Introducing reflection

Java programming dynamics, Part 2: Introducing reflection Use run-time class information to limber up your programming

Document options Print this page E-mail this page Sample code New site feature Plan and design IT solutions with our new Architecture area Rate this page Help us improve this content Level: Intermediate Dennis Sosnoski (dms@sosnoski.com), President, Sosnoski Software Solutions, Inc. 03 Jun 2003 Reflection gives your code access to internal information for classes loaded into the JVM and allows you to write code that works with classes selected during execution, not in the source code. This makes reflection a great tool for building flexible applications. But watch out -- if used inappropriately, reflection can be costly. In Part 2 of his series on Java platform internals, software consultant Dennis Sosnoski provides an introduction to using reflection, as well as a look at some of the costs involved. You'll also find out how the Java Reflection API lets you hook into objects at run time. In "Java programming dynamics, Part 1," I gave you an introduction to Java programming classes and class loading. That article described some of the extensive information present in the Java binary class format. This month I cover the basics of using the Java Reflection API to access and use some of that same information at run time. To help keep things interesting even for developers who already know the basics of reflection, I'm including a look at how reflection performance compares with direct access. Using reflection is different from normal Java programming in that it works with metadata -- data that describes other data. The particular type of metadata accessed by Java language reflection is the description of classes and objects within the JVM. Reflection gives you run-time access to a variety of class information. It even lets you read and write fields and call methods of a class selected at run time. Reflection is a powerful tool. It lets you build flexible code that can be assembled at run time without requiring source code links between components. But some aspects of reflection can be problematic. In this article, I'll go into the reasons why you might not want to use reflection in your programs, as well as the reasons why you would. After you know the trade-offs, you can decide for yourself when the benefits outweigh the drawbacks. Beginners' class The starting point for using reflection is always a java.lang.Class instance. If you want to work with a predetermined class, the Java language provides an easy shortcut to get the Class instance directly: Don't miss the rest of this series Part 1, "Classes and class loading" (April 2003) Part 3, "Applied reflection" (July 2003) Part 4, "Class transformation with Javassist" (September 2003) Part 5, "Transforming classes on-the-fly" (February 2004) Part 6, "Aspect-oriented changes with Javassist" (March 2004) Part 7, "Bytecode engineering with BCEL" (April 2004) Part 8, "Replacing reflection with code generation" (June 2004) Class clas = MyClass.class; When you use this technique, all the work involved in loading the class takes place behind the scenes. If you need to read the class name at run time from some external source, however, this approach isn't going to work. Instead, you need to use a class loader to find the class information. Here's one way to do that: // "name" is the class name to load Class clas = null; try { clas = Class.forName(name); } catch (ClassNotFoundException ex) { // handle exception case } // use the loaded class If the class has already been loaded, you'll get back the existing Class information. If the class hasn't been loaded yet, the class loader will load it now and return the newly constructed class instance. Back to top Reflections on a class The Class object gives you all the basic hooks for reflection access to the class metadata. This metadata includes information about the class itself, such as the package and superclass of the class, as well as the interfaces implemented by the class. It also includes details of the constructors, fields, and methods defined by the class. These last items are the ones most often used in programming, so I'll give some examples of working with them later in this section. For each of these three types of class components -- constructors, fields, and methods -- the java.lang.Class provides four separate reflection calls to access information in different ways. The calls all follow a standard form. Here's the set used to find constructors: Constructor getConstructor(Class[] params) -- Gets the public constructor using the specified parameter types Constructor[] getConstructors() -- Gets all the public constructors for the class Constructor getDeclaredConstructor(Class[] params) -- Gets the constructor (regardless of access level) using the specified parameter types Constructor[] getDeclaredConstructors() -- Gets all the constructors (regardless of access level) for the class Each of these calls returns one or more java.lang.reflect.Constructor instances. This Constructor class defines a newInstance method that takes an array of objects as its only argument, then returns a newly constructed instance of the original class. The array of objects are the parameter values used for the constructor call. As an example of how this works, suppose you have a TwoString class with a constructor that takes a pair of Strings, as shown in Listing 1: Listing 1. Class constructed from pair of strings public class TwoString { private String m_s1, m_s2; public TwoString(String s1, String s2) { m_s1 = s1; m_s2 = s2; } } The code shown in Listing 2 gets the constructor and uses it to create an instance of the TwoString class using Strings "a" and "b": Listing 2. Reflection call to constructor Class[] types = new Class[] { String.class, String.class }; Constructor cons = TwoString.class.getConstructor(types); Object[] args = new Object[] { "a", "b" }; TwoString ts = cons.newInstance(args); The code in Listing 2 ignores several possible types of checked exceptions thrown by the various reflection methods. The exceptions are detailed in the Javadoc API descriptions, so in the interest of conciseness, I'm leaving them out of all the code examples. While I'm on the topic of constructors, the Java programming language also defines a special shortcut method you can use to create an instance of a class with a no-argument (or default) constructor. The shortcut is embedded into the Class definition itself like this: Object newInstance() -- Constructs new instance using default constructor Even though this approach only lets you use one particular constructor, it makes a very convenient shortcut if that's the one you want. This technique is especially useful when working with JavaBeans, which are required to define a public, no-argument constructor. Fields by reflection The Class reflection calls to access field information are similar to those used to access constructors, with a field name used in place of an array of parameter types: Field getField(String name) -- Gets the named public field Field[] getFields() -- Gets all public fields of the class Field getDeclaredField(String name) -- Gets the named field declared by the class Field[] getDeclaredFields() -- Gets all the fields declared by the class Despite the similarity to the constructor calls, there's one important difference when it comes to fields: the first two variants return information for public fields that can be accessed through the class -- even those inherited from an ancestor class. The last two return information for fields declared directly by the class -- regardless of the fields' access types. The java.lang.reflect.Field instances returned by the calls define getXXX and setXXX methods for all the primitive types, as well as generic get and set methods that work with object references. It's up to you to use an appropriate method based on the actual field type, though the getXXX methods will handle widening conversions automatically (such as using the getInt method to retrieve a byte value). Listing 3 shows an example of using the field reflection methods, in the form of a method to increment an int field of an object by name: Listing 3. Incrementing a field by reflection public int incrementField(String name, Object obj) throws... { Field field = obj.getClass().getDeclaredField(name); int value = field.getInt(obj) + 1; field.setInt(obj, value); return value; } This method starts to show some of the flexibility possible with reflection. Rather than working with a specific class, incrementField uses the getClass method of the passed-in object to find the class information, then finds the named field directly in that class. Methods by reflection The Class reflection calls to access method information are very similar to those used for constructors and fields: Method getMethod(String name, Class[] params) -- Gets the named public method using the specified parameter types Method[] getMethods() -- Gets all public methods of the class Method getDeclaredMethod(String name, Class[] params) -- Gets the named method declared by the class using the specified parameter types Method[] getDeclaredMethods() -- Gets all the methods declared by the class As with the field calls, the first two variants return information for public methods that can be accessed through the class -- even those inherited from an ancestor class. The last two return information for methods declared directly by the class, without regard to the access type of the method. The java.lang.reflect.Method instances returned by the calls define an invoke method you can use to call the method on an instance of the defining class. This invoke method takes two arguments, which supply the class instance and an array of parameter values for the call. Listing 4 takes the field example a step further, showing an example of method reflection in action. This method increments an int JavaBean property defined with get and set methods. For example, if the object defined getCount and setCount methods for an integer count value, you could pass "count" as the name parameter in a call to this method in order to increment that value. Listing 4. Incrementing a JavaBean property by reflection public int incrementProperty(String name, Object obj) { String prop = Character.toUpperCase(name.charAt(0)) + name.substring(1); String mname = "get" + prop; Class[] types = new Class[] {}; Method method = obj.getClass().getMethod(mname, types); Object result = method.invoke(obj, new Object[0]); int value = ((Integer)result).intValue() + 1; mname = "set" + prop; types = new Class[] { int.class }; method = obj.getClass().getMethod(mname, types); method.invoke(obj, new Object[] { new Integer(value) }); return value; } To follow the JavaBeans convention, I convert the first letter of the property name to uppercase, then prepend get to construct the read method name and set to construct the write method name. JavaBeans read methods just return the value and write methods take the value as the only parameter, so I specify the parameter types for the methods to match. Finally, the convention requires the methods to be public, so I use the form of lookup that finds public methods callable on the class. This example is the first one where I've passed primitive values using reflection, so let's look at how this works. The basic principle is simple: whenever you need to pass a primitive value, just substitute an instance of the corresponding wrapper class (defined in the java.lang package) for that type of primitive. This applies to both calls and returns. So when I call the get method in my example, I expect the result to be a java.lang.Integer wrapper for the actual int property value. Reflecting arrays Arrays are objects in the Java programming language. Like all