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Introspection

Introspection. the process by which a builder tool finds out which properties, methods, and events a bean supports. searching for classes and methods that follow certain naming conventions by querying the BeanInfo of a class

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Introspection

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  1. Introspection • the process by which a builder tool finds out which properties, methods, and events a bean supports. • searching for classes and methods that follow certain naming conventions • by querying the BeanInfo of a class • BeanInfo is an interface whose methods allow the interrogation of a beans properties, methods and events • associated class that implements the BeanInfo interface • Homework : read about the BeanInfo interface • [Not part of your exam] Good Java Programming

  2. JVM Basics • Use of java compiler (javac) to generate a binary format • Stored in files with a .class extension. • The bytecode is machine code for a special kind of computer • JVM • The binary class format is defined by the JVM • This is called a virtual machine • because it's been designed for implementation in software, rather than hardware. • Every JVM used to run Java platform applications is built around an implementation of this machine. Good Java Programming

  3. JIT: Just in Time • EarlyJVMs were basically interpreters for the virtual machine bytecode. • Difference between an Interpreter and Compiler? • Homework • Hint: Interpreting code is much slower than executing native code • just-in-time (JIT) translation. The JIT technique compiles Java bytecode to native code before executing it for the first time • giving much better performance for repeated executions. • Current JVMs use adaptive techniques to monitor program execution and selectively optimize heavily used code. • What is the 90/10 rule? • What is Java HotSpot? Homework • [Not included in the exam] • How can the HotSpot be “warmed”? Good Java Programming

  4. Detour: linking in C/C++ Lab1.exe: lab1.o g++ lab1.o -o lab1.exe lab1.o: lab1.cpp g++ -c lab1.cpp clean: rm *.o Which line represents the linking step? Good Java Programming

  5. Linking: Java vs C/C++ • C/C++ require linking step • compile + link: to get an executable • This linking process merges code from separately compiled source files, along with shared library code, to form an executable program. • In Java, the classes generated by the compiler generally remain just as they are until they're loaded into a JVM. • A JAR is just a container for the class files. Good Java Programming

  6. Loading a Java class • java -verbose:class AnyClassName • Example: java –verbose:class Driver • prints a trace of the class loading process • Homework • run this command on bingsuns for any Java class generated in the assignment so far • java –verbose:class Driver • NOT included in the Exam • [Not included in the exam] Good Java Programming

  7. Java Reflection • Provides user code access to internal information for classes loaded into the JVM • Allows design of user code that works with classes selected during execution, not in the source code. • Great tool for building flexible applications. • High performance penalty • Reflection is used by JavaBeans Introspector • Package: java.lang.reflect Good Java Programming

  8. Getting Started with Reflection • Given the following code, write code in any programming language to invoke a method, setSenderName, on an object of type stored in the string “className” String className = “ChatMessage”; Good Java Programming

  9. Class in Java • The Class object provides hooks for reflection. It has metadata information on • interfaces implemented by the class. • details of the constructors • fields • methods defined by the class. Good Java Programming

  10. Getting Started with Reflection • Get a hold of java.lang.Class instance Class class = ChatMessage.class; • The class is automatically loaded with this statement Good Java Programming

  11. Reading a Class at run-time • Suppose just the name of the class is available as a string Class javaClass = null; String nameOfClass = “ChatMessage”; try { javaClass = Class.forName(nameOfClass); } catch (ClassNotFoundException ex) { // take care of the exception } Good Java Programming

  12. Get a list of methods in a Class • getDeclaredMethods(Class className); • examples taken from java.sun.com try { Class c = Class.forName(“ChatMessage”); Method m[] = c.getDeclaredMethods(); for (int i = 0; i < m.length; i++) System.out.println(m[i].toString()); } } catch (Throwable e) { System.err.println(e); // take care of the exception } Good Java Programming

  13. Parameters of a Method • getParameterTypes() on Method Class paramList[] = m.getParameterTypes(); Good Java Programming

  14. Fields • getDeclaredFields() on Class try { Class cls = Class.forName(“ChatMessage"); Field fieldlist[] = cls.getDeclaredFields(); for (int i = 0; i < fieldlist.length; i++) { Field fld = fieldlist[i]; System.out.println("name = " + fld.getName()); System.out.println("type = " + fld.getType()); } } Good Java Programming

  15. Invoking a method // need to get the Class information Object obj = someClass.newInstance(); // need to get the correct “Method” // need to get the correct parameters Object result = meth.invoke(obj, params); Good Java Programming

  16. Invoking a method String clsName = “ChatMessage”; Class cls = Class.forName(clsName); Class[] signature = new Class[1]; signature[0] = Integer.class; Method meth = cls.getMethod(methodName, signature); Object obj = cls.newInstance(); Object[] params = new Object[1]; params[0] = new Integer(7); Object result = meth.invoke(obj, params); Good Java Programming

  17. Invoking a method: new version // to avoid the warning that says using the –Xlint flag String clsName = “ChatMessage”; Class<?> cls = Class.forName(clsName); Class<?>[] signature = new Class<?>[1]; signature[0] = Integer.class; Method meth = cls.getMethod(methodName, signature); Object obj = cls.newInstance(); Object[] params = new Object[1]; params[0] = new Integer(7); Object result = meth.invoke(obj, params); Good Java Programming

  18. Invoking a method Given the following String clsName = “ServerDetails”; use reflection to invoke a method that is equivalent to: ServerDetails sd = new ServerDetails(); sd.setHostName(“bingsuns”); Good Java Programming

  19. Java Reflection: calling a constructor Class[] types = new Class[] { String.class, String.class }; Constructor cons = ChatMessage.class.getConstructor(types); Object[] args = new Object[] { "a", "b" }; ChatMessage ts = (ChatMessage)cons.newInstance(args); Good Java Programming

  20. Details of a Fields // given objectInstance, an instance of “ChatMessage” Class chatMessageClass= objectInstance.getClass(); Field[] fieldList = chatMessageClass.getFields(); for (int j=0; j<fieldList.length; j++) { Class fieldClass = fieldList[j].getType(); String fieldName = fieldList[j].getName(); Object fieldObject = fieldList[j].get(objectInstance); } Good Java Programming

  21. Classwork • Given the following information about a class • invoke the following: • one setX method (takes a string and returns void) • on getX method (takes void and returns string) • for every data-member “x” in the class, there is a setX and getX method • You are given the following: • String className = “TestObject”; Good Java Programming

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