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Advanced Programming Rabie A. Ramadan [email protected] 5 A single threaded program class ABC { …. public void main(..) { … .. } begin body end } 2 A Multithreaded Program Main Thread start Thread A start start Thread B Thread C Threads may switch or exchange data/results 3 Single and Multithreaded Processes threads are light-weight processes within a process Single-threaded Process Multiplethreaded Process Threads of Execution Multiple instruction stream Single instruction stream Common Address Space 4 Multithreaded Server: For Serving Multiple Clients Concurrently Server Process Client 1 Process Server Threads Internet Client 2 Process 5 Web/Internet Applications: Serving Many Users Simultaneously PC client Internet Server Local Area Network PDA 6 Multithreaded Applications Modern Applications need Threads (ex1): Editing and Printing documents in background. Printing Thread Editing Thread Multithreaded/Parallel File Copy reader() { - - - - - - - - lock(buff[i]); read(src,buff[i]); unlock(buff[i]); - - - - - - - - } buff[0] buff[1] writer() { - - - - - - - - - lock(buff[i]); write(src,buff[i]); unlock(buff[i]); - - - - - - - - - } Cooperative Parallel Synchronized Threads 8 8 What are Threads? A piece of code that run in concurrent with other threads. Each thread is a statically ordered sequence of instructions. Threads are being extensively used to express concurrency on both single and multiprocessors machines. Programming a task having multiple threads of control – Multithreading or Multithreaded Programming. 9 Java Threads Java has built in thread support for Multithreading • • • Synchronization Thread Scheduling Inter-Thread Communication: • currentThread • yield • sleep • resume start run stop setPriority getPriority suspend Java Garbage Collector is a low-priority thread. 10 Threading Mechanisms... Create a class that extends the Thread class Create a class that implements the Runnable interface Thread MyThread (objects are threads) [a] Runnable Thread MyClass (objects with run() body) [b] 11 1st method: Extending Thread class Create a class by extending Thread class and override run() method: class MyThread extends Thread { public void run() { // thread body of execution } } Create a thread: MyThread thr1 = new MyThread(); Start Execution of threads: thr1.start(); Create and Execute: new MyThread().start(); 12 An example class MyThread extends Thread { public void run() { System.out.println(" this thread is running ... "); } } class ThreadEx1 { public static void main(String [] args ) { MyThread t = new MyThread(); t.start(); } } 13 Creating a Task and Thread Warning: old way(s), new ways First, if you have a thread object, you can call start() on that object • Makes it available to be run • When it’s time to run it, Thread’s run() is called So, create a thread using “old” (not good) way We won’t do this! Not good design • Write class that extends Thread, e.g. MyThread • Define your own run() • Create a MyThread object and call start() on it 2nd method: Threads by implementing Runnable interface Create a class that implements the interface Runnable and override run() method: class MyThread implements Runnable { ..... public void run() { // thread body of execution } } Creating Object: MyThread myObject = new MyThread(); Creating Thread Object: Thread thr1 = new Thread( myObject ); Start Execution: thr1.start(); 15 An example class MyThread implements Runnable { public void run() { System.out.println(" this thread is running ... "); } } class ThreadEx2 { public static void main(String [] args ) { Thread t = new Thread(new MyThread()); t.start(); } } 16 Life Cycle of Thread new start() I/O completed ready Time expired/ interrupted notify() sleeping waiting resume() blocked dispatch sleep() wait() suspend() running Block on I/O completion stop() dead 17 A Program with Three Java Threads Write a program that creates 3 threads 18 Three threads example class A extends Thread { public void run() { for(int i=1;i<=5;i++) { System.out.println("\t From ThreadA: i= "+i); } System.out.println("Exit from A"); } } class B extends Thread { public void run() { for(int j=1;j<=5;j++) { System.out.println("\t From ThreadB: j= "+j); } System.out.println("Exit from B"); } } 19 class C extends Thread { public void run() { for(int k=1;k<=5;k++) { System.out.println("\t From ThreadC: k= "+k); } System.out.println("Exit from C"); } } class ThreadTest { public static void main(String args[]) { new A().start(); new B().start(); new