objects, they have classes. If you have an array, you can get the class of that array using the standard getClass method, just as with any other object. However, getting the class without an existing instance works differently than for other types of objects. Even after you have an array class there's not much you can do with it directly -- the constructor access provided by reflection for normal classes doesn't work for arrays, and arrays don't have any accessible fields. Only the base java.lang.Object methods are defined for array objects. The special handling of arrays uses a collection of static methods provided by the java.lang.reflect.Array class. The methods in this class let you create new arrays, get the length of an array object, and read and write indexed values of an array object. Listing 5 shows a useful method for effectively resizing an existing array. It uses reflection to create a new array of the same type, then copies all the data across from the old array before returning the new array. Listing 5. Growing an array by reflection public Object growArray(Object array, int size) { Class type = array.getClass().getComponentType(); Object grown = Array.newInstance(type, size); System.arraycopy(array, 0, grown, 0, Math.min(Array.getLength(array), size)); return grown; } Back to top Security and reflection Security can be a complex issue when dealing with reflection. Reflection is often used by framework-type code, and for this you may want the framework to have full access to your code without concern for normal access restrictions. Yet uncontrolled access can create major security risks in other cases, such as when code is executed in an environment shared by untrusted code. Because of these conflicting needs, the Java programming language defines a multi-level approach to handling reflection security. The basic mode is to enforce the same restrictions on reflection as would apply for source code access: Access from anywhere to public components of the class No access outside the class itself to private components Limited access to protected and package (default access) components There's a simple way around these restrictions, though -- at least sometimes. The Constructor, Field, and Method classes I've used in the earlier examples all extend a common base class -- the java.lang.reflect.AccessibleObject class. This class defines a setAccessible method that lets you turn the access checks on or off for an instance of one of these classes. The only catch is that if a security manager is present, it will check that the code turning off access checks has permission to do so. If there's no permission, the security manager throws an exception. Listing 6 demonstrates a program that uses reflection on an instance of the Listing 1 TwoString class to show this in action: Listing 6. Reflection security in action public class ReflectSecurity { public static void main(String[] args) { try { TwoString ts = new TwoString("a", "b"); Field field = clas.getDeclaredField("m_s1"); // field.setAccessible(true); System.out.println("Retrieved value is " + field.get(inst)); } catch (Exception ex) { ex.printStackTrace(System.out); } } } If you compile this code and run it directly from the command line without any special parameters, it'll throw an IllegalAccessException on the field.get(inst) call. If you uncomment the field.setAccessible(true) line, then recompile and rerun the code, it will succeed. Finally, if you add the JVM parameter -Djava.security.manager on the command line to enable a security manager, it will again fail, unless you define permissions for the ReflectSecurity class. Back to top Reflection performance Reflection is a powerful tool, but suffers from a few drawbacks. One of the main drawbacks is the effect on performance. Using reflection is basically an interpreted operation, where you tell the JVM what you want to do and it does it for you. This type of operation is always going to be slower than just doing the same operation directly. To demonstrate the performance cost of using reflection, I prepared a set of benchmark programs for this article (see Resources for a link to the full code). Listing 7 shows an excerpt from the field access performance test, including the basic test methods. Each method tests one form of access to fields -- accessSame works with member fields of the same object, accessOther uses fields of another object accessed directly, and accessReflection uses fields of another object accessed by reflection. In each case, the methods perform the same computations -- a simple add/multiply sequence in a loop. Listing 7. Field access performance test code public int accessSame(int loops) { m_value = 0; for (int index = 0; index < loops; index++) { m_value = (m_value + ADDITIVE_VALUE) * MULTIPLIER_VALUE; } return m_value; } public int accessReference(int loops) { TimingClass timing = new TimingClass(); for (int index = 0; index < loops; index++) { timing.m_value = (timing.m_value + ADDITIVE_VALUE) * MULTIPLIER_VALUE; } return timing.m_value; } public int accessReflection(int loops) throws Exception { TimingClass timing = new TimingClass(); try { Field field = TimingClass.class. getDeclaredField("m_value"); for (int index = 0; index < loops; index++) { int value = (field.getInt(timing) + ADDITIVE_VALUE) * MULTIPLIER_VALUE; field.setInt(timing, value); } return timing.m_value; } catch (Exception ex) { System.out.println("Error using reflection"); throw ex; } } The test program calls each method repeatedly with a large loop count, averaging the time measurements over several calls. The time for the first call to each method is not included in the average, so initialization time isn't a factor in the results. In the test runs for this article, I used a loop count of 10 million for each call, running on a 1GHz PIIIm system. My timing results with three different Linux JVMs are shown in Figure 1. All tests used the default