Java Reflection Concept and Working

1. Advanced Java ProgrammingLecture-02: Reflection Aatif Kamal, Dept. of Computing Aatif.kamal@seecs.edu.pk Study Source: http://download.oracle.com/javase/tutorial/reflect/

2. Reflection Overview Reflection API The “Class" class Class methods Array class Member interface Method class invoking a method, throwing exceptions Field class accessible objects

3. Background Programs are just another kind of data Source code is text Manipulate it line by line, or by parsing expressions Compiled programs are data, too Integers and strings are bytes in memory that you interpret a certain way Instructions in methods are just bytes too No reason why a program can't inspect itself

4. How objects work Every object is either a reference or primitive type. object class Point { public Point(int x, int y) {  } public getX() {} public getY() {} protected int x, y; } 5 x 3 y class {…} Point(int,int) {…} getX() {…} getY()

5. 5 Defining Reflection Introduced in Java 1.1. Called the "Java Core Reflection API" Reflection is commonly used by programs which require the ability to examine or modify the runtime behavior of applications running in the Java virtual machine Allows you to find out information about any object, including its methods and fields, at run time. Can no longer get away with this called an enabling technology because it supports Java language elements such as: Java Beans, Serialization, and Remote Method Invocation (RMI). JBoss, Tomcat, Eclipse, etc. are reflection-based

6. 6 Reflection Can be Used To . . . construct new class instances and new arrays access and modify fields of objects and classes invoke methods on objects and classes access and modify elements of arrays

7. Uses of Reflection Extensibility Features An application may make use of external, user-defined classes by creating instances of extensibility objects using their fully-qualified names. Class Browsers and Visual Development Environments A class browser needs to be able to enumerate the members of classes. Visual development environments can benefit from making use of type information available in reflection to aid the developer in writing correct code. Debuggers and Test Tools Debuggers need to be able to examine private members on classes. Test harnesses can make use of reflection to systematically call a discoverable set APIs defined on a class, to insure a high level of code coverage in a test suite.

8. Drawbacks of Reflection Reflection is powerful, but should not be used indiscriminately. If it is possible to perform an operation without using reflection, then it is preferable to avoid using it. Performance Overhead Because reflection involves types that are dynamically resolved, certain Java virtual machine optimizations can not be performed. Slower performance than their non-reflective counterparts, should be avoided in sections of code which are called frequently in performance-sensitive applications. Security Restrictions Reflection requires a runtime permission which may not be present when running under a security manager. Code which has to run in a restricted security context, such as in an Applet.

9. Drawbacks of Reflection Exposure of Internals Since reflection allows code to perform operations that would be illegal in non-reflective code, Accessing private fields and methods, the use of reflection can result in unexpected side-effects, Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.

10. Class c = Class.forName(“java.lang.String”); Object o = c.newInstance(); Reflection - Classes Reflection is a dynamic language feature Used to query object and class information static Class Class.forName(String className) Obtain a java.lang.Class object i.e. Class.forName(“java.lang.String”) gets an object corresponding to class String Object Class.newInstance() Object constructor in disguise Create a new object of a given class This makes a new empty string.

11. Running Example Most typical use of reflection: Take a class name, make a Class object Create object of that class, cast and use it Statically convert Class.newInstancenew T() 1. String className = ...; 2. Class c = Class.forName(className); 3. Object o = c.newInstance(); 4. T t = (T) o; new T1(); new T2(); ...

12. 12 Reflection API Package java.lang.reflect Field class get name and access for an arbitrary field Method class get info about an arbitrary method (and invoke it) Constructor class get info about an arbitrary constructor (and invoke it) Classclass creates new instances of Field, Method, and Constructor Array class static methods to create and get info about arbitrary arrays ….. With the exception of java.lang.reflect.ReflectPermission, none of the classes in java.lang.reflect have public constructors.

13. java.lang.Class For every type of object, the Java virtual machine instantiates an immutable instance of java.lang.Class Class objects represent a loaded class Provides methods to examine the runtime properties of the object its methods its fields its superclass the interfaces it implements whether it's an array Provides the ability to create new classes and objects. Entry point for all of the Reflection APIs

14. 14 Obtaining a Class Object To get to these classes, it is necessary to invoke appropriate methods on Class. At compile time, using the symbolic class name: Class c1 = String.class; Class c2 = Employee[].class; At runtime, by calling getClass( ) on any object: Class c3 = obj.getClass( ); At runtime, by passing a class name (string) to the forName( ) static method: Class c = Class.forName( "java.util.Date" );

15. Retrieving Class Objects - Examples Object.getClass() the simplest way to get its Class is to invoke Object.getClass() Class c = "foo".getClass();  Returns the Class for String Class c = System.console().getClass();  the Class corresponding to java.io.Console. enum E { A, B }; Class c = A.getClass(); A is is an instance of the enum E; thus getClass() returns the Class corresponding to the enumeration type E. byte[] bytes = new byte[1024]; Class c = bytes.getClass(); Since arrays are Objects, it is also possible to invoke getClass() on an instance of an array. The returned Class corresponds to an array with component type byte.

