Advanced programming for Java platform. Introduction

Transcription

1 Advanced programming for Java platform Introduction

2 About course Petr Hnětynka continuation of "Java (NPRG013)" basic knowledge of Java is expected (in the scope of NPRG013) 0/2 Z

3 Zápočet creating a project agreeing a topic till Friday 17 th May 2013 by appropriately complex topic non-trivially exploiting a covered technology the project should be submitted till the end of June the latest deadline Friday 27 th September :00 (noon) submission by ; only if it is necessary the project is shown personally

4 Course synopsis In-depth view of the Java language Overview and history of the Java platform Tips and tricks with the Java type system Reflection API Generics, annotations ClassLoaders, Security Distributed technologies: RMI,... Component model JavaBeans JEE: EJB, Servlets, JSP, Spring,... JME: CLDC, MIDP, JavaCard Other Java technologies: Java APIs for XML, JDBC, JMX,... Android

5 Literature, links Everything about Java ( Java tutorial ( Java Language Specification Thinking in Java

6 Java Basic overiew, history,...

7 Java object oriented (almost) all is object interpreted source code (.java) compiled to the bytecode bytecode (.class) interpreted by the virtual machine platform independent programs run in the virtual machine

8 Historie JDK JDK Inner classes Java 2 platform 2000 JDK 1.2, 1.3 changes in libraries only JDK Assert JDK changes in the language generics annotations... JDK JDK small changes in the language JDK big changes in the language

9 Java platform JSE standard edition JEE enterprise edition JME micro edition

10 Performance originally (~ JDK 1.1, 1998) Java programs 6 times slower than C now: Just-In-Time (JIT) compilation during launching the program is compiled to native code native code is executed slow start, then fast performance ~ comparable with native applications big memory consumption

11 Java implementations Oracle (Sun Microsystems) "official" implementation Windows, Solaris, Linux Blackdown (do 1.5) open-source implementation of JVM for Linux based on the code of Sun JVM IBM IBM JDK Jikes Jikes compiler open-source fast strict compatibility with Java (till 1.5) Jikes RVM

12 Java implementations Jikes RVM Research Virtual Machine open-source for testing extensions written in Java "self-hosting" does need another JVM to run boot-image writer a Java program, which is executed in an existing JVM boot-image loader a program written in C++ does not support complete Java API

13 Java implementations OpenJDK open-source supported by Oracle (Sun Microsystems) official implementation created from OpenJDK

14 Other... (mostly historical ) GNU classpath an implementation of core Java libraries Guavac compiler Kaffe virtual machine Apache Harmony open source Java SE...

15 Android zdroj:

16 Bck2brwsr Java running in a browser Project goals Create small Java capable to boot fast and run in 100% of modern browsers including those that have no special support for Java. Demonstrate that Java has benefits over JavaScript when creating larger HTML5 applications...

17 Popularity zdroj:

18 Popularity zdroj:

19 JAVA Type systems, threads,...

20 Type system strongly typed language classes primitive types (int, boolean, char,...) "everything" is in a class no global variables, functions,... static methods and fields can seen as global elements

21 Test public class InitTest { static int i = 1; { i+=2; }; static { i++; }; public static void main(string argv[]) { System.out.println(i); System.out.println(new InitTest().i); }; }; The program prints out: a) 2 4 b) 1 3 c) 3 5 d) 4 4 e) cannot be compiled

22 Solution correct is a) 2 4 {... } in the class body initializer executed when an instance is created used for initialization of anonymous inner classes static {... } static initializer executed during class loading to VM can access only static elements of the class

23 Test 2 public class InitTest { static int i = 1; { i+=2; }; public InitTest() { System.out.println(i++); } static { i++; }; public static void main(string argv[]) { System.out.println(i); System.out.println(new InitTest().i); }; }; Results: a) b) c) 2 4 5

24 Solution of test 2 correct us C) the initializer is executed before execution of a constructor first, a superclass is initialized initializers and constructors

25 Exceptions and initializers initializers can throw only exceptions that are defined in constructors there must be at least one constructor initializers of anonymous inner classes can throw any exceptions the class is instantiated just once no problem to catch/declare the exceptions

26 Static initializers have to terminate without an exception otherwise cannot be compiled run in the order as in the source file cannot contain return otherwise cannot be compiled

27 Arrays array ~ object variables ~ references can be casted to Object Object a1[] = new Object[10]; Object a2[] = new Object[5]; a1[0]=a2; a2[0]=a1;

28 Threads Java Language Specification defines the "memory model" defines possible behavior of a program (regarding threads) for a given program and execution flow it determines whether the flow is legal volatile class fields always consistent threads can have copied non-volatile fields in own memory because of speed changes of non-volatile field need not be immediately visible to other threads assigning (and reading) primitive types <= 32 bits atomic i.e. long and double are not atomic but volatile long and double are atomic assigning (and reading) references atomic

29 Threads package java.util.concurrent.atomic classes AtomicInteger AtomicIntegerArray AtomicLong they offer atomic access the compareandset() method...