C().start(); } What is the expected output ? } 20 Run 1 threads [1:76] java ThreadTest From ThreadA: i= 1 From ThreadA: i= 2 From ThreadA: i= 3 From ThreadA: i= 4 From ThreadA: i= 5 Exit from A From ThreadC: k= 1 From ThreadC: k= 2 From ThreadC: k= 3 From ThreadC: k= 4 From ThreadC: k= 5 Exit from C From ThreadB: j= 1 From ThreadB: j= 2 From ThreadB: j= 3 From ThreadB: j= 4 From ThreadB: j= 5 Exit from B 21 Run2 threads [1:77] java ThreadTest From ThreadA: i= 1 From ThreadA: i= 2 From ThreadA: i= 3 From ThreadA: i= 4 From ThreadA: i= 5 From ThreadC: k= 1 From ThreadC: k= 2 From ThreadC: k= 3 From ThreadC: k= 4 From ThreadC: k= 5 Exit from C From ThreadB: j= 1 From ThreadB: j= 2 From ThreadB: j= 3 From ThreadB: j= 4 From ThreadB: j= 5 Exit from B Exit from A 22 Thread Priority In Java, each thread is assigned priority, which affects the order in which it is scheduled for running. The threads so far had same default priority (NORM_PRIORITY) and they are served using FCFS policy. • Java allows users to change priority: • ThreadName.setPriority(intNumber) • • • MIN_PRIORITY = 1 NORM_PRIORITY=5 MAX_PRIORITY=10 23 Accessing Shared Resources Applications Access to Shared Resources need to be coordinated. • Printer (two person jobs cannot be printed at the • • same time) Simultaneous operations on your bank account. Can the following operations be done at the same time on the same account? • Deposit() • Withdraw() • Enquire() 24 Online Bank: Serving Many Customers and Operations PC client Internet Bank Server Local Area Network Bank Database PDA 25 26 Shared Resources If one thread tries to read the data and other thread tries to update the same data, it leads to inconsistent state. This can be prevented by synchronising access to the data. Use “Synchronized” method: • public synchronized void update() •{ •} •… 27 the driver: 3rd Threads sharing the same object class InternetBankingSystem { public static void main(String [] args ) { Account accountObject = new Account (); Thread t1 = new Thread(new MyThread(accountObject)); Thread t2 = new Thread(new YourThread(accountObject)); Thread t3 = new Thread(new HerThread(accountObject)); t1.start(); t2.start(); t3.start(); // DO some other operation } // end main() } 28 Shared account object between 3 threads class MyThread implements Runnable { Account account; public MyThread (Account s) { account = s;} public void run() { account.deposit(); } } // end class MyThread class YourThread implements Runnable { Account account; public YourThread (Account s) { account = s;} public void run() { account.withdraw(); } } // end class YourThread class HerThread implements Runnable { Account account; public HerThread (Account s) { account = s; } public void run() {account.enquire(); } } // end class HerThread account (shared object) 29 Monitor (shared object access): serializes operation on shared object class Account { // the 'monitor' int balance; // if 'synchronized' is removed, the outcome is unpredictable public synchronized void deposit( ) { // METHOD BODY : balance += deposit_amount; } public synchronized void withdraw( ) { // METHOD BODY: balance -= deposit_amount; } public synchronized void enquire( ) { // METHOD BODY: display balance. } } 30 Thread concurrency/operation models The master/worker model The peer model A thread pipeline 31 The master/worker model Program Resources Workers taskX Files Database s Master taskY Input (Stream) main ( ) Disks taskZ Special Devices 32 The peer model Program Input Resources Workers taskX Files Database s taskY Disks taskZ Special Devices 33 A thread pipeline Program Filter Threads Stage 1 Stage 2 Stage 3 Input (Stream) Resources Files Files Files Database s Disks Database s Disks Database s Disks Special Devices Special Devices Special Devices 34 Java Serialization 35 So you want to save your data… Common problem: • You’ve built a large, complex object • Spam/Normal statistics tables • Game state • Database of student records • Etc… • Want to store on disk and retrieve later • Or: want to send over network to another Java process In general: want your objects to be persistent 36 Answer 1 You’ve got file I/O nailed, so… Write a set of methods for saving/loading each class that you care about public class MyClass { public void saveYourself(Writer o) throws IOException { … } public static MyClass loadYourself(Reader r) throws IOException { … } } 37 Coolnesses of Approach 1 Can produce arbitrary file formats Know exactly what you want to store and get back/don’t store extraneous stuff Can build file formats