Welcome!

Java IoT Authors: Elizabeth White, Pat Romanski, Liz McMillan, William Schmarzo, Stackify Blog

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
    SYS-CON Events announced today that N3N will exhibit at SYS-CON's @ThingsExpo, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. N3N’s solutions increase the effectiveness of operations and control centers, increase the value of IoT investments, and facilitate real-time operational decision making. N3N enables operations teams with a four dimensional digital “big board” that consolidates real-time live video feeds alongside IoT sensor data a...
    Mobile device usage has increased exponentially during the past several years, as consumers rely on handhelds for everything from news and weather to banking and purchases. What can we expect in the next few years? The way in which we interact with our devices will fundamentally change, as businesses leverage Artificial Intelligence. We already see this taking shape as businesses leverage AI for cost savings and customer responsiveness. This trend will continue, as AI is used for more sophistica...
    Real IoT production deployments running at scale are collecting sensor data from hundreds / thousands / millions of devices. The goal is to take business-critical actions on the real-time data and find insights from stored datasets. In his session at @ThingsExpo, John Walicki, Watson IoT Developer Advocate at IBM Cloud, will provide a fast-paced developer journey that follows the IoT sensor data from generation, to edge gateway, to edge analytics, to encryption, to the IBM Bluemix cloud, to Wa...
    What is the best strategy for selecting the right offshore company for your business? In his session at 21st Cloud Expo, Alan Winters, U.S. Head of Business Development at MobiDev, will discuss the things to look for - positive and negative - in evaluating your options. He will also discuss how to maximize productivity with your offshore developers. Before you start your search, clearly understand your business needs and how that impacts software choices.
    SYS-CON Events announced today that Fusic will exhibit at the Japan External Trade Organization (JETRO) Pavilion 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. Fusic Co. provides mocks as virtual IoT devices. You can customize mocks, and get any amount of data at any time in your test. For more information, visit https://fusic.co.jp/english/.
    SYS-CON Events announced today that Massive Networks, that helps your business operate seamlessly with fast, reliable, and secure internet and network solutions, has been named "Exhibitor" of 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. As a premier telecommunications provider, Massive Networks is headquartered out of Louisville, Colorado. With years of experience under their belt, their team of...
    SYS-CON Events announced today that Enroute Lab will exhibit at the Japan External Trade Organization (JETRO) Pavilion 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. Enroute Lab is an industrial design, research and development company of unmanned robotic vehicle system. For more information, please visit http://elab.co.jp/.
    SYS-CON Events announced today that MIRAI Inc. will exhibit at the Japan External Trade Organization (JETRO) Pavilion 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. MIRAI Inc. are IT consultants from the public sector whose mission is to solve social issues by technology and innovation and to create a meaningful future for people.
    SYS-CON Events announced today that Mobile Create USA will exhibit at the Japan External Trade Organization (JETRO) Pavilion 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. Mobile Create USA Inc. is an MVNO-based business model that uses portable communication devices and cellular-based infrastructure in the development, sales, operation and mobile communications systems incorporating GPS capabi...
    SYS-CON Events announced today that Interface Corporation will exhibit at the Japan External Trade Organization (JETRO) Pavilion 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. Interface Corporation is a company developing, manufacturing and marketing high quality and wide variety of industrial computers and interface modules such as PCIs and PCI express. For more information, visit http://www.i...
    SYS-CON Events announced today that Keisoku Research Consultant Co. will exhibit at the Japan External Trade Organization (JETRO) Pavilion 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. Keisoku Research Consultant, Co. offers research and consulting in a wide range of civil engineering-related fields from information construction to preservation of cultural properties. For more information, vi...
    There is huge complexity in implementing a successful digital business that requires efficient on-premise and cloud back-end infrastructure, IT and Internet of Things (IoT) data, analytics, Machine Learning, Artificial Intelligence (AI) and Digital Applications. In the data center alone, there are physical and virtual infrastructures, multiple operating systems, multiple applications and new and emerging business and technological paradigms such as cloud computing and XaaS. And then there are pe...
    SYS-CON Events announced today that SIGMA Corporation will exhibit at the Japan External Trade Organization (JETRO) Pavilion 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. uLaser flow inspection device from the Japanese top share to Global Standard! Then, make the best use of data to flip to next page. For more information, visit http://www.sigma-k.co.jp/en/.
    SYS-CON Events announced today that B2Cloud 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. B2Cloud specializes in IoT devices for preventive and predictive maintenance in any kind of equipment retrieving data like Energy consumption, working time, temperature, humidity, pressure, etc.
    Agile has finally jumped the technology shark, expanding outside the software world. Enterprises are now increasingly adopting Agile practices across their organizations in order to successfully navigate the disruptive waters that threaten to drown them. In our quest for establishing change as a core competency in our organizations, this business-centric notion of Agile is an essential component of Agile Digital Transformation. In the years since the publication of the Agile Manifesto, the conn...
    While some developers care passionately about how data centers and clouds are architected, for most, it is only the end result that matters. To the majority of companies, technology exists to solve a business problem, and only delivers value when it is solving that problem. 2017 brings the mainstream adoption of containers for production workloads. In his session at 21st Cloud Expo, Ben McCormack, VP of Operations at Evernote, will discuss how data centers of the future will be managed, how th...
    SYS-CON Events announced today that NetApp has been named “Bronze Sponsor” of 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. NetApp is the data authority for hybrid cloud. NetApp provides a full range of hybrid cloud data services that simplify management of applications and data across cloud and on-premises environments to accelerate digital transformation. Together with their partners, NetApp em...
    SYS-CON Events announced today that Nihon Micron will exhibit at the Japan External Trade Organization (JETRO) Pavilion 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. Nihon Micron Co., Ltd. strives for technological innovation to establish high-density, high-precision processing technology for providing printed circuit board and metal mount RFID tags used for communication devices. For more inf...
    SYS-CON Events announced today that Suzuki Inc. will exhibit at the Japan External Trade Organization (JETRO) Pavilion 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. Suzuki Inc. is a semiconductor-related business, including sales of consuming parts, parts repair, and maintenance for semiconductor manufacturing machines, etc. It is also a health care business providing experimental research for...
    SYS-CON Events announced today that Ryobi Systems will exhibit at the Japan External Trade Organization (JETRO) Pavilion 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. Ryobi Systems Co., Ltd., as an information service company, specialized in business support for local governments and medical industry. We are challenging to achive the precision farming with AI. For more information, visit http:...