settings for each JVM. Figure 1. Field access times The logarithmic scale of the chart allows the full range of times to be displayed, but lessens the visual impact of the differences. In the case of the first two sets of figures (the Sun JVMs), the execution time using reflection is over 1000 times greater than that using direct access. The IBM JVM does somewhat better by comparison, but the reflection method still takes more than 700 times as long as the other methods. There were no significant differences in times between the other two methods on any JVM, though the IBM JVM did run these almost twice as fast as the Sun JVMs. Most likely, this difference reflects the specialized optimizations used by the Sun Hot Spot JVMs, which tend to do poorly in simple benchmarks. Besides the field access time tests, I did the same sort of timing test for method calls. For method calls, I tried the same three access variations as for field access, with the added variable of using no-argument methods versus passing and returning a value on the method calls. Listing 8 shows the code for the three methods used to test the passed-and-returned value form of the calls. Listing 8. Method access performance test code public int callDirectArgs(int loops) { int value = 0; for (int index = 0; index < loops; index++) { value = step(value); } return value; } public int callReferenceArgs(int loops) { TimingClass timing = new TimingClass(); int value = 0; for (int index = 0; index < loops; index++) { value = timing.step(value); } return value; } public int callReflectArgs(int loops) throws Exception { TimingClass timing = new TimingClass(); try { Method method = TimingClass.class.getMethod ("step", new Class [] { int.class }); Object[] args = new Object[1]; Object value = new Integer(0); for (int index = 0; index < loops; index++) { args[0] = value; value = method.invoke(timing, args); } return ((Integer)value).intValue(); } catch (Exception ex) { System.out.println("Error using reflection"); throw ex; } } Figure 2 shows my timing results for method calls. Here again, reflection is much slower than the direct alternative. The differences aren't quite as large as for the field access case, though, ranging from several hundred times slower on the Sun 1.3.1 JVM to less than 30 times slower on the IBM JVM for the no-argument case. The test performance for reflection method calls with arguments are substantially slower than the calls with no arguments on all JVMs. This is probably partially because of the java.lang.Integer wrapper needed for the int value passed and returned. Because Integers are immutable, a new one needs to be generated for each method return, adding considerable overhead. Figure 2. Method call times Reflection performance was one area of focus for Sun when developing the 1.4 JVM, which shows in the reflection method call results. The Sun 1.4.1 JVM shows greatly improved performance over the 1.3.1 version for this type of operation, running about seven times faster in my tests. The IBM 1.4.0 JVM again delivered even better performance for this simple test, though, running two to three times faster than the Sun 1.4.1 JVM. I also wrote a similar timing test program for creating objects using reflection. The differences for this case aren't nearly as significant as for the field and method call cases, though. Constructing a simple java.lang.Object instance with a newInstance() call takes about 12 times longer than using new Object() on the Sun 1.3.1 JVM, about four times longer on the IBM 1.4.0 JVM, and only about two times longer on the Sun 1.4.1 JVM. Constructing an array using Array.newInstance(type, size) takes a maximum of about two times longer than using new type[size] for any tested JVM, with the difference decreasing as the array size grows. Back to top Reflection summary Java language reflection provides a very versatile way of dynamically linking program components. It allows your program to create and manipulate objects of any classes (subject to security restrictions) without the need to hardcode the target classes ahead of time. These features make reflection especially useful for creating libraries that work with objects in very general ways. For example, reflection is often used in frameworks that persist objects to databases, XML, or other external formats. Reflection also has a couple of drawbacks. One is the performance issue. Reflection is much slower than direct code when used for field and method access. To what extent that matters depends on how reflection is used in a program. If it's used as a relatively infrequent part of the program's operation, the slow performance won't be a concern. Even the worst-case timing figures in my tests showed reflection operations taking only a few microseconds. The performance issues only become a serious concern if reflection is used in the core logic of performance-critical applications. A more serious drawback for many applications is that using reflection can obscure what's actually going on inside your code. Programmers expect to see the logic of a program in the source code, and techniques such as reflection that bypass the source code can create maintenance problems. Reflection code is also more complex than the corresponding direct code, as can be seen in the code samples from the performance comparisons. The best ways to deal with these issues are to use reflection sparingly -- only in the places where it really adds useful flexibility -- and document its use within the target classes. In the next installment, I'll give a more detailed example of how to use reflection. This example provides an API for processing command line arguments to a Java application, a tool you may find useful for your own applications. It also builds on the strengths of reflection while avoiding the weaknesses. Can reflection simplify your command line processing? Find out in Part 3 of Java programming dynamics.