Chair of Software Engineering. Languages in Depth Series: Java Programming. Prof. Dr. Bertrand Meyer. Exercise Session 10

Transcription

1 Chair of Software Engineering Languages in Depth Series: Java Programming Prof. Dr. Bertrand Meyer Exercise Session 10

2 Today s Exercise Session Pattern of the Day Proxy Quizzes 2

3 Proxy Pattern Structural pattern A proxy provides a surrogate or placeholder for another object to control access to it. One reason for controlling access to an object is to defer the full cost of creation/initialization; e.g., deferred loading of images: until the image is loaded, the editor is dealing with an image proxy object. Another reason might be to inject additional code before and/or after a method; e.g., a proxy object that counts the number of method calls. 3

4 Proxy - Structure Proxy Maintains a reference to a real subject Provides an interface identical to Subject s Controlls access to the real subject; may be also responsible for creation/deletion Other responsibilities, depending on the proxy type (see applicability) 4

5 Proxy - Structure Subject Defines the common interface for RealSubject and Proxy so that Proxy can be used anywhere a RealSubject is expected. RealSubject Defines the real object that the proxy represents 5

6 Proxy - Applicability Proxy is applicable whenever there is a need for a more sophisticated object reference than a simple pointer Remote proxy Represents an object in a different address space. Virtual proxy Creates expensive objects on demand Protection proxy Control access to RealSubject (e.g., KernelProxies, locking) Smart reference Perform additional actions on access Reference counting, synchronization, debug-output etc. 6

7 Proxy - Example (Subject and RealSubject) public interface ITable { public Object getelementat(int row, int column); public void setelementat(object element, int row, int column ); public int getnumberofrows(); public class Table implements ITable {... public Object getelementat(int row, int column) {... public void setelementat(object element, int row, int column ) {... public int getnumberofrows() {... 7

8 Proxy - Example II public class RowLockTableProxy implements ITable { Table realtable; Integer[] locks; public RowLockTableProxy(Table tolock) { realtable = tolock; locks = new Integer[toLock.getNumberOfRows()]; for (int row = 0; row < tolock.getnumberofrows(); row++) locks[row] = new Integer(row); public Object getelementat(int row, int column) { synchronized (locks[row]) { return realtable.getelementat(row, column); public void setelementat(object element, int row, int column) { synchronized (locks[row]) { return realtable.setelementat(element, row, column); public int getnumberofrows() { return realtable.getnumberofrows(); 8

9 Proxy Consequences The Proxy pattern introduces a level of indirection when accessing an object. Potential uses: A remote proxy can bridge different (remote) address spaces A virtual proxy can perform optimization such as creating objects on demand Protection and smart proxies allow additional housekeeping tasks on object access And many other potential usages Copy-on-write objects Caching Firewall proxies (protect from bad clients or targets) 9

10 Proxy Related Patterns Adapter An adapter provides a different interface to the adapted object, whereas the proxy provides the same Decorator Very similar implementations, but different purpose Decorators add one or more responsibilities to an object, whereas a proxy controls access to an object Flyweight The flyweight pattern is often combined with the proxy to minimize memory footprint by sharing data between similar objects 10

11 Hidden Fields In this program, a subclass field has the same name as a superclass field. What does the program print? class Base { public String classname = "Base"; class Derived extends Base { private String classname = "Derived"; public class PrivateMatter { public static void main(string[] args) { System.out.println(new Derived().className); 11

12 Hidden Fields In this program, a subclass field has the same name as a superclass field. What does the program print? class Base { public String classname = "Base"; class Derived extends Base { private String classname = "Derived"; public class PrivateMatter { public static void main(string[] args) { System.out.println(new Derived().className); Compile Error: PrivateMatter.java:11: classname has private access in Derived System.out.println(new Derived().className); ^ 12

13 Hidden Fields One might think that class Derived won't compile, because the variable classname in Derived has more restrictive access than it does in Base. Had classname been an instance method, Derived.className() would have overridden Base.className(), and the program really would have been illegal. Because classname is a field, Derived.className just hides Base.className. Hiding fields can define different access modifiers or even types than the hidden field (but they should not ). The compilation error occurs in class PrivateMatter where Derived.className is accessed. Although Base has a public field classname, this field is not inherited into Derived because it is hidden by Derived.className. But this field is declared private and therefore is not accessible. 13

14 Unwelcome Guest The program in this puzzle models a system that attempts to read a user ID from its environment, defaulting to a guest user if the attempt fails. The author of the program was faced with a situation whereby the initializing expression for a static field could throw an exception. Because Java doesn't allow static initializers to throw checked exceptions, the initialization must be wrapped in a try-finally block. What does the following program print? 14

