their session at @ThingsExpo, Shyam Varan Nath, Principal Architect with GE, and Sudip Majumder, senior director of development at Oracle, discussed the technical details of the connected airline baggage and related social media solutions. These IoT applications will enhance travelers' journey experience and drive efficiency for the airlines and the airports.
    In 2014, Amazon announced a new form of compute called Lambda. We didn't know it at the time, but this represented a fundamental shift in what we expect from cloud computing. Now, all of the major cloud computing vendors want to take part in this disruptive technology. In his session at 20th Cloud Expo, John Jelinek IV, a web developer at Linux Academy, will discuss why major players like AWS, Microsoft Azure, IBM Bluemix, and Google Cloud Platform are all trying to sidestep VMs and containers...
    SYS-CON Events announced today that Catchpoint, a leading digital experience intelligence company, has been named “Silver Sponsor” of SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Catchpoint Systems is a leading Digital Performance Analytics company that provides unparalleled insight into your customer-critical services to help you consistently deliver an amazing customer experience. Designed for digital business, C...
    With major technology companies and startups seriously embracing IoT strategies, now is the perfect time to attend @ThingsExpo 2016 in New York. 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 June 6-8, 2017, at the Javits Center in New York City, New York, is co-located with 20th Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry p...
    In his General Session at 17th Cloud Expo, Bruce Swann, Senior Product Marketing Manager for Adobe Campaign, explored the key ingredients of cross-channel marketing in a digital world. Learn how the Adobe Marketing Cloud can help marketers embrace opportunities for personalized, relevant and real-time customer engagement across offline (direct mail, point of sale, call center) and digital (email, website, SMS, mobile apps, social networks, connected objects).
    Things are changing so quickly in IoT that it would take a wizard to predict which ecosystem will gain the most traction. In order for IoT to reach its potential, smart devices must be able to work together. Today, there are a slew of interoperability standards being promoted by big names to make this happen: HomeKit, Brillo and Alljoyn. In his session at @ThingsExpo, Adam Justice, vice president and general manager of Grid Connect, will review what happens when smart devices don’t work togethe...
    "There's a growing demand from users for things to be faster. When you think about all the transactions or interactions users will have with your product and everything that is between those transactions and interactions - what drives us at Catchpoint Systems is the idea to measure that and to analyze it," explained Leo Vasiliou, Director of Web Performance Engineering at Catchpoint Systems, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York Ci...
    The 20th International Cloud Expo has announced that its Call for Papers is open. Cloud Expo, to be held June 6-8, 2017, at the Javits Center in New York City, brings together Cloud Computing, Big Data, Internet of Things, DevOps, Containers, Microservices 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 ...
    "Tintri was started in 2008 with the express purpose of building a storage appliance that is ideal for virtualized environments. We support a lot of different hypervisor platforms from VMware to OpenStack to Hyper-V," explained Dan Florea, Director of Product Management at Tintri, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
    20th Cloud Expo, taking place June 6-8, 2017, at the Javits Center in New York City, NY, 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.
    SYS-CON Events announced today that Super Micro Computer, Inc., a global leader in Embedded and IoT solutions, will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 7-9, 2017, at the Javits Center in New York City, NY. Supermicro (NASDAQ: SMCI), the leading innovator in high-performance, high-efficiency server technology, is a premier provider of advanced server Building Block Solutions® for Data Center, Cloud Computing, Enterprise IT, Hadoop/Big Data, HPC and E...
    SYS-CON Events announced today that Linux Academy, the foremost online Linux and cloud training platform and community, will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Linux Academy was founded on the belief that providing high-quality, in-depth training should be available at an affordable price. Industry leaders in quality training, provided services, and student certification passes, its goal is to c...
    IoT is at the core or many Digital Transformation initiatives with the goal of re-inventing a company's business model. We all agree that collecting relevant IoT data will result in massive amounts of data needing to be stored. However, with the rapid development of IoT devices and ongoing business model transformation, we are not able to predict the volume and growth of IoT data. And with the lack of IoT history, traditional methods of IT and infrastructure planning based on the past do not app...
    Fact is, enterprises have significant legacy voice infrastructure that’s costly to replace with pure IP solutions. How can we bring this analog infrastructure into our shiny new cloud applications? There are proven methods to bind both legacy voice applications and traditional PSTN audio into cloud-based applications and services at a carrier scale. Some of the most successful implementations leverage WebRTC, WebSockets, SIP and other open source technologies. In his session at @ThingsExpo, Da...
    In the next five to ten years, millions, if not billions of things will become smarter. This smartness goes beyond connected things in our homes like the fridge, thermostat and fancy lighting, and into heavily regulated industries including aerospace, pharmaceutical/medical devices and energy. “Smartness” will embed itself within individual products that are part of our daily lives. We will engage with smart products - learning from them, informing them, and communicating with them. Smart produc...
    Why do your mobile transformations need to happen today? Mobile is the strategy that enterprise transformation centers on to drive customer engagement. In his general session at @ThingsExpo, Roger Woods, Director, Mobile Product & Strategy – Adobe Marketing Cloud, covered key IoT and mobile trends that are forcing mobile transformation, key components of a solid mobile strategy and explored how brands are effectively driving mobile change throughout the enterprise.
    Smart Cities are here to stay, but for their promise to be delivered, the data they produce must not be put in new siloes. In his session at @ThingsExpo, Mathias Herberts, Co-founder and CTO of Cityzen Data, discussed the best practices that will ensure a successful smart city journey.
    WebRTC sits at the intersection between VoIP and the Web. As such, it poses some interesting challenges for those developing services on top of it, but also for those who need to test and monitor these services. In his session at WebRTC Summit, Tsahi Levent-Levi, co-founder of testRTC, reviewed the various challenges posed by WebRTC when it comes to testing and monitoring and on ways to overcome them.
    For basic one-to-one voice or video calling solutions, WebRTC has proven to be a very powerful technology. Although WebRTC’s core functionality is to provide secure, real-time p2p media streaming, leveraging native platform features and server-side components brings up new communication capabilities for web and native mobile applications, allowing for advanced multi-user use cases such as video broadcasting, conferencing, and media recording.
    The best-practices for building IoT applications with Go Code that attendees can use to build their own IoT applications. In his session at @ThingsExpo, Indraneel Mitra, Senior Solutions Architect & Technology Evangelist at Cognizant, provided valuable information and resources for both novice and experienced developers on how to get started with IoT and Golang in a day. He also provided information on how to use Intel Arduino Kit, Go Robotics API and AWS IoT stack to build an application tha...