Reflections on Java Reflection

출처 :http://www.onjava.com/pub/a/onjava/2007/03/15/reflections-on-java-reflection.html

Reflections on Java Reflection

byRuss Olsen
03/15/2007

In ordinary life, a reflection is what you see when you look in the mirror. In the world of programming,reflectionis what you call it when a program looks at and possibly even modifies its own structure. The Java reflection API allows you to do exactly that by giving you a window into the fundamental features of the language -- classes and fields and methods -- via an ordinary Java API. Understanding reflection will help you understand the tools that you use every day. How does Eclipse manage to do all that helpful auto-completion of method names? How does Tomcat go from a class name in aweb.xmlfile to a running servlet fielding web requests? And how does Spring do all of that magic dependency injection stuff? In your own programs, you can use reflection to write code that is more flexible and dynamic: with reflection your program can cope gracefully with classes it has never met before.

Conjuring Up A Class

As I said, the fundamental idea of reflection is to give you an ordinary Java API into the inner workings of your program. Since the most basic idea in Java is the class (try to write a Java program without one), a good place to start is by looking at theClassclass. No, that is not a typing stutter: you can actually get hold of an object which contains everything you might ever need to know about a Java class. That object will have a type ofClass.Once you have aClassinstance, you can extract all kinds of information about the class from it, including the name of the class, whether it is public or abstract or final, and even its super-class.

Enough theory: let's turn our reflection microscope on the following, very simpleEmployeeclass:

package com.russolsen.reflect;public class Employee{   public String _firstName;   public String _lastName;   private int _salary;      public Employee()   {      this( "John", "Smith", 50000);   }    public Employee(String fn, String ln, int salary)   {      _firstName = fn;      _lastName = ln;      _salary = salary;   }         public int getSalary()   {      return _salary;   }      public void setSalary(int salary)   {      _salary = salary;   }      public String toString()    {      return "Employee: " + _firstName +  " "             + _lastName + " " + _salary;   }}

The easiest way to get a class object is to just ask an object instance for its class with thegetClassmethod. The code below creates aEmployeeinstance, asks for its class, and prints out various bits of information about the class:

package com.russolsen.reflect;import java.lang.reflect.Modifier;public class GetClassExample{   public static void main(String[] args)   {       Employee employee = new Employee();            Class klass = employee.getClass();            System.out.println( "Class name: " + klass.getName());      System.out.println(             "Class super class: " + klass.getSuperclass());            int mods = klass.getModifiers();      System.out.println(             "Class is public: " + Modifier.isPublic(mods));      System.out.println(             "Class is final: " +  Modifier.isFinal(mods));      System.out.println(             "Class is abstract: " + Modifier.isAbstract(mods));    }}

Run the code above and you will see something like:

Class name: com.russolsen.reflect.EmployeeClass super class: class java.lang.ObjectClass is public: trueClass is final: falseClass is abstract: false

As the example shows, getting the class name and super-class is as easy as calling some accessors. If you are interested in knowing if the class is public or abstract or final, the process is only slightly more complicated. All of this information comes packaged up in a singleintreturned bygetModifiers.Fortunately, Java supplies you with theModifierclass, which is contains a bunch of static methods that will help you make sense of the number returned bygetModifiers.

CallinggetClasson an instance is not the only way to get hold of a class object. You can also get it directly from a class name:

Class klass = Employee.class;

The third way of getting hold of a class is the interesting one: you can create aClassobject from a string -- well only if that string happens to contain the name of a class. The way to do this is to call theforNameclass method onClass:

Class klass = Class.forName("com.russolsen.reflect.Employee");            System.out.println( "Class name: " + klass.getName());      System.out.println(             "Class super class: " + klass.getSuperclass());            // Print out the rest...

One thing to keep in mind aboutforNameis that you need to supply the full, complete with the package, name of the class. Plain old "Employee" will not do, it has to be "com.russolsen.reflect.Employee". WithforNamewe begin to get a glimpse at some of the fundamental power (and coolness) of the reflection API: you can start with a class name in a string and end up with the class.