30 Synchronization "monitor" a lock and waiting threads queue associated with each instance a lock and waiting threads queue associated with each class command/modifier synchronized command synchronized (expression) Block expression must evaluate to a reference before the Block is to be executed, the thread must obtain the lock on the instance specified by the expression after the Block is finished, the lock is released

31 Synchronization modifier of a method synchronized in the signature of the method behaves in the same manner like the command synchronized the thread also before execution of the method must obtain the lock on the instance after the method is finished, the lock is released static synchronized methods manipulates the lock associated with the class mutually excluded are only synchronized methods and blocks if a thread has obtained the lock on an instance other threads can use fields of the instance and call non-synchronized methods of the instance

32 wait & notify wait, notify, notifyall defined in java.lang.object void wait() can be called only when the calling thread has obtained the lock on the given instance (i.e. in a synchronized section) or throws the exception IllegalMonitorStateException puts the thread to the queue of waiting threads, and releases the lock on the instance other threads can obtain the lock, i.e. enter synchronized sections

33 wait & notify the thread is in the queue of waiting threads until the notify or notifyall method is called void notify() "wakes up" a thread from the queue (if the queue is not empty) can be called only from synchronized sections jinak výjimka IllegalMonitorStateException the waked up thread continues after it obtains the lock (i.e. after the tread, which holded the lock (and called notify) leaves the synchronized section) void notifyall() "wakes up" all threads from the queue the threads can continue after they obtain the lock

34 wait & notify three wait methods void wait() void wait(int milis) void wait(int milis, int nanos) threads stay in the queue till waked up or the given time has elapsed waiting in the wait() can interrupted (the method interrupt()) the exception InterruptedException is thrown wait, notify, and notifyall are final the method sleep() does not releases the lock

35 Synchronization java.util.concurrent semaphores locks synchronized queues atomic operations...

36 Visibility in classes is it possible to change element's visibility in children? e.g. class A { public void foo() {} } class B extends { private void foo() {} } visibility cannot be restricted but can be increased why if it would be possible class A { public void foo() {} } class B extends { private void foo() {} } then the following code would be possible A a = new B(); a.foo();

37 Changes in the language Java 5 static import auto-boxing and auto-unboxing new for cycle generics enum methods with variable number of parameters (printf) annotations (metadata)

38 Java 7, 8 Java 6 changes in libraries only Java small changes in the language Java 8 probably summer 2013 bigger changes in the language Java 8 will be revolution, not evolution Mark Reinhold, konference JavaOne, říjen 2011 but September 2012 plans to include modules dropped

39 Java 7 changes changes in syntax support for dynamic languages (a new instruction in bytecode) changes in NIO Nimbus (Swing LaF) new version of JDBC...

40 Java 7 changes in syntax expressing constants binary constants 0b underscores in numerical literals 1_000_000 String type in the switch String month;... switch (month) { case "January": case "February":... }

41 Java 7 changes in syntax operator <> simplified instantiation of generics type in <> is automatically inferred ex. List<String> list = new ArrayList<>(); List<List<String>> list = new ArrayList<>(); List<List<List<String>>> list = new ArrayList<>(); Map<String, Collection<String>> map = new LinkedHashMap<>(); question Why <> is necessary? I.e. why is not sufficient List<String> list = new ArrayList();

42 Java 7 changes in syntax the interface AutoClosable and extended try ex: class Foo implements AutoClosable {... public void close() { } } try ( Foo f1 = new Foo(); Foo f2 = new Foo() ) {... } catch ( ) {... } finaly {... } at the end of try (normally or by an exception), close() is always called on all the objects in the try declaration called in the reverse order than declared

43 Java 7 changes in syntax multi- catch for several exceptions ex: try {... } catch (Exception1 Exception2 ex) {... } better type control during re-throw

44 Is the following code correct private void foo(int i) throws Ex1, Ex2 { try { if (i < 0) { throw new Ex1(); } else { throw new Ex1(); } } catch (Exception ex) { throw ex; } } in Java 7 yes in Java 6 no the compiler prints out an error here

45 What will be in 8 Lambda expressions (Closures, anonymous methods) examples (int x, int y) -> x + y (x, y) -> x - y () -> 42 (String s) -> System.out.println(s) x -> 2 * x c -> { int s = c.size(); c.clear(); return s; }

46 Lambda usage interface Consumer<T> { void accept(t t); } current code pointlist.foreach(new Consumer() { public void accept(point p) { p.move(p.y, p.x); } }); with lambda expressions pointlist.foreach(p -> p.move(p.y, p.x));

47 Lambda a lambda expression is an instance of a functional interface an interface with a single method public interface Runnable { void run(); } public interface ActionListener { void actionperformed(actionevent e); } public interface Comparator<T> { int compare(t o1, T o2); boolean equals(object obj); } a lambda expression does not contain information about which functional interface it is implementing lambda expressions are objects Runnable r = () -> {}; Object o = r;

48 What will not be in 8 Modules