CS5233 Components Models and Engineering

Transcription

1 CS5233 Components Models and Engineering (Komponententechnologien) Master of Science (Informatik) Reflection Seite 1

2 Java Reflection Reflection Reflection is when you see yourself. Why would you like to see yourself? To adapt yourself to some special need. Refection in programming: a running program sees itself / manipulates itself. Refection uses metadata: Data about components of the running program. Reflection is essential for each Java-based run-time component technology. Seite 2

3 Java Reflection / Metadata Metadata Metadata = data about the program and it's parts Metadata access and usage through the following classes classes: java.lang.class<t> fields: java.lang.reflect.field constructors: java.lang.reflect.constructor<t> methods: java.lang.reflect.method arrays: java.lang.reflect.array modifiers: java.lang.reflect.modifier Seite 3

4 Java Reflection / Metadata Metadata usage examples Example: Access class-related metadata of an object. package reflectionp; public class Hello { public static void main(string[] args) { Hello hello = new Hello(); Class<?> clazz = hello.getclass(); System. out.println("class " + clazz + " was loaded by " + clazz.getclassloader()); Example: Use metadata to create an object package reflectionp; public class Hello { void sayhello() { System.out.println("Hello"); public static void main(string[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException { Class<?> helloclass = Class. forname ("reflectionp.hello" ); Hello hello = (Hello) helloclass.newinstance(); hello.sayhello(); Seite 4

5 Java Reflection / Metadata of Classes Class.forName Class.forName(String name [, boolean initialize, ClassLoader loader ]) create class object from class name initialize (default = true) initialize the class loader (default = current loader = this.getclass().getclassloader() ) Class-objects for primitve types primitive types have related class-objects: int.class, byte.class, may not be used to create objects (of course) may be used to check parameter or result types public class Hello { void sayhello() { System. out.println("hello"); public static void main(string[] args) throws ClassNotFoundException, SecurityException, NoSuchMethodException { Class<?> clazz = Class. forname ("reflectionp.hello"); Method m = clazz.getdeclaredmethod( "sayhello"); if (m.getreturntype().equals( void.class)) { System. out.println("sayhello is a void method" ); Seite 5

6 Java Reflection / Metadata of Classes Static and dynamic access to Class-objects Class<?> object.getclass () dynamic access to the class of an object returns the class of an object an instance of class Class Class. class static access to an instance of class Class Example: Class<String> classobject1 = String. class; String astring1 = classobject1.newinstance(); Class<?> classobject2 = Class. forname ("java.lang.string" ); String astring2 = (String) classobject2.newinstance(); System. out.println(astring1.getclass()); System. out.println(astring2.getclass()); static access to class-object generics allowed dynamic access to class-object generic not allowed (cast instead) class java.lang.string class java.lang.string Seite 6

7 Java Reflection / Metadata of Classes Static and dynamic checks on a class object instanceof <static type expression> expression of type boolean check of an object against a static type expression ClassObject. isinstance ( object ) method of class Class dynamic equivalent for instanceof check of an object against a class denoted by a Class object (a dynamically resolved type) Class<?> clazza = Class. forname ("reflectionp.animal" ); Class<?> clazzc = Class. forname ("reflectionp.cow" ); Animal Object o = clazzc.newinstance(); if (o instanceof Animal) { // OK o is an Animal Cow System. out.println(o + " is an Animal"); if (o instanceof Class.forName("reflectionP.Animal") ) { // Syntax-ERROR System. out.println(o + " is an Animal"); if (clazza.isinstance(o)) { // OK o is an Animal Possible only if System. out.println(o + " is an Animal"); we may refer statically to if (clazzc.isinstance(o)) { // OK o is a Cow class Animal! System. out.println(o + " is a Cow"); Seite 7

8 Java Reflection / Metadata of Classes Static and dynamic casts (<type>) object expression of static type <type> casts an object to a statically denoted type ClassObject. cast (object ) method of class Class casts object to the class dynamically represented by the ClassObject dynamic equivalent of a cast Animal Class<?> clazza = Class. forname ("reflectionp.animal" ); Class<?> clazzc = Class. forname ("reflectionp.cow" ); Class<?> clazzt = Class. forname ("reflectionp.tiger"); Cow Tiger Object o = clazzc.newinstance(); Animal Animal Object Object a1 a2 a2 a3 = = = = (Animal) o; // OK (Class.forName("reflectionP.Animal")) o; // Syntax ERROR clazza.cast(o); clazzt. cast(o); // Class cast exeception Possible only if we may refer statically to class Animal! Seite 8

9 Java Reflection / Metadata of Classes assubclass classobject. assubclass (anotherclassobject ) casts classobject to represent a subclass of anotherclassobject usually you work with statically known interfaces and dynamically loaded implementations of the interface sometimes however you have to feed a method with a class-object of a certain subtype Interface MilkProvider is implemented by class ("unchecked") static Class<? extends MilkProvider> getmp1(string classname) throws ClassNotFoundException { Class<?> clazzc = Class. forname (classname); return (Class<? extends MilkProvider>) clazzc; static cast: not type save static Class<? extends MilkProvider> getmp2(string classname) throws ClassNotFoundException { Class<?> clazzc = Class. forname (classname); return clazzc.assubclass(milkprovider.class); assubclass to safely cast to type bound Class<? extends MilkProvider> clazz = getmp 1/2 ("reflectiontest.cow" ); MilkProvider mp = clazz.newinstance(); mp.milk(); same usage for both versions Seite 9