Instances Instantly

Getting hold of a class and finding all about it is interesting and useful in its own right, but actuallydoingsomething with it is where reflection gets really exciting. The most obvious thing to do with aClassobject is to make a new instance of that class. The simple way to do this is with thenewInstancemethod. To show all this in action, the little program below takes a command line argument (which should contain a class name) creates a class from it and then makes a new instance of that class:

package com.russolsen.reflect;public class NewInstanceExample{   public static void main(String[] args)      throws ClassNotFoundException,      InstantiationException, IllegalAccessException   {      Class klass = Class.forName(args[0]);      Object theNewObject = klass.newInstance();      System.out.println("Just made: " + theNewObject);   }}

Run the code above with "com.russolsen.reflect.Employee" as an argument and you will manufacture a brand newEmployeeobject:

Just made: Employee: John Smith 50000

Run it again, but this time feed it "java.util.Date" and you will get:

Just made: Tue Feb 27 20:25:20 EST 2007

Think about how much flexibility you get with just a few lines of code: the program above really knows nothing aboutEmployeeorDateand yet it can create a new instance of either. This is a different way of doing Java.

Beyond the no-args Constructor

It turns out that calling theClass.newInstancemethod is identical to doing an ordinarynewwith no arguments. But what happens if you callnewInstanceon a class that lacks a no argument constructor? Nothing good: you are in for an unpleasantInstantiationException.

The good news is that you can dynamically create new instances of a class that requires constructor arguments -- you just have to work a bit harder. What you need to do is to find the constructor that you need from the class and call it with the right arguments. The way to find the constructor of your dreams is to call thegetConstructormethod with a description of the constructor that you are looking for. What you will get back is aConstructorobject, which you can use to create a new instance.

Let's see how this all works in code:

Class klass = Class.forName("com.russolsen.reflect.Employee");      Class[] paramTypes = {            String.class,             String.class,             Integer.TYPE };            Constructor cons = klass.getConstructor(paramTypes);            System.out.println( "Found the constructor: " + cons);            Object[] args = {             "Fred",             "Fintstone",             new Integer(90000) };            Object theObject = cons.newInstance(args);      System.out.println( "New object: " + theObject);

The only difference between the constructors are the parameters they take, so the way that you tellgetConstructorwhich constructor you are looking for is to pass it an array ofClass's, one for each parameter. The example above goes looking for a constructor that takes two strings and anint.Once you have aConstructor,creating a new object instance with it is very simple: you just call thenewInstancemethod, passing in another array, this time an array full of the actual argument values.

There is a nice little land mine hidden ingetConstructor.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 primitiveintor its uncle, a object of classjava.lang.Integer?If you are looking for a constructor that takes the object wrapper type, something likejava.lang.Integer,just use the wrapper class,Integer.classIf you mean the primitiveintyou will need useInteger.TYPE,which is a sort of placeholder for the class that the primitive lacks. All of the wrapper classes have a staticTYPEfield which you can use to indicate a primitive of that type.

Digging Deeper into A Class

As you saw in the first example, aClassobject 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 thegetMethodsmethod:

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

getMethodsreturns an array ofMethodobjects, 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.InterruptedExceptionFound a method: public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedExceptionFound a method: public final void java.lang.Object.wait() throws java.lang.InterruptedExceptionFound 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()

SincegetMethodstakes a client's view of the class, the array will containsallof the public methods, including those defined in the class itself and in its super class and its super-duper class and so on up toObject.

If you are interested in a single method, you can usegetMethod(note that it is singular), which works a lot likegetConstructor,except that you need to pass in a method name as well as the parameter types. The code below looks for a method calledsetSalarythat takes a singleintparameter:

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 theMethodobject 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 thesetSalarymethod on anEmployeeobject 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 typingtheObject.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 thesetSalarymethod 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 togetMethods, getFieldsreturns an array ofFieldobjects, 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);      }

SinceEmployeehas exactly two public fields, you get a two member array back:

Class name: com.russolsen.reflect.EmployeeFound field: public java.lang.String com.russolsen.reflect.Employee._firstNameFound field: public java.lang.String com.russolsen.reflect.Employee._lastName

You can also look for a specific field by name with thegetFieldmethod:

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

Once you have aFieldobject, getting the field value is as simple as calling thegetmethod, while setting the value is just a call tosetaway:

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_salaryfield change before your very eyes:

Old first name is: TomNew 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 theMethodinstance that goes with the method you want to call. You can't get this fromgetMethod;it only returns public methods. The way to get hold of a private (or protected) method is to usegetDeclaredMethod.WhilegetMethodtakes a client's view of a class and only returns public methods,getDeclaredMethodreturnsallof the methods declared by one class. In the example below, we use it to get at theMethodobject for the very privateremoveRangemethod on thejava.util.ArrayListclass:

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 aprivatemethod, calling it is a simple matter of clicking off the finalsetAccessablesafety and having at it:

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

TheremoveRangemethod 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 ajava.utilclass. Would you want to make a habit of circumventing the clear intent of the code and callingprivatemethods? 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

• Reflection tutorial
• In depth article about reflection from IBM's developerWorks

Russ Olsen