to interface w/ other codes/programs • XML • Tab-delimited/spreadsheet • etc. If your classes are nicely hierarchical, makes saving/loading simple 38 Saving/Loading Recursive Data Structs public interface Saveable { // implemented by many classes public void saveYourself(Writer w) throws IOException; // should also have this // public static Object loadYourself(Reader r) // throws IOException; // but you can’t put a static method in an interface in Java } 39 Painfulnesses of Approach 1 This is called recursive descent parsing (and formatting) If all you want to do is store/retrieve data, do you really need to go to all of that effort? Fortunately, no. Java provides a shortcut that takes a lot of the work out. 40 Approach 2: Using Databases Most Client-Server applications use a RDBMS as their data store while using an object-oriented programming language for development Objects must be mapped to tables in the database and vice versa Applications generally require the use of SQL statements embedded in another programming language “Impedance mismatch” – the data you want to save might not be well structured to save in database 41 Approach 3: Enter Serialization... Serialization is the process of transforming an in-memory object to a byte stream. Deserialization is the inverse process of reconstructing an object from a byte stream to the same state in which the object was previously serialized. “Serializing out” and “serializing in” are also used. 42 Serialization basics The requirements for serialization are straightforward: • Only class instances rather than primitive types can be • • serialized. For an object to be serializable, its class or some ancestor must implement the empty Serializable interface. An empty interface is called a marker interface. 43 Serialization basics The syntax for serialization is straightforward: • An object is serialized by writing it to an ObjectOutputStream. • An object is deserialized by reading it from an ObjectInputStream. 44 Serialization code FileOutputStream out = new FileOutputStream( “save.ser” ); ObjectOutputStream oos = new ObjectOutputStream( out ); oos.writeObject( new Date() ); oos.close(); 45 Deserialization code FileInputStream in = new FileInputStream( “save.ser” ); ObjectInputStream ois = new ObjectInputStream( in ); Date d = (Date) ois.readObject(); ois.close(); 46 Things that you don’t want to save Sometimes, you want to explicitly not store some non-static data • Computed values that are cached simply for convenience/speed • Passwords or other “secret” data that shouldn’t be written to disk Java provides the “transient” keyword. transient foo==don’t save foo public class MyClass implements Serializable { private int _primaryVal=3; // is serialized private transient int _cachedVal=_primaryVal*2; // _cachedVal is not serialized } 47 Graphs Serialization works by examining the variables of an object and writing primitives datatypes like numbers and characters to a byte stream. It also caters to the situation where an object is inside another object. If an object has a reference to an object which has a reference to another object, they are all saved together. The set of all objects referenced is called a graph of objects and object serialization converts entire graphs to byte form. Graphs Gotchas: #1 -- Efficiency For tables , it is not necessarily efficient, and may even be wrong By default, Java will store the entire internal _table, including all of its null entries! Now you’re wasting space/time to load/save all those empty cells Plus, the hashCode()s of the keys may not be the same after deserialziation -- should explicitly rehash them to check. Gotchas: #2 -- Backward compatibility Suppose that you have two versions of class Foo: Foo v. 1.0 and Foo v. 1.1 The public and protected members of 1.0 and 1.1 are the same; the semantics of both are the same So Foo 1.0 and 1.1 should behave the same and be interchangable BUT... The private fields and implementation of 1.0 and 1.1 are different What happens if you serialize with a 1.0 object and deserialize with a 1.1? Or vice versa? Backward compat, cont’d. Issue is that in code, only changes to the public or protected interfaces matter With serialization, all of a sudden, the private data members (and methods) count too Have to be very careful to not muck up internals in a way that’s inconsistent with previous versions E.g., changing the meaning, but not name of some data field Backward compat, cont’d Example: // version 1.0 public class MyClass { MyClass(int arg) { _dat=arg*2; } private