10 Java Reflection / Metadata of Classes isclassproperty Checks classobject. isproperty() methods check properties of classes represented by a classobject properties checks are: isannotation() isannotationpresent(class<? extends Annotation> annotationclass) isanonymousclass() isarray() isassignablefrom(class<?> cls) isenum() isinstance(object obj) isinterface() islocalclass() ismemberclass() isprimitive() issynthetic() Seite 10

11 Java Reflection / Arrays Arrays Class-objects for array types array types have related class-objects, that are referenced through codings: [Lclass-name; Array of class-name objects [code Array of primitive-values according to the code as may [-s as dimensions Codings for primitive types: I ~ int, J ~ long, B ~ byte, F ~ float, D ~ double, Z ~ boolean access and use metadata of arrays using java.lang.reflect.array create array-objects with Array.newInstance(Class<?> componenttype, int length) access component type using java.lang.class java.lang.class.getcomponenttype int[] a = (int[]) Array. newinstance (int.class, 2); a[0] = 1; Array. set (a, 1, 2); System. out.println(a[1] + (Integer)Array. get (a, 0)); Class<?> clazz = a.getclass().getcomponenttype(); int[] b = (int[]) Array.newInstance( clazz, 2); b[0] = 1; Seite 11

12 Java Reflection / Arrays Arrays Example Hello[] ha = new Hello[2]; System. out.println(ha.getclass()); //->class [LreflectionP.Hello; Class<?> clazz1 = Class. forname ("[I"); Object o1 = clazz1.newinstance(); //ERROR java.lang.instantiationexception int[][] aa = (int[][]) Array. newinstance (Class. forname ("[I"), 2); aa[0] = (int[]) Array. newinstance (int.class, 2); aa[1] = (int[]) Array. newinstance (int.class, 3); aa = (int[][]) Array.newInstance(Class.forName( "[[I"), 2); Hello[][] hh = (Hello[][]) Array.newInstance(Class.forName( "[LreflectionP.Hello;" ), 1); hh[0] = new Hello[1]; hh[0][0] = (Hello) Class.forName( "reflectionp.hello").newinstance(); Class.newInstance may not be used to create arrayinstances but it may be used to create an array-class object that is used as component type by Array.newInstance Seite 12

13 Java Reflection / Constructors Constructors default constructor is invoked by Class.newInstance other constructors: lookup with Class.getConstructor invoked via Constructor.newInstance Class<?> clazz = Class. forname ("reflectionp.name" ); Object o1 = clazz.newinstance(); // newinstance calls the default constructor //create constructor object from parameter types Constructor<?> constructor = clazz.getconstructor( new Class[]{String. class); //create instance using constructor object and arguments Object o2 = constructor.newinstance( new Object[]{"Karla"); System. out.println(o1); //-> Hugo System. out.println(o2); //-> Karla package reflectionp; class Name { private String name; public Name() { name = "Hugo"; // must be public to be used via reflection public Name(String name){ this.name = public String tostring() { return name; Seite 13

14 Java Reflection / Methods Methods lookup public method with class.getmethod in class or a superclass lookup any method with class.getdeclaredmethod in class invocation with method.invoke Class<?> clazz = Class. forname ("reflectionp.name" ); Object o = clazz.newinstance(); // tostring with no parameters Method m = clazz.getmethod("tostring", new Class[]{); System. out.println( m.invoke(o, new Object[]{) ); package reflectionp; class Name { private String name; public Name() { name = "Hugo"; // must be public to be used via reflection public Name(String name){ this.name = public String tostring() { return name; Seite 14

15 Java Reflection / Methods class Person { private String name; public Person() { name = "Hugo"; public Person(String name){ this.name = public String tostring() { return "Person "+ name; Methods / Example class Student extends Person { public Student() { super("carla"); public Student(String name){ public String tostring() { return "Cand. inf. "+ super.tostring(); Class<?> clazz = Class. forname ("reflectionp.student" ); Object o = clazz.newinstance(); Method m = clazz.getmethod( "tostring", new Class[]{); System. out.println( m.invoke(o, new Object[]{)); Method superm = Object. class.getdeclaredmethod("tostring", new Class[]{); System. out.println( superm.invoke(o, new Object[]{)); dynamic binding! Cand. inf. Person Carla Cand. inf. Person Carla Seite 15

16 Java Reflection / Fields Fields lookup public field with class.getfield in class or a superclass lookup any field with class.getdeclaredfield in class invocation with method.invoke Class<?> clazz = Class. forname ("reflectionp.student"); Object o = clazz.newinstance(); Field f = clazz.getsuperclass().getdeclaredfield( "name"); f.set(o, "Egon"); // ERROR: private, not accessible f.setaccessible( true); // set field to accessible f.set(o, "Egon"); Method m = clazz.getmethod("tostring", new Class[]{); System. out.println( m.invoke(o, new Object[]{) ); classes Student and Person as above Seite 16

17 Java Reflection / Modifiers Modifiers Modifiers are represented as bits in an integer Modifiers are accessed using method getmodifiers() on a java.lang.class -object java.lang.reflect.method -object java.lang.reflect.field -object Check for modifiers using static methods of java.lang.reflect.modifier e.g. Class<?> clazz = Class. forname ("reflectionp.student"); int modifiers = clazz.getmodifiers(); if (Modifier. isinterface (modifiers)) { System. out.println(clazz + " is an interface"); else { System. out.println(clazz + " is not interface" ); Modifiers are: abstract, static, final, interface, native, private, protected, public, static, strictfp, synchronized, transient, volatile Seite 17