16. Retrieving Class Objects - Examples The .class Syntax If the type is available but there is no instance then it is possible to obtain a Class by appending ".class" to the name of the type. This is also the easiest way to obtain the Class for a primitive type. boolean b; Class c = b.getClass(); // compile-time error Class c = boolean.class; // correct Class c = java.io.PrintStream.class; Class c = int[ ][ ][ ].class;

17. Retrieving Class Objects - Examples Class.forName() If the fully-qualified name of a class is available, it is possible to get the corresponding Class using the static method Class.forName() Class driverObject = Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver"); Class c = Class.forName("com.duke.MyLocaleServiceProvider") TYPE Field for Primitive Type Wrappers Two ways .class syntax, more convenient and the preferred way Class c = boolean.class;Class c1 = String.class;Class c2 = Employee[].class; All primitive types & void has a wrapper class in java.lang, used for boxing of primitive types to reference types. Each wrapper class contains a field named TYPE which is equal to the Class for the primitive type being wrapped. Class c = Double.TYPE;

18. 18 Class Methods(1 of 4) public String getName( ); Returns the name of the class referred to by the Class object. public booleanisInterface( ); Returns true if the Class object refers to an interface. public booleanisArray( ); Returns true if the Class object refers to an array type. public Class getSuperclass( ); Returns the superclass of the current Class object.

19. 19 Class Methods (2 of 4) public Class[] getInterfaces( ); Returns array of interface classes implemented by this class. public Class[] getClasses( ); Returns array of inner classes within this class. public Object newInstance( ); Creates and returns an instance of this class. public static Class forName( String name ); Returns a Class object corresponding to a class name (static method)

20. 20 Class Methods (3 of 4) public Constructor[] getConstructors( ); Returns an array of all public constructors in the current class. (import java.lang.reflect.Constructor) public Method[] getDeclaredMethods( ); Returns an array of all public and private methods declared in the current class or interface. (import java.lang.reflect.Method) public Method[] getMethods( ); Returns an array of all public methods in the current class, as well as those in all superclasses and superinterfaces.

21. Code Examples ReflectMethod.java & ReflectMethod2.java Class, Parent Class, Package name Show all methods name, signatures – use getMethods() Bypassing Abstraction SpecificMethodInfoDemo.java Showing method directly using getMethod() Bridge method Concept SampleInvoke .java,RunMethod.java & InovkeMain.java How to execute a method – using Method.invoke() Recursion through Reflection PrivateMethod.java Executing private methods of a class

22. 22 Member Interface Implemented by Constructor, Method, and Field Class getDeclaringClass( ) returns the Class object representing the class or interface that declares the member or constructor represented by this Member. intgetModifiers( ) returns the Java language modifiers for the member or constructor represented by this Member, as an integer. String getName( ) returns the simple name of the underlying member or constructor represented by this Member.

23. 23 Using a Method Object Using a Method object, you can... get its name and parameter list and invoke the method Obtain a Method from a signature, or get a list of all methods. To specify the signature, create an array of Class objects representing the method’s parameter types. Array will be zero-length if no parameters Special Class objects for primitives

24. 24 Representing the Primitive Types Special Class objects representing the eight primitive types: Byte.TYPE, Character.TYPE, Integer.TYPE, Long.TYPE, Short.TYPE, Double.TYPE, Float.TYPE, Boolean.TYPE Void Class type: Void.TYPE Also Class types for arrays, such as ... class type for int[ ] is Integer.TYPE[].class class type for int[ ][ ] is Integer.TYPE[][].class

25. 25 Method Class public class Method implements Member { public Class getReturnType( ); public Class[] getParameterTypes( ); public String getName( ); public intgetModifiers( ); public Class[] getExceptionTypes( ); public Object invoke( Object obj, Object[] args); } The modifiers are stored as a bit pattern; class Modifier has methods to interpret the bits.

