Click here to close now.

Welcome!

Java Authors: Liz McMillan, Carmen Gonzalez, Elizabeth White, Yeshim Deniz, Navrup Johal

Related Topics: Java

Java: 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
    SYS-CON Events announced today that Ciqada will exhibit at SYS-CON's @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Ciqada™ makes it easy to connect your products to the Internet. By integrating key components - hardware, servers, dashboards, and mobile apps - into an easy-to-use, configurable system, your products can quickly and securely join the internet of things. With remote monitoring, control, and alert messaging capability, you will meet your customers' needs of tomorrow - today! Ciqada. Let your products take flight. For more inform...
    WebRTC is an up-and-coming standard that enables real-time voice and video to be directly embedded into browsers making the browser a primary user interface for communications and collaboration. WebRTC runs in a number of browsers today and is currently supported in over a billion installed browsers globally, across a range of platform OS and devices. Today, organizations that choose to deploy WebRTC applications and use a host machine that supports audio through USB or Bluetooth can use Plantronics products to connect and transit or receive the audio associated with the WebRTC session.
    The best mobile applications are augmented by dedicated servers, the Internet and Cloud services. Mobile developers should focus on one thing: writing the next socially disruptive viral app. Thanks to the cloud, they can focus on the overall solution, not the underlying plumbing. From iOS to Android and Windows, developers can leverage cloud services to create a common cross-platform backend to persist user settings, app data, broadcast notifications, run jobs, etc. This session provides a high level technical overview of many cloud services available to mobile app developers, includi...
    SYS-CON Media announced today that @WebRTCSummit Blog, the largest WebRTC resource in the world, has been launched. @WebRTCSummit Blog offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. @WebRTCSummit Blog can be bookmarked ▸ Here @WebRTCSummit conference site can be bookmarked ▸ Here
    There are lots of challenges in IoT around secure, scalable and business friendly infrastructure for enterprises. For large corporations, IoT implementations are one of the top priorities of the decade. All industries are seeing a competitive need to sustain by investing in IoT initiatives. The value addition comes from improved customer service, innovative product and additional revenue streams. The data from these IP-connected devices can be leveraged for a variety of business applications as well as responsive action controls. The various architectural building blocks of an IoT ...
    Health care systems across the globe are under enormous strain, as facilities reach capacity and costs continue to rise. M2M and the Internet of Things have the potential to transform the industry through connected health solutions that can make care more efficient while reducing costs. In fact, Vodafone's annual M2M Barometer Report forecasts M2M applications rising to 57 percent in health care and life sciences by 2016. Lively is one of Vodafone's health care partners, whose solutions enable older adults to live independent lives while staying connected to loved ones. M2M will continue to gr...
    Chuck Piluso will present a study of cloud adoption trends and the power and flexibility of IBM Power and Pureflex cloud solutions. Speaker Bio: Prior to Data Storage Corporation (DSC), Mr. Piluso founded North American Telecommunication Corporation, a facilities-based Competitive Local Exchange Carrier licensed by the Public Service Commission in 10 states, serving as the company's chairman and president from 1997 to 2000. Between 1990 and 1997, Mr. Piluso served as chairman & founder of International Telecommunications Corporation, a facilities-based international carrier licensed by t...
    Dave will share his insights on how Internet of Things for Enterprises are transforming and making more productive and efficient operations and maintenance (O&M) procedures in the cleantech industry and beyond. Speaker Bio: Dave Landa is chief operating officer of Cybozu Corp (kintone US). Based in the San Francisco Bay Area, Dave has been on the forefront of the Cloud revolution driving strategic business development on the executive teams of multiple leading Software as a Services (SaaS) application providers dating back to 2004. Cybozu's kintone.com is a leading global BYOA (Build Your O...
    As enterprises move to all-IP networks and cloud-based applications, communications service providers (CSPs) – facing increased competition from over-the-top providers delivering content via the Internet and independently of CSPs – must be able to offer seamless cloud-based communication and collaboration solutions that can scale for small, midsize, and large enterprises, as well as public sector organizations, in order to keep and grow market share. The latest version of Oracle Communications Unified Communications Suite gives CSPs the capability to do just that. In addition, its integration ...