int _dat; } // version 1.1 public class MyClass { MyClass(int arg) { _dat=arg*3; } // NO-NO! private int _dat; } Backward compat, cont’d: Java helps as much as it can Java tracks a “version number” of a class that changes when the class changes “substantially” • Fields changed to/from static or transient • Field or method names changed • Data types change • Class moves up or down in the class hierarchy Trying to deserialize a class of a different version than the one currently in memory throws InvalidClassException Java Reflection 55 What is Reflection Reflection: the process by which a program can observe and modify its own structure and behavior at runtime. Based on RTTI (Run-Time Type Identification): • • RTTI: allows programs to discover at runtime and use at runtime types that were not known at their compile time Non-RTTI / Traditional approaches: • • assume all types are known at compile time Polymorphism in OO languages: is a particular case of very limited RTTI Kinds of tasks specific to Reflection Inspection: analyzing objects and types to gather information about their definition and behavior. • • Find the run-time type information of an object Find information about a type (supertypes, interfaces, members) • Dynamic type discovery Manipulation: uses the information gained through inspection to change the structure/behavior: • • • create new instances of new types discovered at runtime dynamically invoke discovered methods • Late binding: the types and methods used by a program are not known at compile-time The most one could imagine to do in a reflective language: restructure types and objects on the fly ! How is Reflection implemented? Reflective capabilities need special support in language and compiler ! • • Java: java.lang.reflection .NET: System.Reflection Reflection case study: Reflection in Java Class java.lang.reflect.Class • It is the entry point for all of the Reflection API • For each new class in a program a “Class” object is created. • Provides methods to examine the runtime properties of the object including its members and type information. • Provides the ability to create new objects of this type. The Reflection Logical Hierarchy in Java Object compiled class file Class Field Method Constructor Member Retrieving a Class object Object.getClass(): If an instance of an object is available, then the simplest way to get its Class is to invoke Object.getClass(). Class c = "foo".getClass(); .class: 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.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 cString = Class.forName("java.lang.String;"); Inspecting a Class After we obtain a Class object myClass, we can: Get the class name String s = myClass.getName() ; Get the class modifiers int m = myClass.getModifiers() ; bool isPublic = Modifier.isPublic(m) ; bool isAbstract = Modifier.isAbstract(m) ; bool isFinal = Modifier.isFinal(m) ; Test if it is an interface bool isInterface = myClass.isInterface() ; Get the interfaces implemented by a class Class [] itfs = myClass.getInterfaces() ; Get the superclass Class super = myClass.getSuperClass() ; Some ways to do introspection java.lang.Class • Class.getMethods () // returns array of method objects • Class.getConstructor (Class[ ] parameterTypes) • returns the constructor with those parameters java.lang.reflect.Array • Array.NewInstance (Class componentType, int length) java.lang.reflect.Field java.lang.reflect.Method All of the above require the existence of run-time objects that describe methods and classes 63 Beans In Java 64 What is a Bean? A Java Bean is a reusable software component that works with Java. More specifically: a Java Bean is a reusable software component that can be visually manipulated in builder tools. 65 Reusable Software Components Designed to apply the power and benefit of reusable, interchangeable parts from other industries to the field of software construction. Reusable software components can be simple like familiar push buttons, text fields list boxes, scrollbars, dialogs 66 Beans, Widgets, Controls, and Components If you come from a Windows background, you probably think in terms of visual controls, possibly Visual Basic Extensions (VBXs) or OLE Controls (OCXs) and now Active X Controls. If you're more accustomed to environments like X Windows, you probably think in terms of toolkits or widgets. 