Take an in-depth look at the Java Reflection API

In last month's "Java In-Depth," I talked about introspection and ways in which a Java class with access to raw class data could look "inside" a class and figure out how the class was constructed. Further, I showed that with the addition of a class loader, those classes could be loaded into the running environment and executed. That example is a form of static introspection. This month I'll take a look at the Java Reflection API, which gives Java classes the ability to perform dynamic introspection: the ability to look inside classes that are already loaded.

The utility of introspection

One of Java's strengths is that it was designed with the assumption that the environment in which it was running would be changing dynamically. Classes are loaded dynamically, binding is done dynamically, and object instances are created dynamically on the fly when they are needed. What has not been very dynamic historically is the ability to manipulate "anonymous" classes. In this context, an anonymous class is one that is loaded or presented to a Java class at run time and whose type was previously unknown to the Java program.

Anonymous classes

Supporting anonymous classes is hard to explain and even harder to design for in a program. The challenge of supporting an anonymous class can be stated like this: "Write a program that, when given a Java object, can incorporate that object into its continuing operation." The general solution is rather difficult, but by constraining the problem, some specialized solutions can be created. There are two examples of specialized solutions to this class of problem in the 1.0 version of Java: Java applets and the command-line version of the Java interpreter.

Java applets are Java classes that are loaded by a running Java virtual machine in the context of a Web browser and invoked. These Java classes are anonymous because the run time does not know ahead of time the necessary information to invoke each individual class. However, the problem of invoking a particular class is solved using the Java class java.applet.Applet.

Common superclasses, like Applet, and Java interfaces, like AppletContext, address the problem of anonymous classes by creating a previously agreed upon contract. Specifically, a runtime environment supplier advertises that she can use any object that conforms to a specified interface, and the runtime environment consumer uses that specified interface in any object he intends to supply to the run time. In the case of applets, a well-specified interface exists in the form of a common superclass.

The downside of a common superclass solution, especially in the absence of multiple inheritance, is that the objects built to run in the environment cannot also be used in some other system unless that system implements the entire contract. In the case of the Applet interfaces, the hosting environment has to implement AppletContext. What this means for the applet solution is that the solution only works when you are loading applets. If you put an instance of a Hashtable object on your Web page and point your browser to it, it would fail to load because the applet system cannot operate outside its limited range.

In addition to the applet example, introspection helps to solve a problem I mentioned last month: figuring out how to start execution in a class that the command-line version of the Java virtual machine has just loaded. In that example, the virtual machine has to invoke some static method in the loaded class. By convention, that method is named main and takes a single argument -- an array of String objects.

The motivation for a more dynamic solution

The challenge with the existing Java 1.0 architecture is that there are problems that could be solved by a more dynamic introspection environment -- such as loadable UI components, loadable device drivers in a Java-based OS, and dynamically configurable editing environments. The "killer app," or the issue that caused the Java Reflection API to be created, was the development of an object component model for Java. That model is now known as JavaBeans.

User interface components are an ideal design point for an introspection system because they have two very different consumers. On the one hand, the component objects are linked together to form a user interface as part of some application. Alternatively, there needs to be an interface for tools that manipulate user components without having to know what the components are, or, more importantly, without access to the components' source code.

The Java Reflection API grew out of the needs of the JavaBeans user interface component API.

What is reflection?

Fundamentally, the Reflection API consists of two components: objects that represent the various parts of a class file, and a means for extracting those objects in a safe and secure way. The latter is very important, as Java provides many security safeguards, and it would not make sense to provide a set of classes that invalidated those safeguards.

The first component of the Reflection API is the mechanism used to fetch information about a class. This mechanism is built into the class named Class. The special class Class is the universal type for the meta information that describes objects within the Java system. Class loaders in the Java system return objects of type Class. Up until now the three most interesting methods in this class were:

• forName, which would load a class of a given name, using the current class loader

• getName, which would return the name of the class as a String object,which was useful for identifying object references by their class name

• newInstance, which would invoke the null constructor on the class (if it exists) and return you an object instance of that class of object

To these three useful methods the Reflection API adds some additional methods to class Class. These are as follows:

• getConstructor, getConstructors, getDeclaredConstructor
• getMethod, getMethods, getDeclaredMethods
• getField, getFields, getDeclaredFields
• getSuperclass
• getInterfaces
• getDeclaredClasses

In addition to these methods, many new classes were added to represent the objects that these methods would return. The new classes mostly are part of the java.lang.reflect package, but some of the new basic type classes (Void, Byte, and so on) are in the java.lang package. The decision was made to put the new classes where they are by putting classes that represented meta-data in the reflection package and classes that represented types in the language package.

Thus, the Reflection API represents a number of changes to class Class that let you ask questions about the internals of the class, and a bunch of classes that represent the answers that these new methods give you.

How do I use the Reflection API?

The question "How do I use the API?" is perhaps the more interesting question than "What is reflection?"