    The IoT Bootcamp is coming to Cloud Expo | @ThingsExpo on June 9-10 at the Javits Center in New York. Instructor. Registration is now available at http://iotbootcamp.sys-con.com/ Instructor Janakiram MSV previously taught the famously successful Multi-Cloud Bootcamp at Cloud Expo | @ThingsExpo in November in Santa Clara. Now he is expanding the focus to Janakiram is the founder and CTO of Get Cloud Ready Consulting, a niche Cloud Migration and Cloud Operations firm that recently got acquired by Aditi Technologies. He is a Microsoft Regional Director for Hyderabad, India, and one of the f...
    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!
    While not quite mainstream yet, WebRTC is starting to gain ground with Carriers, Enterprises and Independent Software Vendors (ISV’s) alike. WebRTC makes it easy for developers to add audio and video communications into their applications by using Web browsers as their platform. But like any market, every customer engagement has unique requirements, as well as constraints. And of course, one size does not fit all. In her session at WebRTC Summit, Dr. Natasha Tamaskar, Vice President, Head of Cloud and Mobile Strategy at GENBAND, will explore what is needed to take a real time communications ...
    SYS-CON Media announced today that @ThingsExpo Blog launched with 7,788 original stories. @ThingsExpo Blog offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. @ThingsExpo Blog can be bookmarked. The Internet of Things (IoT) is the most profound change in personal and enterprise IT since the creation of the Worldwide Web more than 20 years ago.
    The world's leading Cloud event, Cloud Expo has launched Microservices Journal on the SYS-CON.com portal, featuring over 19,000 original articles, news stories, features, and blog entries. DevOps Journal is focused on this critical enterprise IT topic in the world of cloud computing. Microservices Journal offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. Follow new article posts on Twitter at @MicroservicesE
    SYS-CON Events announced today that robomq.io will exhibit at SYS-CON's @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. robomq.io is an interoperable and composable platform that connects any device to any application. It helps systems integrators and the solution providers build new and innovative products and service for industries requiring monitoring or intelligence from devices and sensors.
    Containers and microservices have become topics of intense interest throughout the cloud developer and enterprise IT communities. Accordingly, attendees at the upcoming 16th Cloud Expo at the Javits Center in New York June 9-11 will find fresh new content in a new track called PaaS | Containers & Microservices Containers are not being considered for the first time by the cloud community, but a current era of re-consideration has pushed them to the top of the cloud agenda. With the launch of Docker's initial release in March of 2013, interest was revved up several notches. Then late last...
    Wearable technology was dominant at this year’s International Consumer Electronics Show (CES) , and MWC was no exception to this trend. New versions of favorites, such as the Samsung Gear (three new products were released: the Gear 2, the Gear 2 Neo and the Gear Fit), shared the limelight with new wearables like Pebble Time Steel (the new premium version of the company’s previously released smartwatch) and the LG Watch Urbane. The most dramatic difference at MWC was an emphasis on presenting wearables as fashion accessories and moving away from the original clunky technology associated with t...
    SYS-CON Events announced today that Litmus Automation will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Litmus Automation’s vision is to provide a solution for companies that are in a rush to embrace the disruptive Internet of Things technology and leverage it for real business challenges. Litmus Automation simplifies the complexity of connected devices applications with Loop, a secure and scalable cloud platform.
    In 2015, 4.9 billion connected "things" will be in use. By 2020, Gartner forecasts this amount to be 25 billion, a 410 percent increase in just five years. How will businesses handle this rapid growth of data? Hadoop will continue to improve its technology to meet business demands, by enabling businesses to access/analyze data in real time, when and where they need it. Cloudera's Chief Technologist, Eli Collins, will discuss how Big Data is keeping up with today's data demands and how in the future, data and analytics will be pervasive, embedded into every workflow, application and infra...
    From telemedicine to smart cars, digital homes and industrial monitoring, the explosive growth of IoT has created exciting new business opportunities for real time calls and messaging. In his session at @ThingsExpo, Ivelin Ivanov, CEO and Co-Founder of Telestax, shared some of the new revenue sources that IoT created for Restcomm – the open source telephony platform from Telestax. Ivelin Ivanov is a technology entrepreneur who founded Mobicents, an Open Source VoIP Platform, to help create, deploy, and manage applications integrating voice, video and data. He is the co-founder of TeleStax, a...