15 Unwelcome Guest public class UnwelcomeGuest { public static final long GUEST_USER_ID = -1; private static final long USER_ID; static { try { USER_ID = getuseridfromenvironment(); catch (IdUnavailableException e) { USER_ID = GUEST_USER_ID; System.out.println("Logging in as guest"); private static long getuseridfromenvironment() throws IdUnavailableException { throw new IdUnavailableException(); // Simulate an error public static void main(string[] args) { System.out.println("User ID: " + USER_ID); class IdUnavailableException extends Exception { IdUnavailableException() { 15

16 Unwelcome Guest public class UnwelcomeGuest { public static final long GUEST_USER_ID = -1; private static final long USER_ID; static { try { USER_ID = getuseridfromenvironment(); catch (IdUnavailableException e) { USER_ID = GUEST_USER_ID; System.out.println("Logging in as guest"); private static long getuseridfromenvironment() throws IdUnavailableException { throw new IdUnavailableException(); // Simulate an error public static void main(string[] args) { System.out.println("User ID: " + USER_ID); class IdUnavailableException extends Exception { IdUnavailableException() { Compile Error: UnwelcomeGuest.java:10: variable USER_ID might already have been assigned USER_ID = GUEST_USER_ID; ^ 16

17 Unwelcome Guest USER_ID is a blank final, a final field whose declaration lacks an initializer Determining whether a program can perform more than one assignment to a blank final is a hard problem. It is equivalent to the classic halting problem, which is known to be unsolvable in general Java takes a conservative approach : A blank final field can be assigned only at points in the program where it is definitely unassigned Therefore, there are some provably safe programs must be rejected Possible solution: turn the offending field from a blank final into an ordinary final, replacing the static initializer block with a static field initializer: public class UnwelcomeGuest {... private static final long USER_ID = getuseridorguest(); private static long getuseridorguest() { try {... catch {

18 Serialization The following program creates an object and checks that it obeys a class invariant. Then the program serializes the object, deserializes it, and checks that the deserialized copy also obeys the invariant. Does it? If not, why not? 18

19 Serialization public class SerialQuizz { public static void main(string[] args) { Sub sub = new Sub(666); sub.checkinvariant(); Sub copy = (Sub) deepcopy(sub); copy.checkinvariant(); // Copies its argument via serialization static public Object deepcopy(object obj) { try { ByteArrayOutputStream bos = new ByteArrayOutputStream(); new ObjectOutputStream(bos).writeObject(obj); ByteArrayInputStream bin = new ByteArrayInputStream(bos.toByteArray()); return new ObjectInputStream(bin).readObject(); catch (Exception e) { throw new IllegalArgumentException(e); 19

20 Serialization class Super implements Serializable { final Set<Super> set = new HashSet<Super>(); final class Sub extends Super { private int id; public Sub(int id) { this.id = id; set.add(this); // Establish invariant public void checkinvariant() { if (!set.contains(this)) throw new AssertionError("invariant violated"); public int hashcode() { return id; public boolean equals(object o) { return (o instanceof Sub) && (id == ((Sub)o).id); 20

21 Serialization class Super implements Serializable { final Set<Super> set = new HashSet<Super>(); final class Sub extends Super { Runtime Exception: Exception in thread "main" AssertionError private int id; at Sub.checkInvariant(SerialKiller.java:41) at SerialKiller.main(SerialKiller.java:10) public Sub(int id) { this.id = id; set.add(this); // Establish invariant public void checkinvariant() { if (!set.contains(this)) throw new AssertionError("invariant violated"); public int hashcode() { return id; public boolean equals(object o) { return (o instanceof Sub) && (id == ((Sub)o).id); 21

22 Serialization The error is caused by the readobject method of HashSet. This method can indirectly invoke an overridden method on an uninitialized object. What happens when we deserialize the Sub instance in the program: First, the Super fields of the Sub instance are serialized The only such field is set, which contains a reference to a HashSet. Each HashSet instance uses a HashMap with the hash set's elements as keys. The HashSet class has a readobject method that creates an empty HashMap and inserts a key-value mapping for each element in the set, using the map's put method. This method calls hashcode on the key to determine its bucket. In our program, the only key is the Sub instance whose set field is currently being deserialized. The id field of this instance has yet to be initialized, so it contains 0. Therefore, Sub is placed in the wrong bucket. 22

23 Serialization No good general solution: The serialization system violates the rule that you must not invoke an overridable method of a class from its constructor or pseudoconstructor Either delay population of the hash set until the validateobject callback Or ensure that contents of HashSet, HashMap, and Hashtable do not refer back on it, directly or indirectly. 23

24 Questions? 24

25 Exercise Session 11 Pattern of the Day Decorator Quizzes 25