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Reflections on Java Reflection
Pages: 1, 2, 3

There is a nice little land mine hidden in getConstructor. When you are specifying the parameter types for the constructor you are looking for, you need to carefully distinguish between a primitive argument and the related object based wrapper. Does the constructor take as an argument the primitive int or its uncle, a object of class java.lang.Integer? If you are looking for a constructor that takes the object wrapper type, something like java.lang.Integer, just use the wrapper class, Integer.class If you mean the primitive int you will need use Integer.TYPE, which is a sort of placeholder for the class that the primitive lacks. All of the wrapper classes have a static TYPE field which you can use to indicate a primitive of that type.



Digging Deeper into A Class

As you saw in the first example, a Class object can supply you with all of the basic information about a class, things like its name and super class. But you can go way beyond this name, rank and serial number level of information. You can, for example, find out about all of the public methods supported by a class with the getMethods method:

      Class klass = Class.forName("com.russolsen.reflect.Employee");
           
      Method[] methods = klass.getMethods();
      
      for(Method m : methods )
      {
         System.out.println( "Found a method: " + m );
      }

getMethods returns an array of Method objects, one for each public method that instances of the class will answer to:

Found a method: public java.lang.String com.russolsen.reflect.Employee.toString()
Found a method: public int com.russolsen.reflect.Employee.getSalary()
Found a method: public void com.russolsen.reflect.Employee.setSalary(int)
Found a method: public native int java.lang.Object.hashCode()
Found a method: public final native java.lang.Class java.lang.Object.getClass()
Found a method: public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException
Found a method: public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException
Found a method: public final void java.lang.Object.wait() throws java.lang.InterruptedException
Found a method: public boolean java.lang.Object.equals(java.lang.Object)
Found a method: public java.lang.String java.lang.Object.toString()
Found a method: public final native void java.lang.Object.notify()
Found a method: public final native void java.lang.Object.notifyAll()

Since getMethods takes a client's view of the class, the array will contains all of the public methods, including those defined in the class itself and in its super class and its super-duper class and so on up to Object.

If you are interested in a single method, you can use getMethod (note that it is singular), which works a lot like getConstructor, except that you need to pass in a method name as well as the parameter types. The code below looks for a method called setSalary that takes a single int parameter:

      Class klass = Class.forName("com.russolsen.reflect.Employee");
      Class[] paramTypes = {Integer.TYPE };
      Method setSalaryMethod = 
         klass.getMethod("setSalary", paramTypes);
           
      System.out.println( "Found method: " + setSalaryMethod);

Calling a method via reflection turns out to be very similar calling a constructor. All you need is the Method object that we talked about earlier, an array of arguments to pass to the method and the instance whose method you are calling. The code below calls the setSalary method on an Employee object to give our man a raise:

      Class klass = Class.forName("com.russolsen.reflect.Employee");

      Class[] paramTypes = {Integer.TYPE };
      Method setSalaryMethod = 
         klass.getMethod("setSalary", paramTypes);
      
      Object theObject = klass.newInstance();
      Object[] parameters = { new Integer(90000) };
      
      setSalaryMethod.invoke(theObject, parameters);

Why would you bother going through all of this instead of just typing theObject.setSalary(90000)? Well, take another look at the code above. Other than the string on the first line, the program above is completely generic: you could use it to call the setSalary method on any instance of any class. With a little rearrangement, you can use the code above to call any method on any class.

Messing with Fields

You are not limited to just calling methods via reflection: you also have full reign over fields. Similar to getMethods, getFields returns an array of Field objects, one for each public instance field supplied by the class or its super classes:

      Class klass = Class.forName("com.russolsen.reflect.Employee");
      
      System.out.println( "Class name: " + klass.getName());
      
      Field[] fields = klass.getFields();
      
      for(Field f : fields )
      {
         System.out.println( "Found field: " + f);
      }

Since Employee has exactly two public fields, you get a two member array back:

Class name: com.russolsen.reflect.Employee
Found field: public java.lang.String com.russolsen.reflect.Employee._firstName
Found field: public java.lang.String com.russolsen.reflect.Employee._lastName

You can also look for a specific field by name with the getField method:

      Field field = klass.getField("_firstName");
      System.out.println("Found field: " + field);

Once you have a Field object, getting the field value is as simple as calling the get method, while setting the value is just a call to set away:

      
      Object theObject = new Employee("Tom", "Smith", 25);
      
      Class klass = Class.forName("com.russolsen.reflect.Employee");
      
      Field field = klass.getField("_firstName");
      
      Object oldValue = field.get(theObject);
      System.out.println( "Old first name is: " + oldValue);
      
      field.set( theObject, "Harry");
      Object newValue = field.get(theObject);
      System.out.println( "New first name is: " + newValue);

Run the code above and watch the value of the _salary field change before your very eyes:

Old first name is: Tom
New first name is: Harry

Breaking the Rules

No look at reflection can be complete without talking about how to break one of the most sacred of all the rules in Java. Everyone knows that you cannot call a private method from outside of the class that defines it. Right? Well you can't if you stick to conventional techniques, but with reflection you can do darn near anything.

The first thing you need to call a private method is the Method instance that goes with the method you want to call. You can't get this from getMethod; it only returns public methods. The way to get hold of a private (or protected) method is to use getDeclaredMethod. While getMethod takes a client's view of a class and only returns public methods, getDeclaredMethod returns all of the methods declared by one class. In the example below, we use it to get at the Method object for the very private removeRange method on the java.util.ArrayList class:

      ArrayList list = new ArrayList();
      list.add("Larry");
      list.add("Moe");
      list.add("Curley");

      System.out.println("The list is: " + list);

      Class klass = list.getClass();

      Class[] paramTypes = { Integer.TYPE, Integer.TYPE };
      Method m = klass.getDeclaredMethod("removeRange", paramTypes);

      Object[] arguments = { new Integer(0), new Integer(2) };
      m.setAccessible(true);
      m.invoke(list, arguments);
      System.out.println("The new list is: " + list);

Once you have a private method, calling it is a simple matter of clicking off the final setAccessable safety and having at it:

The list is: [Larry, Moe, Curley]
The new list is: [Curley]

The removeRange method appears to remove the given range of items from the list. This is pretty strong voodoo: you just reached in and called a private method on a java.util class. Would you want to make a habit of circumventing the clear intent of the code and calling private methods? No! But such things have been known to come in handy in a pinch.

Conclusion

Reflection allows your programs to do things that seem to defy the rules of Java. You can write code that can find out all about a class to which it has never been properly introduced. And your code can act on that dynamically discovered knowledge: it can create new instances, call methods and set and get the value of fields. In extreme circumstances you can even mess with the private internals of a class. A good understanding of reflection is what you need to grasp the workings of some of the more sophisticated tools of the Java world; it is also what you need to write programs that go beyond what "ordinary" Java programs can do.

Resources

Russ Olsen is currently a senior engineer with FGM, where he builds information systems with both J2EE and Rails. Russ spends a lot of his otherwise free time writing about technology.


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