67 Beans or Class Libraries What is the difference between a Java Bean and an instance of a normal Java class? Beans from typical Java classes is introspection. Tools that recognize predefined patterns in method signatures and class definitions can "look inside" a Bean to determine its properties and behavior. Method signatures within Beans must follow a certain pattern in order for introspection tools to recognize how Beans can be manipulated, both at design time, and run time. 68 Basic Bean Concepts Beans share certain common defining features. • • • • • Support for introspection allowing a builder tool to analyze how a bean works. Support for customization allowing a user to alter the appearance and behavior of a bean. Support for events allowing beans to fire events, and informing builder tools about both the events they can fire and the events they can handle. Support for properties allowing beans to be manipulated programmatically, as well as to support the customization mentioned above. Support for persistence allowing beans that have been customized in an application builder to have their state saved and restored. 69 JavaBean Rules A JavaBean must have a public, no-argument constructor (a default constructor). The JavaBean class attributes must be accessed via accessor and mutator methods that follow a standard naming convention (getXxxx and setXxxx, isXxxx for boolean attributes. This allows frameworks to automate operations on attribute values. The JavaBean class should be serializable. This allows Java applications and frameworks to save, store, and restore the JavaBean’s state. 70 Writing a Simple JavaBean 71 Simple Bean Test 72 The Java™ Platform Java Technology Enabled Devices Java Technology Enabled Desktop Umesh Bellur Workgroup Server High-End Server The JavaTM Platform Java 2 Platform Micro Edition (J2METM) Optional Packages Optional Packages Java 2 Enterprise Edition (J2EE) Java 2 Standard Edition (J2SE) Personal Basis Profile Personal Profile Foundation Profile CDC JVM MIDP CLDC KVM * Under development in JCP Java Card APIs CardVM J2EE 1.4 APIs and Technologies J2SE (improved) JAX-RPC (new) Web Service for J2EE J2EE Management J2EE Deployment JMX 1.1 JMS 1.1 JTA 1.0 Servlet 2.4 JSP 2.0 EJB 2.1 JAXR Connector 1.5 JACC JAXP 1.2 JavaMail 1.3 JAF 1.0 Java EE 5 and 6 JAX-WS 2.0 & JSR 181 Java Persistence EJB 3.0 JAXB 2.0 JavaSever Faces 1.2 – new to Platform JSP 2.1 – Unification w/ JSF 1.2 StAX – Pull Parser – new to Platform ‘Enterprise’ in J2EE ‘Programming in the large’ and ‘enterprise computing’ : differ from small-scale and academic computing • Lots of users and the application has an ‘extended life’ • Deployed on heterogeneous computing environments • Needs to have versioning mechanism • Developed by a team of developers over long time • Maintainability, Flexibility, Reusability are major issues Difficulties • Needs to support transactions, resource-pooling, security, threading, persistence, life-cycle management etc… • System programming at the expense of business logic • Developers have to become specialists • Proprietary APIs result in non-portable code Need for special solutions to manage complexity • Proprietary frameworks and middleware • Need for standard APIs for enterprise computing • Multi-tiered architecture in enterprise applications J2EE Platform Architecture Component • A component is an application level software unit. • The J2EE platform supports the following types of components : • Applets, • Application clients, • Web components and • Enterprise Java Beans (EJBs) Container • All J2EE components depend on the runtime support of a system-level entity called a container. • Containers provide components with services such as • life cycle management, • security, • deployment • threading Servlet & JSP (JavaServer Pages) What is a Servlet? Java™ objects which extend the functionality of a HTTP server Dynamic contents generation Better alternative to CGI, ISAPI, etc. • Efficient • Platform and server independent • Session management • Java-based Servlet vs. CGI Request Request CGI1 CGI1 Request RequestCGI2 CGI2 Request Request CGI1 CGI1 Request Request Servlet1 Servlet1 Request Request Servlet2 Servlet2 Request Servlet1 Child Childfor forCGI1 CGI1 CGI CGI Based Based Webserver Webserver Child Childfor forCGI2 CGI2 Child Childfor forCGI1 CGI1 Servlet Servlet Based BasedWebserver Webserver JVM JVM Servlet1 Servlet1 Servlet2 Servlet2 What is JSP Technology? Enables separation of business logic from presentation • Presentation is in the form of HTML or XML • Business logic is implemented as Java Beans or • custom tags Better maintainability, reusability Extensible via custom tags Builds on Servlet technology