The Reflection API is symmetric, which means that if you are holding a Class object, you can ask about its internals, and if you have one of the internals, you can ask it which class declared it. Thus you can move back and forth from class to method to parameter to class to method, and so on. One interesting use of this technology is to find out most of the interdependencies between a given class and the rest of the system.

A working example

On a more practical level, however, you can use the Reflection API to dump out a class, much as my dumpclass class did in last month's column.

To demonstrate the Reflection API, I wrote a class called ReflectClass that would take a class known to the Java run time (meaning it is in your class path somewhere) and, through the Reflection API, dump out its structure to the terminal window. To experiment with this class, you will need to have a 1.1 version of the JDK available.

Note: Do not try to use a 1.0 run time as it gets all confused, usually resulting in an incompatible class change exception.

The class ReflectClass begins as follows:

import java.lang.reflect.*;
import java.util.*;
public class ReflectClass {

As you can see above, the first thing the code does is import the Reflection API classes. Next, it jumps right into the main method, which starts out as shown below.

   
        public static void main(String args[]) {
        Constructor   cn[];
        Class         cc[];
        Method        mm[];
        Field         ff[];
        Class         c = null;
        Class         supClass;
        String      x, y, s1, s2, s3;
        Hashtable classRef = new Hashtable();
        if (args.length == 0) {
            System.out.println("Please specify a class name on the command line.");
            System.exit(1);
        }
        try {
            c = Class.forName(args[0]);
        } catch (ClassNotFoundException ee) {
            System.out.println("Couldn't find class '"+args[0]+"'");
            System.exit(1);
        }

The method main declares arrays of constructors, fields, and methods. If you recall, these are three of the four fundamental parts of the class file. The fourth part is the attributes, which the Reflection API unfortunately does not give you access to. After the arrays, I've done some command-line processing. If the user has typed a class name, the code attempts to load it using the forName method of class Class. The forName method takes Java class names, not file names, so to look inside the java.math.BigInteger class, you simply type "java ReflectClass java.math.BigInteger," rather than point out where the class file actually is stored.

Identifying the class's package

Assuming the class file is found, the code proceeds into Step 0, which is shown below.

        /*
         * Step 0: If our name contains dots we're in a package so put
         * that out first.
         */
        x = c.getName();
        y = x.substring(0, x.lastIndexOf("."));
        if (y.length() > 0) {
            System.out.println("package "+y+";\n\r");
        }

In this step, the name of the class is retrieved using the getName method in class Class. This method returns the fully qualified name, and if the name contains dots, we can presume that the class was defined as part of a package. So Step 0 is to separate the package name part from the class name part, and print out the package name part on a line that starts with "package...."

Collecting class references from declarations and parameters

With the package statement taken care of, we proceed to Step 1, which is to collect all of the other class names that are referenced by this class. This collection process is shown in the code below. Remember that the three most common places where class names are referenced are as types for fields (instance variables), return types for methods, and as the types of the parameters passed to methods and constructors.

        ff = c.getDeclaredFields();
        for (int i = 0; i < ff.length; i++) {
            x = tName(ff[i].getType().getName(), classRef);
        }

In the above code, the array ff is initialized to be an array of Field objects. The loop collects the type name from each field and process it through the tName method. The tName method is a simple helper that returns the shorthand name for a type. So java.lang.String becomes String. And it notes in a hashtable which objects have been seen. At this stage, the code is more interested in collecting class references than in printing.

The next source of class references are the parameters supplied to constructors. The next piece of code, shown below, processes each declared constructor and collects the references from the parameter lists.

        
cn = c.getDeclaredConstructors();
        for (int i = 0; i < cn.length; i++) {
            Class cx[] = cn[i].getParameterTypes();
            if (cx.length > 0) {
                for (int j = 0; j < cx.length; j++) {
                    x = tName(cx[j].getName(), classRef);
                }
            }
        }

As you can see, I've used the getParameterTypes method in the Constructor class to feed me all of the parameters that a particular constructor takes. These are then processed through the tName method.

An interesting thing to note here is the difference between the method getDeclaredConstructors and the method getConstructors. Both methods return an array of constructors, but the getConstructors method only returns those constructors that are accessible to your class. This is useful if you want to know if you actually can invoke the constructor you've found, but it isn't useful for this application because I want to print out all of the constructors in the class, public or not. The field and method reflectors also have similar versions, one for all members and one only for public members.

The final step, shown below, is to collect the references from all of the methods. This code has to get references from both the type of the method (similar to the fields above) and from the parameters (similar to the constructors above).

        mm = c.getDeclaredMethods();
        for (int i = 0; i < mm.length; i++) {
            x = tName(mm[i].getReturnType().getName(), classRef);
            Class cx[] = mm[i].getParameterTypes();
            if (cx.length > 0) {
                for (int j = 0; j < cx.length; j++) {
                    x = tName(cx[j].getName(), classRef);
                }
            }
        }

In the above code, there are two calls to tName -- one to collect the return type and one to collect each parameter's type.