1. How Java Programs Work MIS 3023 Business Programming Concepts II The University of Tulsa Professor: Akhilesh Bajaj All slides in this presentation ©Akhilesh Bajaj, 2008. All rights reserved

2. Objectives Understand how Programs are Executed on a Computer Understand how a Java TM program works Let’s get started!

3. How Programs Work on a Computer Levels of Computer Programming Languages: -First Generation Language: Machine Code 1101110100111100 1001101110101010 -Second Generation Language: Assembler Load X into register a -Third Generation Language: Pascal, Fortran, COBOL, C, C++, Java X = 7; -Fourth Generation Language: Visual Basic, Oracle Developer Use a mouse & widgets to create a GUI that executes interface events

4. How Programs Work on a Computer The CPU (Central Processing Unit) executes the instruction set (machine code) Every program needs to be compiled and ultimately fed to the CPU in the CPU’s language (the instruction set) Different CPUs have different instruction sets (e.g., X86 versus Motorola) The quality of the instruction set can play a bigger role in the performance than other numbers like clock speed. A compiler is a computer program that translates the source code into either machine code or an intermediate language.

5. How Programs Work on a Computer Once the source code has been compiled into an executable (like a.exe or.bat file on the PC) it can be executed by the CPU. First, the executable is loaded into main memory (RAM). One of the registers in the CPU is the PC (program counter). This has the main memory address of the next instruction to be executed. The CPU fetches the first instruction, executes it, and the PC is updated with the address of the next instruction (which is usually the next line, but may be a jump to another procedure).

6. How Programs Work on a Computer In addition to instructions, the program also has data (variables, etc) How are these data stored usually? The data locations (referenced by variable names in the source program) are also in main memory (RAM). The CPU needs to be able to fetch these data locations, load them into registers and then perform calculations with them. At the hardware level, the CPU is nothing but a a large logical circuit, that, if fed a certain logical input, generates a predetermined output. The set of all possible inputs is the instruction set of the CPU. The CPU does not “understand” the instructions we feed it in any sense. It simply responds mechanistically to inputs and produces the same output each time for a given input.

7. How Programs Work on a Computer Figure From our Text Book: Shows the Fetch-and-Execute Cycle

8. How Programs Work on a Computer For every programming language, the compiler is a program that has a “dictionary” that translates every possible source code statement into a corresponding set of instructions at the lower level (machine code or some intermediate language). Every program executes as a process. The task manager in Windows shows us the processes that are running on our machine. The CPU can also be interrupted by devices like the keyboard and secondary storage (hard disk). This allows multi-programming: E.g., One process is executing, but it needs to read a file from the hard disk. The CPU sends a request to the hard disk manager, and then puts this process in its process queue, takes the next process and executes it. Once the first process’s data is available, the CPU gets the interrupt, takes that first process back in, and continues to execute it.

9. How Java Programs Work One of the strong points of Java is that it is compile once-run anywhere. How can we do this, if different CPUs (say the X86 CPU in PCs versus the Motorola CPU in Macintosh versus a SUN SPARC CPU) have different instruction sets? Indirection: Create an intermediate layer of instructions using a 2-stage compiler. The first stage takes the.java file and converts it into a.class file of the same name. This is done using the javac program that comes as part of the java SDK. We used this in our first lab to create HelloWorld.class. The code in the.class file is called Java bytecode.

10. How Java Programs Work This Java bytecode is then run on the JVM (Java Virtual Machine). The JVM is an interpreter, that goes through each line in the bytecode and translates it into the set of instructions for that particular CPU. Clearly different CPUs need different JVMs. Once the JVM is built for that CPU, it will execute any.class file. So, the same.class files will work on any machine that has a JVM. Advantages: -Programmers do not have to distribute source code, only.class files -Programmers do not have to rewrite code if we switch from say PC to apple machines (reuse same.class files) -The JVM can act as a security buffer when code is downloaded from the network (prevents.class files from doing bad things)

11. How Java Programs Work The JVM is called by the command java that we used in our first lab. So >java HelloWorld executed the.class file and printed out “ Hello World ”. In order to use Java, we need to make sure our CLASSPATH environment variable includes the /lib subdirectory that is part of the SDK. This subdirectory contains all the extensive library classes that we can use. One of the major benefits of Java is that it has an extensive set of these classes whose code can be reused. The CLASSPATH should also have the. in it, so we can also use classes we created that are in the current working directory Finally, the PATH environment variable should have the /bin subdirectory in the SDK, since the java and javac programs are found there.

12. We learnt how programs are executed We learnt how the java environment works Supporting material is in sections 1.1-1.3 in our online textbook. CONCLUSION