Welcome!

Java IoT Authors: Yeshim Deniz, Liz McMillan, Mano Marks, Elizabeth White, 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
    Bert Loomis was a visionary. This general session will highlight how Bert Loomis and people like him inspire us to build great things with small inventions. In their general session at 19th Cloud Expo, Harold Hannon, Architect at IBM Bluemix, and Michael O'Neill, Strategic Business Development at Nvidia, discussed the accelerating pace of AI development and how IBM Cloud and NVIDIA are partnering to bring AI capabilities to "every day," on-demand. They also reviewed two "free infrastructure" pr...
    In his keynote at @ThingsExpo, Chris Matthieu, Director of IoT Engineering at Citrix and co-founder and CTO of Octoblu, focused on building an IoT platform and company. He provided a behind-the-scenes look at Octoblu’s platform, business, and pivots along the way (including the Citrix acquisition of Octoblu).
    Data is an unusual currency; it is not restricted by the same transactional limitations as money or people. In fact, the more that you leverage your data across multiple business use cases, the more valuable it becomes to the organization. And the same can be said about the organization’s analytics. In his session at 19th Cloud Expo, Bill Schmarzo, CTO for the Big Data Practice at Dell EMC, introduced a methodology for capturing, enriching and sharing data (and analytics) across the organization...
    The explosion of new web/cloud/IoT-based applications and the data they generate are transforming our world right before our eyes. In this rush to adopt these new technologies, organizations are often ignoring fundamental questions concerning who owns the data and failing to ask for permission to conduct invasive surveillance of their customers. Organizations that are not transparent about how their systems gather data telemetry without offering shared data ownership risk product rejection, regu...
    Grape Up is a software company, specialized in cloud native application development and professional services related to Cloud Foundry PaaS. With five expert teams that operate in various sectors of the market across the USA and Europe, we work with a variety of customers from emerging startups to Fortune 1000 companies.
    Financial Technology has become a topic of intense interest throughout the cloud developer and enterprise IT communities. Accordingly, attendees at the upcoming 20th Cloud Expo at the Javits Center in New York, June 6-8, 2017, will find fresh new content in a new track called FinTech.
    SYS-CON Events announced today that Interoute, owner-operator of one of Europe's largest networks and a global cloud services platform, has been named “Bronze Sponsor” of SYS-CON's 20th Cloud Expo, which will take place on June 6-8, 2017 at the Javits Center in New York, New York. Interoute is the owner-operator of one of Europe's largest networks and a global cloud services platform which encompasses 12 data centers, 14 virtual data centers and 31 colocation centers, with connections to 195 add...
    Multiple data types are pouring into IoT deployments. Data is coming in small packages as well as enormous files and data streams of many sizes. Widespread use of mobile devices adds to the total. In this power panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, panelists will look at the tools and environments that are being put to use in IoT deployments, as well as the team skills a modern enterprise IT shop needs to keep things running, get a handle on all this data, and deli...
    The age of Digital Disruption is evolving into the next era – Digital Cohesion, an age in which applications securely self-assemble and deliver predictive services that continuously adapt to user behavior. Information from devices, sensors and applications around us will drive services seamlessly across mobile and fixed devices/infrastructure. This evolution is happening now in software defined services and secure networking. Four key drivers – Performance, Economics, Interoperability and Trust ...
    The Internet of Things is clearly many things: data collection and analytics, wearables, Smart Grids and Smart Cities, the Industrial Internet, and more. Cool platforms like Arduino, Raspberry Pi, Intel's Galileo and Edison, and a diverse world of sensors are making the IoT a great toy box for developers in all these areas. In this Power Panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, panelists discussed what things are the most important, which will have the most profound e...
    @ThingsExpo has been named the Most Influential ‘Smart Cities - IIoT' Account and @BigDataExpo has been named fourteenth by Right Relevance (RR), which provides curated information and intelligence on approximately 50,000 topics. In addition, Right Relevance provides an Insights offering that combines the above Topics and Influencers information with real time conversations to provide actionable intelligence with visualizations to enable decision making. The Insights service is applicable to eve...
    SYS-CON Events announced today that Grape Up will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct. 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Grape Up is a software company specializing in cloud native application development and professional services related to Cloud Foundry PaaS. With five expert teams that operate in various sectors of the market across the U.S. and Europe, Grape Up works with a variety of customers from emergi...
    SYS-CON Events announced today that Hitachi, the leading provider the Internet of Things and Digital Transformation, 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. Hitachi Data Systems, a wholly owned subsidiary of Hitachi, Ltd., offers an integrated portfolio of services and solutions that enable digital transformation through enhanced data management, governance, mobility and analytics. We help globa...
    SYS-CON Events announced today that SoftLayer, an IBM Company, has been named “Gold Sponsor” of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2016, at the Javits Center in New York, New York. SoftLayer, an IBM Company, provides cloud infrastructure as a service from a growing number of data centers and network points of presence around the world. SoftLayer’s customers range from Web startups to global enterprises.
    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 compute, storage and networking technologies, 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. 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/...
    Amazon has gradually rolled out parts of its IoT offerings in the last year, but these are just the tip of the iceberg. In addition to optimizing their back-end AWS offerings, Amazon is laying the ground work to be a major force in IoT – especially in the connected home and office. Amazon is extending its reach by building on its dominant Cloud IoT platform, its Dash Button strategy, recently announced Replenishment Services, the Echo/Alexa voice recognition control platform, the 6-7 strategic...
    Judith Hurwitz is president and CEO of Hurwitz & Associates, a Needham, Mass., research and consulting firm focused on emerging technology, including big data, cognitive computing and governance. She is co-author of the book Cognitive Computing and Big Data Analytics, published in 2015. Her Cloud Expo session, "What Is the Business Imperative for Cognitive Computing?" is scheduled for Wednesday, June 8, at 8:40 a.m. In it, she puts cognitive computing into perspective with its value to the busin...
    Cognitive Computing is becoming the foundation for a new generation of solutions that have the potential to transform business. Unlike traditional approaches to building solutions, a cognitive computing approach allows the data to help determine the way applications are designed. This contrasts with conventional software development that begins with defining logic based on the current way a business operates. In her session at 18th Cloud Expo, Judith S. Hurwitz, President and CEO of Hurwitz & ...
    Cybersecurity is a critical component of software development in many industries including medical devices. However, code is not always written to be robust or secure from the unknown or the unexpected. This gap can make medical devices susceptible to cybersecurity attacks ranging from compromised personal health information to life-sustaining treatment. In his session at @ThingsExpo, Clark Fortney, Software Engineer at Battelle, will discuss how programming oversight using key methods can incre...