26. 26 Method Examples Retrieve the name of a method: Method meth = c1.getDeclaredMethods( ); String name = meth.getName( ); Retrieve an array of parameter types: Class parms[] = meth.getParameterTypes( ); Retrieve a method's return type: Class retType = meth.getReturnType( );

27. 27 Method.invoke( ) public Object invoke(Object obj, Object[] args) If the parameters or return types are primitives, they are wrapped using one of the eight wrapper classes. example: Integer.TYPE The first parameter to invoke is the controlling object (use null for static methods) The second parameter is the parameter list array of objects Disadvantages to using invoke( ): executes more slowly than static invocation you have to handle all checked exceptions you lose lots of compile-time checks

28. 28 Exceptions and Invoke( ) If invoked method throws an exception, invoke( ) will throw an InvocationTargetException get the actual exception by calling getException Lots of other exceptions to worry about before you call invoke: Did class load? ClassNotFoundException Was method found? NoSuchMethodException Can you access method? IllegalAccessException

29. 29 Example: Invoking main( ) Calling: main( String[] args ) Simplified, with no error checking: Class cl = Class.forName( className ); Class[] paramTypes = new Class[] { String[].class }; Method m = cl.getDeclaredMethod( "main", paramTypes ); Object[] args = new Object[] { new String[] { "Breathing", "Fire" } } m.invoke( null, args );

30. 30 Invoking a Constructor Call getConstructor( ), then call newInstance( ) catch InstantiationException Class c1 = Class.forName("Villain"); Class[] paramTypes = new Class[] {String.class,Integer.TYPE }; Constructor m = c1.getConstructor( paramTypes ); Object[] arguments = new Object[] { "Darth Vader", new Integer(20) }; Villan v = (Villan) m.newInstance(arguments);

31. 31 Member Interface Implemented by Constructor, Method, and Field Class getDeclaringClass( ) returns the Class object representing the class or interface that declares the member or constructor represented by this Member. intgetModifiers( ) returns the Java language modifiers for the member or constructor represented by this Member, as an integer. String getName( ) returns the simple name of the underlying member or constructor represented by this Member.

32. Code Examples ReflectMethod.java & ReflectMethod2.java Class, Parent Class, Package name Show all methods name, signatures – use getMethods() Bypassing Abstraction SpecificMethodInfoDemo.java Showing method directly using getMethod() Bridge method Concept SampleInvoke .java,RunMethod.java & InovkeMain.java How to execute a method – using Method.invoke() Recursion through Reflection PrivateMethod.java Executing private methods of a class

33. 33 The Array Class import java.lang.reflect.Array; public class Array { // all static methods: public int getLength( Object arr ); public Object newInstance( Class elements, int length ); public Object get( Object arr, int index ); public void set( Object arr, int index, Object val ); // Various specialized versions, such as... public int getInt( Object arr, int index ); public void setInt( Object arr, int index, int val ); }

34. 34 Array Samples Canine[] kennel = new Canine[10]; . int n = Array.getLength( kennel ); // set the contents of an array element Array.set( kennel, (n-1), new Canine( "Spaniel" ) ); // get an object from the array, determine its class, // and display its value: Object obj = Array.get( kennel, (n-1) ); Class c1 = obj.getClass( ); System.out.println( c1.getName( ) + "-->" + obj.toString( ) );

35. 35 Two Ways to Declare an Array // first: Canine kennel = new Canine[10]; // second: Class c1 = Class.forName( "Canine" ); Canine kennel = (Canine[]) Array.newInstance( c1, 10 ); next: expanding an Array

36. 36 Example: Expanding an Array Problem statement: write a function that receives an arbitrary array, allocates storage for twice the size of the array, copies the data to the new array, and returns the new array

37. 37 Example: Expanding an Array Why won't this code work? public static Object[] doubleArrayBad( Object[] arr ) { intnewSize = arr.length * 2 + 1; Object[] newArray = new Object[ newSize ]; for( inti = 0; i < arr.length; i++ ) newArray[ i ] = arr[ i ]; return newArray; }

38. 38 Example: Expanding an Array Ans: This method always returns an array of Object, rather than the type of the array being copied. public static Object[] doubleArrayBad( Object[] arr ) { intnewSize = arr.length * 2 + 1; Object[] newArray = new Object[ newSize ]; for( inti = 0; i < arr.length; i++ ) newArray[ i ] = arr[ i ]; return newArray; }

39. 39 Example: Expanding an Array Use reflection to get the array type: public Object[] doubleArray( Object[] arr ) { Class c1 = arr.getClass( ); if( !c1.isArray( ) ) return null; intoldSize = Array.getLength( arr ); intnewSize = oldSize * 2 + 1; Object[] newArray = (Object[]) Array.newInstance( c1.getComponentType( ), newSize ); for( inti = 0; i < arr.length; i++ ) newArray[ i ] = arr[ i ]; return newArray; } see ReflectionArrayTestjava

40. Assignment 02 Extension One to Assignment 01 Send a class object from one machine to another machine On recipient side through reflection show complete description of object Class, Parent class, Package, interface implemented Class Properties, their access modifier, datatype, name & value Class methods, list all methods, access modifiers, static/non static, return Type, Name of method, Input parameters, exceptions And execute any static method object on the recipient side. Save this assignment code as different version.

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