1 Java object model Part 3: Serialization & Reflection

2 Serialization Denotes the process of storing an object to a stream without converting to an external representataion Serialization allows an object to be persistent: that is, to exist separate from the program that created it When an object is serialized, it is transformed into a sequence of bytes; this can later be restored to the original object

3 Java object serialization In order for an object to be serialized, it must implement the Serializable interface Like Cloneable, Serializable is devoid of methods To serialize an object, invoke the writeObject() method of the ObjectOutputStream class; to deserialize, invoke the readObject() method of ObjectInputStream

4 Example // write data to output file: MyClass[] students = new MyClass[12];... ObjectOutputStream outs = new ObjectOutputStream (new FileOutputStream(“myfile.dat”)); outs.writeObject(students); outs.close(); // read data from input file: ObjectInputStream ins = new ObjectInputStream (new FileInputStream(“myfile.dat”)); MyClass students = (MyClass[]) ins.readObject(); ins.close();

5 Serialization & dependent objects Serialization automatically takes into account any additional referenced objects - follows all references contained in the object being serialized & serializes them Thus, any objects contained within a serializable object must also implement Serializable

6 Standard Java classes & serialization Many standard classes implement Serializable Examples: –String –ArrayList: means entire list of objects could be stored in a single operation

7 The transient modifier We may wish to exclude some information when serializing an object: –passwords or other confidential data –extraneous data (whether or not a particular graphical object was in a selected state, e.g.) Reserved word transient can be used to modify variable declaration; such a variable will not be represented when the object is serialized

8 Reflection Mechanism by which a program can analyze its objects & their capabilities at runtime reflection classesJava API includes several reflection classes, described on next slide

9 Reflection Classes Class: describes a type Package: describes a package Field: describes field; allows inspection, modification of all fields Method: describes method, allows its invocation on objects Constructor: describes constructor, allows its invocation Array: has static methods to analyze arrays

10 Class class object Includes class name and superclass of an object, as we have already seen; also includes: –interface types class implements –package of class –names & types of all fields –names, parameter types, return types of all methods –parameter types of all constructors

11 Class class methods getSuperClass() returns Class object that describes superclass of a given type; returns null is type is Object or is not a class getInterfaces() returns array of Class objects describing interface types implemented or extended; if type doesn’t implement or extend any interfaces, returns array of length 0 (only returns direct superinterfaces)

12 Class class methods getPackage() returns Package object; Package has getName() method which returns String containing package name getDeclaredFields() returns array of Field objects declared by this class or interface –includes public, private, protected & package- visible fields –includes both static & instance fields –does not include superclass fields

13 Field class methods getName(): returns String containing field name getType(): returns Class describing field type getModifiers(): returns an int whose various bits are set to indicate whether field is public, private, protected, static, or final: use static Modifier methods isPublic, isPrivate, isProtected, isStatic, isFinal to test this value

14 Example - prints all static fields of java.lang.Math Field[] fields = math.class.getDeclaredFields(); for (int x = 0; x < fields.length; x++) if(Modifier.isStatic(fields[x].getModifiers())) System.out.println(fields[x].getName());

15 More Class class methods getDeclaredConstructors() returns array of Constructor objects describing class constructors Constructor object has method getParameterTypes that returns an array of Class objects describing the constructor’s parameters

16 Example: printing Rectangle constructor information Constructor cons = Rectangle.class.getDeclaredConstructors(); for (int=0; x < cons.length; x++) { Class [] params = cons[x].getParameterTypes(); System.out.println(“Rectangle(”); for (int y=0; y < params.length; y++) { if(y > 0) System.out.print(“, ”); System.out.print(params[y].getName()); } System.out.println(“)”); }

17 Output from example Rectangle() Rectangle(java.awt.Rectangle) Rectangle(int, int, int, int) Rectangle(int, int) Rectangle(java.awt.Point, java.awt.Dimension) Rectangle(java.awt.Point) Rectangle(java.awt.Dimension)

18 Class class’s getDeclaredMethods() method Returns array of Method objects Method object methods include: –getParameterTypes(): returns array of parameter types –getName(): returns method name –getReturnType(): returns Class object describing return value type

19 Obtaining single Method or Constructor objects Class’s getDeclaredMethod() (note the singular) returns a Method object if supplied with a method name and array of parameter objects: Method m = Rectangle.class.getDeclaredMethod(“contains”, new Class[]{int.class, int.class}); For Constructor object: Constructor c = Rectangle.class.getDeclaredConstructor(new Class[] {});

20 Method methods invoke(): can be used to call a method described by a Method object - need to: –supply implicit parameter (null for static methods) –supply array of explicit parameter values (need to wrap primitive types) –if method returns a value, invoke returns an Object; need to cast or unwrap return value, as appropriate

21 Example - saying hello the hard way import java.lang.reflect.*; import java.io.*; public class SayHello { public static void main(String[]args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException { Method m = PrintStream.class.getDeclaredMethod (“println”, new Class[] {String.class}); m.invoke(System.out, new Object[] {“Hello!}); }

22 Example with return value Method m = Math.class.getDeclaredMethod(“sqrt”, new Class[] {double.class}); Object r = m.invoke(null, new Object[] {new Double(10.24)}); double x = ((Double) r).doubleValue();

23 Inspecting objects Can use reflection mechanism to dynamically look up object fields as program runs To allow access to a field, call its setAccessible() method; example: Class c = object.getClass(); Field f = c.getDeclaredField(name); f.setAccessible(true);

24 Inspecting objects Once granted access, you can read and write any field of the object: Object value = f.get(object); f.set(object, value); Notes: –f must be a Field that describes a field of object; otherwise, get and set throw exception –if field type is primitive, get returns & set expects a wrapper –if field is static, supply null for object

25 Inspecting array elements Field allows read/write access to arbitrary field of object; Array works similarly on array objects –for array a with index x: –get() method: Object value = Array.get(a,x) –set() method: Array.set(a,x,value);

26 Inspecting array elements Finding length of array a: int n = Array.getLength(a); Creating new array (twice as large) with static method newInstance(): Object newArray = Array.newInstance( a.getClass().getComponentType(), 2 * Array.getLength(a) + 1); System.arraycopy(a, 0, newArray, 0, Array.getLength(a)); a = newArray;